NEAR-INFRARED SPECTROSCOPY ANALYSIS OF CEREBRAL OXYGENATION CHANGES IN RESPONSE TO ACUTE INTERMITTENT ALTITUDE EXPOSURE

Abstract

Near-infrared spectroscopy allows the non-invasive monitoring of tissue oxygenation and is widely used in clinical research. Changes in oxyhaemoglobin to total haemoglobin content are used to calculate the tissue oxygenation index as a percentage (Boushel et al., 2001: Scandinavian Journal of Medicine and Science in Sports, 11, 213 – 222). However, near infrared spectroscopy is not widely available to assess acute haemodynamic responses to intermittent moderate altitude exposure in healthy individuals. Therefore, the purpose of this study was to investigate acute changes in cerebral oxygenation in response to intermittent hypoxic exposure at moderate altitude in physically active adults. After receiving institutional ethical approval, seven male sports science students (mean age 26.1 years, s=7.44; stature 1.77 m, s=4.67; body mass 75.1 kg, s=12.6) participated in two resting tests, the first breathing normoxic air for 5 min to record baseline values and the second breathing normobaric hypoxic air for 15 min to simulate an altitude of 3048 m. A near-infrared spectrometer (NIRO 200, Hamamatsu Photonics, Germany) was used to determine oxygenation. An optode was placed on the right temple and secured using tape and a bandage. Statistical significance was set at P≤0.05 and statistical differences were analysed using a one-sample t-test. A significant difference (P≤0.001) in the cerebral tissue oxygenation index was observed between normoxic and hypoxic conditions (mean 67.7%, s=0.39 and 65.6%, s=1.23, respectively). The tissue oxygenation index decreased by only a further 1.4% between minutes 5 and 15 during hypoxia. Values for the normoxic cerebral tissue oxygenation index were comparable to those previously reported (Imray et al., 2000: Clinical Science, 98, 159 – 164). Acute normobaric hypoxia results in hyperventilation, causing a decrease in the partial pressure of arterial carbon dioxide and vasoconstriction of cerebral vessels (Tobias, 2006: Expert Review of Medical Devices, 3, 235 – 243). This may have resulted in a slight reduction in cerebral blood flow, which may partially explain the lower oxygenation values observed. The present study found that decreases in the acute cerebral tissue oxygenation index were greatest between 2 and 5 min after the onset of moderate altitude intermittent hypoxia. Near infrared spectroscopy provides a suitable means of haematological analysis during intermittent hypoxia and could help develop more effective altitude training strategies. Additionally, it could be used to aid interpretation of early stage responses to acute mountain sickness. Future research could focus on acute blood flow kinetics and ventilatory responses to establish their relationships with cerebral oxygenation during intermittent hypoxic exposure in healthy physically active individuals.