A middle-aged man presented with dyspnea for 1 month. His medical history included familial hypertrophic cardiomyopathy and implantable cardioverter-defibrillator implantation for sinus bradycardia and advanced atrioventricular (AV) block and to prevent sudden cardiac death. DDD pacing was set to 60 bpm with postventricular atrial refractory period (PVARP) of 350 ms. Prolonged AV delay of 350 ms was needed because of severe atrial conduction delay. Physical examination showed signs of heart failure—peripheral edema and S3 gallop. Echocardiography showed reduced left ventricular systolic function; however, transmitral flow was not of restrictive pattern (Figure 1A). Atrial contraction due to ventriculoatrial (VA) conduction during systole was suspected based on the pulmonary venous (PV) flow and verified on intracardiac electrocardiogram (Figure 1B, C). We thought that heart failure was caused by masked systolic atrial contraction. An accessory pathway, which was elucidated electrophysiologically via an implantable cardioverter-defibrillator, caused VA conduction (Figure 2A, B). The patient refused curative abolition of the pathway. VA conduction disappeared in an AV delay of 150 ms, but diastolic atrial contraction also disappeared (Figure 3A) and delayed systolic atrial contraction appeared by the PV flow (Figure 3B). The AV delay of 280 ms caused preserved diastolic atrial contraction without VA conduction (Figure 4). Follow-up after 1 year showed resolution of heart failure symptoms.