To diminish the work of breathing, some demand valve systems are equipped with inspiratory pressure support (IPS). The purpose of this study was to evaluate the work performed during spontaneous breathing using the Siemens Servo 900C ventilator (SVC) at the minimal IPS level available, and comparing it with a demand valve ventilator without IPS, the Ohmeda CPU1 (CPU1), and with a home-built continuous flow system (CFS). We found a larger minute ventilation and inspiratory peak flow with the SVC and the CFS than with the CPU1 (p less than 0.05). When the work of breathing was measured at the airway opening, we found that the CFS led to the least amount of work (0.17 +/- 0.05 J.L-1, p less than 0.05). Additionally, this work was strikingly less for SVC than for CPU1 (0.22 +/- 0.06 versus 0.42 +/- 0.10 J.L-1, p less than 0.001) as a result of a higher flow supplied by the SVC. By contrast, the work measured using the esophageal pressure, i.e., including the work dissipated on the lung and airways, was significantly reduced with the CFS (1.34 +/- 0.45 J.L-1, p less than 0.05), but surprisingly not different between SVC and CPU1 (1.49 +/- 0.57 versus 1.54 +/- 0.51 J.L-1). We conclude that the absence of a demand valve in CFS involves the lowest work of breathing. Likewise, the high flow capability in SVC reduces the work necessary to overcome the circuit and demand valve resistances.(ABSTRACT TRUNCATED AT 250 WORDS)