The validity and reliability of continuous-wave near-infrared spectroscopy for the assessment of leg blood volume during an orthostatic challenge

Near-infrared spectroscopy (NIRS) can potentially be used to assess the cardiovascular autonomic system by monitoring orthostatic challenge-induced shifts in lower limb blood volume. However, in order to be of clinical utility the test must be valid, reliable, and relatively simple to conduct. Purpose To induce lower limb blood volume shifts using a 10 min 70° head-up tilt, and: (1) in the soleus, determine the validity of an inexpensive continuous wave (cw)-NIRS device by comparing to a criterion frequency-domain (fd-) NIRS device, (2) determine the between-day reliability of soleus assessments obtained from cw-NIRS and fd-NIRS; and, (3) compare the between-day reliability for fd-NIRS assessments obtained at the soleus (standard) and gastrocnemius (simpler alternative). Methods Fifteen non-smoking healthy adults were tested on 3 different mornings, under standardized conditions, separated by a maximum of 7 days. Total haemoglobin concentration (tHb) was continuously monitored bi-laterally in the medial soleus using cw-NIRS and fd-NIRS. For site comparison, tHb was measured in the medial gastrocnemius using fd-NIRS. Results (1) The area under the curve (AUC) for cw-NIRS and fd-NIRS assessments at the soleus were not significantly different (p = 0.619). (2) The criterion (0.75) intra-class correlation coefficient (ICC) was exceeded for both cw-NIRS and fd-NIRS. (3) The criterion ICC was exceeded for both soleus and gastrocnemius assessments. Conclusion Continuous-wave NIRS can be used to monitor orthostatic stress-induced shifts in lower leg blood volume with acceptable validity and reliability. This orthostatic test may present a relatively simple and inexpensive approach for assessing the cardiovascular autonomic nervous system.