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To investigate the incidence of abnormal exercise blood pressure responses in hypertrophic cardiomyopathy (HCM) and the potential role of hemodynamic instability as a mechanism of sudden death, 129 consecutive patients with HCM underwent maximal symptom-limited treadmill exercise testing with blood pressure recording. Four patterns of blood pressure response were observed. Forty-three patients had significant exercise hypotension, with either a continuous fall in systolic blood pressure (n=5) from the start of exercise or a sudden fall in systolic blood pressure (20–100 mm Hg; mean, 40 mm Hg) from the peak value (n=38), 23 patients had a normal response during exercise but an abnormal blood pressure response in the recovery period, and the remaining 62 patients demonstrated a normal blood pressure response. Patients with exercise hypotension were younger (33±14 versus 46±14 years) and more of them had a family history of HCM and sudden death compared with those with a normal blood pressure response (15 of 43 versus 6 of 62 patients). Similarly, the 23 patients with abnormal recovery blood pressure responses were younger (43±16 versus 46±14 years) and had a higher incidence of a family history of sudden death (10 of 24 versus 6 of 62 patients). Left ventricular cavity dimensions were smaller in those with exercise hypotension, but 11 other clinical, echocardiographic, and arrhythmic variables were similar. To assess the mechanism of exercise hypotension, 14 patients who demonstrated exercise hypotension and 14 symptomatic patients with a normal exercise blood pressure response underwent invasive hemodynamic exercise testing. In the hypotensive group, cardiac index increased from 2.6 to 9.5 1/min/m2 at peak exercise similar to the increase found in patients with a normal blood pressure response. Cardiac index at peak exercise was marginally higher in the hypotensive group at peak exercise. Systemic vascular resistance was similar at rest and after 2 minutes of exercise, but resistance was significantly lower in the hypotensive group at peak exercise (428±185 versus 744±187 dynes/sec/cm−5; p = 0.0001). Exercise hypotension is common in HCM (33%) and is due to a fall in systemic vascular resistance occurring despite a rising cardiac index. The association of hemodynamic instability, young age, and an adverse family history emphasizes the potential role of hemodynamic collapse as an initiating mechanism of sudden death in HCM. Prospective evaluation, particularly of young patients, is warranted.