This study investigated the acute hemodynamic, respiratory, and metabolic effects of blood contact with an extracorporeal membrane oxygenation (ECMO) circuit before initiation of oxygenation or ventilation in an animal model. The results showed non-significant increases in pulmonary vascular resistance and decreases in cardiac output and left ventricle stroke work after 30 minutes of ECMO circulation without gas flow through the membrane. There were also non-significant increases in the alveolar-arterial oxygen gradient and reductions in arterial oxygen saturation and pressure. Overall, the changes observed were modest and non-significant, suggesting blood contact with the primed ECMO circuit resulted in minimal systemic or metabolic alterations in this experimental model.