André Aubert

Heart Rate Variability in Athletes

This review examines the influence on heart rate variability (HRV) indices in athletes from training status, different types of exercise training, sex and ageing, presented from both cross-sectional and longitudinal studies. The predictability of HRV in over-training, athletic condition and athletic performance is also included. Finally, some recommendations concerning the application of HRV methods in athletes are made.The cardiovascular system is mostly controlled by autonomic regulation through the activity of sympathetic and parasympathetic pathways of the autonomic nervous system. Analysis of HRV permits insight in this control mechanism. It can easily be determined from ECG recordings, resulting in time series (RR-intervals) that are usually analysed in time and frequency domains. As a first approach, it can be assumed that power in different frequency bands corresponds to activity of sympathetic (0.04–0.15Hz) and parasympathetic (0.15–0.4Hz) nerves. However, other mechanisms (and feedback loops) are also at work, especially in the low frequency band.During dynamic exercise, it is generally assumed that heart rate increases due to both a parasympathetic withdrawal and an augmented sympathetic activity. However, because some authors disagree with the former statement and the fact that during exercise there is also a technical problem related to the non-stationary signals, a critical look at interpretation of results is needed.It is strongly suggested that, when presenting reports on HRV studies related to exercise physiology in general or concerned with athletes, a detailed description should be provided on analysis methods, as well as concerning population, and training schedule, intensity and duration. Most studies concern relatively small numbers of study participants, diminishing the power of statistics. Therefore, multicentre studies would be preferable.In order to further develop this fascinating research field, we advocate prospective, randomised, controlled, long-term studies using validated measurement methods. Finally, there is a strong need for basic research on the nature of the control and regulating mechanism exerted by the autonomic nervous system on cardiovascular function in athletes, preferably with a multidisciplinary approach between cardiologists, exercise physiologists, pulmonary physiologists, coaches and biomedical engineers.

Noninvasive assessment of seasonal variations in cardiac structure and function in cyclists.

Noninvasive studies were performed in 12 male bicylists in the competitive season (CS) and in the resting season (RS) and in 12 matched control subjects to determine seasonal variations in cardiac structure and function in athletes and to compare the data with those of nonathletes. In athletes, peak oxygen uptake was 6% lower in the RS than the CS; the RS value was 40% higher than in nonathletes. The echocardiograms of athletes showed a higher left ventricular total diameter at end-diastole in the CS than in the RS; this difference was due to a greater septal and posterior wall thickness, with unchanged internal diameter. On the ECG, R-wave voltages were larger in the CS in leads I, V5 and V6. Athletes had greater left ventricular dimension and wall thickness than nonathletes, and their ratio of wall thickness to internal radius of the left ventricle was higher. Various echo- and mechanocardiographic indexes of left ventricular function were determined. During the RS, the athletes had a lower percent shortening and maximal velocity of left ventricular internal diameter, lower maximal and minimal velocities of the endocardium of the posterior wall, a longer preejection period and a larger ratio of preejection period to left ventricular ejection time. These findings are probably related to a greater left ventricular end-systolic stress, an index of myocardial afterload, in the RS.We conclude that cyclists in the CS, compared with nonathletes, have greater left ventricular internal dimension and increased wall thickness, with similar left ventricular function. During the RS, internal dimension does not change from the level in the CS, but wall thickness is somewhat reduced and left ventricular function is slightly depressed, most likely because of a higher afterload in RS.

The analysis of heart rate variability in unrestrained rats. Validation of method and results

An experimental setting and software were developed to evaluate cardiac autonomic function in unrestrained rats. Subcutaneously implanted ECG electrodes and an indwelling venous catheter were tunneled to a tail cuff in five rats. The ECG was A/D converted at 1000 Hz. After peak detection, a time series of RR intervals was obtained. Programs for the analysis of heart rate variability (HRV) were implemented in LabVIEW. Statistical properties were determined in the time domain. After cubic spline function curve fitting, resampling at 0.1 s and test for stationarity, power spectral analysis was performed on sampled records of 30 min duration after applying a sliding Hanning window (Welch method: 256 points (duration 25.6 s), 50% overlap and 0.039 Hz resolution). Algorithms were tested with simulated signals consisting of isolated frequency components, which were retrieved at their exact locations. Physiological validation of the system was performed by, beta-adrenergic and cholinergic blockade and by forced breathing at a fixed rate. Measurements were performed on five unrestrained rats under basal conditions. Mean RR was 174.2 +/- 3.6 ms; S.D., 13.3 +/- 4.6 ms; rMSSD, 5.2 + /- 1.2 ms; pNN10, 3.5 +/- 1.9% and pNN5, 18.7 +/- 6.4%. Low (0.19-0.74 Hz) and high frequency (0.78-2.5 Hz) power were determined (and also percent of low to total and high to total): 18.42 +/- 10.74 ms2 (22.9 +/- 6.5%) and 15.66 +/- 5.56 ms2 (19.9 +/- 2.7%), and the ratio low/high: 1.16 +/- 0.39. In conclusion, HRV analysis programs were developed and thoroughly tested through simulations and in vivo, under basal conditions and after pharmacological blockades. Using this software, HRV data from unrestrained rats were obtained.

Effects of aerobic training intensity on resting, exercise and post-exercise blood pressure, heart rate and heart-rate variability

We aimed to investigate the effects of endurance training intensity (1) on systolic blood pressure (SBP) and heart rate (HR) at rest before exercise, and during and after a maximal exercise test; and (2) on measures of HR variability at rest before exercise and during recovery from the exercise test, in at least 55-year-old healthy sedentary men and women. A randomized crossover study comprising three 10-week periods was performed. In the first and third period, participants exercised at lower or higher intensity (33% or 66% of HR reserve) in random order, with a sedentary period in between. Training programmes were identical except for intensity, and were performed under supervision thrice for 1 h per week. The results show that in the three conditions, that is, at rest before exercise, during exercise and during recovery, we found endurance training at lower and higher intensity to reduce SBP significantly (P<0.05) and to a similar extent. Further, SBP during recovery was, on average, not lower than at rest before exercise, and chronic endurance training did not affect the response of SBP after an acute bout of exercise. The effect of training on HR at rest, during exercise and recovery was more pronounced (P<0.05) with higher intensity. Finally, endurance training had no significant effect on sympathovagal balance. In conclusion, in participants at higher age, both training programmes exert similar effects on SBP at rest, during exercise and during post-exercise recovery, whereas the effects on HR are more pronounced after higher intensity training.

Steep fall in cardiac output is main determinant of hypotension during drug-free and nitroglycerine-induced orthostatic vasovagal syncope

BACKGROUND How much of the hypotension occurring during postural syncope is cardiac output-mediated and how much can be ascribed to a fall in systemic vascular resistance are unknown. The contribution of both determinants may be influenced by the use of vasoactive drugs. OBJECTIVE The purpose of this study was to assess the determinants of hypotension during drug-free and nitroglycerine (NTG)-induced vasovagal presyncope in routine tilt table testing. METHODS In this retrospective study, a total of 56 patients (37 female; age 36 +/- 19 years) with suspected vasovagal syncope and a positive tilt test at two clinical centers were selected. In 29 patients, presyncope was provoked by 0.4 mg sublingual NTG, administered in the 60 degrees head-up tilt position. In the other 27 patients, presyncope was provoked by passive tilt alone. Finger arterial pressure was monitored continuously, and left ventricular stroke volume was computed from pressure pulsations. RESULTS After NTG administration, heart rate rose, and peak heart rate was similar in all patients. Use of NTG did not affect circulatory patterns precipitating a vasovagal response. On average in all patients, marked hypotension was mediated by an approximately 50% fall in cardiac output, whereas systemic vascular resistance was well maintained until presyncope. CONCLUSION Hypotension during routine tilt testing is cardiac output-mediated, and the mechanism appears independent of the use of 0.4 mg sublingual NTG. The study data challenge the conventional idea of systemic vasodilation as the overriding cause of hypotension during postural syncope.

Cardiovascular function and basics of physiology in microgravity

Space exploration is a dream of mankind. However, this intriguing environment is not without risks. Life, and the human body, has developed all over evolution in the constant presence of gravity, especially from the moment on when living creatures left the ocean.When this gravitational force is no longer acting on the body, drastic changes occur. Some of these changes occur immediately, others progress only slowly. In the past 40 years of human space flight (first orbital flight by Yuri Gagarin on 12 April, 1961) several hazards for the human body have been identified. Bone mineral density is lost, muscle atrophy and cardiovascular deconditioning occur; pulmonary function, fluid regulating systems of the body, the sensory and the balance system are all disturbed by the lack of gravity.These changes in human physiology have to be reversed again when astronauts return to earth. This can cause adaptation problems, especially after long-duration space flights. Also the reaction of human physiology to radiation in space poses a huge risk at this moment. In this review the accent will be on cardiovascular function in space: how normal function is modified to reach a new equilibrium in space after short- and long-duration exposure to microgravity. In order to make long-duration space flight possible the mechanisms of this physiological adaptation must be understood to full extent. Only with this knowledge, effective countermeasures can be developed.

EFFECTS OF AUTONOMIC BLOCKADE ON NON‐LINEAR CARDIOVASCULAR VARIABILITY INDICES IN RATS

1 The present study assesses the effects of autonomic blockade (a‐ and b‐adrenoceptor and cholinergic) on cardiovascular function studied by heart rate variability (HRV), blood pressure variability (BPV) and baroreflex sensitivity in rats using non‐linear dynamics. Little is known about the influence of pharmacological autonomic nervous system interventions on non‐linear cardiovascular regulatory indices. 2 In 13 conscious rats, heart rate and aortic blood pressure were measured continuously before, during and after autonomic blockade with atropine, phentolamine and propranolol. Non‐linear scaling properties were studied using 1/f slope, fractal dimension and long‐ and short‐term correlation. Non‐linear complexity was described with correlation dimension, Lyapunov exponent and approximate entropy. Non‐linear indices were compared with linear time and frequency domain indices. 3 b‐Adrenoceptor blockade did not alter the non‐linear characteristics of HRV and BPV, although low‐frequency power of HRV was depressed. a‐Adrenoceptor blockade decreased the scaling behaviour of HRV, whereas cholinergic blockade decreased the complexity of the non‐linear system of HRV. For BPV, the scaling behaviour was increased during a‐adrenoceptor blockade and the complexity was increased during cholinergic blockade. The linear indices of HRV and BPV were decreased. 4 The present results indicate that the b‐adrenoceptor system has little involvement in the generation of non‐linear HRV and BPV in rats. 5 a‐Adrenoceptor blockade mostly influenced the scaling properties of the time series, whereas cholinergic blockade induced changes in the complexity measures. 6 The absence of the baroreflex mechanism can trigger a compensatory feed‐forward system increasing the complexity of BPV.

Wavelet decomposition analysis of heart rate variability in aerobic athletes

Heart rate variability (HRV) can be quantified, among others, in the frequency domain using digital signal processing (DSP) techniques. The wavelet transform is an alternative tool for the analysis of non-stationary signals. The implementation of perfect reconstruction digital filter banks leads to multi resolution wavelet analysis. Software was developed in LabVIEW. In this study, the average power was compared at each decomposition level of a tachogram, containing the consecutive RR-intervals of two groups of subjects: aerobic athletes and a control group. Compared to the controls, the aerobic athletes showed an increased power in all frequency bands. These results are in accordance with values obtained by spectral analysis using the Fourier transform, suggesting that wavelet analysis could be an appropriate tool to evaluate oscillating components in HRV, but in addition to classic methods, it also gives a time resolution.

Impact of age on the vasovagal response provoked by sublingual nitroglycerine in routine tilt testing

NTG (nitroglycerine) is used in routine tilt testing to elicit a vasovagal response. In the present study we hypothesized that with increasing age NTG triggers a more gradual BP (blood pressure) decline due to a diminished baroreflex-buffering capacity. The purpose of the present study was to examine the effect of NTG on baroreflex control of BP in patients with distinct age-related vasovagal collapse patterns. The study groups consisted of 29 patients (16-71 years old, 17 females) with clinically suspected VVS (vasovagal syncope) and a positive tilt test. Mean FAP (finger arterial pressure) was monitored continuously (Finapres). Left ventricular SV (stroke volume), CO (cardiac output) and SVR (systemic vascular resistance) were computed from the pressure pulsations (Modelflow). BRS (baroreflex sensitivity) was estimated in the time domain. In the first 3 min after NTG administration, BP was well-maintained in all patients. This implied an adequate arterial resistance response to compensate for steeper reductions in SV and CO with increasing age. HR (heart rate) increased and the BRS decreased after NTG administration. The rate of mean FAP fall leading to presyncope was inversely related to age (r=0.51, P=0.005). Accordingly, patients with a mean FAP fall >1.44 mmHg/s (median) were generally younger compared with patients with a slower mean FAP-fall (30+/-10 years compared with 51+/-17 years; P=0.001). The main determinant of the rate of BP fall on approach of presyncope was the rate of fall in HR (r=0.75, P<0.001). It was concluded that, in older patients, sublingual NTG provokes a more gradual BP decline compared with younger patients. This gradual decline cannot be ascribed to failure of the baroreflex-buffering capacity with increasing age. Age-related differences in the laboratory presentation of a vasovagal episode depend on the magnitude of the underlying bradycardic response.

Different evolutions in heart rate variability after heart transplantation: 10-year follow-up

Background. After heart transplantation, the donor heart is extrinsically denervated. No input of sympathetic or vagal nerves can influence the heart rate, resulting in a flat power spectrum of the beat-to-beat variability. The occurrence and the significance of reinnervation remain controversial. Methods and Results. We monitored the evolution of heart rate variability (HRV) after heart transplantation, starting from a few weeks postoperatively up to 10 years after surgery. Twenty–four-hour Holter recordings of 216 heart-transplant patients were analyzed using time and frequency domain analysis of HRV. Analysis of all data revealed an increase in 24-hour and night-time total power starting from 2 years after transplantation. Low-frequency oscillations calculated over the total 24 hours, day- and nighttime increased significantly starting from year 4 and onward (year 4–8: P<0.005). No evolution was found in high-frequency power. Subgroup analysis revealed a group with a clear spectral component (n=16), a group with a small component (n=124), and a group with a flat spectrum (n=76). Only the first group revealed an evolution in both high- and low-frequency power. Conclusion. These results indicate three different types of evolution in HRV, with reinnervating patterns present in only a minority of the patients. The vast majority of the patients show no signs of reinnervation.