Osteopathic Manipulative Treatment and Its Relationship to Autonomic Nervous System Activity as Demonstrated by Heart Rate Variability

For most osteopathic physicians the validation of osteopathic manipulative treatment (OMT) has been largely observational and based on patient outcomes such as improvement in pain scales, range of motion, and other empiric measures.1,2 However, the osteopathic profession has long recognized a relationship between the autonomic nervous system and the function of the body in health and disease, although there is relatively little quantitative data evaluating the relationship between manipulation and the autonomic nervous system.3,4

A theoretical basis for the action of OMT and its effect in the body has been advanced based on autonomic activation causing concomitant vasodilatation, smooth muscle relaxation, and increased blood flow, resulting in improved range of motion, decrease in pain perception, or change in tissue. Until recently this association remained largely a theoretical consideration due to the inability to accurately measure autonomic activity directly. Over the past two decades indirect methods have been developed and refined to provide noninvasive markers of autonomic balance,5,6 with heart rate variability (HRV) being commonly used. HRV is based on the inherent variation of the R-to-R intervals of a standard electrocardiogram (ECG), with these variations largely due to changes in autonomic balance at the sinus node.6-8

Spectral analysis of heart rate variability has been used to study autonomic balance in humans, and it is generally accepted that the high frequency (HF) component (0.15-0.4 Hz) can be used as a marker for vagal modulation of heart rate. Although it is tempting to use the low frequency (LF) component (0.04-0.15 Hz) as a marker for sympathetic activity, its specificity is less clear. Pagani and colleagues9,10 have hypothesized that when the LF component is expressed in normalized units (LFnu) it becomes a better marker of sympathetic modulation of heart rate. For most studies using spectral analysis, the LF/HF ratio is used and considered by many to be a good index of sympathovagal balance.6,7,9,10

The confidence given to the LF/HF ratio accurately reflecting autonomic balance is significantly influenced by experimental design. A tilt protocol involving postural change from horizontal to upright can be used to calibrate the change in the LF/HF ratio which occurs between the two positions and thus set a physiological range for sympathetic and vagal modulation of heart rate. An experimental procedure then can be implemented where comparisons are made of the changes in the LF/HF ratios that occur when the body is shifted from the horizontal to the upright position under conditions with application of an intervention versus without the intervention. In this manner, an experimentally mediated change in LF/HF ratio (i.e., with intervention) can be calibrated against a physiologically relevant change in ratio (i.e., without intervention).

This approach was used by these investigators in a pilot study (n=9 healthy, adult volunteers, 3 females and 6 males) which showed that the LF/HF ratio changed from a mean of 1.75+1.40 (mean+SD) in the horizontal position to a mean of 6.00+1.20 in the 50-degree head-up position. This change reflects an increase in sympathetic tone. Mean heart rate in these subjects increased from 61+7 bpm to 78+2 bpm in the head-up position. The subjects then were treated in the 50-degree head-up position with an OMT procedure, cervical myofascial release, which is thought to increase vagal tone. After the procedure was applied, the LF/HF ratio decreased back down to 1.75+1.58, even though the subjects were still in the head-up position. These data support the initial hypothesis that specific OMT procedures can modulate vagal tone, and also provide information relating to the significance of the LF/HF change. That is, the application of OMT reversed the increase in the ratio that occurs in the 50-degree head-up position.