Analysis of Heart Rate Variability after a Ranger Training Course

Transcription

1 MILITARY MEDICINE, 169, 8:583, 2004 Analysis of Heart Rate Variability after a Ranger Training Course Guarantor: COL Jean-Claude Jouanin Contributors: COL Jean-Claude Jouanin; ASC Caroline Dussault; ICT Michel Pérès; ICT Pascale Satabin; LT COL Christophe Piérard; GEN Charles Yannick Guézennec We studied the effects of prolonged physical activities on resting heart rate variability (HRV) during a training session attended by 23 cadets of the French military academy. This course lasts 1 month and is concluded by a 5-day field exercise simulation with physical and psychological stress. Data collection took place before (B) and immediately at the end (E) of the course. It included HRV recordings during a stand test (5 minutes lying down and 5 minutes standing), with a Polar R-R monitor, followed by blood sampling to assay plasma testosterone. The results (B and E) showed that the testosterone level fell by approximately %, indicating a high level of fatigue. During the stand test, the total power (TP) of the HRV spectrum increased in a supine position. The TP of B was 5,515.7 ms 2 (SE, 718.4) and of E was ms 2 (SE, 2,539.2; p 0.001). High-frequency (HF) normalized values increased and low-frequency (LF) normalized values fell, regardless of position (HF normalized values and LF normalized values: supine, p 0.01, p 0.05; standing, p 0.05, p 0.01, respectively). LF:HF ratio fell 66.2 (SE, 12.9%; p 0.01) in a lying position. During the time-domain analysis of HRV, differences between adjacent normal R-R intervals more than 50 milliseconds, expressed as a percentage, and differences between the coupling intervals of adjacent normal RR intervals increased in the lying position (p 0.001). These results as a whole suggest that parasympathetic nervous system activity increases with fatigue. Introduction ntense physical training with incomplete recovery times may I result in marked fatigue. Fry et al. 1 reported that some authors have attempted to contrast two types of fatigue on the basis of the study of cardiovascular responses: orthosympathetic and parasympathetic fatigue. Fatigue with an orthosympathetic dominant is believed to be characterized by an increase in resting heart rate (HR) and blood pressure (BP). This form occurs mainly in young people involved in strength-speed activities. The parasympathetic dominant form is believed to be characterized by increased parasympathetic activity and/or orthosympathetic inhibition, with a drop in HR and BP. This form occurs mainly in older people involved in endurance activities and is considered to reflect exhaustion of the autonomic nervous system in general. For the last several years, resting HR variability (HRV) has been studied with a noninvasive, reliable method that makes it possible to show changes in the sympathovagal balance. Various authors have shown in follow-up studies of healthy subjects that intensive training modifies HRV Department of Aerospace Physiology, Institute of Aerospace Medicine of the Army Health Department, BP 73, Brétigny-Sur-Orge Cedex, France. This work was presented at the Fifth European Federation of Autonomic Societies Meeting, May 22 24, 2003, Toulouse, France. This manuscript was received for review in March The revised manuscript was accepted for publication in August Reprint & Copyright by Association of Military Surgeons of U.S., (for review, see Carter et al. 2 ) and that the fatigue associated with an increased training load in athletes may also do so. 3 6 In the latter situation, changes in resting HRV have been described during the tilt test with subjects in a supine or standing position. 3,6 These observations of athletes who voluntarily increased their training load for several weeks have been attributed to a modification of the sensitivity of the cardiovascular system to parasympathetic stimulation 3 or an increase in parasympathetic activity because of intensive endurance training. 5,6 In military settings, some training situations can extend for several weeks and thus can create conditions conducive to the onset of chronic fatigue. These training courses combine intense and sustained physical activity, food and fluid intake deficits (more or less voluntary), induced sleep disruption, and psychological stress. These various constraints can induce neuroendocrine changes that are expressed by pronounced fatigue and diminished capacity to adapt to stress. 7 9 Thus, as training continues and fatigue accumulates, the sympathovagal balance may shift and resting HRV may provide evidence of the cardiovascular effects of this change. The principal aim of this work was to determine whether HRV serves as an indicator of fatigue. To test this hypothesis, we conducted an experiment during a ranger training course at the French National Center for Ranger Training to see whether changes in the resting sympathovagal balance were associated with a state of prolonged fatigue. Materials and Methods A group of 23 students from the French military academy of St Cyr-Coëtquidan (age, years; height, cm; weight, kg) agreed to participate in the experiment after being informed of the protocol, which was conducted in compliance with the recommendations of the French ethics committee (Faculty of Medicine, Paris V, France). All subjects were male cadets from the French military academy, healthy, and in good physical condition. HRV was recorded with a Polar R-R monitor (Polar Electro, Kempele, Finland). A single observer measured resting BP with a sphygmomanometer throughout the experiment. Blood samples, taken from a vein in the crook of the elbow and immediately centrifuged and frozen at 180 C, were used for a plasma testosterone assay by the radioimmunoassay method (I 125 RIA kit; Diasorin, Stillwater, MN). The normal range was 10 to 30 nmol L 1 for testosterone. Fifteen days before the ranger training course began, the subjects underwent a reference stand test at the medical center of the Military Academy at Coëtquidan. Changing position is classically used with HRV spectral analysis to study the effects of posture on sympathovagal balance. 10 The test took place in the morning between 6:00 am and 8:00 am, with groups of five subjects in a quiet room isolated from the bustle of the medical center. During the stand test, the subjects were supine for at 583

2 584 Analysis of Heart Rate Variability least 5 minutes and then rose and remained standing for at least 5 minutes. The subjects were spontaneously breathing because when respiration was not controlled, respiratory frequency R-R interval (RRi) fluctuations bore no significant relation to tonic vagal-cardiac nerve activity. 11,12 Electrocardiogram (ECG) derivations were collected with two pregelled ECG electrodes (Ag/AgCl; Maersk Medical, Gol, UK) placed on the thorax under the mid-clavicular line and attached to the portable data recorder of the Polar R-R monitor. BP was measured on the right arm in a supine and then standing position, before and after the stand test. The mean BP (MBP) was obtained by the following calculation: MBP (mm Hg) diastolic BP (DBP) (systolic BP [SBP] DBP)/3. Blood samples were taken at the conclusion of the stand test, after the subjects had rested in a supine position for 5 minutes. The ranger training course took place in June It included instructional sessions varied with periods of intensive physical training in which aerobic activities such as traversing several kilometers (25 35 km) on foot with a backpack weighing roughly 10 kg (personal effects, food, and arms) alternated with anaerobic activities (house-to-house fighting, individual combat, climbing, water-crossing, night fighting, etc.). These instruction periods were combined with situations of psychological stress and sleep disruption, which was induced intentionally by the instructors and led to accumulated fatigue because recovery was never possible. After 3 weeks of instruction, the training course entered its concluding phase with a ranger field exercise lasting 5 days. Globally, this combat course resembles the conditions studied by Opstad, 13 and the subjects physical activity can be described as submaximal (corresponding to 35% of maximal oxygen uptake), prolonged, and fatiguing. At the end of the training course (upon arrival from the 5-day field exercise), a stand test was performed and blood samples were taken between 6:00 am and 8:00 am in the same conditions as for the reference testing at the Coëtquidan medical center. Analysis of Results The R-R intervals recorded by the Polar monitor were transferred to a portable computer and were subsequently analyzed in the laboratory, after we examined the recordings visually for possible artifacts associated with movement and muscle contractions. The spectral analysis included data recorded for the subject in the supine and standing positions, but not the 30 seconds of recording that followed the change of position. The analysis considered the time and frequency domains. The HRV time-domain analysis studied the standard deviations of the RRi (SD-RRi), the percentage of successive RRi whose difference exceeded 50 milliseconds (pnn50), and the calculated as the root mean square of the differences between the coupling intervals of adjacent RRi (rmssd). All were computed for each data set. The SD-RRi, pnn50, and rmssd are thought to represent variations of the high frequencies (HF) of HRV, i.e., parasympathetic nervous system modulation. 14 The spectral (frequency-domain) analysis of HRV makes it possible to study HR oscillations. To have sufficient precision for measurement of HRV, we used a fast Fourier transformation algorithm, with a plotting frequency of 2 Hz. The results were analyzed in accordance with the Task Force recommendations, i.e., frequency groups were defined as follows: below 0.04 Hz for very low frequency (VLF), 0.04 to 0.15 Hz for LF, and 0.15 to 0.40 Hz for HF. To assess the relative influence of the LF and HF values compared with the total power (TP) of the spectrum (milliseconds squared), they were expressed as normalized data (nu; HFnu HF/TP VLF; LFnu LF/TP VLF, or HFnu HF/LF HF; LFnu LF/LF HF). Changes in the TP of the spectrum and in the power of the HF spectrum are thought to represent modulations in parasympathetic activity, whereas the course of the power of the LF spectrum is considered to be associated with changes in the parasympathetic and sympathetic nervous systems. Consequently, the LF:HF ratio would thus be a useful reflection of the sympathovagal balance or to reflect the sympathetic nervous system modulations. 14 Statistical Analysis The results are expressed as the means SE. During the stand test, the effect of both factors, training and body position, on changes in HRV were studied with Friedman repeated-measures analysis of variance on ranks. All pairwise multiple comparison procedures used a Tukey test. Student s t test for matched values was used to compare the testosterone levels before and after the ranger training course. The significance threshold was set at p Results At the end of ranger training course, the subjects had lost a moderate amount of weight ( kg before, kg after, p 0.05), and their body mass index ( weight/height 2 ) was unchanged ( before, after, not significant). Table I reports the cardiovascular variables (RRi and BP) at rest. At the end of the ranger training course, the duration of the RRi increased for both positions. This corresponded to a decrease in resting HR of 12% during a supine position and 8% during a standing position. SBP, DBP, and MBP measured in the supine position were lower at the end of the course than TABLE I TIME COURSE OF THE RRI, SD-RRI, SBP, DBP, AND MBP IN SUPINE POSITION AND IN UPRIGHT POSITION BEFORE AND AFTER RANGER TRAINING Before Ranger Training After Ranger Training Supine RRi (ms) 1, , c SD-RRi b SBP (mm Hg) D b DBP (mm Hg) D b MBP (mm Hg) D b Upright RRi upright (ms) d b,d SD-RRi d (NS) SBP (mm Hg) D c DBP (mm Hg) D (NS) MBP (mm Hg) D a a p b p c p d p compared with upright.

3 Analysis of Heart Rate Variability 585 before it. In the standing position, HR, SBP, and MBP all dropped after training, but DBP did not change significantly. Hormone Assays At the end of the course, plasma testosterone had dropped by % ( nmol L 1 before, and nmol L 1 after; p 0.001). Time and Frequency Analysis of HRV during the Stand Test Time-Domain Analysis RRi variability (SD-RR) increased approximately 32% in the supine position (Table I), and pnn50 and rmssd increased in the supine but not the standing position (Fig. 1). Similarly, at the end of the course, pnn50 (p 0.001) and rmssd (p 0.001) were all lower in the standing than the supine position (Fig. 1). The variance analysis showed a training effect on pnn50 and rmssd during the supine position (p 0.05). Fig. 2. TP of the HRV spectra. Before (- - -) and after ( ) ranger training course (, p 0.01). Frequency-Domain Analysis The frequency-domain analysis shows that at the end of the course, the TP of the supine HRV frequency spectrum had increased from the reference values (Fig. 2). In both positions, LFnu decreased (Fig. 3a), whereas HFnu increased (Fig. 3b). Nonetheless, the LF:HF ratio fell by % in the supine position (p 0.01) and was not affected during standing (Fig. 3c). When we compare the data for standing and supine positions, in all cases, position has an effect on the HRV component (p 0.001). We note two things: first, the TP of the frequency spectrum fell at the end of the course during standing (p 0.001; Fig. 2); second, in standing position, LFnu (p 0.001) was higher, HFnu (p 0.001) was lower, and the LF:HF (p 0.001) ratio was accordingly higher (Fig. 3). However, variance analysis Fig. 3. Spectral analysis of standardized frequency components during the stand test. (a) LF components (LFnu); (b) HF components (HFnu); and (c) LF:HF ratio. Before (- - -) and after ( ) ranger training course (, p 0.05 and, p 0.01, respectively). showed a training effect on TP of HRV spectra in the supine position (p 0.05), but training had no effect on the HRV component in the standing position. Fig. 1. Time-domain analysis of HRV. (a) Time course of pnn50 and (b) root mean square of the rmssd. Before (- - -) and after ( ) ranger training course (, p 0.001). Discussion Our study showed a modification in sympathovagal balance in the early period of recovery from submaximal prolonged fa-

4 586 Analysis of Heart Rate Variability tiguing physical activity. That is, as Aakwaag et al., 7 Opstad, 8 and Guézennec et al. 9 described during a combat course of 5 days duration, the 1-month ranger training course was associated with psychological stress, elevated energy expenditure, caloric deficit expressed by moderate weight loss, and disruption of biological rhythms, sleep deprivation in particular. The blood samples taken within 1 hour of the end of the concluding field exercise showed that the plasma testosterone level had decreased 28.6%. This drop in testosterone is consistent with descriptions of the influence of stress and work load 7,13,9 or other intensive physical activity. 15,16 Indeed, suppressions in testosterone concentrations that may last several days have been reported after long-duration submaximal activity 15,9 or intense resistance exercise. 17 Recently, a significant decrease in testosterone serum concentrations was reported in prolonged strenuous physical activity after a 3-week military training program. 18 This confirms that decreased testosterone is a biological marker for fatigue in this type of training. From a cardiovascular point of view, this state of pronounced fatigue is expressed by a diminution of resting HR and BP. HR and MBP were lower during the postcourse stand test compared with the precourse values, regardless of whether the subject was lying down or standing up (Table I). These results are consistent with parasympathetic fatigue. 1 Similarly, compared with the reference values, the TP of the spectrum was higher in the supine position at the end of the course and the standardized HF values were higher for subjects in either position (Fig. 3). These results are consistent with increased parasympathetic activity, whereas the diminution of the standardized LF values in both positions and the change in the LF:HF ratio in the supine position expresses a diminution in sympathetic activity after training. The data from the time-domain analysis, that is, SD-RRi, pnn50, and rmssd, provide indices of modulation of parasympathetic activity in the supine position (Table I; Fig. 1). However, these at-rest values may be associated with the effects of the endurance training our subjects underwent, which may have diminished sympathetic activity or increased parasympathetic modulation. 2,19 In our study, the training effect on the parasympathetic drive occurred during the supine position, but was not evident in the standing position, whereas resting DBP, time- and frequency-domain analysis (SD-RRi, pnn50, rmssd, and TP and LF:HF ratio, respectively), did not differ significantly. Moreover, we note that the literature on the effects of training on HRV in healthy subjects sedentary or active is still being discussed. Portier et al. 6 observed a change in resting HRV in marathoners whose endurance training increased (60%) considerably for 12 weeks: standardized LF decreased and HF increased in the supine position. Hedelin et al. 5 described a crosscountry skier whose HRV increased along with the TP of the spectrum and HF in the supine position. Accordingly, fatigue appears to be accompanied by an increase in parasympathetic activity rather than a decrease of sympathetic activity. Hautala et al. 20 described the recovery phase among a group of skiers after a cross-country skiing competition: HF decreased in the 24 hours after the maximum exercise and then rebounded 48 hours after the event, indicating an increase in parasympathetic activity. Yamamoto et al. 4 observed a change in resting HRV with increased parasympathetic activity from the first week of training of a group of cyclists who overtrained for 6 weeks. These authors consider that in the context of acute fatigue, resting HRV depends on the subjects aerobic fitness. In contrary, Uusitalo et al. 3,21 sought to induce overtraining among nine women athletes by increasing the volume of their training for 6 to 9 weeks at an intensity of 70 to 90% of VO 2 max, by 130%, and at an intensity less than 70 to 90% of VO 2 max, by 100%. In that study, the HRV spectral analysis during a tilt test showed that the LF power spectrum increased in the supine position. Cardiovascular response to the tilt test in the study may have been attenuated by the increase in sympathetic cardiac activity, because the power spectrum of the HF, the best marker of parasympathetic modulations, did not change. These authors suggest that this result may indicate a diminution in baroreflex sensitivity, associated with an increase in plasma volume and in ventricular compliance with intensive training. Hedelin et al. 22 studied nine competitive canoeists (six men and three women) who increased their training load by 50% for 6 days; their VO 2 max decreased, as did their maximum lactate threshold and their exercising HR. The lower exercising HR may be associated with increased plasma volume during the exercise. In this case, during the tilt test, the spectral analysis of HRV showed no modifications in the power spectrum of resting LF or HF. In our study, we might hypothesize that modification of resting HRV for a long period would require combining an increase in the intensity and duration of training with a reduction in the recovery periods. Indeed, in the early 5-day ranger training course recovery, it is unlikely to assess the increase in RRi to the sole endurance training effect. In previous human studies, 23,24 it appears that an increase in VO 2 max of approximately 12 ml min 1 kg 1 in addition to a long-lasting endurance training (several months or years) would be required to obtain preponderance of parasympathetic control on HR, but does not necessarily improve HRV. 25 In the experiments by Hedelin et al. 22 and Uusitalo et al., 3 some subjects were certainly able to recover at night and were not exhausted. HRV is very sensitive to circadian rhythm and the power spectrum for HF and LF increases at night. This indicates an increase in parasympathetic activity and, to some extent, in sympathetic activity. 26 Sleep deprivation could increase the effects of peripheral fatigue on central cardiovascular regulation. However, during the ranger training course, our subjects practiced intense physical activity day and night and had only 2 or 3 hours for recovery. Therefore, in further studies, it could be suitable to evaluate the decrease in aerobic fitness at the end of 5-day ranger training course. In this situation of prolonged physical and psychological stress, the threshold of sensitivity of the sympathovagal balance may change and thereby attenuate the cardiovascular response to physical and psychological stress described by Herd. 27 After 5 days of ranger field exercise, Opstad 5 observed a divergence between the changes in peripheral blood catecholamines and in exercising HR. He hypothesized that the sensitivity of the adrenergic receptors during the field exercise might diminish or a concomitant activation of the parasympathetic system might make up for the increase in sympathetic activity. Moreover, taking into account the results obtained in humans in after fatiguing exercise, 28 muscle chemoreceptors efferents might contribute in the central regulation of autonomic nervous system activity. On a practical point of view, our results are in accordance

5 Analysis of Heart Rate Variability with the study of Radespiel-Tröger et al. 29 who compared a Polar R-R monitor with a reference ECG for measurement of HRV. Indeed, high-risk subjects or athletes could potentially benefit from the widespread use of an inexpensive HRV measuring device. In addition to this clinical application, the use of this method in a military context, during a stand test, could be sufficient to allow a screening of initial fatigue by army medical staff. However, these preliminary results obtained in real field conditions need to be strengthened by further laboratory studies to validate HRV evolution during stand test as a tool for monitoring military training and recovery to avoid excessive fatigue that could impair the performance of soldiers. 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