[Indirect determination of the intrapulmonary pressure course in the lung model in high frequency ventilation]

One of the determinants of intrapulmonary pressure during machine ventilation at a given time constant of the respiratory system is the duration of expiration. At high frequencies of ventilation with short expiration times substantial gas trapping can occur with end-expiratory increase in transpulmonary pressure (inadvertent PEEP). The occlusion technique allows measurement of the intrapulmonary pressure at the airway opening because of equilibration throughout the respiratory tract. The complete intrapulmonary pressure curve can be obtained, if occlusion takes place at progressively increasing intervals after the beginning of a breath. We present a computer-assisted method for measuring the occlusion pressure at defined time points throughout the respiratory cycle. A lung simulator is ventilated by a Sechrist ventilator. The tube leading to the simulator is occluded every 5 breaths, the time point of the occlusion being advanced progressively into each breath. Occlusion pressure is compared to intrapulmonary pressure measured directly by an intrapulmonary probe. All of this is controlled by a personal computer. We are able to demonstrate that, at ventilation frequencies of up to 600/min pressure curves measured indirectly correspond sufficiently well with pressures recorded directly in the lung model. An automatic evaluation of the measurements is possible even with a tube leakage of up to 70%.