Method and apparatus for controlling a ventilator

Abstract

Method and apparatus for controlling a ventilator are described. The invention can be used to control mechanical ventilators as well as respiratory assist devices such as CPAP machines. The apparatus receives input data indicative of patient's oxygen level. A controller determines peep, or CPAP, and F_IO2, on the basis of data indicative of the patient's oxygen level. In an alternative embodiment, the apparatus further receives input data indicative of patient's carbon dioxide levels, respiratory elastance and airway resistance, and barometric pressure. The controller further utilizes the said input data to determine the optimal values of tidal volume and breathing frequency for a next breath of the patient, and uses the respiratory elastance and airway resistance data to determine any necessary adjustments in the I:E ratio. The controller also applies safety rules, detects and corrects artifacts, and generates warning signals when needed.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application

$P_{{\mathrm{aO}}_2}=\frac{-\ln \left[1-{\left(S_{{\mathrm{pO}}_2}\right)}^{0.5}\right]}{0.046}+\mathrm{CP}$

Where P_aCO2 and P_aO2 are arterial partial pressures of CO₂ and O₂ respectively, P_etCO2 is the end-tidal partial pressure of CO₂ measured by the CO₂ sensor, and K₁ is the difference between the arterial partial pressure of CO₂ and the end-tidal partial pressure of CO₂. K₁ can be measured in advance and depending on the patient's conditions, it can be adjusted to set the desired P_aCO2 of the patient. S_pO2 is the arterial hemoglobin oxygen saturation of the patient measured by a pulse oximeter and CP is an added correction factor which is used to correct and shift P_aO2 based on the patient's measured blood pH level. If the patient's blood pH level is in the 7.45-7.55 range, CP is set to zero. Otherwise, CP needs to be adjusted by +3.5 mm Hg per every −0.1 deviation in pH from the above range. After the calculation of P_aCO2 and P_aO2, their values are compared to defined minimum acceptable levels to determine whether there has been any measurement artifact in step 104. If any artifact is detected, the calculated value is discarded and the previous calculated value is resumed. In the next step at 106, if P_aCO2 and/or P_aO2 are not within certain defined ranges, alarms are transmitted to the output ports. In the step that follows at 108, if the calculated P_aCO2 and P_aO2 values are both lower than their minimum threshold limits (which are different from the minimum acceptable values used in step 104), the possibility of pulmonary embolism is assumed, predefined levels of ventilation and breathing frequency are provided, and an alarm is generated in steps 110 and 112, and the program returns to A. However, if the calculated P_aCO2 and P_aO2 values are not found to be simultaneously lower than their minimum threshold levels in 108, then the effect of CO₂ on the required ventilation is calculated in step 114:
V_C=C₁.P_aCO2−C₂

Where V_C is the ratio of alveolar ventilation as the net effect of CO₂ to the resting value of ventilation, C₁ is the sensitivity factor of the controller to CO₂ (e.g., C₁=0.405) and C₂ is a constant (e.g., C₂=14.88).

Next, in step 116, the P_aO2 value is compared to a high threshold limit of 104 mm Hg. If P_aO2 is greater than or equal to this threshold value, the effect of oxygen on ventilation is set to zero in 118, and the next step at 122 is followed. Otherwise, if P_aO2 is found to be less than the threshold value in step 116, then control is passed to step 120 in which the effect of oxygen on the required ventilation is calculated by using the following equation:
V_O=(4.72×10⁻⁹)(104−P_aO2)^4.9

Where V_O is the ratio of alveolar ventilation as the net effect of oxygen to the resting value of ventilation. It is recognized that the above equations are based on the use of a capnograph and a pulse oximeter to measure the carbon dioxide and oxygen levels of the patient respectively. If other measurement techniques are utilized to provide data indicative of said levels, then other alternative equations may be used to determine the required ventilation for the patient, without deviating from the scope and the essential attributes of the invention.

In the next step at 122, the effect of increase in the metabolic rate ratio, MRR, (i.e. rate of metabolism/basal rate of metabolism), on ventilation is calculated by using the following equation:
V_M=0.988(MRR−1)

Where V_M is the ratio of alveolar ventilation as the net effect of increase in the metabolic rate ratio, MRR, to the resting value of ventilation, and MRR is an input to the algorithm. In the next step at 124, total alveolar ventilation for the next breath is calculated:
V_A=(V_A at rest)(V_C+V_O+V_M)

Where V_A is alveolar ventilation in liters/minute and V_A at rest is the alveolar ventilation at rest which is input and stored in the software. In the next step at 126, the physiological dead space of the patient, and the total dead space including that of the equipment are calculated, if not provided in advance, as follows:
V_D=(0.1698V_A/60)+0.1587
V_Dt=V_D+V_ED

In these equations, V_D is the patient's dead space in liters, V_ED is the equipment dead space due to the tubes and connections, and V_Dt is the total dead space. It should be noted that the constant factors in these equations are based on measured experimental values for adults and can therefore be different for individual patients. Also, for other patient populations, they need to be adjusted. For example the constant factor of 0.1587 should change to a much smaller value for infants (e.g., 2.28×10⁻³). In the next step at 128, data indicative of barometric pressure and the patient's airway resistance (or the air viscosity factor in the lungs) and respiratory elastance are read from the input ports. The barometric pressure data which is affected mostly by the altitude, is used as a reference pressure (for the purpose of calibration) in the invention.

In the next step at 130, the optimal frequency for the next breath is computed. This calculation is based on minimization of the respiratory work rate and is done in order to stimulate natural breathing, provide a more comfortable breathing pattern to the patient, and thereby, expedite the weaning process in assisted ventilation. The following equation, which is a modified version of an equation derived in 1950 by Otis et al. to describe the control of breathing frequency in mammals, is used to calculate the optimal breathing frequency in the invention:

$f=\frac{-K^{\prime }{V}_D+\sqrt{{\left(K^{\prime }{V}_D\right)}^2+4K^{\prime }{K}^{″}{\Pi }^2{V}_{AR}{V}_D}}{2K^{″}{\Pi }^2{V}_D}$

where f is the optimum breathing frequency in breaths/second, V_AR is the alveolar ventilation in liters/second and is equal to V_A/60, K′ is the respiratory elastance (reciprocal of respiratory compliance) in cm H₂O/liter and K″ is the airway resistance in cm H₂O/liter/second. Next in step 132, the required minute ventilation and tidal volume are calculated:
V_E=V_A+60fV_Dt
V_T=V_A/60f+V_Dt

Where V_E represents total minute ventilation in liters/minute and V_T is tidal volume in liters. In the next step at 134, additional safety rules are applied. If breathing frequency, f, tidal volume, V_T, or minute ventilation are not within prescribed safe ranges, their values are limited and adjusted.

In the next step 136 which follows, the breathing frequency is compared with an upper limit value F_max. This upper limit frequency is defined as:
F_max=⅕τ

Where τ is the respiratory time constant and is equal to K″/K′. If in step 136, the breathing frequency is found to be higher than F_max, then in the next step at 138, its value is reduced to F_max (in which case V_T is also adjusted according to procedures in steps 132 and 134), and step 140 is followed. Otherwise, if the computed breathing frequency is less than or equal to F_max, it does not need further adjustment and the program is transferred to step 140. In step 140, the expiration time, T_E, is compared to 2.5 times τ. If it is found to be less than 2.5 τ, then step 142 is followed and the I:E ratio (the ratio of the inspiratory time to the expiratory time) is adjusted, so that T_E becomes at least equal to 2.5 τ. Otherwise, if T_E is found to be greater than or equal to 2.5 τ in step 140, it does not need to be adjusted (i.e. the adjustment value is zero) and the program is transferred to step 144. The reason for the adjustments in the breathing frequency and T_E in steps 138 and 142 mentioned above, is to provide sufficient time for exhalation based on the patient's respiratory time constant and to avoid build up of intrinsic positive end-expiratory pressure (PEEPi).

In step 144 that follows, the calculated values for ventilation, breathing frequency, and the adjustment in the I:E ratio for the next breath are provided to the output ports. At this point, if the ventilator is in the pressure control/assist mode, the inspiratory pressure is calculated by using the following equation:
P_m=K′V_T+PEEP

where P_m is the inspiratory pressure in cm H₂O. Thereafter, the control data indicative of P_m is also provided to an output port and the routine is held for the duration of the next breathing cycle. After the delay is passed, the program returns to the beginning of the loop at A.

It should be noted that the major portion of the procedure depicted in FIG. 2 to calculate the optimal breathing frequency and tidal volume of the breaths of a patient and controlling them automatically, has been described in U.S. Pat. No. 4,986,268. In the present invention, the necessary adjustments in the I:E ratio are controlled automatically as described above, and the levels of F_IO2 and PEEP are automatically controlled by another algorithm which is described next.

Referring to FIGS. 3a-3i, there is illustrated a flow chart of a control algorithm which is operated upon by the digital processor. This algorithm is either run by itself, or in an alternative embodiment of the invention, it is run in parallel to the algorithm of FIGS. 2a-2c described above. The purpose of this algorithm is to automatically control the levels of F_IO2 and PEEP provided to the patient on the ventilator and thereby improve the patient's oxygenation. The method depicted in FIGS. 3a-3i can be used for patients on mechanical ventilation or those on respiratory assist devices receiving CPAP treatment. Depending on the type of the ventilatory treatment, the term PEEP in the flow chart is meant to be interchangeable with CPAP.

As is seen, at the start of the flow chart, the desired set point for arterial partial pressure of oxygen of the patient is defined in step 200. This is done on the basis of the patient's conditions and his/her underlying illness. Then in the next step at 202, the initial value of F_IO2 is set and transmitted to the output port.

In step 204 that follows next, the initial value of PEEP is set and transmitted to an output port. The initial value of PEEP can be set by using different options. For certain patient groups such as COPD patients, the initial PEEP can be chosen to be 80% to 85% of the intrinsic PEEP (PEEPi) which needs to be measured in advance. For some other patient groups such as ARDS patients, the initial PEEP setting can be chosen to be 3-4 cm H₂O above the lower inflection pressure point of the inspiratory (or the expiratory) pressure volume curve of the patient. This value can either be calculated by the lung mechanics calculator and PV monitor unit and provided automatically to the digital processor via an input port, or the calculated value of the pressure can be provided manually by the clinician either through one of the input ports or via software. The third option is that the clinician arbitrarily decides an initial setting for PEEP and provides it to the digital processor, preferably via software. After setting the initial PEEP value in 204, the next step in 206 is followed. At this point, a time parameter (e.g., TP) for PEEP adjustment is defined and initially set to zero. The purpose of defining this parameter is to guarantee that PEEP adjustments are done only after a certain time has elapsed since the latest adjustment, thereby giving enough time to an adjustment in PEEP to make an impact on the patient's oxygenation.

In step 208 which follows next, another parameter, AP, for PEEP adjustment is defined. If this parameter is set to zero, then PEEP is controlled manually and only F_IO2 is automatically adjusted. If AP is set to one, then both F_IO2 and PEEP are automatically controlled.

In the next step 210, the threshold values for arterial hemoglobin oxygen saturation, S_pO2, (or alternatively for arterial partial pressure of oxygen) are defined. In a preferred practice of the invention, four threshold values are defined for S_pO2 and they are set at 90%, 93%, 95%, and 97% respectively. However, the threshold values may differ for different patients. They should be defined based on the patient's conditions and the desired levels of oxygenation.

Next, program control passes to step 212 in which a loop indicator (e.g., LI) is defined and is set to 1.5, and the main loop starts at A′.

In the next step in 214, the patient's S_pO2 data is read from one of the input ports, and in step 216, the arterial partial pressure of oxygen is calculated from the S_pO2 data as:

$P_{{\mathrm{aO}}_2}=\frac{-\ln \left[1-{\left(S_{{\mathrm{pO}}_2}\right)}^{0.5}\right]}{0.046}+\mathrm{CP}$

Where P_aO2 is the arterial partial pressure of oxygen, and CP is an added correction factor which is used to shift P_aO2 based on the patient's measured blood pH level. If the patient's blood pH is within 7.45-7.55, then CP is set to zero. Otherwise, for every +0.1 deviation in pH from this range, CP is adjusted by −3.5 mm Hg as was also mentioned in the description of FIG. 2 earlier.

In step 218 that follows next, the calculated partial pressure of oxygen, P_aO2, is compared with a minimum acceptable value. This is done to detect artifacts in the measurement of S_pO2. If the calculated P_aO2 is found to be less than the minimum acceptable value, then control passes to step 220 in which an artifact is assumed and an alarm is generated. Then step 222 is performed in which the S_pO2 data is discarded and the previous value Of P_aO2 in the memory is resumed and step 224 is followed. However, if in 218, the calculated P_aO2 is found to be greater than or equal to the minimum acceptable value, its value is accepted and control passes to step 224.

In step 224, S_pO2 is compared to a minimum safe value, which is the first threshold value defined previously in step 210 (e.g., 90%). If S_pO2 is less than or equal to the minimum safe value, loop B is started in 226 and the loop indicator, LI, is set to 2.5. Then in step 228, F_IO2 is increased stepwise (i.e. in a step-like arrangement) to a high value, F1, (e.g., 60%), and an alarm is generated in 230. Control then passes to loop F at which the procedure of PEEP adjustment begins as will be described later. However, if S_pO2 is found to be higher than the minimum safe value in step 224, control passes to 232 where S_pO2 is compared to a second threshold value (e.g., 93%). If S_pO2 is less than the second threshold value, then steps 234 and 236 are followed in which the loop indicator, LI, is examined and compared to 2. If LI is less than 2, control passes to another loop E which will be described later. If LI is greater than or equal to 2, the next step in 238 is performed in which LI is compared to 3. If LI is less than 3, control passes to loop B (where F_IO2 was set high at F1, e.g., 60%), otherwise, the program transfers to step 240. In this step, LI is compared to 4. If it is less than 4, control passes to loop C; otherwise, the program transfers to loop D (loops C and D will be described later).

Back to step 232, if S_pO2 is found to be higher than or equal to the 2^nd threshold value (e.g., 93%), then steps 242 and 244 are followed in which LI is compared to 2. If it is less than 2, control passes to loop E. Otherwise, in the next step at 246, LI is compared to 3. If less than 3, loop C is defined and started at 248, and LI is set to 3.5. Then in step 250, F_IO2 is set stepwise at a moderately high value, F2 (e.g., 45%), and control transfers to loop F in which the procedure of PEEP adjustment is followed. However, if in step 246, LI is found to be greater than or equal to 3, control passes to step 252 in which LI is compared to 4. If LI is less than 4, then S_pO2 is compared to a third threshold value (e.g., 95%) in step 254. If S_pO2 is less than the third threshold value, control passes to loop C in which F_IO2 was set at a moderately high level, F2 (e.g., 45%). Otherwise, if S_pO2 is found to be higher than or equal to the third threshold value in 254, then the next step in 256 is followed in which loop D is defined and started and LI is set to 4.5. Next in step 258, F_IO2 is set stepwise at a slightly high level, F3 (e.g., 30%), and control passes to loop F.

Back to step 252, if LI is found to be greater than or equal to 4, then S_pO2 is compared to a 4^th threshold value (e.g., 97%) in step 260. If S_pO2 is less than the 4^th threshold value, control passes to loop D in which F_IO2 was set at a slightly high value, F3 (e.g., 30%). Otherwise, if S_pO2 is higher than or equal to the 4^th threshold value in 260, then loop E is started in 262 and LI is set to 1.5. In loop E, a proportional, integral, derivative (PID) control procedure is performed to adjust F_IO2 (PID control is a control technique comprising proportional, integral, and derivative terms). In the next step at 264, using the P_aO2 set point defined in step 200, the proportional, differential, and integral components of error are calculated as follows:
Y₁(k)=P_aO2(set-point)−P_aO2
Y₂(k)=[Y₁(k)−Y₁(k−1)]/T
Y₃(k)=Y₃(k−1)+TY₁(k)

In the above equations, Y₁(k), Y₂(k), and Y₃(k) represent the proportional, differential, and integral components of error in P_aO2 respectively, and T is a sampling interval.

In step 266 that follows, the required F_IO2 is calculated by using the following equations:
E(k)=αY₁(k)+βY₃(k)+γY₂(k)
G(k)=E(k)+0.21

Where E(k) is an error function, α, β, and γ are the PID coefficients, and G(k) is the required F_IO2. In a preferred practice of the invention, T is set to 0.75 seconds, and α, β, and γ are set to 6.45×10⁻⁵, 3.22×10⁻⁵, and 7.29×10⁻⁶ respectively. These parameters were tuned to minimize steady-state oscillations and to keep the overshoot/undershoot in the F_IO2 response of the PID controller below 25% of the total change. It is also recognized that other error correction schemes can be used to determine F_IO2. As long as those schemes reduce the error in the oxygen level of the patient in a similar way as described above, they will be within the scope of the present invention.

In the next step in 268, the calculated value of F_IO2 is compared with a minimum of 0.21 (i.e. 21%). If the F_IO2 value is less than 21%, in step 270 which follows, it is set to a minimum of 21% and control passes to loop F. However, if in 268, F_IO2 is found to be greater than or equal to 21%, control passes to step 272 in which F_IO2 is compared to a maximum allowed value (e.g., 80%). If F_IO2 is less than or equal to the maximum allowed value, the next step in 274 is followed where the calculated value of F_IO2 is sent to the output port and control passes to step 276. In this step F_IO2 is compared to 60%. If it is less than 60%, control passes to loop F. Otherwise, an alarm is generated in 278 and then control transfers to loop F.

Back to step 272, if the calculated value of F_IO2 is found to be higher than the maximum allowed value, it is reduced to the maximum value in step 280, an alarm is generated, and then control transfers to loop F.

Up to the beginning of loop F at step 282, the focus of control is on automatic control of F_IO2. As shown, two different mechanisms are incorporated in the control process of F_IO2 in a preferred practice of the invention. One, a rapid stepwise control scheme which responds instantly to fast declines in S_pO2, and the other, a more finely controlled PID algorithm that provides fine control of F_IO2 in the absence of sharp and hazardous declines in S_pO2. The stepwise controller in a preferred practice of the invention has three loops, each with its defined minimum and maximum S_pO2 threshold levels. These three loops were shown respectively at B, C, and D, and the PID control loop was shown at E in the flow chart of FIG. 3. The controller switches from the PID control to the rapid stepwise algorithm only if rapid declines in S_pO2 are detected. Once in the stepwise mode, the controller continuously checks S_pO2, and if it rises, the controller reduces F_IO2 to minimize the exposure of the patient to high and toxic levels of F_IO2. The controller is designed to correct hypoxemia within seconds and to avoid hyperoxemia. As shown, the controller detects artifacts in the measurement of S_pO2, discards the artifacts, and generates alarms when appropriate. The algorithm also enables clinicians to define the desired oxygenation levels for different patients. This is done by defining an appropriate P_aO2 set point, by setting the threshold values for S_pO2, and by adjusting the correction parameter, CP, in accordance with the measured pH levels in the patient's blood as described above.

After the determination of the required F_IO2, the procedure of adjusting the PEEP value is started at F in step 282. In this step, the ratio of PEEP/F_IO2 is calculated. Then in 284, the control parameter AP, which was defined in step 208, is examined. If it is less than 1, it means that PEEP is not adjusted automatically and it is instead adjusted manually by the operator. In this case, the controller merely watches the PEEP/F_IO2 ratio and generates warning signals, if the ratio is either too low or too high. In step 286, the ratio is compared to a minimum allowed value (e.g., 0.12). If it is less than the minimum value, an alarm is generated in 288 and control passes to I (which will be described later). However, if the PEEP/F_IO2 ratio is found to be equal to or greater than the minimum value in step 286, then the next step in 290 is performed where the ratio is compared to a maximum allowed value (e.g., 0.22). If the ratio is less than or equal to the maximum value, control passes to I. Otherwise, an alarm is generated in step 292 and then control is transferred to I.

Back to step 284, if AP is not less than 1, it means that PEEP should be calculated and automatically adjusted. Therefore, the automatic PEEP adjustment control loop is started next at G at step 294. In the step 296 that follows, the PEEP/F_IO2 ratio is compared to a minimum allowed value (e.g., 0.12). If it is less than the minimum, the procedure at H is started and it is examined how long ago the last adjustment in PEEP was made. In step 300 that follows, the time parameter, TP, is compared to a defined fixed interval, T1, for example 240 seconds. If TP is less than 240 seconds, it means that the last PEEP adjustment was made less than 4 minutes ago. Then the procedure at J is started. Control passes to step 302 in which no change is made in PEEP and the time parameter, TP, is increased by a fixed amount (e.g., 0.75 seconds):
TP_(new)=TP_(old)+0.75

Thereafter, control passes to I. However, if in step 300, it is found that TP is equal to or greater than 240 seconds, it means that the last adjustment in PEEP was made at least 4 minutes ago or longer. Therefore, control passes to step 304. In this step, TP is set back to zero. Then in 306 that follows, PEEP is increased by a fixed amount (e.g., 2 cm H₂O):
PEEP_(new)=PEEP_(old)+2 cm H₂O

Thereafter, control passes to I.

Back to step 296, if the PEEP/F_IO2 ratio is not found to be less than the minimum allowed value, control transfers to step 308. In this step the PEEP/F_IO2 ratio is compared to a maximum allowed value (e.g., 0.22). If the ratio is not less than the maximum value, step 310 is next performed. At this point, the PEEP/F_IO2 ratio is compared to a slightly higher value than the maximum, RG, (e.g., 0.24). If it is not greater than this value, control passes to J. Otherwise; step 312 is performed in which the time parameter, TP, is compared to the fixed interval of 240 seconds. If TP is less than 240 seconds, control passes to J. Otherwise; TP is set back to zero in step 314, and PEEP is reduced by a fixed amount (e.g., 2 cm H₂O) in step 316:
PEEP_(new)=PEEP_(old)−2 cm H₂O

Thereafter, control passes to I. In step 318 at I, the routine is held for a fixed interval (e.g., 0.75 seconds) and then control returns to the beginning of the main loop at A′.

Back to step 308, if the PEEP/F_IO2 ratio is found to be less than the maximum allowed limit (e.g., 0.22), the step 320 is next followed. In this step S_pO2 is compared to a predefined minimum allowed value (e.g., 92%). If it is higher than or at least equal to the predefined minimum value, the PEEP level is not changed and control passes to J. However, if in 320, S_pO2 is found to be less than the predefined minimum value, then control passes to H, where it is determined whether at least 4 minutes have passed since the last PEEP adjustment, and if so, PEEP is increased by a fixed amount (e.g., 2 cm H₂—O) as was shown earlier.

In performing the automatic PEEP adjustments, the PEEP/F_IO2 is kept within a clinically acceptable range. As shown above, if the PEEP/F_IO2 is too low, PEEP is increased by a fixed increment (e.g., 2 cm H₂O). Also, if the PEEP/F_IO2 ratio is within the acceptable range and S_pO2 is low, then PEEP is increased by a fixed increment (e.g., 2 cm H₂O) to improve patient's oxygenation. On the other hand, if the PEEP/F_IO2 ratio increases beyond a maximum defined value, the program reduces PEEP in fixed amounts (e.g., 2 cm H₂O). In any case, the interval between two successive PEEP adjustments is at least equal to a fixed period (e.g., 240 seconds), to allow for the changes in PEEP to have an observable and measurable impact on the patient's oxygenation.

It should be noted that the above examples for the incremental step size for PEEP adjustment (e.g. 2 cm H₂O) and the minimum and maximum values for the ratio of PEEP/F_IO2, are indicated for patients receiving ventilatory treatment in a more acute clinical setting such as the intensive care or a constant care unit of a hospital. Smaller incremental adjustments (e.g. 1 cm H₂O) and more conservative ranges for the ratio of PEEP (or CPAP)/F_IO2 may be adopted if the invention is used to improve the breathing and oxygenation of more stable, spontaneously breathing patients.

FIG. 4 illustrates in detail, a preferred circuit diagram of the Signal Generator Circuit, 46, and the alarm circuit 54. The preferred component types and values are shown in the chart below:

	Component	Type/Value
	IC1	DAC0802LCN
	IC2	LM741CN
	IC3	SN7400N
	C1	0.1	μF
	C2	0.03	μF
	D1	1N4148
	R1	5.1	kΩ
	R2	50	kΩ pot
	R3	10	kΩ pot
	R4	2.7	kΩ
	R5	330	Ω

There has been described a method and apparatus for controlling a ventilator. The invention utilizes data indicative of measured oxygen levels of the patient to automatically control F_IO2, and PEEP (or CPAP). In an alternative embodiment, the invention further uses the respiratory mechanics data (i.e. respiratory elastance and airway resistance) to automatically make the necessary adjustments in the I:E ratio of the patient on the ventilator. It further incorporates the features of U.S. Pat. No. 4,986,268 and uses data indicative of measured levels of oxygen and the respiratory mechanics data of the patient, along with data indicative of barometric pressure (as a reference calibrating pressure), and data indicative of measured carbon dioxide level of the patient to automatically control the breathing frequency and tidal volume of breaths of the patient on the ventilator. The invention also detects and corrects artifacts in the measured oxygen and carbon dioxide data and applies safety rules. In its different embodiments, the invention can improve total and/or assist ventilatory treatments provided to different patient groups.

The present invention may be embodied in other specific forms without departing from the scope and the essential attributes thereof. Therefore, reference should be made to the appended claims rather than to the foregoing specification, with regard to the scope of the invention.

Claims

1. An apparatus for automatically controlling a ventilator comprising:

first means for processing data indicative of at least a measured oxygen level of a patient, and for providing output data indicative of:

required concentration of oxygen in inspiratory gas of the patient (F_IO2) and positive end-expiratory pressure (peep) for a next breath of the patient;

wherein F_IO2 is determined to reduce the difference between the measured oxygen level of the patient and a desired value;

wherein peep is determined to keep a ratio of peep/F_IO2 within a prescribed range and, while keeping the ratio within the prescribed range, to keep the measured oxygen level of the patient above a predefined value; and

second means, operatively coupled to the first means, for providing control signals, based on the output data provided by the first means, to the ventilator;

wherein the control signals provided to the ventilator automatically control peep, and F_IO2, for a next breath of the patient.

2. The apparatus of claim 1, wherein the first means comprises a programmable microcomputer.

3. The apparatus of claim 2, further comprising

an alarm unit;

wherein the first means further determines whether there has been an artifact in the measured oxygen levels and replaces and/or corrects the data determined to be based on the artifact; and

wherein the second means further provides an alarm control signal to the alarm unit to warn of the artifact in the measured oxygen levels.

4. The apparatus of claim 2, further comprising

an alarm unit;

wherein the first means further determines whether the measured oxygen levels are outside a prescribed range; and

wherein the second means further provides an alarm control signal to the alarm unit to warn of the measured oxygen level of the patient being outside a prescribed range.

5. The apparatus of claim 2, further comprising an analog to digital (A/D) converter connected to an input of the first means for converting analog signals from an oxygen sensor, indicative of the oxygen level of the patient, to digital data.

6. The apparatus of claim 5, wherein the oxygen sensor is a pulse oximeter measuring arterial hemoglobin oxygen saturation in the patient's blood.

7. The apparatus of claim 2, wherein data indicative of the lower inflection pressure point on an inspiratory or expiratory pressure volume curve of the patient (LIP) is provided to the first means.

8. The apparatus of claim 7, wherein the data indicative of LIP is supplied by a monitor operatively coupled to the first means.

9. The apparatus of claim 2, wherein data indicative of the patient's measured intrinsic positive end-expiratory pressure (PEEPi) is provided to the first means.

10. The apparatus of claim 9, wherein the data indicative of PEEPi is supplied by a monitor operatively coupled to the first means.

11. The apparatus of claim 2, wherein the programmable microcomputer further comprises a program means for determining from the input data: the patient's arterial partial pressure of oxygen; the required F_IO2; the required peep; for a next breath of the patient.

12. The apparatus of claim 11, wherein the program means further determines, from the input data: whether there has been an artifact in the data indicative of the measured oxygen level of the patient, and wherein the program means further replaces and/or corrects the data based on the artifact and generates a warning signal in the event the artifact is determined.

13. The apparatus of claim 2, wherein data corresponding to a set point for arterial partial pressure of oxygen, threshold values for the oxygen level of the patient, and a correction factor for oxygen based on measured blood pH levels of the patient are entered manually and stored in a software program.

14. The apparatus of claim 2, wherein the first means further processes input data indicative of respiratory elastance, respiratory airway resistance, barometric pressure, and measured carbon dioxide levels of the patient, and based upon the input data, provides digital output data indicative of required ventilation, optimum breathing frequency, and required adjustment in the ratio of inspiration time to expiration time (I:E) for a next breath of the patient, and; wherein the second means further generates additional control signals to the ventilator based on the output data of the first means; wherein the additional control signals to the ventilator control tidal volume and frequency of inhaled gas provided to the patient by the ventilator and effect necessary adjustments in the ratio of I:E for a next breath of the patient.

15. The apparatus of claim 14, wherein the input data indicative of respiratory elastance and airway resistance of the patient are supplied to the first means by one or more monitors coupled to the first means.

16. The apparatus of claim 14, wherein the input data indicative of respiratory elastance and airway resistance of the patient are entered manually and stored in a software program.

17. The apparatus of claim 14, wherein the input data indicative of the measured oxygen level of the patient and the measured carbon dioxide level of the patient are provided to the first means by one or more monitors coupled to the first means.

18. The apparatus of claim 17, wherein the input data indicative of the measured oxygen level of the patient is provided by a pulse oximeter measuring arterial hemoglobin oxygen saturation of the patient, and the input data indicative of the measured carbon dioxide level of the patient is provided by an exhaled gas analyzer detecting end-tidal partial pressure of carbon dioxide or end-tidal concentration of carbon dioxide in exhaled gas of the patient.

19. The apparatus of claim 17, wherein, based on data indicative of measured oxygen and carbon dioxide levels of the patient, the first means detects an artifact in the data, discards the data having the artifact, resumes a previous value of the data in a memory, and provides a warning instruction signal; and wherein the second means generates a warning control signal that is supplied to an alarm unit that generates an alarm signal.

20. The apparatus of claim 17, wherein, based on data indicative of measured carbon dioxide and oxygen levels of the patient, the first means detects a potential pulmonary embolism and produces digital output data indicative of predefined levels of ventilation and breathing frequency and a required adjustment in the I:E ratio, and provides a warning instruction signal; and wherein the second means generates a warning control signal.

21. The apparatus of claim 17, further comprising program means for determining from the input data: (i) partial pressures of oxygen and carbon dioxide in arterial blood of the patient; (ii) presence of artifact(s) in the data indicative of the measured oxygen and carbon dioxide levels of the patient, and in case of artifact detection, replacing and/or correcting the data and corresponding partial pressure value(s); (iii) net effects of oxygen and carbon dioxide on alveolar ventilation; (iv) total required alveolar ventilation; (v) optimal frequency of breathing; (vi) required ventilation; (vii) required adjustment in the I:E ratio; (viii) required F_IO2; and (ix) required peep; for a next breath of the patient.

22. The apparatus of claim 21, wherein data corresponding to a set point for arterial partial pressure of oxygen, an adjustment factor for carbon dioxide level of the patient, threshold levels for oxygen level of the patient, and a correction factor for oxygen based on measured blood pH levels of the patient, are entered manually and stored in a software program.

23. The apparatus of claim 14, wherein the input data indicative of barometric pressure is supplied to the first means by one or more monitors coupled to the first means.

24. The apparatus of claim 14, wherein the input data indicative of barometric pressure is entered manually and stored in hardware.

25. The apparatus of claim 14, wherein the input data indicative of barometric pressure is entered manually and stored in a software program.

26. The apparatus of claim 14, wherein the first means also receives and processes data indicative of the patient's metabolic rate ratio.

27. The apparatus of claim 26, wherein the data indicative of the patient's metabolic rate ratio is supplied to the first means by a monitor coupled to the first means.

28. The apparatus of claim 26, wherein the data indicative of the patient's metabolic rate ratio is entered manually and stored in a software program.

29. A method for automatically controlling a ventilator comprising the steps of:

(a) measuring an oxygen level of a patient and providing a data signal indicative of the measured oxygen level;

(b) determining: (i) required concentration of oxygen in an inspiratory gas of the patient, F_IO2, based on the data signal indicative of the measured oxygen level of the patient and to reduce the difference between the measured oxygen level of the patient and a desired value; (ii) required positive end-expiratory pressure, peep, wherein a ratio of peep/F_IO2 is maintained within a prescribed range, and to keep the measured oxygen level of the patient above a predefined value; and

(c) providing data signals indicative of the required F_IO2 and the required peep based upon the determining of step (b), for automatically controlling F_IO2 and peep for a next breath of the patient.

30. The method of claim 29, wherein step (b) further comprises determining, from the data indicative of the measured oxygen level in (a), whether there has been an artifact in the measured oxygen level, and replacing and/or correcting the data signal in (a) in the event the artifact is determined.

31. The method of claim 29, wherein the data signal indicative of measured oxygen level of the patient is in analog form and is converted to digital form before the determining of step (b), and wherein the providing of step (c) further comprises converting the data signals from digital to analog form.

32. The method of claim 31, wherein the measuring of the oxygen level of the patient comprises measuring an arterial hemoglobin oxygen saturation of the patient via pulse oximetry.

33. The method of claim 32, wherein an arterial partial pressure of oxygen of the patient is derived from the arterial hemoglobin oxygen saturation of the patient measured by the pulse oximeter.

34. The method of claim 33, wherein the following equation is used to calculate the arterial partial pressure of oxygen (P_aO2) of the patient from the arterial hemoglobin oxygen saturation data (S_pO2) measured by pulse oximetry:

35. The method of claim 34, further comprising:

comparing P_aO2 to a minimum acceptable value, and,

if P_aO2 is found to be less than the minimum acceptable value:

discarding P_aO2 and a latest measured S_pO2 data;

resuming previous values of P_aO2 and S_pO2; and

generating a warning signal.

36. The method of claim 29, wherein data corresponding to the lower inflection pressure point on an inspiratory or expiratory pressure volume curve of the patient (LIP) is also provided in step (a), and an initial value for peep is set equal to LIP plus 0 to 8 cm H₂O and the initial value for peep is provided in step (b).

37. The method of claim 36, wherein the data corresponding to LIP is supplied by a monitor.

38. The method of claim 29, wherein data corresponding to the measured intrinsic peep of the patient (PEEPi) is also provided in step (a), and an initial value for peep is set between 80% and 100% of PEEPi and the initial value for peep is provided in step (b).

39. The method of claim 38, wherein the data corresponding to PEEPi is supplied by a monitor.

40. The method of claim 29, wherein an initial value for peep is determined by the operator and is manually provided.

41. The method of claim 29, wherein the required concentration of oxygen in the inspiratory gas of the patient (F_IO2) is calculated by using a stepwise control scheme and/or by using a proportional-integral-derivative (PID) technique.

42. The method of claim 41, wherein using a PID technique comprises comparing S_pO2 obtained by pulse oximetry to a defined minimum safe value, and wherein using the PID technique continues while S_pO2 is greater than the defined minimum safe value.

43. The method of claim 41, wherein using a PID technique comprises comparing S_pO2 obtained by pulse oximetry to a defined minimum safe value, and wherein, if S_pO2 is found to be less than or equal to the defined minimum safe value, a stepwise control scheme is followed that comprises the steps of:

raising F_IO2 stepwise to avoid hypoxemia,

allowing F_IO2 to remain high until S_pO2 rises to a second threshold value,

lowering F_IO2 stepwise,

comparing S_pO2 to a third threshold value,

lowering F_IO2 stepwise upon S_pO2 rising to the third threshold value,

comparing S_pO2 to a fourth threshold value,

returning control to the PID technique upon S_pO2 rising to the fourth threshold value.

44. The method of claim 41, wherein the difference between a P_aO2 set point and the P_aO2 of the patient is reduced by using a PID control procedure according to the following equations:

Y₁(k)=P_aO2(set-point)−P_aO2

Y₂(k)=[Y₁(k)−Y₁(k−1)]/T

Y₃(k)=Y₃(k−1)+TY₁(k)

E(k)=αY₁(k)+βY₃(k)+γY₂(k)

G(k)=E(k)+0.21

where Y₁(k), Y₂(k), and Y₃(k) are the proportional, derivative, and integral components of error, respectively, E(k) is an error function, T is a sampling interval, G(k) is the required F_IO2, and parameters α, β, and γ are PID coefficients.

45. The method of claim 41, wherein the determining of required peep of the patient comprises the following procedure:

comparing the peep/F_IO2 ratio to a defined minimum allowed value,

increasing peep by a fixed incremental value if the peep/F_IO2 ratio is lower than the defined minimum allowed value and the time elapsed since the last adjustment in peep is longer than or equal to a fixed defined interval T1,

comparing the peep/F_IO2 ratio with a defined maximum allowed value if the peep/F_IO2 ratio is not less than the defined minimum allowed value,

comparing S_pO2 with a defined value if the peep/F_IO2 ratio is less than the defined maximum allowed value,

increasing peep by a fixed incremental value if S_pO2 is less than the defined value and the time elapsed since the last adjustment in peep is longer than or equal to T1,

if the peep/F_IO2 ratio is not less than the defined maximum allowed value, comparing the peep/F_IO2 ratio to a value higher than the defined maximum allowed value, RG, whereby if the peep/F_IO2 ratio is higher than RG, and the time elapsed since the last adjustment in peep is greater than or equal to T1, decreasing peep by a fixed incremental amount.

46. An apparatus for automatically controlling a ventilator for a next breath of a patient comprising:

first means for processing input data indicative of at least a measured oxygen level of the patient, and for providing output data indicative of:

required concentration of oxygen in inspiratory gas of the patient (F_IO2) and required positive end-expiratory pressure (peep) for a next breath of the patient,

wherein F_IO2 is determined to reduce the difference between the measured oxygen level of the patient and a desired value;

wherein a ratio of peep/F_IO2 is calculated and peep is determined to keep said ratio of peep/F_IO2 within one prescribed range;

wherein peep is determined without using a proportional-integral-derivative (PID) control procedure and while peep is determined to keep the ratio of peep/F_IO2 within one prescribed range, peep is determined to also keep the measured oxygen level of the patient above a predefined value; and

second means, operatively coupled to the first means, for providing control signals, based on the output data provided by the first means, to the ventilator for peep and to a mixer regulator, operatively coupled to the ventilator, for F_IO2;

wherein the control signals automatically control peep and F_IO2 for a next breath of the patient.

47. The apparatus of claim 46, wherein the first means comprises a programmable microcomputer.

48. The apparatus of claim 47, further comprising an alarm unit;

wherein the first means further determines whether there has been an artifact in the measured oxygen level and in response to determining an artifact is present, replaces the measured oxygen level with the artifact with a most recent measured oxygen level without artifact; and

wherein the second means further provides an alarm control signal to the alarm unit in response to the determination by the first means that there has been an artifact to warn of an artifact in the current measured oxygen level.

49. The apparatus of claim 47, further comprising an alarm unit;

wherein the first means further determines whether the measured oxygen level is outside a prescribed range; and

wherein the second means further provides an alarm control signal to the alarm unit in response to the determination by the first means that the measured oxygen level is outside the prescribed range to warn of the measured oxygen level of the patient being outside the prescribed range.

50. The apparatus of claim 47, wherein the input data indicative of at least the measured oxygen level of the patient processed by the first means includes arterial hemoglobin oxygen saturation data measured by a pulse oximeter operatively coupled to the first means.

51. The apparatus of claim 47, wherein data indicative of a lower inflection pressure point on an inspiratory or expiratory pressure volume curve of the patient (LIP) is provided to the first means.

52. The apparatus of claim 51, wherein the data indicative of LIP is supplied by a monitor operatively coupled to the first means.

53. The apparatus of claim 47, wherein data indicative of the patient's measured intrinsic positive end-expiratory pressure (PEEPi) is provided to the first means.

54. The apparatus of claim 53, wherein the data indicative of PEEPi is supplied by a monitor operatively coupled to the first means.

55. The apparatus of claim 47, wherein the programmable microcomputer further comprises a program means for determining from the input data:

a patient's arterial partial pressure of oxygen;

the required F_IO2; and

the required peep

56. The apparatus of claim 55, wherein the program means further determines, from the input data:

whether there has been an artifact in the measured oxygen level of the patient, and

wherein the program means, in response to determining an artifact is present, further replaces the measured oxygen level with the artifact with a most recent measured oxygen level without artifact, and

the apparatus further comprising alarm means for generating a warning signal in the event the artifact is determined to be present.

57. The apparatus of claim 47, wherein data corresponding to a set point for arterial partial pressure of oxygen, threshold values for arterial hemoglobin oxygen saturation level of the patient, and a correction factor for arterial partial pressure of oxygen based on measured blood pH levels of the patient are entered manually and stored in a software program.

58. The apparatus of claim 47, wherein input data indicative of at least a measured oxygen level of the patient which is processed by the first means further includes input data indicative of respiratory elastance, respiratory airway resistance, barometric pressure, and measured carbon dioxide level of the patient, and based upon the input data, the first means provides digital output data indicative of required ventilation, optimum breathing frequency, and required adjustment in the ratio of inspiration time to expiration time (I:E) for a next breath of the patient;

wherein the second means further generates additional control signals to the ventilator based on the digital output data of the first means; and

wherein the additional control signals to the ventilator control tidal volume and frequency of inhaled gas provided to the patient by the ventilator and effect necessary adjustments in the ratio of I:E for a next breath of the patient.

59. The apparatus of claim 58, wherein the input data indicative of respiratory elastance and airway resistance of the patient are supplied to the first means by one or more monitors coupled to the first means.

60. The apparatus of claim 58, wherein the input data indicative of respiratory elastance and airway resistance of the patient are entered manually and stored in a software program.

61. The apparatus of claim 58, wherein the input data indicative of the measured oxygen level of the patient and the measured carbon dioxide level of the patient are provided to the first means by one or more monitors coupled to the first means.

62. The apparatus of claim 61, wherein the input data indicative of the measured oxygen level of the patient is provided by a pulse oximeter measuring arterial hemoglobin oxygen saturation of the patient, and the input data indicative of the measured carbon dioxide level of the patient is provided by an exhaled gas analyzer detecting end-tidal partial pressure of carbon dioxide or end-tidal concentration of carbon dioxide in exhaled gas of the patient.

63. The apparatus of claim 61, wherein, based on the input data indicative of the measured oxygen level and the measured carbon dioxide level of the patient, the first means detects an artifact in the input data indicative of the measured oxygen level and/or the measured carbon dioxide level of the patient, discards the input data indicative of the measured oxygen level and/or the measured carbon dioxide level of the patient having the artifact, and resumes a value of data indicative of the most recent measured oxygen level without artifact and/or the most recent measured carbon dioxide level without artifact stored in a memory; and wherein the second means generates a warning control signal that is supplied to an alarm unit that generates an alarm signal.

64. The apparatus of claim 61, wherein, based on the input data indicative of the measured carbon dioxide level and the measured oxygen level of the patient, the first means detects a potential pulmonary embolism and produces digital output data indicative of predefined levels of ventilation and breathing frequency and a required adjustment in the I:E ratio, and provides a warning instruction signal; and wherein the second means generates a warning control signal in response to the warning instruction signal provided by the first means.

65. The apparatus of claim 61, wherein the first means comprises a first control program means for determining from the input data:

(i) partial pressures of oxygen and carbon dioxide in arterial blood of the patient;

(ii) presence of artifact(s) in the input data indicative of the measured oxygen level and/or carbon dioxide level of the patient, and in case of artifact detection, replacing the input data indicative of the measured oxygen level and/or carbon dioxide level of the patient with the artifact and corresponding partial pressure value with data indicative of most recent measured oxygen level without an artifact and/or the measured carbon dioxide level without artifact and corresponding partial pressure value(s);

(iii) net effects of oxygen and carbon dioxide on alveolar ventilation;

(iv) total required alveolar ventilation;

(v) the optimal frequency of breathing;

(vi) the required ventilation; and

(vii) the required adjustment in the I:E ratio

for a next breath of the patient; and

a second control program means for determining:

(i) the required F_IO2; and

(ii) the required peep

for a next breath of the patient.

66. The apparatus of claim 65, wherein data corresponding to a set point for the arterial partial pressure of oxygen, an adjustment factor for the measured carbon dioxide level of the patient, threshold levels for arterial hemoglobin oxygen saturation level of the patient, and a correction factor for the arterial partial pressure of oxygen based on measured blood pH levels of the patient are entered manually and stored in a software program.

67. The apparatus of claim 58, wherein the input data indicative of barometric pressure is supplied to the first means by one or more monitors coupled to the first means.

68. The apparatus of claim 58, wherein the input data indicative of barometric pressure is entered manually and stored in hardware.

69. The apparatus of claim 58, wherein the input data indicative of barometric pressure is entered manually and stored in a software program.

70. The apparatus of claim 58, wherein the first means also receives and processes data indicative of the patient's metabolic rate ratio.

71. The apparatus of claim 70, wherein the data indicative of the patient's metabolic rate ratio is entered manually and stored in a software program.

72. A method for automatically controlling a ventilator for a next breath of a patient comprising the steps of:

(a) measuring an oxygen level of the patient and providing a data signal indicative of the measured oxygen level;

(b) determining for a next breath of the patient:

(i) required concentration of oxygen in an inspiratory gas of the patient, F_IO2, based on the data signal indicative of the measured oxygen level of the patient, to reduce the difference between the measured oxygen level of the patient and a desired value; and

(ii) required positive end expiratory pressure, peep,

wherein a ratio of peep/F_IO2 is calculated and peep is determined to keep said ratio of peep/F_IO2 within one prescribed range,

wherein peep is determined without using a proportional-integral-derivative (PID) control procedure, and

while peep is determined to keep the ratio of peep/F_IO2 within one prescribed range, peep is determined to also keep the measured oxygen level of the patient above a predefined value; and

(c) providing data signals indicative of the required F_IO2 and the required peep, based upon the determining of step (b), for automatically controlling F_IO2 and peep for a next breath of the patient.

73. The method of claim 72, wherein step (b) further comprises determining, from the data signal indicative of the measured oxygen level in step (a), whether there has been an artifact in the measured oxygen level, and, in the event an artifact is determined, replacing the measured oxygen level with the artifact with a most recent measured oxygen level without artifact.

74. The method of claim 72, wherein the measuring of the oxygen level of the patient comprises measuring an arterial hemoglobin oxygen saturation of the patient via pulse oximetry.

75. The method of claim 74, wherein an arterial partial pressure of oxygen of the patient is derived from the arterial hemoglobin oxygen saturation of the patient (S_pO2) measured by a pulse oximeter.

76. The method of claim 75, wherein the following equation is used to calculate the arterial partial pressure of oxygen (P_aO2) of the patient from the arterial hemoglobin oxygen saturation data (S_pO2) measured by pulse oximetry:

77. The method of claim 76, further comprising: comparing P_aO2 to a minimum acceptable value, and, if P_aO2 is found to be less than the minimum acceptable value, discarding the P_aO2 found to be less than the minimum acceptable value and a latest measured S_pO2 data corresponding thereto; resuming a most recent value of P_aO2 found to be equal to or greater than the minimum acceptable value and corresponding value of S_pO2; and generating, by a signal generator, a warning signal.

78. The method of claim 72, wherein data corresponding to the lower inflection pressure point on an inspiratory or expiratory pressure volume curve of the patient (LIP) is also provided in step (a), and an initial value for peep is set equal to LIP plus 0 to 8 cm H₂O and the initial value for peep is provided in step (b).

79. The method of claim 78, wherein the data corresponding to LIP is supplied by a monitor.

80. The method of claim 72, wherein data corresponding to a measured intrinsic peep of the patient (PEEPi) is also provided in step (a), and an initial value for peep is set between 80% and 85% of PEEPi and the initial value for peep is provided in step (b).

81. The method of claim 80, wherein the data corresponding to PEEPi is supplied by a monitor.

82. The method of claim 72, wherein an initial value for peep is determined by the operator and is manually provided.

83. The method of claim 72, wherein the required concentration of oxygen in the inspiratory gas of the patient (F_IO2) in step (b)(i) is calculated by using a stepwise control scheme and/or by using a proportional-integral-derivative (PID) control procedure.

84. The method of claim 83, wherein using the PID control procedure comprises comparing S_pO2 obtained by pulse oximetry to a defined minimum safe value, and wherein using the PID control procedure continues while S_pO2 is greater than the defined minimum safe value.

85. The method of claim 83, wherein using the PID control procedure comprises comparing S_pO2 obtained by pulse oximetry to a defined minimum safe value, and wherein, if S_pO2 is found to be less than or equal to the defined minimum safe value, the stepwise control scheme is followed and comprises the steps of:

raising F_IO2 stepwise to avoid hypoxemia,

allowing F_IO2 to remain high until S_pO2 rises to a second threshold value,

lowering F_IO2 stepwise,

comparing S_pO2 to a third threshold value,

lowering F_IO2 stepwise upon S_pO2 rising to the third threshold value,

comparing S_pO2 to a fourth threshold value,

returning control to the PID control procedure upon S_pO2 rising to the fourth threshold value.

86. The method of claim 83, wherein the difference between a P_aO2 set point and a P_aO2 of the patient is reduced by using the PID control procedure according to the following equations:

Y₁(K)=P_aO2 (set-point)−P_aO2

Y₂(K)=[Y₁(K)−Y₁(K−1)]/T

Y₃(K)=Y₃(K−1)+TY₁(K)

E(K)=αY₁(K)+βY₃(K)+γY₂(K)

G(K)=E(K)+0.21

where Y₁(k), Y₂(k), and Y₃(k) are the proportional, derivative, and integral components of error, respectively, E(k) is an error function, T is a sampling interval, G(k) is the required F_IO2, and parameters α, β, and γ are PID coefficients.

87. The method of claim 83, wherein the determining of required peep of the patient comprises the following procedure:

comparing the peep/F_IO2 ratio to a defined minimum allowed value,

increasing peep by a fixed incremental value if the peep/F_IO2 ratio is lower than the defined minimum allowed value and a time elapsed since the last adjustment in peep is longer than or equal to a fixed defined interval T1,

comparing the peep/F_IO2 ratio with a defined maximum allowed value if the peep/F_IO2 ratio is not less than the defined minimum allowed value,

comparing S_pO2 with a defined value if the peep/F_IO2 ratio is less than the defined maximum allowed value,

increasing peep by a fixed incremental value if S_pO2 is less than the defined value and the time elapsed since the last adjustment in peep is longer than or equal to T1, if the peep/F_IO2 ratio is not less than the defined maximum allowed value, comparing the peep/F_IO2 ratio to a value higher than the defined maximum allowed value, RG, whereby if the peep/F_IO2 ratio is higher than RG, and the time elapsed since the last adjustment in peep is greater than or equal to T1, decreasing peep by a fixed incremental amount.