Intrathoracic Airway Closure Impacts CO₂ Signal and Delivered Ventilation during Cardiopulmonary Resuscitation.

Abstract

End-tidal CO₂ (EtCO₂) is used to monitor cardiopulmonary resuscitation (CPR), but it can be affected by intrathoracic airway closure. Chest compressions induce oscillations in expired CO₂, and this could reflect variable degrees of airway patency. To understand the impact of airway closure during CPR, and the relationship between the capnogram shape, airway closure, and delivered ventilation. This study had three parts: 1) a clinical study analyzing capnograms after intubation in patients with out-of-hospital cardiac arrest receiving continuous chest compressions, 2) a bench model, and 3) experiments with human cadavers. For 2 and 3, a constant CO₂ flow was added in the lung to simulate CO₂ production. Capnograms similar to clinical recordings were obtained and different ventilator settings tested. EtCO₂ was compared with alveolar CO₂ (bench). An airway opening index was used to quantify chest compression-induced expired CO₂ oscillations in all three clinical and experimental settings. A total of 89 patients were analyzed (mean age, 69 ± 15 yr; 23% female; 12% of hospital admission survival): capnograms exhibited various degrees of oscillations, quantified by the opening index. CO₂ value varied considerably across oscillations related to consecutive chest compressions. In bench and cadavers, similar capnograms were reproduced with different degrees of airway closure. Differences in airway patency were associated with huge changes in delivered ventilation. The opening index and delivered ventilation increased with positive end-expiratory pressure, without affecting intrathoracic pressure. Maximal EtCO₂ recorded between ventilator breaths reflected alveolar CO₂ (bench). During chest compressions, intrathoracic airway patency greatly affects the delivered ventilation. The expired CO₂ signal can reflect CPR effectiveness but is also dependent on airway patency. The maximal EtCO₂ recorded between consecutive ventilator breaths best reflects alveolar CO₂.