Efficacy of Positive vs Negative Pressure Ventilation in Unloading the Respiratory Muscles: Methods (4)
Positive pressure ventilation in the control mode was delivered through a nasal CPAP mask via a volume cycled ventilator (Lifecare, Portable Lifecare PLV 100, Lafayette, CO). Between each ventilator session, the subjects resumed QB for 10 to 15 minutes to allow the breathing pattern to return to baseline conditions. The order of assisted ventilation was randomly assigned. Because of the effect of PaC02 on respiratory drive, initial ventilator adjustments of Vt, flow rate, and frequency during NPV and PPV were made to achieve Ve and etCOa similar to that recorded during the QB. If iEMG activity persisted at equal etCO, levels, ventilator pressures were increased to reduce iEMG. This was usually a problem with NPV during which negative pressures to —40 cm H20 were generally unsatisfactory in reducing iEMG.
The subjects were repeatedly encouraged to relax in order to allow the ventilator to control breathing. If patients indicated that the ventilatory volumes were insufficient, these were adjusted until the patient felt comfortable. At the end of each session of ventilatory support, the patient was disconnected and measurements during spontaneous breathing were repeated. It should be emphasized that the subjects remained awake during the study and kept their mouths closed to prevent escape of air. We believe, therefore, that the volume recordings accurately reflect the true volume changes. The entire study lasted from IV2 to two hours. buy ortho tri-cyclen online
Analysis of Data
Comparisons were made using the averages of the last 3 minutes of the two ventilator sessions with QB. The values during quiet breathing, PPV and NPV were compared by an analysis of variance (ANOVA) for repeated measures. Post hoc comparisons of significant differences were performed using the Tukey-A test (CRUNCH Software, CRUNCH-3, Oakland, CA).