Capnometry is a cost-efficient method of monitoring carbon dioxide (CO₂) in healthcare settings, especially during mechanical ventilation and cardiopulmonary bypass. However, the utility of capnometers for studying CO₂ elimination in a longer-duration, clinical extracorporeal membrane oxygenation (ECMO) has been poorly characterized. This report presents data from 10 venovenous ECMO patients at Vanderbilt University Medical Center between April and June 2021. Exhaust CO₂ was intermittently measured from a membrane oxygenator’s gas outlet, and arterial blood samples were simultaneously drawn from the patient. Clinical variables including ECMO and ventilator management, patient demographics, and outcomes were recorded. Exhaust CO₂ and other study variables were recorded at 233 discrete time points from the 10 patients. The median duration of ECMO support was 9 days. Sweep gas flow rate demonstrated a strong, negative correlation with exhaust CO₂ concentration, from both univariate and multivariate regression analyses (adjusted β = − 5.556 for sweep < 4 L/min, p < 0.001). Blood flow, oxygenator duration of use, and arterial CO₂ demonstrated weaker relationships with exhaust CO₂ (adjusted β = 2.341, − 0.258, 0.175, respectively), which agrees with the clinical understanding about CO₂ management during ECMO. This descriptive study provides foundation for future research to precisely define the clinical role of ECMO exhaust capnometry in the management of the sweep gas flow and the overall therapy.