The present case report describes a patient with an artificial mitral valve and dual chamber pacemaker implanted due to perioperative complete atrio-ventricular block. One year later an upgrade to cardiac resynchronization therapy (CRT) combined with ICD function was performed due to significant progression of heart failure symptoms. Beneficial effects of CRT are demonstrated, but unfavourable haemodynamic consequences of right atrial appendage pacing are also underlined. Important interatrial conduction delay during atrial paced rhythm resulted in a significant time difference between optimal sensed and paced atrio-ventricular delay (AVD). This report provides a practical outline how to determine the interatrial delay and the sensed-paced AVD offset under echocardiography in patients treated with CRT.