Comparison of hemodynamic adaptation to orthostatic stress in patients with hypertrophic cardiomyopathy with or without syncope and in vasovagal syncope.

This study was designed to investigate whether, in patients with hypertrophic cardiomyopathy (HC), tilt-induced volume unloading triggers a peripheral reflex similar to that seen in patients with a history of vasovagal syncope or rather acts through an intrinsic cardiac mechanism secondary to diastolic dysfunction. Thirty-seven patients with HC (10 with and 27 without a history of syncope), 10 patients with vasovagal syncope, and 9 controls underwent 70 degrees head-up tilt for 45 minutes during continuous radionuclide monitoring of left ventricular function. We focused on the initial 5 minutes into the tilt test, well before symptoms occurred, to exclude that the observed hemodynamic changes were the consequence rather than the cause of syncope. HC patients with previous syncope and vasovagal patients experienced significant hypotension after the initial 5 minutes of tilt. Only HC patients with a history of syncope had a significant decrease in cardiac output, which began at the initial stage of the test. Systemic vascular resistance decreased in vasovagal patients, but increased in the HC syncopal group. Baseline peak filling rate was lower (2.4 +/- 0.5 vs 3.3 +/- 1.1 stroke counts/s, p = 0.03) and a "pseudonormal" or a restrictive pattern of left ventricular filling was more frequent (70% vs 26%, p = 0.02) in HC patients with than without a history of syncope. Thus, significant hypotension or frank syncope during orthostatic stress in HC patients with a history of syncope is due to an early decrease in cardiac output, which occurs well before the onset of symptoms; such impaired hemodynamic adaptation seems to be related to diastolic dysfunction.