Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) can result in left-ventricular distension and the development of pulmonary edema. We present the case of a 13-year-old girl who presented with cardiogenic shock. VA-ECMO was initiated, but after 6 days, severe left-ventricular distension resulted in decreased VA-ECMO flows. With guidance by bedside transesophageal echocardiography, a percutaneous atrial transseptal cannula was placed and connected to the venous circuit, thus decompressing the left ventricle. The patient improved, was weaned from VA-ECMO 5 days later, and was discharged from the hospital. Bedside transseptal catheter insertion is an effective method of left-ventricular decompression.