BOSTON, MASSACHUSETTS. Heart rate variability (the variation in the interval between heart beats) is a powerful indicator of the state of the autonomic nervous system (ANS). The variation in the heart beat interval is usually measured via a 5-minute electrocardiogram or 24-hour Holter monitoring. The original and still commonly used measure for the variation is referred to as SDNN which is the standard deviation of the heart beat intervals, that is, the square root of the variance. Most scientific work on heart rate variability (HRV) now uses power spectral density (PSD) analysis to relate the relatively simple measurement of beat to beat variability to the state of the autonomic nervous system. PSD analysis uses a mathematical technique (fast Fourier transform) to determine how the power (variance in heart beat interval) is distributed across different frequency bands. There is now general agreement that the power in the low frequency band (LF) from 0.04 to 0.15 Hz (cycles/second) is an indication of sympathetic (adrenergic) branch activity and that the power in the high frequency band (HF) from 0.15 to 0.40 Hz is primarily an indication of parasympathetic (vagal) activity. It follows that the ratio of LF/HF is a measure of the balance of the autonomic nervous system with a higher number indicating an excess of adrenergic activity and a lower number indicating an excess of vagal activity.

Other important measures derived from HRV analysis include the Poincare ratio and the short-term fractal scaling exponent (DFA1) which are related to sinoatrial node firing patterns. A low Poincare ratio and/or a high DFA1 correspond to a less erratic heart beat (more normal sinoatrial firing).

A low heart rate variability has been implicated in sudden cardiac death, ventricular fibrillation, angina, heart attack, atherosclerosis, and other heart-related problems. HRV analysis has been used extensively in the study of atrial fibrillation. LAF episodes can be divided into two groups – those that are preceded by an increase in LF power and a decrease in HF power consistent with an increase in sympathetic (adrenergic) tone, and those that are preceded by a decrease in LF power and an increase in HF power consistent with an increase in parasympathetic (vagal) tone. The changes in HRV are apparent at least 5 minutes before the actual episode.

A team of researchers from Harvard Medical School, Washington University School of Medicine, Wake Forest University School of Medicine, and the University of Washington now reports that the consumption of oily fish and fish oils strongly influences HRV. Their study involved 4465 older men and women (average age at enrolment of 72 years) who were enrolled in 1989-1990 and then followed for 10 years. At enrolment all participants were given a resting 12-lead ECG or a 24-hour Holter monitor recording and information about their intake of fish and fish oils over the past year was obtained. The researchers observed a significant correlation between the intake of broiled or baked fish (especially tuna) and HRV. They also observed a strong correlation between plasma levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and HRV.

Study participants with a high broiled/baked fish intake experienced a greater HRV (higher SDNN) than those with a lower intake. They also showed lower LF power (lower adrenergic stimulation of the ANS) and higher HF power (increased vagal dominance of the ANS), thus resulting in a lower LF/HF ratio, again suggestive of vagal dominance. High fish consumers also had a lower Poincare ratio and a higher DFA1 indicating a more stable sinoatrial firing pattern. These correlations were also evident when comparing HRV variables with the intake of EPA + DHA. The researchers did not observe any correlation between HRV and the intake of fried (non-fatty) fish.

During the follow-up period, 542 deaths occurred related to cardiovascular causes (1.1%/person-year). The researchers found that high values of SDNN and DFA1 were associated with a 1.1% and 8.4% respectively reduced risk of cardiovascular death, while a low Poincare ratio was associated with a 5.9% risk reduction. They conclude that an increased intake of oily fish (or EPA + DHA) have significant beneficial effects on parameters influencing HRV, specifically an increase in vagal tone, modulation of adrenergic-mediated baroreceptor activity, and improved sinoatrial node function. NOTE: The average daily intake of EPA + DHA ranged from 47 mg to 927 mg.

Mozaffarian, D, et al. Dietary fish and omega-3 fatty acid consumption and heart rate variability in US adults. Circulation, Vol. 117, March 4, 2008, pp. 1130-37

Editor's comment: The confirmation that a high intake of EPA + DHA is associated with a decreased risk of cardiovascular death is indeed encouraging. However, the finding that a high oily fish/fish oil consumption is associated with vagal (parasympathetic) dominance may be less welcome news to vagal afibbers, especially since a high fish oil intake has also been associated with a lower resting heart rate.