George E. Billman

Clinical Prevention of Sudden Cardiac Death by n-3 Polyunsaturated Fatty Acids and Mechanism of Prevention of Arrhythmias by n-3 Fish Oils

This review will be limited specifically to the beneficial prevention by the n-3 polyunsaturated fatty acids (PUFAs) of arrhythmic deaths, including sudden cardiac death, which annually causes some 300 000 deaths in the United States and millions more worldwide. We will also show that the growing body of positive clinical studies is supported by what has been learned in animal and laboratory studies regarding the mechanism by which n-3 PUFAs prevent cardiac arrhythmias. See p 2632 Figure 1 shows the 2 essential classes of PUFAs, the n-6 (ω6) and n-3 (ω3) classes. Both classes are “essential” because we cannot make them in our bodies. They must come in our diets, and they are essential for normal growth, development, and optimal function of brain, heart, and probably other systems. The parent fatty acid of the n-6 class, linoleic acid (C18:2n-6; LA) has 18 carbon atoms in its acyl chain, and the first C=C double bond is 6 carbons back from the methyl end of the fatty acid, hence the “n-6” appellation. In the bodies of animals, including humans, LA can be elongated and desaturated through a series of enzymatic steps to form arachidonic acid (C20:4n-6; AA). AA is the source of the n-6 eicosanoids that result from oxygenation of AA by cyclooxygenase, lipoxygenase, and epoxygenase enzymes to form prostaglandins, leukotrienes, lipoxines, and P-450 compounds, which in many instances are potent cell messengers. Figure 1. Two classes of essential PUFAs. In the chloroplast of green plants, algae, and phytoplankton, LA can be further desaturated in the n-3 position to yield α-linolenic acid (C18:3 n-3; ALA), the 18-carbon parent fatty acid of the n-3 class. ALA can be further elongated and desaturated by the same enzymes that convert n-6 LA to AA to form the 20-carbon n-3 analog of AA, namely, eicosapentaenoic acid …

Prevention of Sudden Cardiac Death by Dietary Pure ω-3 Polyunsaturated Fatty Acids in Dogs

Background —Rat diets high in fish oil have been shown to be protective against ischemia-induced fatal ventricular arrhythmias. Increasing evidence suggests that this may also apply to humans. To confirm the evidence in animals, we tested a concentrate of the free fish-oil fatty acids and found them to be antiarrhythmic. In this study, we tested the pure free fatty acids of the 2 major dietary ω-3 polyunsaturated fatty acids in fish oil: cis-5,8,11,14,17-eicosapentaenoic acid (C20:5ω-3) and cis-4,7,10,13,16,19-docosahexaenoic acid (C22:6ω-3), and the parent ω-3 fatty acid in some vegetable oils, cis-9,12,15-α-linolenic acid (C18:3ω-3), administered intravenously on albumin or a phospholipid emulsion. Methods and Results —The tests were performed in a dog model of cardiac sudden death. Dogs were prepared with a large anterior wall myocardial infarction produced surgically and an inflatable cuff placed around the left circumflex coronary artery. With the dogs running on a treadmill 1 month after the surgery, occlusion of the left circumflex artery regularly produced ventricular fibrillation in the control tests done 1 week before and after the test, with the ω-3 fatty acids administered intravenously as their pure free fatty acid. With infusion of the eicosapentaenoic acid, 5 of 7 dogs were protected from fatal ventricular arrhythmias ( P <0.02). With docosahexaenoic acid, 6 of 8 dogs were protected, and with α-linolenic acid, 6 of 8 dogs were also protected ( P <0.004 for each). The before and after control studies performed on the same animal all resulted in fatal ventricular arrhythmias, from which they were defibrillated. Conclusions —These results indicate that purified ω-3 fatty acids can prevent ischemia-induced ventricular fibrillation in this dog model of sudden cardiac death.

The LF/HF ratio does not accurately measure cardiac sympatho-vagal balance.

Power spectral analysis of the beat-to-beat variations of heart rate or the heart period (R–R interval) has become widely used to quantify cardiac autonomic regulation (Appel et al., 1989; Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996; Berntson et al., 1997; Denver et al., 2007; Thayler et al., 2010; Billman, 2011). This technique partitions the total variance (the “power”) of a continuous series of beats into its frequency components, typically identifying two or three main peaks: Very Low Frequency (VLF) <0.04 Hz, Low Frequency (LF), 0.04–0.15 Hz, and High Frequency (HF) 0.15–0.4 Hz. It should be noted that the HF peak is shifted to a higher range (typically 0.24–1.04 Hz) in infants and during exercise (Berntson et al., 1997). The HF peak is widely believed to reflect cardiac parasympathetic nerve activity while the LF, although more complex, is often assumed to have a dominant sympathetic component (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology, 1996; Berntson et al., 1997; Billman, 2011). Based upon these assumptions, Pagani and co-workers proposed that the ratio of LF to HF (LF/HF) could be used to quantify the changing relationship between sympathetic and parasympathetic nerve activities (i.e., the sympatho-vagal balance) (Pagani et al., 1984, 1986; Malliani et al., 1991) in both health and disease. However, this concept has been challenged (Kingwell et al., 1994; Koh et al., 1994; Hopf et al., 1995; Eckberg, 1997; Houle and Billman, 1999; Billman, 2011). Despite serious and largely under-appreciated limitations, the LF/HF ratio has gained wide acceptance as a tool to assess cardiovascular autonomic regulation where increases in LF/HF are assumed to reflect a shift to “sympathetic dominance” and decreases in this index correspond to a “parasympathetic dominance.” Therefore, it is vital to provide a critical assessment of the assumptions upon which this concept is based.

Heart Rate Variability – A Historical Perspective

Heart rate variability (HRV), the beat-to-beat variation in either heart rate or the duration of the R–R interval – the heart period, has become a popular clinical and investigational tool. The temporal fluctuations in heart rate exhibit a marked synchrony with respiration (increasing during inspiration and decreasing during expiration – the so called respiratory sinus arrhythmia, RSA) and are widely believed to reflect changes in cardiac autonomic regulation. Although the exact contributions of the parasympathetic and the sympathetic divisions of the autonomic nervous system to this variability are controversial and remain the subject of active investigation and debate, a number of time and frequency domain techniques have been developed to provide insight into cardiac autonomic regulation in both health and disease. It is the purpose of this essay to provide an historical overview of the evolution in the concept of HRV. Briefly, pulse rate was first measured by ancient Greek physicians and scientists. However, it was not until the invention of the “Physician’s Pulse Watch” (a watch with a second hand that could be stopped) in 1707 that changes in pulse rate could be accurately assessed. The Rev. Stephen Hales (1733) was the first to note that pulse varied with respiration and in 1847 Carl Ludwig was the first to record RSA. With the measurement of the ECG (1895) and advent of digital signal processing techniques in the 1960s, investigation of HRV and its relationship to health and disease has exploded. This essay will conclude with a brief description of time domain, frequency domain, and non-linear dynamic analysis techniques (and their limitations) that are commonly used to measure HRV.

Prevention of ischemia-induced cardiac Sudden death by n−3 polyunsaturated fatty acids in dogs

The objective of this study was to obtain functional information associated with the prevention by n−3 polyunsaturated fatty acids (PUFA) of ischemia-induced fatal cardiac ventricular arrhythmias in the intact, conscious, exercising dog. Thirteen dogs suceptible to ischemia-induced ventricular fibrillation were prepared surgically by ligation of their anterior descending left coronary artery and placement of an inflatable cuff around their left circumflex artery. After 4 wk of recovery, exercise-plus-ischemia tests were performed without and then with an intravenous infusion of an emulsion of free n−3 PUFA just prior to occluding the left circumflex artery while the animals were running on a treadmill. One week later the exercise-plus-ischemia test was repeated but with a control infusion replacing the emulsion of n−3 PUFA. The infusion of the free n−3 PUFA in quantities of 1.0 to 10 g prevented ventricular fibrillation in 10 of the 13 dogs tested (P<0.005), apparently without esterification of the PUFA into membrane phospholipids. The antiarrhythmic effect of the n−3 PUFA was associated with slowing of the heart rate, shortening of the QT-interval (electrical action potential duration), reduction of left ventricular systolic pressure, and prolongation of the electrocardiographic atrial-ventricular conduction time (P-R interval). These effects are comparable with those we have reported in studies with cultured neonatal rat cardiac myocytes.

Low-frequency component of the heart rate variability spectrum: a poor marker of sympathetic activity

The low-frequency component of the heart rate variability spectrum (0.06-0.10 Hz) is often used as an accurate reflection of sympathetic activity. Therefore, interventions that enhance cardiac sympathetic drive, e.g., exercise and myocardial ischemia, should elicit increases in the low-frequency power. Furthermore, because an enhanced sympathetic activation has been linked to an increased propensity for malignant arrhythmias, one might also predict a greater low-frequency power in animals that are susceptible to ventricular fibrillation than in resistant animals. To test these hypotheses, a 2-min coronary occlusion was made during the last minute of exercise in 71 dogs with healed myocardial infarctions: 43 had ventricular fibrillation (susceptible) and 28 did not experience arrhythmias (resistant). Exercise or ischemia alone provoked significant heart rate increases in both groups of animals, with the largest increase in the susceptible animals. These heart rate increases were attenuated by beta-adrenergic receptor blockade. Despite the sympathetically mediated increases in heart rate, the low-frequency power decreased, rather than increased, in both groups, with the largest decrease again in the susceptible animals: 4.0 +/- 0.2 (susceptible) vs. 4.1 +/- 0.2 ln ms2 (resistant) in preexercise control and 2.2 +/- 0.2 (susceptible) vs. 2.9 +/- 0.2 ln ms2 (resistant) at highest exercise level. In a similar manner the parasympathetic antagonist atropine sulfate elicited significant reductions in the low-frequency power. Although sympathetic nerve activity was not directly recorded, these data suggest that the low-frequency component of the heart rate power spectrum probably results from an interaction of the sympathetic and parasympathetic nervous systems and, as such, does not accurately reflect changes in the sympathetic activity.The low-frequency component of the heart rate variability spectrum (0.06-0.10 Hz) is often used as an accurate reflection of sympathetic activity. Therefore, interventions that enhance cardiac sympathetic drive, e.g., exercise and myocardial ischemia, should elicit increases in the low-frequency power. Furthermore, because an enhanced sympathetic activation has been linked to an increased propensity for malignant arrhythmias, one might also predict a greater low-frequency power in animals that are susceptible to ventricular fibrillation than in resistant animals. To test these hypotheses, a 2-min coronary occlusion was made during the last minute of exercise in 71 dogs with healed myocardial infarctions: 43 had ventricular fibrillation (susceptible) and 28 did not experience arrhythmias (resistant). Exercise or ischemia alone provoked significant heart rate increases in both groups of animals, with the largest increase in the susceptible animals. These heart rate increases were attenuated by β-adrenergic receptor blockade. Despite the sympathetically mediated increases in heart rate, the low-frequency power decreased, rather than increased, in both groups, with the largest decrease again in the susceptible animals: 4.0 ± 0.2 (susceptible) vs. 4.1 ± 0.2 ln ms2 (resistant) in preexercise control and 2.2 ± 0.2 (susceptible) vs. 2.9 ± 0.2 ln ms2 (resistant) at highest exercise level. In a similar manner the parasympathetic antagonist atropine sulfate elicited significant reductions in the low-frequency power. Although sympathetic nerve activity was not directly recorded, these data suggest that the low-frequency component of the heart rate power spectrum probably results from an interaction of the sympathetic and parasympathetic nervous systems and, as such, does not accurately reflect changes in the sympathetic activity.

Omega-3 Fatty Acids and Cardiac Arrhythmias: Prior Studies and Recommendations for Future Research A Report from the National Heart, Lung, and Blood Institute and Office of Dietary Supplements Omega-3 Fatty Acids and Their Role in Cardiac Arrhythmogenesis Workshop

Compared with prehistoric times, the ratio of n-6 to n-3 fatty acids in the modern diet has increased ≈10-fold to 20:1.1,2 A substantial body of evidence suggests that n-3 polyunsaturated fatty acids (PUFAs) provide cardiovascular protection and prevent arrhythmias.3–5 This has led to the recommendation by the American Heart Association that all adults eat fatty fish at least 2 times per week and that patients with coronary heart disease (CHD) are advised to consume ≈1 g/d of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) combined.6,7 The evidence base is not entirely consistent, and a number of randomized trials have failed to show a protective effect of n-3 PUFAs against arrhythmias.8–10 This has led to some uncertainty regarding the appropriate recommendations for their use.11 The present review originates from the Omega-3 Fatty Acids and Their Role in Cardiac Arrhythmogenesis Workshop sponsored by the National Heart, Lung, and Blood Institute and the Office of Dietary Supplements on August 29–30, 2005, and includes the findings from the recently published trials. Data from epidemiological studies, randomized clinical trials, animal studies, and basic science mechanistic studies on the role of n-3 PUFAs in arrhythmia prevention are examined. Areas in which the data are conflicting or our current knowledge is lacking are emphasized. Fatty acids are classified by the length of the carbon chain (long chain, n=20 to 22; intermediate chain, n=18) and the number of double bonds (saturated, monounsaturated, polyunsaturated).1,2 For PUFAs, the location of the first double bond relative to the -CH3 or omega (n-) end is given. Long- and intermediate-chain fatty acids must be ingested as part of the diet because they cannot be synthesized by humans and are therefore referred to as essential. The most common dietary fatty acids include (1) the omega-6 linoleic acid …

Prevention of sudden cardiac death by n−3 polyunsaturated fatty acids

There were already several epidemiologic studies that showed eating fish frequently seemed to reduce deaths from coronary heart disease. There were also observational and clinical trials that more specifically showed that the reduction in cardiovascular deaths from eating fish was largely the result of the prevention of sudden cardiac death by n-3 polyunsaturated fatty acids in fish oil. This led me to perform a clinical trial in which all subjects had an implanted cardioverter-defibrillator and were at very high risk of sudden cardiac death. The results of this study and the mechanisms by which n-3 fish oil fatty acids prevent fatal cardiac arrhythmias will be the subject of this review.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Response of Failing Canine and Human Heart Cells to β2-Adrenergic Stimulation

Background Failing human hearts lose β1- but not β2-adrenergic receptors. In canine hearts with tachypacing failure, the ratio of β2- to β1-adrenergic receptors is increased. The present study was designed to determine whether heart failure increases sensitivity to β2-adrenergic stimulation in isolated canine ventricular cardiomyocytes and to verify that myocytes from failing human ventricles contain functional β2-adrenergic receptors. Methods and Results Myocytes from healthy dogs, dogs with tachypacing failure, and human transplant recipients were loaded with fura 2-AM and subjected to electric field stimulation in the presence of zinterol, a highly selective β2-adrenergic agonist. Zinterol significantly increased [Ca2+]i transient amplitudes in all three groups. The failing canine myocytes were significantly more responsive than normal to β2-adrenergic stimulation. We also measured isotonic twitches, indo-1 fluorescence transients, and L-type Ca2+ currents in healthy canine myocytes. Zinterol (10−5 mol...