An electromagnetic valve for inspiratory occlusion pressures E. M. CAMPORESI, M. FEEZOR, J. FORTUNE, F. G. Hall Laboratory for Environmental Research, Duke University, Durham, North Carolina 27710

CAMPORESI, E. M., M. FEEZOR, J. FORTUNE, AND J. SALZANO. An electromagnetic valve for inspiratory occhsion pressures. J. Appl, Physiol.: Respirat. Environ. Exercise Physiol. 45(3): 481-483, 1978. - An electromagnetically powered respiratory valve to occlude a respiratory circuit for short (30-300 ms) periods of the respiratory cycle may be inexpensively constructed from available laboratory instruments and controlled by an electronic circuit. Occlusion of the inspiratory breathing circuit may be repeated at different levels of ventilation without altering slopes or intercepts of CO, rebreathing curves. The early phases of airway occlusion (P,,.,) may therefore be studied in conscious unanesthetized human subjects. carbon dioxide occlusion valve

response;

neural

respiratory

drive;

airways

AND

1) the valve must create a gas-tight seal that results in complete occlusion of the inspiratory airway; 2) occlusion must begin at end-expiratory volume on any desired breath; 3) the valve must have an automatic release mechanism to allow reopening of the airway shortly after 0.1 s from the onset of inspiration; 4) the closure of the valve must be silent so that the subject is unaware of impending inspiratory occlusion; and 5) there must be a way to remotely control the occlusion valve. The magnetic occlusion valve and electronic timing circuit described here fulfill all the requirements and can be inexpensively constructed from easily available laboratory instruments. GENERAL

during an inspiratory effort performed against a complete obstruction of the airway has been proposed as an index of the neurochemical respiratory drive i.n a.nesthetized animals (2, 7) and human subjects (1, 6). In awake subjects the sensation of a suddenly occluded inspiratory pathway in the breathing circu it is associated with a wide range of responses and with a variable duration of the inspiratory effort, but the pressure wave measured from the onset of the inspiration up to 0.15 s is highly reproducible and is una?fected by voluntary responses (7). The occlusion pressure measured at a constant time from the beginning of the inspiration, e.g., at 0.1 s (P,.,), may therefore be used to quantitate the inspiratory effort of conscious subjects in relationship with the ventilatory drive. In conscious human volun .teers , occlusion of the respiratory circuit by manual operation may result in subject discomfort if the airway is kept closed for prolonged times, and it may disrupt normal ventilatory responses, especially at high ventilation levels, as during the latter stages of CO2 rebreathing. There was a need, therefore, for an occlusion apparatus that could be placed in the inspiratory lines of a breathing system and couId meet the following criteria: THE PRESSURE DEVELOPED

OOZl-8987/78/0000-OOOO$Ol.

25 Copyright

0 1978 the American

Physiological

J. SALZANO

DESCRIPTION

The valve was designed to fit the inspiratory lines of a CO,-rebreathing circuit to measure VE, VT, and FIN% on a breath-by-breath basis with use of on-line analog computation (Fig. 1). Inspiratory gas is directed through the center of a circular magnet and into a rigid 500-ml chamber (Fig. 2). Covering the inlet to this chamber is a metal diaphragm suspended from two pins that cause the diaphragm to hang vertically next to the face of the magnet when the valve is in ‘a horizontal position. The airflow is then funneled from the apparatus inb the inspiratory side of a nonrebreathing valve WD = 40 ml). When current is applied to the magnet, the diaphragm is tightly pulled to the face of the magnet creating a gas-tight seal across the inlet tube; an electronic timer in the power supply releases the current after a predetermined period, and the diaphragm opens to allow normal airflow. In actual usage the current is applied during expiration when there is no airflow in the inspiratory side of the system and it is automatically released 125 ms after the onset of inspiration. CONSTRUCTION

A l&in. Society

hole was drilled through the central core of 481

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482

CAMPORESI,

a circular laboratory electromagnet (Central Scientific Co.) consisting of a soft iron outer shell and an inner core separated by a coiled ring electromagnet. The periphery of the central core was grooved and fitted with a 1.75in. O-ring. From a piece of 0.016~in. thick black iron sheeting a 3-in. circular diaphragm was cut so that its outer edges would be flush with the outermost border of the magnet, and this was suspended from two pins placed 1 in, apart on the outer ring of the magnet. The magnet diaphragm assembly was mounted in the bottom of a plastic Erlenmeyer flask. A tube was inserted in the inflow side of the magnet to facilitate connections. All the joints and seams of the valve were

FEEZOR,

Vohe Occlusion

the 2

circuit

used

for

1

I

I

60

65

%oz

CO2

t torr 1

FIG. 5. Posl (cm&O) values at different PACT, obtained in the course of a single rebreathing run in supine position. Regression by dotted line. line of Poe1 on PA coz values is indicated

GOS Flow

FIG.

IO25 tcoz-5

6

55

phragm, imbedded

Valve

FIG. 4. Instantaneous respiratory flow (V) and mouth pressure (Pm) during an occlusion of 120-ms duration. Flow trace suddenly increases from 0 to a peak value at release of magnetic valve. Dotted Line is traced at 100 ms from beginning of inspiration and indicates PoeI. %7 = 0 during occlusion.

I

of respiratory (3, 4)).

I

Insp. 0 0 hp.

Box System

1. Schematic drawing rebreathing (Read technique

SAIZANO

1

I

a0

FIG.

AND

i t

Subject R G Regression tl=

Bag in

FORTUNE,

2. Schematic occlusion-valve diagram: hinged on top, is magnetically attracted in a groove on inner pole of concentric

a soft iron diaagainst the O-ring magnet.

sealed with Silastic or epoxy to assure against leaks. During actual use the flask is mounted in the respiratory circuit with a slight tilt so that the diaphragm is maintained open by gravity, leaving the inspiratory flow unimpeded. Activation of the magnet draws the diaphragm tightly against the O-ring, creating a gastight seal. Termination of the magnet current effects release of the diaphragm within 20 ms. A block diagram of the occlusion-valve control cirI5 set Oefault Timer

Mouth

Vohe ControJ

Pressure

_

Flip-flop

Current , Booster

,Occiusi on Pushbutton

FIG.

3. Block

diagram

of occlusion-valve

control.

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INSPIRATORY

OCCLUSION

VALVE

FIG. 6. Comparison of CO, rebreathing responses in 6 subjects with (ordinate) and without (abscissa) occlusion. A: slope values (A~E/APco& plotted. B: intercepts extrapolated at Pm2 = 55 (VE&. Identity lines are indicated. Each point represents the mean value for 3-6 replications in each subject.

IO No Occlusion

cuitry is presented in Fig. 3. l An occlusion cycle is initiated by pressing the valve-closure push button during exhalation. This sets the valve-control flip-flop, which causes the current-boost amplifier to activate the valve magnet. Sufficient current is provided to maintain good valve seal during the subsequent initial inhalation phase. A mouth-pressure signal obtained from a transducer connected to the mouthpiece (Fig. 1) is applied to an adjustable-trigger threshold circuit, which is set to detect the initial phase of inhalation, usually close to the negative side of zero pressure. At the onset of inhalation, the trigger starts a 30- to 300ms timer, which determines the duration of valve closure. At the end of the desired closure time, the timer resets the valve-control flip-flop, which deactivates the magnet. The magnet is protected from possible damage by a default timer that automatically turns the magnet current off after 15 s when no inhalation is detected, To prevent a spurious opening during use, this 15-s period is reinitiated whenever the magnet is activated. The valve was tested for leakage by activating the magnet and applying suction to the outlet side of the system. Onset of leakage was determined by any discernable fall in the level of a pressure manometer during a 10-s period. With a current of 6.8 A, leakage could be prevented for suction greater than 30 cmH,O; this was greater than any anticipated occlusion pressure. Flow resistance of the valve in the horizontal position is 1 cmH,O for a constant flow of 9.0 l/s, and 1.5 I For schematics of the electronic from Microfiche Publications, P.O. New York, N. Y. 10017.

circuitry, order Box 3513, Grand

Document 03318 Central Station,

20

30 No Occlusion

40

50

cmH,O for a peak flow of 9.0 l/s during sinusoidal flow waves (1 HZ). DISCUSSION

The valve has been used during CO,-rebreathing trials with the standard Read technique (3, 4) on 12 subjects, both at normal atmospheric pressure and under hyperbaric conditions. During each run the airways were occluded approximately every 10 s during the 3min period of rebreathing, yielding up to 18 occlusions at different levels of ventilation. Figure 4 shows the mouth-pressure tracing obtai .ned du.ring a typical occlusion and the computation to obtain a pressure reading 0.1 s after the beginning of inspiration. The correlation between PO.1 values and PACT in the course of a single rebreathing run on a normal subject lying in the supine position is presented in Fig. 5. The short duration of the occlusion period was usually undetected by the subject, especially at high ventilation rates, and the maneuver was never disruptive of the normal breathing patterns. A comparison of the slopes ( A~E/APco~) and extrapolated intercepts (G& of rebreathing curves with and without intermittent airways occlusion is shown in Fig. 6, A and K. The observed scatter falls within the repeatability limits of the test (5) and demonstrates that such short interruption of inspiratory flow did not modify these parameteis of the conventional response to CO,. The apparatus can thus be used to measure inspiratory effort in untrained and unanesthetized subjects. This work was supported Grants HL-07896, HL-I3888, Contract N00014-67-0251. Received

18 January

in part by National Institutes and in part by Office of Naval

1978; accepted

in final

form

28 April

of Health Research 1978.

REFERENCES J. F., N. NEUBURGER, AND I-I. LEVINSON. The ventilatory response to carbon dioxide in asthmatic children, measured by the mouth-occlusion method (PM,,,,). Pediatrics 57: 952959, 1976. 2. GRUNSTEIN, M. M., M. YOUNES, AND J. MXLIC-EMILI. Control of tidal volume and respiratory frequency in anesthetized cats. J. AppZ. Physiol. 35: 463-476, 1973. 3. READ, D. J. C. A clinical method for assessing the ventilatory response to carbon dioxide. Australian Ann. Med. 16: 20-32, 1967. 4. READ, D. J. C., AND J. LEIGH. Blood-brain tissue Pco? relationships and ventilation during rebreathing. J. AppZ. Physiol. 23: 1. COSGROVE,

53-70,1967. 5. SAHN, S. A., LAKSHMINARAYAN,

C. W. ZWILLICH, N. DICK, R. E. MCCULLOUGH,S. AND J. V. VEIL. Variability of ventilatory responses to hypoxia and hypercapnia. J. AppZ. Physiol.: Respirut. Environ.. Exercise Physiol. 43: 1019-1025, 1977. 6. WHITELAW, W. A., J. P. DERENNE, AND J. MILIC-EMILI. Occlusion pressure as a measure of respiratory center output in conscious man. Respiration. PhysioZ. 23: 181-199, 1975. 7. WHITELAW, W. A., AND J. MILIC-EMILI. A new method for evaluating respiratory center output (Abstract). Physiologist 16: 486, 1973.

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