Oxygen cost of breathing for assisted spontaneous breathing modes: Investigation into three states of pulmonary function

Abstract

Objective: We investigated the effects of continuous positive airway pressure (CPAP) and pressure support ventilation (PSV) on the oxygen cost of breathing ( {Mathematical expression}O2resp) for different states of pulmonary function. Additionally {Mathematical expression}O2resp was measured during spontaneous breathing. Design: This was done in a controlled and prospective study. Ventilatory modes were applied randomly. Setting: Measurements were performed in a quiet room on volunteers (VOL) and inpatients treated for chronic obstructive pulmonary disease (COPD). Post-operative patients after aortocoronary bypass surgery (ACB) were studied on the cardio-thoracic intensive care unit just before and after extubation. Patients: Healthy volunteers (n=14), postoperative patients after aorto-coronary bypass surgery (n=15) and patients with COPD (n=9), xFEV1 47.7%) were the objects of study. Interventions: Demand flow CPAP (5 mbar) and PSV (7 mbar, PEEP 5 mbar), using the Hamilton Veolar ventilator, were investigated in comparison to spontaneous breathing. Measurements and results: {Mathematical expression}O2 measured by a Datex Deltatrac metabolic monitor. {Mathematical expression}O2resp was calculated by subtraction of total oxygen uptake {Mathematical expression}O2tot) in controlled mode ventilation (CMV) from that in the respective spontaneous breathing mode. For VOL and COPD patients who were not intubated, a CPAP facemask connected to a short 7.5 mm tube was used as connection to the ventilator. Breathing spontaneously under a canopy system VOL showed a VO2resp of 4.5±4.0% compared to 9.2±3.5% for ACB and 15.4±7.7% for COPD. CPAP changed the VO2resp to 7.8±3.9%, 12.0±4.0% and 9.1±3.6% respectively. PSV reduced the {Mathematical expression}O2resp to 7.9±3.8% in ACB and 7.7±5.5% in COPD. Conclusions: This investigation confirms findings that postoperative patients have a mild increase in {Mathematical expression}O2resp. COPD exhibit the highest increase in VO2resp. Tracheal tubes, masks and CPAP on a demand flow apparatus increases {Mathematical expression}O2resp in volunteers and postoperative patients after cardiac surgery. The same amount of CPAP in contrary reduces {Mathematical expression}O2resp in patients with COPD. Pressure support ventilation can offset the additional {Mathematical expression}O2resp induced by CPAP but at the same level does not further reduce {Mathematical expression}O2resp in COPD patients. © 1995 Springer-Verlag.