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At least 613 records · Page 34Linked to original sources
PubMed2003

The influence of mean heart rate on measures of heart rate variability as markers of autonomic function: a model study.

Some studies have demonstrated that the assessments of autonomic activities from the alterations of heart rate variations (HRVs) after autonomic blockade and during exercise of high intensity by the spectral analysis of HRV seemed inconsistent with actual situation. The inconsistency is probably caused by the contributions of fluctuating magnitudes and mean levels of autonomic activities on HRV having not been clarified. The alterations of HRV after autonomic blockade and during exercise of high intensity using a mathematical model were simulated. The autonomic activity in normal condition was assumed first according to some experimental evidence. Then autonomic activities after sympathetic blockade, vagal blockade and during exercise of high intensity were appropriately adjusted accordingly. The HRVs in response to these given autonomic activities were simulated. We found that the effect on HRV influenced by the mean level of autonomic activity is helpful to explain alterations of HRV in these conditions. After vagal blockade, a largely reduced low frequency (LF) power could be caused by the reduced mean heartbeat interval induced by a decreased mean level of vagal activity. Increased low and high frequency powers after sympathetic blockade could be caused by the increased mean heartbeat interval induced by a decreased mean level of sympathetic activity. A decreased LF power during exercise of high intensity, in addition to the withdrawal of vagal activity, could also be caused by the decreased mean heartbeat interval induced by an increased mean level of sympathetic activity.

Animals↗
PubMed2002

A phenomenon of heart-rate turbulence, its evaluation, and prognostic value.

Identification of high-risk cardiac patients is crucial for stratification strategies and prevention of cardiovascular events, including death. Single ventricular premature beat triggers some oscillations in cardiac cycle duration (the shortening followed by the lengthening of the cycle intervals) in healthy subjects and low-risk patients with ischaemic heart disease and/or heart failure. This phenomenon is called heart-rate turbulence (HRT). It was shown in retrospective and prospective studies that the absence of HRT is associated with increased risk of subsequent mortality in cardiac patients. HRT can be quantified by two variables: turbulence onset (TO), describing an early acceleration phase, and turbulence slope (TS), describing a late deceleration phase of heart rate after ventricular premature beat. Both TO and TS are independent one from another and from other conventional risk predictors. The combination of TO and TS seems to be the strongest Holter-based risk predictor and has some addictive predictive value to left ventricular ejection fraction, heart rate variability, and the averaged diurnal heart rate and baroreflex sensitivity. In addition, HRT has a predictive value in patients treated with beta-blockers and amiodarone. Moreover, it is thought that HRT is mediated by baroreflex and therefore can be used as a non-invasive measure of its sensitivity and autonomic nervous system function. Blunted HRT can be observed in diabetic patients with autonomic dysfunction and in patients with atropine-blocked vagal nerve activity. Moreover, it seems that a diurnal variation of HRT exists because it is better expressed during sleep. However, the use of HRT is limited to patients with dominant sinus rhythm and the presence of single ventricular beat. Nevertheless, the assessment of HRT is an inexpensive and simple method and can be performed with a routine ambulatory 24-hour ECG recording.

Arrhythmias, Cardiac↗
PubMed2003

Circadian profile of cardiac autonomic nervous modulation in healthy subjects: differing effects of aging and gender on heart rate variability.

UNLABELLED: Circadian Profile of Heart Rate Variability. INTRODUCTION: Although heart rate variability (HRV) has been established as a tool to study cardiac autonomic activity, almost no data are available on the circadian patterns of HRV in healthy subjects aged 20 to 70 years. METHODS AND RESULTS: We investigated 166 healthy volunteers (81 women and 85 men; age 42 +/- 15 years, range 20-70) without evidence of cardiac disease. Time-domain HRV parameters were determined from 24-hour Holter monitoring and calculated as hourly mean values and mean 24-hour values. All volunteers were fully mobile, awoke around 7 A.M., and had 6 to 8 hours of sleep. Circadian profiles of vagus-associated HRV parameters revealed a marked day-night pattern, with a peak at nighttime and a plateau at daytime. The characteristic nocturnal peak and the day-night amplitude diminished with aging by decade. Estimates of overall HRV (geometric triangular index [TI], SD of NN intervals [SDNN]) and long-term components of HRV (SD of the averages of NN intervals for all 5-min segments [SDANN]) were low at nighttime and increased in the morning hours. There was a significant decline of 24-hour values of all HRV parameters (P < 0.001) and a strong negative correlation (P < 0.001) with increasing age. Mean 24-hour RR interval (P < 0.001), SDNN, mean SD of NN intervals for all 5-minute intervals (SDNNi), and SDANN (all P < 0.01) were significantly higher in men. Younger men also exhibited significantly higher values for vagus-associated parameters (root mean square successive difference [rMSSD], P < 0.05; SDNNi, P < 0.01); however, gender differences diminished with increasing age. CONCLUSION: Normal aging is associated with a constant decline of cardiac vagal modulation due to a significant decrease of nocturnal parasympathetic activity. The significant gender-related difference of HRV decreases with aging. These findings emphasize the need to determine age-, gender-, and nycthemeral-dependent normal ranges for HRV assessment.

Adaptation, Physiological↗
PubMed1987

Autonomic modulation of aminophylline influence on the electrophysiological effects of adenosine and adenosine triphosphate in the canine heart.

The influence of aminophylline, a competitive antagonist of adenosine, on the chronotropic and dromotropic effects of adenosine and adenosine triphosphate was studied in pentobarbital anaesthetised dogs under various modifications of the autonomic nervous tone. Adenosine and adenosine triphosphate (3 mumol.kg-1 each) were rapidly (greater than or equal to 1 s) injected into the right atrium during both sinus rhythm and right atrial pacing (cycle length 300 ms) before and after infusion of aminophylline (5 mg.kg-1) (n = 21) as well as after increasing doses of aminophylline (n = 10). Some dogs underwent either muscarinic blockade with atropine (0.2 mg.kg-1) (n = 10), or beta adrenergic blockade with propranolol (1 mg.kg-1) (n = 10), or complete autonomic blockade with atropine and propranolol (n = 10). Aminophylline (5 mg.kg-1) antagonised the negative chronotropic and dromotropic effects of adenosine triphosphate and adenosine in dogs pretreated with atropine or atropine plus propranolol but did not affect them in autonomically intact dogs. In addition, the electrophysiological effects of adenosine were antagonised by only the highest doses of aminophylline in autonomically intact dogs and by aminophylline (5 mg X kg-1) in dogs pretreated with propranolol. It was concluded that (a) alteration of the electrophysiological effects of adenosine triphosphate and adenosine by aminophylline is appreciably influenced by the autonomic nervous tone and (b) autonomic blockade is required for the manifestation of the antagonism by aminophylline of the electrophysiological action of adenosine and adenosine triphosphate.

Adenosine↗
PubMedDate not supplied

Overview and methods used for the study of the cardiovascular actions of contrast materials.

Contrast materials cause numerous effects on the cardiovascular system. The major direct actions on the heart are myocardial mechanical and electrophysiological alterations. The major direct action on the peripheral circulation is vasodilation. The indirect effects, for the most part, are reflexly mediated and tend to offset the direct effects. The author reviews the various experimental models and techniques used for assessing the actions of contrast materials on the cardiovascular system. The salient findings from previous studies using various experimental models are discussed.

Animals↗
PubMed1999

Changes in patterns of 24-hr heart rate variability after kidney and kidney-pancreas transplant.

BACKGROUND: Transplantation has been shown to improve cardiorespiratory reflex measures of autonomic function. However, there are limited data on how kidney or kidney-pancreas transplantation influence continuous autonomic modulation of heart rate and the clinical utility of 24-hr heart rate variability (HRV) monitoring. METHODS: Ninety nondiabetic kidney and 30 diabetic kidney-pancreas transplant recipients underwent 24-hr Holter monitoring before and again at 6 and 12 months posttransplantation. Tapes were submitted for determination of HRV including interbeat variability (the proportion of adjacent R-R intervals having a difference <50 msec, the SD of all R-R intervals for the entire recording, and the SD of the averages of R-R intervals calculated over 5-min blocks for the entire recording) which is associated with vagal function, sudden death, and circadian function, respectively. Power spectral analysis quantified total neural, sympathetic, and parasympathetic modulation of the heart in ln(msec2). RESULTS: Nondiabetic kidney recipients showed improvement (P< or =0.05) in the SD of the averages of R-R intervals calculated over 5-min blocks (83.2 vs. 95.7 msec) and the SD of all R-R intervals (94.5 vs. 104.4 msec) by 6 months and all groups showed improvement by 12 months. Kidney-pancreas recipients also showed improved total neural (4.35 vs. 4.64) and sympathetic modulation (2.70 vs. 3.13). Kidney-pancreas recipients had significantly poorer values for each measure (P< or =0.05) at all time points. CONCLUSIONS: Cardiac autonomic neuropathy arises in the presence of uremia and diabetes, with severe dysfunction seen when these conditions occur concomitantly. Improvement in cardiac autonomic function follows both kidney and kidney-pancreas transplantation with more pronounced improvement in the circadian measures. Therefore, circadian measures of 24-hr HRV could be used to monitor the restoration of cardiac autonomic function.

Adult↗
PubMed2004

Nonlinear analysis of the separate contributions of autonomic nervous systems to heart rate variability using principal dynamic modes.

This paper introduces a modified principal dynamic modes (PDM) method, which is able to separate the dynamics of sympathetic and parasympathetic nervous activities. The PDM is based on the principle that among all possible choices of expansion bases, there are some that require the minimum number of basis functions to achieve a given mean-square approximation of the system output. Such a minimum set of basis functions is termed PDMs of the nonlinear system. We found that the first two dominant PDMs have similar frequency characteristics for parasympathetic and sympathetic activities, as reported in the literature. These results are consistent for all nine of our healthy human subjects using our modified PDM approach. Validation of the purported separation of parasympathetic and sympathetic activities was performed by the application of the autonomic nervous system blocking drugs atropine and propranolol. With separate applications of the respective drugs, we found a significant decrease in the amplitude of the waveforms that correspond to each nervous activity. Furthermore, we observed near complete elimination of these dynamics when both drugs were given to the subjects. Comparison of our method to the conventional low-frequency/high-frequency ratio shows that our proposed approach provides more accurate assessment of the autonomic nervous balance. Our nonlinear PDM approach allows a clear separation of the two autonomic nervous activities, the lack of which has been the main reason why heart rate variability analysis has not had wide clinical acceptance.

Adult↗
PubMed1989

Testing of autonomic cardiovascular regulation--methodological considerations.

The different analyses of the results on autonomic nervous function tests were evaluated in 43 male and 32 female diabetic patients and in 24 male and 24 female control subjects, aged 47-67 years, all without any known heart disease. The Valsalva ratio of the first effort did not differ from the mean Valsalva ratio of three efforts. During deep breathing, heart rate variation and max/min R-R interval ratio determined from the first three breathing cycles did not differ from the respective variables calculated from six consecutive breathing cycles. Diastolic blood pressure response to isometric handgrip was greater during the third minute than during the first and the first two minutes. In conclusion, the tests for the evaluation of autonomic nervous function can be simplified without losing their diagnostic value.

Aged↗
PubMed2005

Heart rate variability: measurement and clinical utility.

Electrocardiographic RR intervals fluctuate cyclically, modulated by ventilation, baroreflexes, and other genetic and environmental factors that are mediated through the autonomic nervous system. Short term electrocardiographic recordings (5 to 15 minutes), made under controlled conditions, e.g., lying supine or standing or tilted upright can elucidate physiologic, pharmacologic, or pathologic changes in autonomic nervous system function. Long-term, usually 24-hour recordings, can be used to assess autonomic nervous responses during normal daily activities in health, disease, and in response to therapeutic interventions, e.g., exercise or drugs. RR interval variability is useful for assessing risk of cardiovascular death or arrhythmic events, especially when combined with other tests, e.g., left ventricular ejection fraction or ventricular arrhythmias.

Autonomic Nervous System↗
PubMed1968

The effects of chloralose-urethane and sodium pentobarbitone anaesthesia on the local and autonomic components of the circulatory response to arterial hypoxia.

1. The circulatory and respiratory responses to severe arterial hypoxia were studied in normal rabbits, ;de-efferented' rabbits without functioning autonomic effectors, and atropinized animals before anaesthesia and during chloralose-urethane and sodium pentobarbitone anaesthesia. Net systemic autonomic activity and autonomic activity to the heart was assessed from a comparison of the responses of the various preparations.2. In the normal spontaneously breathing animal each anaesthetic had a similar mode of action, and modified qualitatively the circulatory response present before anaesthesia. In the ;de-efferented' animal the circulatory response was determined by the local effects of hypoxia, and was altered only quantitatively during anaesthesia.3. In the normal unanaesthetized animal the reflex changes in autonomic activity during hypoxia consisted of a large increase in vagal efferent activity, a decrease in cardiac sympatho-adrenal activity, and an increase in total sympatho-adrenal constrictor activity.4 In hypoxia during anaesthesia the vagal efferent activity no longer increased, but the change in sympatho-adrenal activity to heart and systemic circulation was the same as before anaesthesia in the spontaneously breathing animal. During anaesthesia with controlled ventilation systemic sympatho-adrenal activity increased further, and bradycardia again developed. The bradycardia was now due exclusively to reduction in cardiac sympathetic activity and not to an increase in vagal efferent activity.

Animals↗
PubMed1990

Vagal stimulation attenuates sympathetic enhancement of left ventricular function.

We studied the effects of stimulation of the vagal and sympathetic cardiac nerves on left ventricular contraction and relaxation in 16 anesthetized dogs. In each experiment, we paced the ventricles at a fixed rate of 120 beats/min, and we kept the systemic arterial pressure constant. We used the maximum rates of left ventricular pressure rise (dP/dtmax) and fall (dP/dtmin) as our indexes of ventricular contraction and relaxation. Sympathetic nerve stimulation at a frequency of 6 Hz increased dP/dtmax by 802 mmHg/s and raised dP/dtmin by 847 mmHg/s above the control level. The effect of vagal nerve stimulation was directly dependent on the level of sympathetic stimulation. In the absence of sympathetic stimulation, vagal nerve stimulation at a frequency of 5 Hz decreased dP/dtmax by 187 mmHg/s and reduced dP/dtmin by 199 mmHg/s below the control level. When the sympathetic stimulation was 6 Hz, vagal nerve stimulation at 5 Hz decreased dP/dtmax by 513 mmHg/s and reduced dP/dtmin by 709 mmHg/s. Furthermore, propranolol, in doses of 1.5 mg/kg, abolished any significant vagal effect on ventricular contraction or relaxation. We conclude that combined sympathetic-vagal stimulation results in a substantial antagonistic interaction such that vagal stimulation significantly attenuates the sympathetic enhancement of left ventricular function.

Animals↗
PubMed1995

Autonomic nervous system activity in weight gain and weight loss.

Studies in both animals and humans indicate that the autonomic nervous system (ANS) responds to changes in systemic energy balance. In the present study, ANS response to weight change was examined by sequential blockade of cardiac autonomic innervation with parasympathetic (atropine) and sympathetic (esmolol) blockers. Change in heart period (interbeat interval) from baseline after atropine defined the amount of parasympathetic control (PC), and the subsequent change after esmolol defined the amount of sympathetic control (SC). In nonobese subjects, weight gain to 10% above initial body weight resulted in a decrease in PC and an increase in SC, and conversely, weight loss to 10% below initial weight resulted in an increase in PC and a decrease in SC. In obese subjects, weight loss resulted in the same pattern of changes in PC and SC. The major changes were in the parasympathetic arm of the ANS. These findings support the hypothesis that the ANS acts to oppose weight change.

Adrenergic beta-Antagonists↗