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

Peter A Keszler,¹ Pankaj Nagaraj,¹ Kabir Abubakar,¹ Martin Keszler<sup>2

1</sup>Georgetown University, Washington, DC, USA; Georgetown University Hospital, Washington, DC, USA; ²Brown University, Women and Infants Hospital of Rhode Island, Providence, RI, USA

Background: Some continuous flow infant ventilators allow independent setting of inspiratory and expiratory circuit flow rate. In the Dräger 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.
Keywords: mechanical ventilation, newborn, bench study, re-breathing, expiratory flow rate, end-tidal CO₂