Autonomic responses to orthostatic stress in head-up tilt testing: relationship to test-induced prolonged asystole

Prolonged asystole is sometimes an extreme manifestation of neurally mediated syncope.

To investigate the mechanism of head-up tilt testing-induced prolonged (life-threatening) cardiac asystole, we measured temporal changes in frequency domain heart rate variability indices in 25 patients with syncope of undetermined etiology.

Head-up tilt testing (80 degrees) was performed in 25 patients for up to 40 min or until asystole or syncope occurred. Three patients (Group 1; 37 +/- 13 years, 1 man 2 women) had an episode of prolonged cardiac asystole (> or = 10 s) during testing, necessitating cardiopulmonary resuscitation. Syncope, but no asystole, was induced in 10 patients (Group 2; 48 +/- 31 years, 6 men, 4 women), and 12 patients (Group 3; 55 +/- 20 years, 5 men, 7 women) failed to show asystole or syncope during testing. Power spectra of low (0.04-0.15 Hz) and high (0.15-0.40 Hz) frequency, and total (0.01-1.00 Hz) frequency spectra were measured in consecutive 2 min segments throughout the test.

Maximally changed values in heart rate, systolic blood pressure, and heart rate variability indices during testing were compared among the three groups (maximally changed values did not include the values during tilt-induced symptoms). High frequency spectra in Groups 2 and 3, but not in Group 1, decreased during the test. High frequency spectra, low frequency spectra, and total spectra in Group 1 were significantly higher than those in Groups 2 and 3 during testing. In Group 1 patients, findings at test-induced asystole were consistent with exaggerated sympathetic and concurrent persistent parasympathetic activity.

Unusual autonomic responses to orthostatic stress can cause prolonged asystole, and this autonomic nerve dysregulation may relate to asystolic episodes associated with cardiovascular collapse.