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

RATIONALE

End-tidal CO (EtCO) is used to monitor cardiopulmonary resuscitation (CPR), but it can be affected by intrathoracic airway closure. Chest compressions induce oscillations in expired CO, and this could reflect variable degrees of airway patency.

OBJECTIVES

To understand the impact of airway closure during CPR, and the relationship between the capnogram shape, airway closure, and delivered ventilation.

METHODS

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 CO flow was added in the lung to simulate CO production. Capnograms similar to clinical recordings were obtained and different ventilator settings tested. EtCO was compared with alveolar CO (bench). An airway opening index was used to quantify chest compression-induced expired CO oscillations in all three clinical and experimental settings.

MEASUREMENTS AND MAIN RESULTS

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. CO 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 EtCO recorded between ventilator breaths reflected alveolar CO (bench).

CONCLUSIONS

During chest compressions, intrathoracic airway patency greatly affects the delivered ventilation. The expired CO signal can reflect CPR effectiveness but is also dependent on airway patency. The maximal EtCO recorded between consecutive ventilator breaths best reflects alveolar CO.