Objective: We investigated the effects of continuous positive airway pressure (CPAP) and pressure support ventilation (PSV) on the oxygen cost of breathing (VO2resp) for different states of pulmonary function. Additionally VO2resp 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: VO2 was measured by a Datex Deltatrac metabolic monitor. VO2resp was calculated by subtraction of total oxygen uptake (VO2tot) 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 VO2resp 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 VO2resp. COPD exhibit the highest increase in VO2resp. Tracheal tubes, masks and CPAP on a demand flow apparatus increases VO2resp in volunteers and postoperative patients after cardiac surgery. The same amount of CPAP in contrary reduces VO2resp in patients with COPD. Pressure support ventilation can offset the additional VO2resp induced by CPAP but at the same level does not further reduce VO2resp in COPD patients.