| Abstract/Notes |
BACKGROUND: The thrust applied during spinal manipulation can stimulate local mechanoreceptors and activate afferent impulses to initiate a somatovisceral reflex. In addition, freeing a spinal fixation by manipulation is believed to normalize transmission of nervous impulses through the affected segment. We postulate that these effects will at least transiently alter autonomic sympathovagal balance, as shown through heart rate variability (HRV). OBJECTIVE: To assess whether there is an acute change in HRV in response to thoracic manipulation. METHODS: HRV was measured in 28 asymptomatic individuals before and after application of a high velocity, low amplitude thrust to the appropriate transverse process, as determined by pre-manipulative palpation, with a diversified pisiform contact. Following a baseline session, each subject was studied in two sessions approximately one week apart. In random order, the upper thorax (T1-T4) was manipulated in one session, and the lower thorax (T8-T12) was manipulated in the other. The biomechanical characteristics of each thrust were measured using a custom designed tri-axial force transducer. HRV was determined from the electrocardiogram (ECG). The subject's breathing was measured with a piezo-electric elastic belt. With the subject in a prone position, the ECG and breathing were recorded for 10 minutes preceding ("pre"), and for 10 minutes following ("post") the manipulation. The low (LF, 0.04 to 0.15 Hz) and high (HF, 0.15 to 0.4 Hz) frequency components of the HRV power spectrum were normalized against the total power minus the ultra-low frequency component (< 0.04 Hz). RESULTS: In normalized units, comparing post-manipulation to pre-manipulation there was no change in LF (41.9 ± 16.7 (SD) vs. 43.9 ± 22.4, p=0.54 upper thoracic; 41.6 ± 19.0 vs. 43.2 ± 19.4, p=0.56 lower thoracic) or HF (48.6 ± 16.6 vs. 47.5 ± 22.1, p=0.69 upper thoracic; 50.4 ± 21.1 vs. 47.2 ± 19.3, p=0.22 lower thoracic). In addition, there was no change in the LF/HF ratio (1.09 ± 0.74 vs. 1.57 ± 1.64, p=0.09 upper thoracic; 1.35 ± 1.81 vs. 1.41 ± 1.56, p=0.67 lower thoracic). However, a "normalizing" response to upper thoracic manipulation was observed in the LF and HF power, as well as in the LF/HF ratio, with post-manipulation values tending more towards their central value. This was evidenced by the interquartile range for the post-manipulation values being less than the pre-manipulation values for LF (20.6 vs. 34.3), HF (24.2 vs. 37.1), and the LF/HF ratio (0.96 vs. 1.60). In addition, the magnitude of the response to upper thoracic manipulation was linearly dependent upon the initial pre-manipulation value (R2=0.45, p=0.0001 for LF; R2=0.44, p=0.0001 for HF; R2=0.79, p<0.0001 for LF/HF). DISCUSSION: These results suggest that autonomic sympathovagal balance can be at least transiently reset by manipulation of the upper, but not lower thoracic spine. Rather than a simple increase or reduction in HRV, the nature of the response to upper thoracic manipulation is of interest. The "normalization" that we observed is suggestive of the longstanding claim that chiropractic manipulation initiates a homeostatic response causing low pre-manipulation values to be increased, and high values to be reduced towards a central value suggestive of a physiologic set-point. Of equal significance is our finding that the strength of the response was linearly dependent upon the magnitude of the initial state, with a progressively greater response being associated with initial values that are further from the central value for this study population. This abstract is reproduced with the permission of the publisher. |
