Flow-volume curves as measurement of respiratory mechanics during ventilatory support: The effect of the exhalation valve

Objective: To assess the feasibility of expiratory flow-volume curves as a measurement of respiratory mechanics during ventilatory support: to what extent is the shape of the curve affected by the exhalation valve of the ventilator? Design: Prospective, comparative study. Setting: Medical intensive care unit of a university hospital. Patients: 28 consecutive patients with various conditions, mechanically ventilated with both the Siemens Servo 900C and 300 ventilators, were studied under sedation and paralysis. Interventions: The ventilator circuit was intermittently disconnected from the ventilator at end-inspiration in order to obtain flow-volume curves with and without the exhalation valve in place. Measurements and results: Peak flow (PEF) and the slope of the flow-volume curve during the last 50% of expired volume (SF50) were obtained both with and without the exhalation valve in place. The exhalation valve caused a significant reduction in peak flow of 0.3 l/s (from 1.27 to 0.97 l/s) with the Siemens Servo 900 C ventilator and of 0.42 l/s (from 1.36 to 0.94 l/s) with the Siemens Servo 300 ventilator (p < 0.001). The SF50 was not affected. Conclusion: In mechanically ventilated patients, the exhalation valve causes a significant reduction in peak flow, but does not affect the SF50. This study further suggests that the second part of the expiratory flow-volume curve can be used to estimate patients' respiratory mechanics during ventilatory support.