Ventilatory and diaphragmatic EMG responses to negative-pressure ventilation in airflow obstruction

To assess the responses of patients with chronic obstructive lung disease (COLD) to negative-pressure ventilation (NPV), we studied eight naive patients with moderate to severe COLD before (control) and during NPV with "low" (-10-cmH2O) and "high" (-30-cmH2O) pressure swings in a Drinker tank respirator. Tidal volume (VT) and minute ventilation (VE) were recorded from a Respitrace and diaphragmatic electromyogram (DEMG) from a bipolar esophageal electrode. During short, 5-min runs of "low" and "high" NPV, VT did not change and VE increased in a borderline significant way at -30-cmH2O NPV. Peak integrated DEMG amplitude did not change with respect to control during short runs of NPV. However, when NPV was maintained for 20-60 min, a significant (though small, 20%) decrease in peak DEMG amplitude was observed with respect to control. By contrast, in a ninth patient habituated to NPV, the decrease in peak DEMG amplitude during a 5-min run of NPV was 60%. Significant increases in arterial PO2 (at -10- and -30-cmH2O NPV) and decreases in arterial PCO2 (at -30-cmH2O NPV) were found during NPV for the whole group of patients. One-to-one phase locking between the respirator and patients was the most common pattern of entrainment observed. However, 1:1 phase locking did not preclude the presence of dissociation between the two pacemakers. We conclude that short runs of NPV in naive patients do not result in changes in DEMG, as opposed to immediate and nearly complete cessation of inspiratory activity in trained patients.