Lung Carbon Dioxide Elimination Corralates With Physiologic Dead Space Volume During Mechanical Ventilatory Support

Authors: MURAT SUNGUR, MUHAMMED GÜVEN

Abstract: Increased mean airway pressure (Paw) predisposes to increased alveolar dead space volume and, hence, physiologic dead space volume (V{Dphys}. This is the result of overdistending alveoli, converting Zone 2 and Zone 3 units to Zone 1 units. Lung carbon dioxide elimination (LCO_2) is a reflection of pulmonary capillary blood flow. It is hypothesized that as Zone 1 units form or V{Dphys} increases, LCO_2 decreases proportionately and eventually PaCO_2 increases. The purpose of this study is to determine if LCO_2 correlates with V{Dphys} during mechanical ventilation. Six sheep (66.3 ± 6.5 kg), anesthetized with sodium thiopental and paralyzed using pancronium, had pulmonary artery and arterial catheters inserted, and were intubated and ventilated [Fraction of inspired oxygen of 1.0, controlled mechanical ventilation]. Acute lung injury was induced by tracheal instillation of hydrochloric acid (pH 2.5, 0.25 mL/kg). Continuous positive airway pressure (CPAP) levels of 5, 10 and 20 cm H_2O were randomly applied. Cardiac output was maintained nearly constant at all CPAP levels. Data from flow/pressure and infrared capnometer sensors, positioned between the endotracheal tube and the \"Y\" piece of the breathing circuit, were directed to a commercially available respiratory monitor (Novametrix), which provided real time display of Paw and LCO_2 (area under the exhaled volume and CO_2 curve integrated over 1 min). V{Dphys} and the physiologic dead space volume to tidal volume ratio (VD/VT), calculated using the single breath CO_2 elimination technique, were also displayed on the monitor. Data were analyzed using regression analysis; alpha was set at 0.05 for statistical significance. Conclusion: CPAP increases Paw, which correlated positively with V{Dphys}. LCO2 correlated negatively and PaCO2 correlated positively with VD/VT. At VD/VT of approximately 0.5, LCO_2 began decreasing and PaCO_2 increasing. LCO_2 is simple to measure, and real time data provides useful clinical information, i.e., a noninvasive inference of changes in V{Dphys} and PaCO_2 following application of positive pressure.

Keywords: Lung carbon dioxide elimination, CPAP, Dead space

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