Effect of valve closure time on the determination of respiratory resistance by flow interruption
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The resistance of the respiratory system to flow may be conveniently assessed by the flow-interruption technique, in which the flow of gas at the mouth of a subject is suddenly interrupted, while the pressure just distal to the point of interruption, is recorded. There is a rapid change in pressure immediately upon interruption, presumably giving the resistive pressure drop across the pulmonary airways. This is followed by a further slow change in pressure reflecting stress relaxation in the respiratory system and possibly gas redistribution between different regions of the lung. The diagnostic potential of the post-occlusion pressure signal is dependent on the airway opening being occluded effectively instantaneously. We present a description of an occlusion valve we have designed and built for performing rapid airway occlusions. We are able to measure the closing characteristics of our valve precisely, and show that its finite closure time of 12 ms causes the initial rapid drop in pressure to be underestimated by about 7 per cent. A simple numerical correction scheme allows us to estimate this pressure drop correctly to within one or two per cent.
KeywordsAirways resistance Linear models of respiratory system
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