The Valsalva maneuver (VM) with beat-to-beat blood pressure and heart rate monitoring are used to evaluate orthostatic intolerance (OI). However, they lack the ability to detect cerebral hemodynamic changes, which may be a cause of OI symptoms. Therefore, we utilized near-infrared spectroscopy during VM. Patients with OI symptoms and normal healthy subjects were recruited. Patients were subgrouped according to VM results: patients with normal VM (NVM) and abnormal VM (AbVM). Oxyhemoglobin (HbO), deoxyhemoglobin, and total hemoglobin changes were measured at four different source–detector distances (SD) (15, 30, 36, and 45 mm), and latency, amplitude, duration, and integrated total signal were calculated. Those parameters were compared between a normal healthy control (HC) group and the two OI patient subgroups. We found that HbO increment latency at 30-mm SD in the HC, NVM, and AbVM groups was as follows: 0.39±0.23 s, 2.79±0.36 s, and 8.14±0.55 s, respectively (p<0.05). Among the four parameters we evaluated, latency of HbO increment was the best marker for differentiating OI.
In this work, we analyzed the clinical applicability of NIRS for use during Quantitative Autonomic Testing (QAT). QAT is a protocol consisting of deep breathing, Valsalva maneuver, and tilt table examination. It is used to diagnose a patient with disorders of the autonomic nervous system (ANS). Disorders of ANS includes orthostatic hyper/hypotension, vasovagal syncope, and postural orthostatic tachycardia syndrome. The results of QAT are typically analyzed with the use of blood pressure and heart rate data, however these metrics may be influenced by factors such as arrhythmia, making the data interpretation and diagnosis difficult for clinicians. We tested our custom built 108-channel NIRS probe on 26 elderly patients during the QAT protocol with various ANS disorders. We found that prefrontal cerebral oxygenation correlated well with blood pressure and heart rate changes for all three tasks, making it a clinically feasible tool for observing ANS functionality. During the Valsalva maneuver, we observed a longer delayed and lower amplitude response of cerebral oxygenation to the prefrontal area in orthostatic intolerant patients. During the tilt table examination, we saw a larger response in cerebral oxygenation and less equal transient cerebral oxygenation during tilt up and tilt down in tilt table examinations that were positive (unhealthy), compared to tilt table examinations that were negative (healthy). Overall, our study showcases NIRS as an enhanced tool for understanding ANS disorders.
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