Is absolute hypovolemia a risk factor for vasovagal response to head-up tilt?
To test the hypothesis that hypovolemia is associated with an increased incidence of vasovagal syncope during head-up tilt (HUT) 45 patients with history of syncope or presyncope were studied. Blood volume (radio-iodinated serum albumin) was determined, then subjects underwent a graded HUT (from 15 degrees-60 degrees HUT) with cuff blood pressure and ECG monitoring. All patients were kept on their own medications during evaluation. Thirty patients (12 male, 18 female, mean age 50 +/- 19 [SD] years) had hypovolemia, defined as blood volume < 90% of lab normal for corresponding sex, while 15 patients (7 male, 8 female, mean age 52 +/- 21 years) were normovolemic with blood volume ranging from 91%-110% of sex-matched normal subjects. The normovolemic patients served as controls. During HUT, a vasovagal response was elicited in 5 of the 30 hypovolemics and in 4 of the 15 normovolemic (16.7% and 26.7%, respectively, P = NS). In those who developed vasovagal response, the changes of heart ra
