Automatic method for measuring mouth occlusion pressure response to carbon dioxide inhalation

Abstract

An index of the output of the respiratory centre may be obtained by measuring the subatmospheric pressure developed in the airway 0·1 s after the start of an inspiration against an occluded airway (P_0.1). This index of respiratory drive is independent of the effects of impaired pulmonary mechanics and a pure measure of central sensitivity to carbon dioxide may be obtained by plotting the P_0.1 response while breathing an increasing concentration of carbon dioxide. In the method described here the airway was occluded for a short time (0·12 s) from the start of inspiration using an electronically operated solenoid valve. The valve produced silent occlusions which were of short duration and did not perturb ventilation. The subatmospheric pressure (P_0·1) and end-tidal carbon dioxide tension (Pe'CO₂) were automatically sampled and plotted on an xy-recorder. The slope of the linear plot of P_0·1 against PE'CO₂ was then derived (P_0·1/PE'CO₂). There was good correlation between P_0·1/PE'CO₂ and the slope of the ventilatory response to carbon dioxide in normal subjects (r=0·91). Morphine produced significant depression (P<0·05) of P_0·1/PE'CO₂ in a group of eight normal healthy volunteers, indicating central depression of carbon dioxide sensitivity. The automatic sampling and plotting of P_0·1 and PE'Co₂ enabled rapid and simple measurements to be made with high precision and no observer bias. This technique has significant advantages over conventional manual methods of deriving P_0·1/PE'CO₂ and is suitable for routine clinical use.