Autonomic Control Of Heart Rate Variability Research Articles

Profound impact of the sleep and circadian system on autonomic control of the heart

Heart rate variability (HRV), is modulated by the autonomic nervous system (ANS) and is an indicator of clinically relevant cardiovascular phenotypes. Vigilance state and endogenous circadian phase are major modulators of the ANS. Sympathetic activity is higher during wakefulness than in sleep, and, within sleep, the sympathetic contribution is greater in rapid-eye-movement (REM) sleep than in deep non-REM sleep. It has been clearly shown that acute sleep deprivation leads to a greater sympathetic influence on the autonomic control of the heart. Endogenous circadian rhythmicity influences autonomic control of HR and the timing of these endogenous rhythms can be altered by extended sleep/rest episodes and associated changes in photoperiod. Circadian control of the heart is not entirely mediated by the sleep–wake-cycle and autonomous modulations are influenced by the circadian system. During continuous wakefulness under controlled behavioural conditions, HRV is not constant but varies with circadian phase such that global HRV is greatest in the early morning hours with a major cardiovascular event due to abrupt changes in the sympatho-vagal-cardiac control during the transition from sleep to wakefulness. The impact of sleep, wakefulness, and circadian phase on autonomic control of HRV is also of interest because the frequency of acute myocardial infarctions shows a marked circadian variation. Understanding the sources of individual differences in HRV and how these relate to inter-individual differences in sleep and circadian physiology may increase the diagnostic use of HRV and may provide new avenues for preventive therapy especially with novel recording technique using at-home sleep monitoring.

Dietary sodium influences the effect of mental stress on heart rate variability

Dietary sodium influences intermediate physiological traits in healthy adults independent of changes in blood pressure. The purpose of this study was to test the hypothesis that dietary sodium affects cardiac autonomic modulation during mental stress. In a prospective, randomized cross-over design separated by 1 month between diets, 70 normotensive healthy young adults (F/M: 44/26, aged 18-38 years) consumed a 5-day low (10 mmol/day), normal (150 mmol), and high (400 mmol) sodium diet followed by heart rate variability (HRV) recordings at rest and during 5-min computerized mental arithmetic. Women were studied in the low hormone phase of the menstrual cycle following each diet. Diet did not affect resting blood pressure, but heart rate (HR) (mean ± SE) was 66 ± 1, 64 ± 1, and 63 ± 1 bpm in low, normal, and high sodium conditions, respectively (analysis of variance P = 0.02). For HRV, there was a main effect of sodium on resting SD of normalized RR intervals (SDNN), square root of the mean squared difference of successive normalized RR intervals (RMSSD), high frequency, low-frequency normalized units (LFnu), and high-frequency normalized units (HFnu) (P < 0.01 for all). The response to low sodium was most marked and consistent with sympathetic activation and reduced vagal activity, with increased LFnu and decreased SDNN, RMSSD, and HFnu compared to both normal and high sodium conditions (P ≤0.05 for all). Dietary sodium-by-mental stress interactions were significant for mean NN, RMSSD, high-frequency power, LFnu, and low frequency/high frequency ratio (P < 0.05 for all). The interactions signify that sodium restriction evoked an increase in resting sympathetic activity and reduced vagal activity to the extent that mental stress caused modest additional disruptions in autonomic balance. Conversely, normal and high sodium evoked a reduction in resting sympathetic activity and incremental increase in resting vagal activity, which were disrupted to a greater extent during mental stress compared to low sodium. We conclude that autonomic control of HRV at rest and during mental stress is altered by dietary sodium in healthy normotensive young adult men and women.

Heart rate variability is altered by dietary sodium: a randomized cross‐over study in healthy young adults

Dietary sodium influences intermediate physiological traits in healthy young adults independent of changes in blood pressure. The purpose of this study was to test the hypothesis that dietary sodium affects cardiac autonomic function. In prospective, randomized cross‐over design separated by 1 month or more between diets, 69 normotensive healthy young adults (M/F: 26/43, ages 18–38 yr) consumed a 5‐day low (10 mmol/day), normal (150 mmol), and high sodium (400mmol) diet prior to a protocol examining cardiovascular traits including heart rate variability (HRV) by time domain and frequency domain analysis (Nevrokard). All women were studied in the low hormone phase of the menstrual cycle. Diet did not affect blood pressure, but HR (mean ± SE) was 68±1, 65±1, and 64±1 in low, normal, and high sodium conditions, resp. (RM ANOVA p &lt; 0.01). For HRV there was a main effect of sodium on SDNN, RMSSD, HF, LFnu, HFnu, and LF/HF ratio (p &lt; 0.01 for all). The response to low sodium was most marked and consistent with sympathetic activation, with increased LFnu and LF/HF ratio, and decreased SDNN, RMSSD, and HFnu compared to both normal and high sodium conditions (p ≤ 0.05 for all). These findings suggest that autonomic control of HRV is altered through changes in dietary sodium without changes in blood pressure. Support: UL1 TR000135, HL‐89331

Effect of pioglitazone on cardiac sympathovagal modulation in patients with type 2 diabetes

This study aims to examine the effect of pioglitazone on potential progression of autonomic damage in addition to changes in control of cardiovascular function in patients with type 2 diabetes (T2DM). Thirty patients with T2DM and 32 healthy subjects participated in the study. Sympathovagal activity, assessed by power spectral analysis (PSA) of R-R intervals variability, and blood pressure (BP) were studied during clinostatism and orthostatism in controls and patients. We have assessed blood pressure control by 24-hour monitoring of ambulatory blood pressure. Patients were treated with pioglitazone (30mg/day) for 6months, and then re-evaluated by PSA for heart rate variability (HRV). Reduced levels of HbA1c (P<0.0001) and urinary albumin (P=0.008) were observed in pioglitazone-treated patients compared to untreated baseline levels. Arterial BP remained unchanged following pioglitazone treatment. T2DM patients had reduced HRV (low-frequency power; LF; P<0.0001 and LF/HF; LF/HF; P<0.0001) at baseline (clinostatism) compared to controls. Baseline clinostatic differences between groups persisted after pioglitazone treatment and no effect of treatment on basal HRV variables was observed. In controls, HF decreased and LF and LF/HF ratio increased in the orthostatic position. A similar effect for HF was observed in patients, but LF and LF/HF did not increase. The normal difference between HF-power in clinostatism versus orthostatism observed for controls (P<0.0001) was restored in patients following pioglitazone treatment (P=0.028). A significant decrease from lying to standing position in orthostatic LF-power (P<0.0001) and LF/HF (P<0.0001) was also observed between patients and controls. Although no differences in autonomic control of HRV were observed between controls and patients with T2DM, significant differences were observed in sympathovagal balance following either clinostatic or orthostatic challenge. These findings provide initial evidence of a potential additional benefit afforded by pioglitazone for the improvement of cardiac sympathovagal balance in T2DM.

WORKING THE NIGHT SHIFT CAUSES INCREASED VASCULAR STRESS AND DELAYED RECOVERY IN YOUNG WOMEN

Shiftwork has been associated with elevated blood pressure (BP) and decreased heart-rate variability (HRV), factors that may increase the long-term risk of cardiovascular-related mortality and morbidity. This study explored the effect of shiftwork on dynamic changes in autonomic control of HRV (cardiac stress), systolic BP and diastolic BP, i.e., SBP and DBP (vascular stress), and recovery in the same subjects working different shifts. By studying the same subjects, the authors could reduce the effect of possible contribution of between-subject variation from genetic predisposition and environmental factors. The authors recruited 16 young female nurses working rotating shifts—day (08:00–16:00 h), evening (16:00–00:00 h), and night (00:00–08:00 h)—and 6 others working the regular day shift. Each nurse received simultaneous and repeated 48-h ambulatory electrocardiography and BP monitoring during their work day and the following off-duty day. Using a linear mixed-effect model to adjust for day shift, the results of the repeated-measurements and self-comparisons found significant shift differences in vascular stress. While working the night shift, the nurses showed significant increases in vascular stress, with increased SBP of 9.7 mm Hg. The changes of SBP and DBP seemed to peak during waking time at the same time on the day off as they did on the working day. Whereas HRV profiles usually returned to baseline level after each shift, the SBP and DBP of night-shift workers did not completely return to baseline levels the following off-duty day (p < .001). The authors concluded that although the nurses may recover from cardiac stress the first day off following a night shift, they do not completely recover from increases in vascular stress on that day. (Author correspondence: jdwang@ntu.edu.tw)

Towards assessing the sympathovagal balance

Exact assessment of the autonomic nervous system's (ANS) activity by means of heart rate variability (HRV) is a long-standing challenge. Although many techniques have been proposed to take up the challenge, none ever proposed a rationale for the approach behind the technique or a satisfying discrimination of the two activities which underlie the autonomic control of HRV. We here propose a new method, providing both an understanding of the discrimination's nature and a framework which we believe leads to a thorough assessment of the sympathovagal balance, as a trajectory between points in a well-chosen space. The methodology assumes tools from scale invariance/covariance physics. The sympathovagal balance is obtained on a beat-to-beat basis with the dynamics portrayed through a trajectory. Furthermore, universal trajectories are sought which would comprehensively describe the effect of atropine and isoproterenol injections on systems underlying the heart pace variations. Non-invasive assessment of the respective activities of the sympathetic and parasympathetic subsystems of the ANS would be possible through cardiac autonomic measurements.

Sympatho-vagal control of heart rate variability in patients treated with suppressive doses of L-thyroxine for thyroid cancer

This study aimed to analyze the autonomic control of heart rate variability (HRV) in subjects receiving chronic l-thyroxine (l-T4) treatment after total thyroidectomy and (131)I therapy for differentiated thyroid carcinoma. Blood pressure (BP) and sympatho-vagal activity (evaluated by power spectral analysis (PSA) of time-domain parameters of HRV) were studied in clinostatism and after orthostatism in 24 healthy controls, and in 12 patients taking l-T4 (125-200 mug/day) to maintain serum TSH levels at <0.01 muIU/ml. The study of HRV by PSA is a non-invasive method of analyzing sympatho-vagal control of HRV by quantifying high-frequency (HF) (0.15-0.4 Hz) and low-frequency (LF) (0.04-0.15 Hz) powers. Patients on L-T4 treatment had undetectable TSH levels, serum free T4 (fT4) above the normal range or at the upper limit in one case, and normal free tri-iodothyronine (fT3) levels. Heart rate and R-R intervals were not different in the two groups, both in clinostatism and in ortostatism. Systolic and mean BP were higher in patients than in controls and were inversely correlated with actual serum fT4 levels. During clinostatism, thyroid patients showed significantly lower LF power (P = 0.035), LF/(LF + HF) (P = 0.008) and LF/HF (P = 0.01) than controls. When patients moved from lying to standing, there was a significantly different decrease in orthostatic LF power (P = 0.001), LF/(LF + HF) (P = 0.044) and LF/HF (P = 0.047) versus controls. Changes in autonomic control of HRV, characterized by decreased sympathetic activity and impaired sympatho-vagal balance with preserved vagal tone, are detectable in patients with hyperthyroxinemia due to suppressive l-T4 therapy and increased systolic and mean, but not diastolic, BP.