AUTONOMIC RESPONSES TO ORTHOSTATIC STRESS IN HEAD-UP TILT TESTING - RELATIONSHIP TO TEST-INDUCED PROLONGED ASYSTOLE

Background: Prolonged asystole is sometimes an extreme manifestation o f neurally mediated syncope. Hypothesis: To investigate the mechanism of head-up tilt testing-induced prolonged (life-threatening) cardiac a systole, we measured temporal changes in frequency domain heart rate v ariability indices in 25 patients with syncope of undetermined etiolog y. Methods: Head-up tilt testing (80 degrees) was performed in 25 pati ents for up to 40 min or until asystole or syncope occurred. Three pat ients (Group 1; 37 +/- 13 years, 1 man, 2 women) had an episode of pro longed cardiac asystole (greater than or equal to 10 s) during testing , necessitating cardiopulmonary resuscitation. Syncope, but no asystol e, was induced in 10 patients (Group 2; 48 +/- 31 years, 6 men, 4 wome n), 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 througho ut the test. Results: Maximally changed values in heart rate, systolic blood pressure, and heart rate variability indices during testing wer e compared among the three groups (maximally changed values did not in clude the values during tilt-induced symptoms). High frequency spectra in Groups 2 and 3, but not in Group I, decreased during the test. Hig h frequency spectra, low frequency spectra, and total spectra in Group 1 were significantly higher than those in Groups 2 and 3 during testi ng. In Group 1 patients, findings at test-induced asystole were consis tent with exaggerated sympathetic and concurrent persistent parasympat hetic activity. Conclusion: Unusual autonomic responses to orthostatic stress can cause prolonged asystole, and this autonomic nerve dysregu lation may relate to asystolic episodes associated with cardiovascular collapse.