Abstract
There are no reports on pressure spreading during pressure support ventilation (PSV) in the lung. We studied changes in alveolar pressure during PSV.
Six mature mongrel dogs were anesthetized with pentobarbital 25mg·kg-1 and intubated with a Univent tracheal tube (I. D. 8.5mm, Fuji Systems, Japan). They were ventilated on continuous positive pressure ventilation (CPPV) mode with the EVITA2 ventilator (Dräger, Germany). Anesthesia was maintained by infusion of pentobarbital 2-3mg·kg-1·hr-1. Alveolar pressure was measured from a capsule which was glued to the surface of the lung in the intercostal space in the right lateral position using the “closed-chest capsule technique” (Bates, 1989). Tracheal pressure was measured from the occlusion catheter of the Univent-tube. Pressure support (PS) levels were set at 5 and 10cmH2O, PEEP 5cmH2O, and O2 concentration 0.21. The rising time of PS was set at about 0.5sec. Pressure-time curves of the circuit of ventilator, trachea and alveoli were recorded on a polygraph simultaneously. Tidal volume, respiratory rate, minute volume and duty ratio were measured with a respiratory monitor (OMR-8101, Nihon Kohden, Japan). P0.1 was measured with EVITA2. During CPPV, the pressure difference between the trachea and alveoli was very small and the time phases of inspiration and expiration were consistent with each other. The velocity of inspiratory and expiratory pressure changes was slower in the trachea and alveoli than in the circuit. Mean compliance and mean resistance of the respiratory system were 19.0 ml·cmH2O-1 and 16.3cmH2O·l-1·sec-1 respectively. During PSV, the pressure difference between the trachea and alveoli was very small, as with CPPV. The pressure waveform of the circuit plateaued during inspiration, but the tracheal and alveolar pressures never showed a plateau. In conclusion, in normal mongrel dogs, the influence of resistance in the lung was negligible on pressure spreading during CPPV and PSV.
