Efficacy of Positive vs Negative Pressure Ventilation in Unloading the Respiratory Muscles: Results (1)We investigated two groups of subjects. The first group consisted of five men with normal pulmonary function. The second group was composed of six male patients with COPD, ex-cigarette smokers (48 ±28 pack years, mean ± SD), and average age of 71 ±5 years (range, 63 to 77 years). The study day FEV, was 1.13 ±0.2 L with a mean FEV/FVC of 44 percent ± 11 percent. Previous arterial blood gases demonstrated an average Pa02 of 78 ± 11 mm Hg and a mean PaC02 of 38 ±2 mm Hg. No subject was hypercarbic (PaC02 >45 mm Hg). Using ANOVA for repeated measures, we found that patients with COPD had higher Ve (p<0.01), lower etC02 (p<0.05), and higher peak Pdi (p<0.05) than normals during QB, NPV and PPV (Table 1). However, the response of the measured indices to assisted ventilation was the same in both groups. Therefore, we pooled the data from both groups to compare the effects of AV on Ve, peak Pdi, and ETC02.

Figure 1 is representative of tracings obtained from one of our patients with COPD during QB, NPV, and PPV The tracings of Pdi, EMG, and expired volume during QB represent baseline data. Comparison of the tracings during AV illustrate minimal reduction in Pdi and EMG during NPV, with much greater reduction during PPV. Another measure of diaphragmatic capture is illustrated by the small variation of the Vt during PPV as compared with the variability during NPV.


Figure 1. Representative tracings during QB, NPV, and PPV sessions in a patient with COPD. Note the larger reduction in Pdi and EMG during PPV After discontinuation of PPV (vertical arrow), return of diaphragmatic activity is demonstrated.