BACKGROUND: Many chiropractors hypothesize that spinal manipulation affects the autonomic nervous system (ANS). However, the ANS responses to chiropractic manipulative therapy are not well documented, and more research is needed to support this hypothesis. This study represents a step toward the development of a reliable method by which to document that chiropractic manipulative therapy does affect the ANS by exploring the use of paced breathing as a way to reduce the inherent variability in ANS measurements. OBJECTIVE: To examine the hypothesis that the variability of ANS measurements would be reduced if breathing were paced to a metronome at 12 breaths/min. SETTING: The study was performed at Parker College Research Institute. Eight normotensive subjects were recruited from the student body and staff. METHODS: Respiration frequency was measured through a strain gauge. A 3-lead electrocardiogram (ECG) was used to register the electric activity of the heart, and arterial tonometry monitors were used to record the left and right radial artery blood pressures. Signals were recorded on an IBM-compatible computer with a sampling frequency of 100 Hz. Normal breathing was used for the first 3 recordings, and breathing was paced to a metronome for the final 3 recordings at 12 breaths/min. Fourier analysis was performed on the beat-by-beat fluctuations of the ECG-determined R-R interval and systolic arterial pressure (SBP). Low-frequency fluctuations (LF; 0.04-0.15 Hz) reflected sympathetic activity, whereas high-frequency fluctuations (HF; 0.15-0.4 Hz) represented parasympathetic activity. Sympathovagal indices were determined from the ratio of the two bandwidths (LF/HF). The coefficient of variation (CV%) for autonomic parameters was calculated ([average/SD] x 100%) to compare breathing normally and breathing to a metronome with respect to variability. One-way analysis of variance was used to detect differences. A value of P < 0.05 was considered statistically significant; all results are presented as average ± SD. RESULTS: Three male and 5 female normotensive subjects were studied. Metronome breathing did not produce any significant changes in blood pressure for the left and right radial arteries, heart rate, or pressure pulse transmission time. Breathing to a metronome increased ECG-HF power (0.25 ± 0.07 vs 0.35 ± 0.09, P < 0.04), decreased ECG-LF/HF (1.08 ± 0.55 vs 0.57 ± 0.35, P < 0.05), and reduced the CV% for ECG-LF (47.6% ± 23.4% vs 23.8% ± 14.6%, P < 0.03), ECG-HF (46.2% ± 14.2% vs 25.8% ± 17.0%, P < 0.03) and ECG-LF/HF (50.1% ± 27.6% vs 23.4% ± 12.3%, P < 0.03) in comparison with normal breathing. Metronome breathing increased the left and right radial artery SBP-HF fluctuations (left, 0.11 ± 0.05 vs 0.30 ± 0.16, P < 0.007; right, 0.09 ± 0.05 vs 0.27 ± 0.15, P < 0.008) and decreased the SBP-LF/HF components (left, 3.42 ± 2.36 vs 1.14 ± 0.88, P > 0.03; right, 3.08 ± 1.77 vs 1.20 ± 0.93, P < 0.02). Metronome breathing did not significantly alter the CV% for SBP-HF, SBP-LF, and SBP-LF/HF. CONCLUSIONS: Metronome breathing increased parasympathetic activity, as evidenced by augmented HF power in the ECG and SBP data. The variability (CV%) of ECG-determined ANS measurements was significantly reduced with paced breathing at 12 breaths/min, but no significant reductions were observed for the SBP-determined ANS measurements. These findings indicate that ECG data are more sensitive than SBP data for future clinical trials.
Keywords: interoception; autonomic nervous system
