Visual- and Vestibular-Autonomic Influence on Short-Term Cardiovascular Regulatory Mechanisms (Synergy Project with Sensorimotor Adaptation Team)
Craig D. Ramsdell, M.D.
Harvard Medical School
This study involved a repeated measures design with 16 healthy human subjects (eight male, eight female) to examine cardiovascular regulatory responses during actual and virtual head-upright tilts. Baroreflex sensitivity was first evaluated with subjects in supine and upright positions during actual tilt-table testing using both neck barocuff and CSI methods. The responses to actual tilts during this first session were then compared to responses during visually induced tilt and/or rotation obtained during a second session.
Effect of actual changes in posture on baroreflex responses. CSI involves the mathematical analysis of second-to-second fluctuations in non-invasively measured heart rate (HR), arterial blood pressure (ABP), and instantaneous lung volume (ILV, respiratory activity) in order to characterize quantitatively the physiologic mechanisms responsible for the couplings between these signals. A random interval breathing protocol (mean rate of 12 breaths per minute, inter-breath intervals randomly varying between one and 15 seconds) is utilized to broaden the frequency content of the recorded physiological signals, thereby facilitating CSI. Using the CSI technique, we have previously observed significant alterations to the autonomically mediated coupling mechanisms with a change in posture from supine to upright, while non-autonomically mediated mechanisms are left essentially unchanged. Further analysis of data from this first session will utilize CSI measurements to confirm this result, and to quantitatively compare the neck barocuff method with CSI in estimating baroreflex sensitivity.
Carotid baroreflex responses were obtained in both supine and head upright tilt positions using the neck barocuff employed according to the method described by Fritsch et al. (1992). This technique allows assessment of vagally mediated carotid baroreceptor-cardiac reflex responses provoked by neck pressure and suction steps during held expiration. Pressure was increased to 40 mmHg for 5 cardiac cycles, reduced by 15 mmHg decrements after each of the next seven R waves to -65 mmHg, and finally returned to ambient levels. Responses from up to four successful repetitions of this stimulus sequence during both supine and upright positions were averaged. R-R intervals were plotted against carotid distending pressure (taken to be systolic minus neck chamber pressures). There were significant differences between male and female subjects for minimum, maximum and control RR interval (p<0.01). For both male and female subjects, there were highly significant decreases (p<0.0001) in minimum, maximum and control RR intervals when subjects were tilted from the supine to upright position. There were not; however, significant differences in either the RR interval ranges or maximum slopes between these positions.
Cardiovascular responses during virtual tilt and/or rotation. A second session with the same subjects was then used to examine the effects of visually induced virtual tilt and/or rotation stimuli in modulating autonomic cardiovascular reflexes. One of the stimuli involved a simple "mirror bed" to provide an illusion of body tilt without rotation. This device involved mounting a mirror over a subject in a supine orientation to align surrounding visual vertical cues with the subject's longitudinal body axis. In addition to the mirror bed, visually induced tilt and/or rotation illusions were elicited by a full-field virtual environment generator at NASA known as the Preflight Adaptation Trainer DOME. The subject was supine with the head positioned near the center of this large spherical DOME. A virtual scene aligned with the longitudinal body axis was then rotated in the subject's pitch, yaw or roll planes to elicit sensations of tilt and/or rotation. The pitch and yaw DOME visual stimuli rotated about an earth horizontal axis producing the paradoxical sense of tilt and rotation. The roll visual stimulus, on the other hand, rotated about an earth vertical axis typically resulting in the sense of rotation without tilt.
The visual conditions were therefore chosen to provide the following combinations of perceived tilt and/or rotation:
Mirror bed - perceived tilt without rotation
DOME Pitch and Yaw - perceived tilt and rotation
DOME Roll - perceived rotation without tilt
Although there was a high degree of variability across subjects, the mean responses reflect the expected combinations of perceived tilt and rotation described above. The mirror bed was rated by subjects to be the most compelling, with the perceived orientation of the head (54.76.7, meanSEM) slightly greater than the perceived orientation of the body (45.05.7). Cardiovascular responses were recorded during two minutes prior to the start of each virtual tilt and during the initial three minutes with eyes open. Although the data appear to be quite variable, there were a few instances when the changes were quite dramatic. For example, rapid decreases in both systolic and diastolic pressure were observed in some subjects at the onset of the virtual tilt similar to the changes in blood pressure to an actual change in body posture on a tilt table.
Our preliminary results suggest that visually induced virtual tilt can elicit at least transient cardiovascular changes in some individuals. Pending further analysis, we expect to find that the degree of change in cardiovascular reflexes will correlate with individual measures of tilt perception. We will further characterize these effects on cardiovascular regulatory mechanisms using CSI, and expect that visually induced tilts will result in reductions in HR baroreflex sensitivity. The significance of these findings is that virtual environment stimuli may be used in the future to enhance cardiovascular and/or vestibular countermeasures for long-duration spaceflight.