AIRWAY OCCLUSION PRESSURE

Airway occlusion pressure has been used in the past two decades for as sessing output of the respiratory controller. It gives a measurement o f a weighted sum of the effect of all respiratory muscles active at a given time and, unlike ventilation or tidal volume, does not depend on the resistance or compliance of the respiratory system. In anesthetiz ed subjects or animals, it gives a tracing of the time course of respi ratory neuromuscular output through the respiratory cycle, modified by elimination of most phasic vagal stretch receptor feedback and perhap s slightly by activation of some chest wall reflexes. The original pos tulate that an occluded inspiration would be isometric and the measure d pressure free from losses due to force-length and force-velocity has been shown to be incorrect. The volume at which occlusion takes effec t, distortions of the chest wall during the maneuver, tonic vagal inpu t, and strength of the muscles must be taken into account when the dat a are interpreted. Brief occlusions [pressure at 0.1 s (P0.1)] are use ful in measuring output in the very first part of inspiration in consc ious subjects but must be treated with a great deal of caution. They a re most reliable when end-expiratory volume remains constant and there are no important phase lags between flow and pressure. Allowance may be necessary for damping of the pressure signal on its passing through the compliant upper airway. Changes in P0.1 may often be due to chang es in the shape of the driving pressure wave without a proportionate c hange in overall output. The technique remains useful when its limitat ions are recognized. Because of its simplicity, it can be easily and u sefully applied to a range of clinical investigations.