Increase of expiratory resistance by the PEEP-valve of the servoventilator

Monitoring lung mechanics and airway pressures during differential lung ventilation (DLV) with emphasis on weaning from DLV

Fifteen polytrauma patients with asymmetric pulmonary contusion were treated with differential lung ventilation (DLV) for a mean of 106 hours (range 24-298, median 83). The differential time constant (Tc), compliance (Ct), inspiratory and expiratory airway resistance (Rawinsp, Rawexp) and peak-airway pressure (Pawpeak) were monitored to evaluate the function of each lung. Values measured after starting DLV were compared to those obtained prior to stopping DLV in order to analyse whether these parameters had returned to symmetrical values when recommencement of conventional mechanical ventilation was considered on clinical parameters and also whether these could be useful criteria for weaning from DLV. The significant difference in Tc of the contused lung compared to the contralateral lung after starting DLV is mainly determined by altered Ct resulting from contusion. During DLV improvement of Ct resulted in identical Tc of both lungs prior to stopping DLV. Changes in the Rawinsp contributed little to changes in Tc. Identical Tc prior to stopping DLV coincided with identical Pawpeak on symmetrical ventilator settings. These data suggest that when less advanced monitoring equipment is available, the differential Pawpeak might be used as a measure of differential lung mechanics in asymmetrical pulmonary contusion.

Jet flow-regulated expiratory resistance to maintain constant CPAP during the entire respiratory phase

We have developed a new continuous positive airway pressure (CPAP) device that consists of a microcomputer, a pressure transducer, and a pair of electronic interface valves. One of these valves creates the inspiratory demand flow, and the other creates the opposing jet flow by acting as an expiratory valve to maintain a constant CPAP. By controlling the two electronic interface valves, the airway pressure can be kept constant during the entire respiratory cycle. We compared our device with CPAP systems supplied with commercially available ventilators: the Puritan-Bennett 7200a, the Bear 5, the Servo 900C, and the CV 2000. A two-chambered spring loaded model lung was used to simulate inspiration and a piston pump model lung to simulate active exhalation. We compared both the inspiratory triggering work (WWIt) and expiratory flow-resistive work (WE) of each ventilator while in CPAP mode by calculating the corresponding areas of the pressure-volume loops using electrical integration. The WWIt of our apparatus and demand-flow ventilators was much smaller than that of the CV 2000. In our device, WE was also much smaller than those of the others. These results indicate that our device can be used for CPAP without causing airway pressure fluctuation, and therefore, without imposing an extra workload on the patient.