Cerebral and circulatory haemodynamics before vasovagal syncope induced by orthostatic stress

Abstract

Vasovagal syncope is usually described as a sudden and transient loss of consciousness that resolves spontaneously. Cardiocirculatory changes are well described during and before syncope. However, changes in the cerebral oxygenation are not well defined. In this study, near-infrared spectroscopy (NIRS) was used to assess the cerebral oxygenation directly during 80 degree head-up (HU) tilt. To simulate central hypovolaemia, 500 ml of blood was drawn from each of 10 healthy subjects. Oxygenation index (OI) was defined as the difference between oxy- and deoxyhaemoglobin concentration. Blood pressure, heart rate and cardiac output were monitored using a finger plethysmographic device. The protocol was divided into two stages, each consisting of a 15-min stabilization period in the supine (SUP) position, 15 min in HU position and another 10 min in SUP position. Between both stages, blood was drawn from the subject. Haemoglobin concentration and haematocrit were measured before and 30 min after withdrawal of blood. No compensatory haemodilution was observed. During HU position in the second stage, six subjects 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) decrease (-1.4 +/- 0.5 microM min-1) compared with NF (-0.18 +/- 0.16 microM min-1). This indicates that the onset of (pre)syncope is preceded by a mismatch 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 visible.

Systemic circulation and cerebral oxygenation during head-up tilt in spinal cord injured individuals

“…Therefore, this study used [HHb] changes as a measure for cerebral oxygenation. [HHb], re¯ecting the change in match between oxygen consumption and delivery, 7,8 was signi®cantly increased in both groups during head-up tilt. Since cerebral oxygen demand is expected to be independent of body position, the increased [HHb] found indicates a decrease in oxygen delivery, ie cerebral blood¯ow, while meeting cerebral oxygen demand as indicated by the general absence of presyncope symptoms.…”

“…This non-invasive method has been described in greater detail in earlier studies. 7,8 Optodes were placed above the left eyebrow, using an interoptode distance of 4.5 cm. A pathlength factor of 6 was used.…”

“…Possibly more relevant than cerebral¯ow detected by transcranial Doppler, near infrared spectroscopy (NIRS) re¯ects the match between oxygen delivery to and consumption by the cerebrum. 7,8 The aim of this study therefore, was to compare AB individuals with SCI individuals with a lesion above T4 regarding changes in cardiovascular variables and cerebral oxygenation induced by tilt.…”

The role of cerebral oxygenation and regional cerebral blood flow on tolerance to central hypovolemia

“…Furthermore, in a study of fainters vs. nonfainters following withdrawal of 500 ml of blood plus head-up tilt (3), the fainters had decreased oxygen extraction in the cerebral tissues (positive oxygenation index, i.e., HbO 2 Ϫ dHb), while the nonfainters had increased oxygen extraction (negative oxygenation index), suggesting that increased tolerance may be due to increased cerebral oxygen extraction. The onset of presyncope is thought to be due to a mismatch between oxygen supply and demand in the brain (3,25), but this hypothesis has not been explored in relation to tolerance to maximal central hypovolemia.…”

Perceived Exertion is Not Necessarily Associated with Altered Brain Activity during Exercise

“…Cerebral oxygenation changes reflect cerebral functional activation (Colier et al, 1997;Colier et al, 1999;Kleinschmidt et al, 1996;Shibuya et al, 2004aShibuya et al, , 2004bShibuya and Tachi, 2006;Shibuya and Kuboyama, 2007;Shibuya et al, 2008;Obrig et al, 1996), and these changes can be measured using several techniques. Near-infrared spectroscopy (NIRS) and functional magnetic resonance imaging enable the noninvasive (Colier et al, 1999;Kleinschmidt et al, 1996;Mehagnoul-Schipper et al, 2000;Obrig et al, 1996, Obrig et al, 2000 In fact, Ide et al (1999) and Ide and Secher (2000) reported that prefrontal cortex activation (oxygenation) increased with increase in exercise intensity.…”