A dual control system for assisting respiration* Yoshinori M i t a r n u r a , Tornohisa M i k a m i , and Katsuyuki Y a m a m o t o Department of Bio-Medical Control, Research Institute of AppliedElectricity, Hokkaido University, Sapporo 060, Japan

M i t s u o Kimura Anritsu Denki Co., Atsugi 243, Japan

Abstract--A dual control system for assisting respiration was developed. The following features were included." (i) ventilation is controlled by the metabolic rate from continuously measured C02 output, (ii) physiologic dead space approximated as a linear function of tidal volume is used to estimate alveolar ventilation, and (iii) oxygen concentration in the inspired gas is regulated by the arterial oxygen saturation continuously measured with an ear oximeter. The ventilator was used on dogs with an inspired gas mixture of 85% N2 and 15% 02. Arterial Pco2 was maintained between 36 and 39 mmHg for a duration exceeding 60 minutes. Although an oscillatory variation was seen in the arterial Po2 due to the adoption of an o n - o f f control mode to regulate the 02 fraction in the inspired gas, it remained between 80 and 135 mmHg. The dual control system o f assisting respiration is capable o f maintaining both the arterial Pco2 and Po2 within normal levels at any level of metabolic rate and any respiratory frequency. K e y w o r d s - - A s s i s t e d respiration, Regulated C02 and 02, Physiologic dead space

1 Introduction

IN PATIENTS with respiratory insufficiency or depressed respiratory centres, respiration must be carefully controlled by external means to maintain both arterial CO2 and 02 homeostasis. Most conventional ventilators have been designed in such a way as to synchronise the driving cycle with the respiratory cycle of the patient and at the same time to maintain the tidal volume or driving pressure constant. Accordingly, the arterial blood gas tension of the patient does not always remain within normal limits, since the minute ventilation of the patient does not necessarily follow the variation of his metabolic rate. Several investigators (FRUM]N et aI., 1959; LAMBERTSEN and WENDEL, 1960; HOLLOMANet al., 1968; KAM1YAMA et al., 1971; COLES et al., 1973) have presented methods of regulating the alveolar CO2 tension at a certain constant level by monitoring the end-tidal C02. However, these methods have the following disadvantages: (i) At a low tidal volume or an increased respiratory rate, the end-tidal CO2 and the mean alveolar C O 2 tension may differ because of contamination of the end-tidal sample by dead-space air (BURTON, 1966; HOLLOMAN et al., 1968). These two tensions may also differ in various pulmonary diseases such as chronic bronchitis, emphysema and pulmonary fibrosis, since all alveoli do not deflate simultaneously during expiration (WEST, 1967). *First received 5th November 1973 and in final form 30th January 1974

846

(ii) In the case o f a large degree of ventilationperfusion ratio inequality, ventilatory regulation by end-expiratory Pco2 maintains the arterial Pco2 at too high a level because of the difference in the alveolar-arterial Pco2. In previous papers (MITAMURA et al., 1971) an optimally controlled ventilator was reported. The following features were included: (i) ventilation was controlled by the patient's metabolic rate from the continuously measured CO2 output (ii) the physiologic dead space, approximated as a linear function of tidal volume, was used to estimate alveolar ventilation. Since the carbon dioxide output is estimated by multiplying the average CO2 concentration of the expired gas by the minute ventilation, it is not important whether all alveoli deflate together or not. Since physiologic dead space is used, alveolar ventilation can be estimated even in the case of uneven ventilation in relation to pulmonary capillary blood flow. An optimally controlled ventilator was shown to be effective for the automatic control of ventilation in dogs in spite of the changes in metabolic rate. The ventilator was also applied to human subjects with respiratory insufficiency. In the majority of the cases, both arterial Pco2 and Po2 w e r e maintained within a normal range under conditions of inhalation of 20~o oxygen. But in some patients with a severe ventilation-perfusion ratio inequality or diffusion impairment, where the arterial P% was

Medical and Biological Engineering

November 1975

below the normal value, and arterial Pco2 was meter (British Oxygen Co.) utilised as a pneumo normal, oxygen therapy was considered to be neces- tachometer (Fig. 2). The phototransistor (NEC sary (KAWAKAMI et al., 1971; MmAMI et al., 1972). PD 6) and a 12 V tungsten lamp are attached to the However, oxygen enrichment of the inspired gas inner wall of the respirometer in parallel and adjacent may produce some undesirable side effects, such as to each other. The diameters of the transistor and ventilatory depression (STADIE et aI., 1944), pul- lamp are 2 m m and 4 mm, respectively. Since the monary irritation (WATERS, 1942; BEAN, 1945; inner diameter of the respirometer is 20 mm, the COMROE et aL, 1965), pulmonary collapse (WINTER occupation ratio of the cross-sectional area of the and SMITH; 1972) and retrolental fibroplasia detector to the inner diameter is a mere 0" 05; hence (DANMAN e t al., 1954). Thus, the oxygen concen- the flow pattern is not disturbed by such obstacles. tration of the inspired gas must be controlled to avoid both hypoxia and hyperoxia. tt is the purpose of this study to develop a dual control system of assisting respiration which will sustain both the arterial Pco2 and Po2 at normal values. 2 Methods

Figure 1 shows a block diagram of the present dual control system for assisting respiration. This control system can be divided into three functional parts as followsi (i) monitor and error-detecting system, which is composed of a continuous monitor of COe concentration in the expired gas, a flowmeter for measuring the inspired gas flow, an ear oximeter for monitoring the arterial oxygen saturation, a comparator for detecting errors between the measured oxygen saturation and a reference input, and a position detector for detecting the initial position of the bellows. (ii) The controller, which consists of two parts. One part is an electronic circuit for computing the optimal tidal volume based on the continuously measured COe output and physiologic dead space of the patients. The other part establishes an on-off component of the error signal of oxygen saturation and feeds this signal to an electromagnetic valve. (iii) The controlled system, which consists of the subject, the pulse motor, the bellows and the electromagnetic valve for changing the mixing ratio of air and oxygen. M o n i t o r and error detector: The average fraction of CO2 in the expired gas is continuously monitored with a Beckman LB-1 medical gas analyser, and the signal is linearised with the lineariser which has seven straight-line segments of gain. The expired gas of the subject, led into the mixing box through a solenoid valve, is mixed sufficiently, and passes out freely through the outlet. The 1 litre capacity of the mixing box is sufficient to obtain the average COe fraction of several expiratory phases. The linearised output of the average fraction of COe is fed into the controller where the patient's CO2 output is computed for the estimation of the required alveolar ventilation. A continuous measurement of the airflow of inspired gas is accomplished by a Wright respiroMedical and Biological Engineering

15%0 z 02

Fig. I Schematic drawing of the dual control system for assisting respiration. When the respirometer detects the f l o w sign of inspiration, the pulse motor drives the bellows until the tidal volume of the patient becomes equal to the optima/ value estimated by the controller, During expiration the rack moves to the right until it reaches the optical coupling system consisted of the fight emitting diode (LED) and phototransistor (PTR). One-way valves in front of the bellows impede the return of the expired gas. The electromagnetic valve in front o f the box closes during the inspiratory phase, The carbon-dioxide gas analyser analyses the sampling gas continuously from the box for the measurement of FEco2. The electromagnetic valve placed between the oxygen tank and the bellows is switched to change the oxygen percentage in the inspired gas. The ear oximeter measures the arterial oxygen saturation continuously, A gas mixture of 15% 02 and 85% N2 was inhaled to induce hypoxia experimentally

Gas passing through the respirometer causes a rotation of a vane, the angular velocity of which is proportional to the volume flow rate of the gas. Each time the blade of the vane passes over the phototransistor, the intensity of light entering the phototransistor changes and an electrical pulse output is generated. As the pulse width may be changed by the angular velocity of the vane, a waveshaping circuit makes all the pulses the same width and height by means of a monostable multivibrator. The inspired gas volume is computed by integrating the pulses from the waveshaping circuit using an integrator. On the other hand, the pulse burst from the waveshaping circuit is used for detecting the initial time of inspiration.

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Arterial oxygen saturation is continuously measured by the ear oximeter (Erma, P W A - 5 , F-660 B-a) and the signal is linearised with the lineariser, which has nine straight-line segments of gain. The output of the lineariser is compared with the reference value of arterial oxygen saturation. Any deviation in the measured arterial oxygen saturation from the reference input will result in an error signal to the controller.

The physiologic dead space has been shown to be well approximated by a linear function of tidal volume (SEVERINGHAUSand STUPFEL,1957; NUNN and H~LL, 1960; WILLIAMSet al., 1966; COOPER, 1967) I7o = k l V r + k 2

.

.

.

.

.

(5)

.

(6)

.

(7)

.

where kl and k2 are constants. The minute ventilation is defined as I)'E = l ? a + f V o

RESPIROMETER

.

= fVr

.

.

.

.

.

.

.

. .

. .

. .

. .

where f is the respiratory frequency. Combining eqns. 4, 5, 6 and 7, the optimal tidal volume can be defined as V r = ( I / f ) { ( 1 / ( 1 - k l ) } . { ( P B - Pnzo)/Pa coz}l?co2 ~,

LAMP 8, PHOTOTRA.

+k21(1 - k~)

Fig. 2 Schematic diagram of the pneumotachometer" The shaded portion is a vane. Gas passing through the respirometer in the direction o f the arrow causes a rotation of a vane. When the light beam from the lamp is cut by the vane. the phototransistor puts out a pulse

During expiration the rack pulls the bellows in such a way as to fill it up with the gas for the next inspiration. When the rack reaches the positiondetecting system consisting of a light-emitting diode (LED) a n d phototransistor (PTR), it stops and remains stationary until the next inspiration occurs Controller: The optimal tidal volume is derived

by the following procedure. When no appreciable amount of CO2 is inspired, CO2 production per minute (l?co2) can be defined as 1)'co2 = FEco2 I?E = {(P~ co2/(PB- Pn2o)} VE 9

(1)

where l?z is the minute ventilation, FE co2 is the average concentration of CO2 in the expired gas, PE co2 is the partial pressure of CO2 in the expired gas, PB is the barometric pressure and Pn2o is the saturated-water-vapour pressure at body temperature. The physiologic dead space Vo is defined as 11. = {(P. co2 - PE co2)/P, co2} Vr

(2)

where P . co: is the arterial partial pressure of CO2 and VT is a tidal volume. The alveolar ventilation is defined as lYa = (1 --V./VT)IYE

. . . . . .

(3)

Combining eqns. 1, 2 and 3, the required alveolar ventilation which will maintain the homeostasis of P , co~ at a certain level of the metabolic rate can be defined as

PA= 848

.

(8)

Fcco 2

{(PB-- PH2O)/(Pa co2}

Pco2

,

(4)

S~ECT

9

vr

v

. vE

ULTIPLIER PERIOD

Ps" PH=O

(VT)opt ~

A/f opt

Va

L___3 Fig. 3 Block diagram of the controller The first-order lag system has a time constant of lOs.

This can be realised in the controller as shown in Fig. 3. The output of the pneumotachometer is fed into the integrator where the tidal volume is computed. The air-flow signal is also used for the computation o f the respiratory period. Breath-by-breath division of tidal volume by the respiratory period gives I?E. A n important consideration in the calculation of the CO2 output is the transport delay of the expired gas from the patient's mouth to the COz analyser. Furthermore, the CO2 fraction in the box is not the average fraction of a single expiration, but of several expirations. Carbon-dioxide output must be computed by multiplying I)'E and FE co2 for the same expiration cycle. Thus the l)'n signal must be delayed and averaged in accordance with the FE co2 signal. As the expired gas leaves the outlet of the box continuously, FE co2 is influenced greatly by the present expiration and only slightly by former expirations. A first-order lag system is used for this purpose. The required alveolar ventilation is obtained from the continuously measured CO2 output based on eqn. 4. The optimal tidal volume is computed by multiplying l?a by the respiratory period

M e d i c a l and Biological Engineering ~:

N o v e m b e r 1975

during the expiration. The time limiter initiates inspiration by changing the state of RS-FF 2 when apnoea continues beyond the setting time. The controller for supplying various concentrations of inspired 02 to automatically make corrections for deviations in Sao2 cannot be designed strictly by the method of linear control theory, since the subject introduces nonlinearities into the regulator. Thus the problem of controller design NAND4 was solved experimentally. Although various control NANDI NANO5 modes can be considered, an on-off control system INSR EXP, SIGNAL was employed here for its simplicity. If the patient's .... arterial oxygen saturation is below the reference value, the controller adds oxygen to the inspired gas NAND5 NAND7 Q by opening the electromagnetic valve located between the oxygen tank and the inspiratory circuit. The electromagnetic valve is opened only during expiration and is synchronised with the respiratory cycle. Moreover, the valve is switched when the Q line voltage is zero by using a zero-crossing voltage detector (TOLL, 1972), because the noise produced IRESPIROMETER I [CiREtll-'f] ([~J by switching disturbs the normal operation of the controller. Fig. 4 Circuit diagram of the control system for the Figure 5 shows a schematic drawing of the conpulse motor Vr signal is obtained from Vrnpt troller, and also illustrfftes the expected waveforms illustrated in Fig. 3. (/e signal can be adjusted which correspond to the points indicated by the manually to reduce the difference between the inspiratory flow rate of the patient and letters. The inputs to the controller are the reference values of arterial So2, the arterial So2 signals respirator. Expiratory and inspiratory signals are fed to the pulse motor. The setting time obtained from the lineariser of the oximeter and the in the time limiter is adjustable from 5 to lOs. respiratory phase signal D obtained from the R-S flip-flop 2 in Fig. 4. Values of 1 of the signal D Figure 4 shows the circuit of the controller. During indicate inspiration and values of 0 indicate expiration. Operational amplifier ov 1 establishes the error the time period between the end of expiration and before the next inspiration, the R-s flip-flop signal between the So2 of the patient and the reference (RS-FF 2) maintains 0 (low state). Thus the NAND value, and relays it to the next amplifier oP 2. The circuit NAND 4 cannot send the inspiratory signals output signal of the o f 2 is essentially a square wave, t o the pulse motor. When the patient attempts to since it works as a comparator. The output signal inspire, the monostable multivibrator NM 3 generates C goes to 1 or 0 depending on whether the oxygen pulse sequences according to the rotation of the vane in the inspired gas is sufficient or not. The output in the respirometer. The output of MM 3 changes signal E of the NAND N 7 shows I only during RS-rr 2 from the 0 to 1 (high state), which initiates expiration when the So2 of the patient falls below inspiration. Once inspiration occurs, it continues the reference value. The diode bridge D 1 rectifies until the real tidal volume of the patient (output of the alternating current of the line and generates the output signal A. Accordingly, the comparator the integrator) equals the optimal value. When these two signals coincide, MM 2 generates a trigger consisting of a transistors T 1, T 2 and T 3, generates pulse, which changes the state of RS-FF 2 from 1 to 0 positive pulses B every time the line voltage crosses and terminates the forced inspiration. At the same zero. As the NAND logic N 3 a n d N4 acts as an time RS-FF 3 is switched from 1 to 0 by NM 2, which g - s flip-flop, the output F of the transistor T 4 goes allows the NAND 3 to send expiratory signals to the to 1 at the first zero-crossing time from the beginpulse motor. Expiratory signals continue until the ning of the expiration, and goes to 0 at the first rack crosses the optical path in the position detector. zero-crossing time from the end of the expiration. When the rack reaches the position detector, the A state of 1 for this pulse allows the closure of the state of MM 1 changes from 0 to 1, which terminates relay contact, resulting in turning on the thyristor, D 3. A state of 0 opens the relay contact, which shuts the backward movement of the pulse motor: RS--FF 3 controls the electronic switch which is off the thyristor. Thus the electromagnetic valve placed parallel with the capacitor in the integrator. placed between the oxygen tank and the bellows is During the inspiration, the output of 1 of RS--FF 3 switched when the line voltage crosses the zero. As opens the electronic switch, which enables integra- the flow rate of oxygen is the dominant factor which tion. The state of 0 of RS-r~ 3 closes the electronic determines the fraction of inspired Oz, this was determined experimentally. switch and maintains the integrator at a null output and then dividing by 1 / ( l - k 1 ) and by adding k2/(1-kj). The subject is forced to inhale by the ventilator until the tidal volume of the patient coincides with the optimal value. When the subject's apnoea exceeds the setting time, the time limiter automatically initiates the next inspiration. The setting time is adjustable from 5 to 10 s.

~

Medical and Biological Engineering G

November 1975

849

Controlled system: A pinion attached to the shaft of the pulse motor moves the bellows attached to the rack in both directions. The displacement of the bellows is regulated by the controller according to the estimated tidal volume. The maximum stroke of the bellows is 1" 2 litres, corresponding to 120 m m of displacement. The inspiratory flow r a t e can be adjusted manually by changing the input voltage to the pulse-frequency modulator in the controller (Fig. 4). This adjustment is useful for reducing the difference between the inspiratory flow rate of the patient and the respirator.

3 Results

A dual control system for assisting respiration was applied to dogs anaesthetised with intravenous pentobarbital (30 mg/kg) without any premedication. The tracheas of all the dogs were intubated with cuffed end-tracheal tubes. Silicone-rubber catheters were inserted into the descending aorta through the femoral artery. Arterial Pco2 and Po~ were continuously measured with a mass spectrometer (MEDSPECT MS--8). A gas mixture of 85~o N2 and 1 5 ~ 02 was inhaled throughout the experiment to induce hypoxia. In the first stage, only the *ISV

§ -15V ~r

~ ~

" {V'XcVVVVV T2

8L c

L2A_Lk

~g

1

N2 N3 N4

o]

I

E

j:

"

I' fOK

IK

IOK

I O O K ~ (

so. S

o

look

,

'

I =

r

.

I

~

Fig. 5 Schematic drawing o f the controller for the regulation of oxygen concentration in inspired gas OP 1, OP 2 : B B 3 0 5 8 T 1--T5:2SC372 N 1 - - N 7." M 5 3 2 0 0 D I : M B - 4 , D 2 : SD34, D 3 : A C O 6 B R

The subject can breathe spontaneously through a one-way valve as illustrated in Fig. 1. Once the ventilator is triggered by the flow indication of inspiration, the pulse motor drives the bellows until the tidal volume becomes equal to the estimated optimal value. If the subject wishes to breathe more than the optimal tidal volume, he may, whereas the bellows remains where the tidal volume equals the optimal value. A solenoid valve shuts off the expiratory airway during the phase of inspiration to complete the positive pressure breathing. 850

VALV ~s

&

A dual control system for assisting respiration.

A dual control system for assisting respiration* Yoshinori M i t a r n u r a , Tornohisa M i k a m i , and Katsuyuki Y a m a m o t o Department of Bio...
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