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Olfactory Evoked Brainstem Autonomic Responses in Subjects with Normal Sensations of Smell

      Background: Quantifiable olfactory-evoked brainstem autonomic responses are not well documented in clinical physiology. We present a preliminary feasibility study of objective measurement of brainstem response to olfaction. Aim: We studied olfactory-evoked brainstem autonomic responses in real time in healthy subjects using a novel neurophysiological method. Methods: Six healthy adults were administered a standard odourous 4% dilution of n-butanol, while continuous brainstem regulated cardiovascular parameters (Heart Rate (HR), R-R interval, Cardiac Vagal Tone (CVT) measured in units of a Linear Vagal Scale (LVS), Cardiac Sensitivity to Baroreflex (CSB), Systolic and Diastolic Blood Pressure (SBP, DBP), Mean Arterial Pressure (MAP)) were being recorded in real time using the NeuroScope. Results: Resting parameters were: HR = 73.3 ± 2.9 beats/min, R-R intervals = 842.7 ± 32.5 ms, CVT = 5.9 ± 1.0 LVS, CSB = 3.6 ± 0.7 ms/mmHg, SBP = 113.7 ± 7.2 mmHg, DBP = 59.0 ± 2.9 mmHg, MAP = 77.2 ± 3.9 mmHg. Response latencies to odours were: 16.2 ± 4.0 s for CVT and 17.2 ± 3.9 s for CSB, with peaks within 24.6 ± 5.7 s for CVT and 25.5 ± 5.3 s for CSB. There was significant increase in CVT (p = 0.029) and CSB (p = 0.004), with peaks of 11.3 ± 4.9 LVS and 9.4 ± 3.9 ms/mmHg respectively, during olfactory stimulation. Sham stimulation did not cause significant responses of CVT nor CSB, and no significant responses were seen in other parameters. Conclusions: We have demonstrated the feasibility of non-invasive clinical evaluation of olfactory-evoked brainstem responses by quantifying the responses of the arterial baroreceptor related cardiovascular reflexes in real time. Other cardiovascular parameters did not respond significantly to our odourous stimulus in these conscious human volunteers.
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