All variables showed the positive effects of PEEP in conjunction with a recruitment maneuver. Optimal PEEP was 10 cm H2O because at this pressure level the highest compliance value in conjunction with the lowest dead space fraction indicated a maximum amount of effectively expanded alveoli. FRC and Pao2 were insensitive to alveolar over-distension.
LUFU is a non-invasive method for the determination of FRC that requires only minor additional equipment and no modification to the ventilator. It can be used in difficult conditions such as breathing patterns with variations from breath to breath. The results of this study show that LUFU is sufficiently reliable and repeatable to warrant its clinical application.
A method has been developed for reliable measurement of the FRC with an oxygen washin/washout technique. This method is sufficiently easy to use to suit for application in intensive care units. It does not require any action by the operator except a manual change of inspired oxygen concentration. Accuracy and sensitivity of the method have been proven sufficient to meet clinical and scientific requirements. Future clinical studies will reveal the applicability of the chosen procedure under clinical conditions.
Inspiratory muscle unloading decreases ventilatory drive. In this study, we examined the time course of this effect in patients with chronic obstructive pulmonary disease receiving two modes of ventilatory support: pressure support ventilation (PSV), during which each cycle was assisted, and biphasic positive airway pressure (BIPAP), set up in such a manner that one spontaneous breath took place between two consecutive pressure-assisted breaths. The first breath following the switch from spontaneous breathing to PSV was associated with an increase in tidal volume (VT) and a drop in mean transdiaphragmatic pressure (mean Pdi) and inspiratory work (WI) performed per liter but with unchanged values of esophageal occlusion pressure at 100 ms (Pes 0.1), diaphragmatic electrical activity (EMGdi), and WI performed by breath. The same phenomena were observed for the assisted breath of BIPAP as compared with the preceding spontaneous breath. During the subsequent breaths of PSV, Pes 0.1, EMGdi, and WI performed per breath decreased progressively up to the sixth to eighth breaths, and VT returned to pre-PSV values. We conclude that in patients with chronic obstructive pulmonary disease the decrease in ventilatory drive associated with PSV takes place from the first breath onwards but requires six to eight breaths to be fully achieved. During BIPAP, as a consequence of the kinetics of the PSV-induced downregulation of ventilatory drive, assisted breaths following spontaneous breaths are characterized by an enhanced inspiratory efficiency.
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