LEFT-VENTRICULAR DIMENSIONS AND AUTONOMIC BALANCE DURING HEAD-UP TILTDIFFER BETWEEN PATIENTS WITH ISOPROTERENOL-DEPENDENT AND ISOPROTERENOL-INDEPENDENT NEURALLY-MEDIATED SYNCOPE

Objectives. This study sought to elucidate differences in mechanisms o f neurally mediated syncope between patients with syncope induced by h ead up tilt alone and those requiring isoproterenol infusion to induce syncope during head-up tilt. Background. Some patients with neurally mediated syncope require isoproterenol to induce syncope during head-u p tilt (isoproterenol dependent), and others do not (isoproterenol ind ependent). Differences in mechanisms between these two groups have not been well elucidated. Methods. A 60 degrees head-up tilt test was per formed in 13 patients with isoproterenol-independent syncope (Group I, mean [+/-SD] age 28 +/- 12 years), 14 patients with isoproterenol-dep endent syncope (Group II, mean age 34 +/- 14 years) and 20 control sub jects without syncope (Group III, mean age 32 +/- 12 years). Left vent ricular size and contractility were determined by echocardiography, an d sympathovagal balance was determined with power spectral analysis of heart rate variability using a maximal entropy method. Results. Group I patients had smaller left ventricular dimensions than Groups II and III during baseline tilt. During head-up tilt with isoproterenol infu sion (0.01 to 0.04 mu g/kg body weight per min), left ventricular dime nsions decreased to the same extent in Groups II and III, but fraction al shortening was greater in Group II than in Group III at the end of the tilt. The ratio of low (0.05 to 0.15 Hz) to high frequency (0.15 t o 1.0 Hz) component became greater in Group I than in Groups II and II I during the last period of baseline tilt, However, the ratio was grea ter in Group II than in Group III during the last period of the tilt w ith isoproterenol. Conclusions. Patients with isoproterenol independen t syncope had an exaggerated decrease In left ventricular size and sym pathetic predominance preceding syncope during head-up tilt. In contra st, in patients with isoproterenol dependent syncope, similar changes in autonomic nervous balance were evident only during isoproterenol in fusion in addition to head-up tilt.