CEREBRAL AND CIRCULATORY HEMODYNAMICS BEFORE VASOVAGAL SYNCOPE INDUCED BY ORTHOSTATIC STRESS

Vasovagal syncope is usually described as a sudden and transient loss of consciousness that resolves spontaneously. Cardiocirculatory change s are well described during and before syncope. However, changes in th e cerebral oxygenation are not well defined. In this study, near-infra red spectroscopy (NIRS) was used to assess the cerebral oxygenation di rectly during 80 degrees head-up (HU) tilt. To simulate central hypovo laemia, 500 ml of blood was drawn from each of 10 healthy subjects. Ox ygenation index (OI) was defined as the difference between oxy- and de oxyhaemoglobin concentration. Blood pressure, heart rate and cardiac o utput were monitored using a finger plethysmographic device. The proto col was divided into two stages, each consisting of a 15-min stabiliza tion period in the supine (SUP) position, 15 min in HU position and an other 10 min in SUP position. Between both stages, blood was drawn fro m the subject. Haemoglobin concentration and haematocrit were measured before and 30 min after withdrawal of blood. No compensatory haemodil ution was observed. During HU position in the second stage, six subjec ts showed signs of presyncope (F) and four did not (NF). A significant difference between F and NF was found in the observation that, before fainting, the OI of F showed a steady and significant (P=0.02) decrea se (-1.4+/-0.5 mu M min(-1)) compared with NF (-0.18+/-0.16 mu M min(- 1)). This indicates that the onset of (pre)syncope is preceded by a mi smatch between oxygen demand and oxygen supply in the cerebrum. Using NIRS enabled us to monitor this mismatch and to predict the onset of a syncope before clear signs in cardiocirculatory variables were visibl e.