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 rate and blood pressure during HUT were not significantly different between hypovolemics and normovolemics, neither at the endpoint (vasovagal response) nor immediately before the development of the vasovagal response. In patients with nonvasovagal events, four types of hemodynamic responses to tilt were observed: normal blood pressure response associated with normal heart rate increase, normal blood pressure response in association with accentuated increase in heart rate, orthostatic hypotension with normal acceleration of heart rate, and orthostatic hypotension with accelerated increase in heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)