Efficacy of Positive vs Negative Pressure Ventilation in Unloading the Respiratory Muscles: Discussion (1)
We have shown that PPV is more effective than NPV in achieving an acute reduction of diaphragmatic activity in the naive awake subject. We interpret the reduction in integrated EMG during PPV to represent a decrease in neural drive. The decrease in peak Pdi and PTI of the diaphragm is consistent with reduced diaphragmatic energy expenditure. Furthermore, the small coefficient of variation of Vt during PPV as compared with NPV is consistent with ventilator capture of the breathing pattern.
We recorded diaphragmatic EMG via surface electrodes placed in the sixth and seventh intercostal spaces. Previous investigators have described that there is a good correlation between surface and esophageal EMG recordings of diaphragmatic activity,- a finding corroborated by our experience (unpublished observations). In addition to the EMG, we also used other measures of diaphragmatic activity such as peak Pdi and PTI. The changes in these two indices were significantly correlated with the change in the iEMG as noted by Rochester et al (r = 0.818 and r = 0.801, respectively) and support the use of surface iEMG as indicative of diaphragmatic activity. ventolin inhalers
Although the AV was applied for short periods, there were large reductions in diaphragmatic activity during PPV During NPV the changes were considerably smaller and not different from the activity during QB. Comparable reductions in iEMG during PPV were described previously, but in this study, measurements were made during non-REM sleep. Our study shows that significant reductions in iEMG, Pdi, and PTI are apparent within a short period of time after initiating nasal PPV in awake naive normal subjects and patients with COPD.