Effect of variable circuit flow rate during the expiratory phase on CO2 elimination

Background: Some continuous flow infant ventilators allow independent setting of inspiratory and expiratory circuit flow rate. In the Drager Babylog 8000+ ventilator, this is called "variable inspiratory, variable expiratory flow" (VIVE). Some clinicians believe that lower expiratory flow decreases expiratory resistance. The minimum expiratory flow rate needed to avoid -re-breathing of carbon dioxide (CO2) has never been established. Objective: We sought to determine if re-breathing becomes evident at the lowest possible expiratory flow rate setting of 1 L/min. Design/methods: We conducted a bench study using end-tidal CO2 (ETCO2) measurement and a 45 mL (90 mL for the "term" model) test lung pre-filled with 100% CO2. We previously showed that the time needed for ETCO2 to be eliminated from the lung is a highly reproducible indicator of efficiency of ventilation. Re-breathing would thus be identified by an increase in the time required for the CO2 to be washed out from the test lung at stable settings of rate and tidal volume (V-T). Using a Babylog 8000+ ventilator in volume guarantee mode with VIVE and a standard ventilator circuit, we tested the effect of decreasing expiratory flow rate under conditions simulating three sizes of patients: extremely low birth weight infant, wt = 600 g (V-T = 3.5 mL, respiratory rate (RR) = 60 breaths min(-1), minute ventilation (MV) = 210 mL/min, expiratory flow rate = 3 L/min, 2 L/min, and 1 L/min), very low birth weight infant, wt = 1.5 kg (V-T = 7 mL, RR = 60 breaths min(-1), MV = 420 mL/min, expiratory flow rate = 4 L/min, 3 L/min, 2 L/min, and 1 L/min), and term infant, wt = 3.6 kg (V-T = 16 mL, RR = 60 breaths min(-1), MV = 960 mL/min, expiratory flow rate = 5 L/min, 4 L/min, 3 L/min, 2 L/min, and 1 L/min). Each measurement was repeated four times and the mean values were compared by analysis of variance for repeated measures. Results: The CO2 elimination times were always within 10% of each other with each repetition. There was no significant increase in the elimination time even at the lowest flows. Conclusion: Clinically significant re-breathing does not occur even with expiratory flow rate at the lowest possible setting of 1 L/min. VIVE is safe to use, although its clinical utility has not been established.