Abstract
In the course of studies on the use of helium in obstructive dyspnea it became evident that varying the pressure of the inhaled atmosphere modified materially the effort required to ventilate the lungs. Inspiration was especially aided by inhaling a helium oxygen mixture under a positive pressure of one to 7 cm. of water in dyspnea such as that encountered in severe asthma. 1 Although moderate positive pressures appeared to have some usefulness in expiration by preventing marked deflation of the smaller bronchi and bronchioles, more marked pressures were found fatiguing. Various pulmotors were tried in which inspiration was achieved by positive pressure and expiration by negative pressure but they had the disadvantage of requiring excessive pressures before the shift from inspiration to expiration was possible. They were adapated to the unconscious rather than the conscious patient.
The basic mechanism we employed was a rebreathing apparatus with positive pressure blowers which forced air through soda-lime and made contact with the patient either with a mouth-piece or a mask. The blower in the inspiratory arm of the apparatus provided positive pressure, whereas the blower in the expiratory arm created a negative pressure, each capable of control and measurement by pressure control valves and gauges. The photo-electric cell was used to operate the solenoid valve which opened and closed depending upon the interruption of a beam of light passing through a photo-tube which contained a delicate vane that swung forward and backward as air was inhaled or exhaled.
The detail of the mechanism is shown in Fig. 1. The light source (F) throws a beam of light through the air current vane (D). The vane may take any one of three positions (a) vertical, when there is no movement of gas in the tube, (b) horizontal and to the left, when gas is moving from right to left as to inspiration, (c) horizontal and to the right, when gas is moving from left to right as in expiration.
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