RELATIONSHIP BETWEEN PEAK AEROBIC POWER AND CEREBRAL HEMODYNAMICS IN PATIENTS WITH TRAUMATIC BRAIN INJURY

Abstract

Patients with traumatic brain injury (TBI) are characterized by premature fatigue that may be related to their low aerobic capacity and reduced cortical activation. Near infrared spectroscopy (NIRS) is a valid, noninvasive technique for determining cerebral oxygenation (Cox) and blood volume (Cbv) during exercise. An increase in Cox and Cbv measured by NIRS is reflective of increased cortical activation. PURPOSE: To examine the inter-relationships among the peak values of the power output, oxygen uptake (peak VO2), Cox and Cbv during incremental cycle exercise to voluntary fatigue in patients with TBI. METHODS: Twenty-one inpatients (17 males, 4 females) with TBI from a rehabilitation center consented to perform a symptom limited incremental exercise test to voluntary fatigue. Cardiorespiratory responses were continuously monitored using an automated metabolic measurement cart (Vmax, Sensormedics, CA) interfaced with an electro-cardiogram. NIRS was used to continuosuly record Cox and Cbv from both the frontal lobes using a dual wave-length instrument (MicroRunman, NIM Inc., PA). Data were averaged over 20 sec intervals for statistical analysis. RESULTS: Regression analysis indicated that oxygen uptake and heart rate increased linearly with work rate while Cox and Cbv increased in a curvilinear manner. Patients attained a peak heart rate that corresponded to 76% of their age predicted maximum heart rate (220 - age), indicating an inability to fully tax their cardiovascular system. The peak values of power (86.4 ± 59.4 Watts), VO2 (20.5 ± 8.1 ml/kg/min) and O2 pulse (11.0 ± 5.3 ml/beat) were considerably lower than those reported for healthy individuals matched for age and gender. A significant (P < .05) relationship was observed between peak power vs absolute peak VO2 (r = 0.76) and relative peak VO2 (r = 0.72). In general, stronger correlations were observed between the hemodynamic variables on the left side and the peak power and peak oxygen uptake as compared to the right side. However, only Cbv on the left side was significantly related to the peak power output (r = 0.51), absolute peak VO2 (r = 0.61) and relative peak VO2 (r = 0.54). CONCLUSIONS: These findings suggest that changes in cerebral blood volume in the left frontal lobe may play an important role in determining the peak exercise capacity in patients with TBI. Support: Alberta Heritage Foundation for Medical