Correlation Between EtCO₂ and PCO₂ in Patients Undergoing Critical Care Transport

End-tidal carbon dioxide (EtCO₂) monitoring is widely used as a surrogate for the partial pressure of carbon dioxide (PCO₂) in critically ill patients receiving manual or mechanical ventilation in prehospital, emergency, and critical care settings. Specific targets for ETCO₂ are a key component of Emergency Medical Services (EMS) protocols, especially for specific patient groups such as those with traumatic brain injury. However, the correlation between EtCO₂ and venous or arterial PCO₂ is uncertain. We aimed to assess the correlation between EtCO₂ and PCO₂ in intubated patients undergoing critical care transport (CCT), and in specific subgroups of patients.

We performed a retrospective review of patients undergoing emergency transport by a multi-state CCT agency. Patients were included if they had an advanced airway and both an EtCO₂ and PCO₂ reading within 5 min of each other. We obtained data on patient demographics, transport characteristics, medical categories, vital signs, lab values, and specific interventions. We performed univariable and multivariable binary logistic regression to assess the association between delta PCO₂ and these characteristics.

We included 6,459 patients (mean age 58.4 years [SD 18.5], 57.1% male), of which a subset of 551 patients had multiple EtCO₂-PCO₂ measurements within 5 min. The median (IQR) initial delta PCO₂ was 12.9 mmHg (7.1, 21.9). 3,967 (61.4%) patients had a delta PCO₂ >10 mmHg and 1,843 (28.5%) had a delta PCO₂ >20 mmHg. We identified an independent association between delta PCO₂ >10 mmHg and age, male sex, interfacility transport, venous sampling site, respiratory rate, hypotension, hypoxia, and thoracostomy. In patients with multiple blood gas measurements, 76% had delta PCO₂ >10 mmHg over the duration of the transport.

We identified substantial differences between EtCO₂ and PCO₂ across patients with medical and traumatic conditions undergoing critical care transport. The PCO₂ assessment should be strongly considered as part of ventilatory management in patients encountered in emergency and critical care settings.