Computer simulation clarifies mechanisms of carbon dioxide clearance during apnoea.

BACKGROUND: Apnoeic oxygenation can come close to matching the oxygen demands of the apnoeic patient but does not facilitate carbon dioxide (CO2) elimination, potentially resulting in dangerous hypercapnia. Numerous studies have shown that high-flow nasal oxygen administration prevents hypoxaemia, and appears to reduce the rate of increase of arterial CO2 partial pressure (PaCO2), but evidence is lacking to explain these effects. METHODS: We extended a high-fidelity computational simulation of cardiopulmonary physiology to include modules allowing variable effects of: (a) cardiogenic oscillations affecting intrathoracic gas spaces, (b) gas mixing within the anatomical dead space, (c) insufflation into the trachea or above the glottis, and (d) pharyngeal pressure oscillation. We validated this model by reproducing the methods and results of five clinical studies on apnoeic oxygenation. RESULTS: Simulated outputs best matched clinical data for model selection of parameters reflecting: (a) significant effects of cardiogenic oscillations on alveoli, both in terms of strength of the effect (4.5 cm H2O) and percentage of alveoli affected (60%), (b) augmented gas mixing within the anatomical dead space, and (c) pharyngeal pressure oscillations between 0 and 2 cm H2O at 70 Hz. CONCLUSIONS: Cardiogenic oscillations, dead space gas mixing, and micro-ventilation induced by pharyngeal pressure variations appear to be important mechanisms that combine to facilitate the clearance of CO2 during apnoea. Evolution of high-flow oxygen insufflation devices should take advantage of these insights, potentially improving apnoeic gas exchange.