© 1997 by European Society of Cardiology
Copyright © 1997, European Society of Cardiology
Acute and persistent effects of a 46-kilometre wilderness trail run at altitude: cardiovascular autonomic modulation and baroreflexes
aDepartment of Internal Medicine, University of Pavia and IRCCS S. Matteo, 27100 Pavia, Italy
bCenter for Exercise and Applied Human Physiology, Johnson Center, University of New Mexico, Albuquerque, NM, USA
cNew Mexico Health Enhancement and Marathon Clinics Research Foundation, Albuquerque, NM, USA
* Corresponding author. Tel.: +39 (382) 502979; fax: +39 (382) 529196.
Received 20 May 1996; accepted 18 December 1996
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Abstract |
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 Top Abstract 1 Introduction2 Methods3 Results4 DiscussionReferences |
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Objective: To test the hypothesis that prolonged physical exercise induces long-lasting effects on blood pressure and heart rate we studied 17 endurance runners before and after the 1995 Sandia Wilderness Crossing Research Run (46 km of rocky trails, average altitude 2500 m). Methods: We evaluated the response of the cardiovascular system to sympathetic stimulation by orthostatism and to sympathetic and parasympathetic carotid baroreceptor stimulations by sinusoidal neck suction at different frequencies (sympathetic activity on blood pressure by low-frequency stimulation, parasympathetic activity on RR interval by high-frequency stimulation). We used power spectral analysis of beat-to-beat RR interval, systolic and diastolic non-invasive blood pressure, in order to quantify the respiratory fluctuations (depending on vagal activity on the RR interval) and the slower non-respiratory fluctuations, depending on sympathetic activity on the blood pressure. Recordings were performed 24 h before, and 30 min, 24 h and 48 h after the run. Results: Thirty minutes after the race we found reduced blood pressure, signs of relative sympathetic predominance (increased RR interval low-frequency/high-frequency ratio from 0.65±0.15 to 1.63±0.37, P<0.05), reduced effect of parasympathetic baroreceptor stimulation (decrease in RR interval high-frequency neck-suction synchronous oscillations, from 5.33±0.34 to 3.55±0.37 ln-ms2, P<0.005), unchanged blood pressure responses to sympathetic stimulations; 24 h after the race, the response to parasympathetic stimulation was increased (to 6.44±0.32 ln-ms2, P<0.0005) compared to baseline (24 h before the race), whereas sympathetic stimulation by neck suction had no longer an effect on blood pressure. Conclusion: The acute effects of prolonged exertion are associated with a relative increase in sympathetic activity. Twenty-four hours after this race an increased sensitivity to vagal and reduced sensitivity to sympathetic baroreflex stimulations was found. In this field study at altitude we found long-lasting effects on cardiovascular autonomic modulation after physical exertion.
KEYWORDS Exercise; Autonomic nervous system; Baroreceptor; Heart rate variability; Blood pressure; Spectrum analysis; Training; Man
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1 Introduction |
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TopAbstract1 Introduction2 Methods3 Results4 DiscussionReferences |
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Many noteworthy contributions to the understanding of adaptations of humans to altitude and exertion have been made in the laboratory where conditions can be rigorously controlled. However, studies of sojourners transiently exposed to the mountain environment give insights, albeit under less rigorously controlled conditions, that cannot be gleaned from laboratory studies. Altitude is not synonymous with just a decrease in barometric pressure and consequent hypoxia as achieved in an altitude chamber, but many additional ambient factors such as temperature, humidity, sensory stimulation, radiation and human interactions during competition play important roles. Thus both laboratory experiments and clinical field studies have their place in defining cardiovascular adaptations to exercise and hypoxia.
The results of studies on the after-effects of exercise on baroreflex activity appear conflicting. Thus a single bout of dynamic exercise increased the responsiveness of the vagal baroreflex. This finding is thought to explain the blood pressure stability found up to 24 h after intense exercise [1]. Signs of cardiac sympathetic activation 24 h after heavy dynamic exercise in sedentary subjects have also been reported. This finding could explain the coexistence of training-induced bradycardia with enhanced sympathetic activity reported in champion athletes, attributed to a slow decay of sympathetic activation resulting from the previous day's training session in these individuals [2].
The majority of well-controlled laboratory studies have dealt with the effects on cardiovascular function of short-lasting maximal exercise at sea level. Little information exists on the immediate and prolonged effects of long duration exercise at altitude in a competitive field setting. Long-distance running at altitude, an increasingly popular athletic activity, offers an opportunity to assess cardiovascular control in well trained individuals who participate in such annual events and train throughout the year for this competition. While it must be emphasized that these athletes are not representative of other groups of regular exercisers or of competitive athletes in general, they nevertheless are an important segment of fitness enthusiasts.
To clarify the time course of the changes in autonomic control of the heart and arterial vessels in response to prolonged exercise at altitude, we studied a group of such athletes before, immediately after, 24 h and 48 h later at the 1995 Sandia Wilderness Crossing Research Run. We evaluated the autonomic modulation of the heart and vessels by spectral analysis of RR interval and blood pressure variability, supine and during sympathetic activation induced by orthostatic challenge [3, 4]. In addition, we measured the sensitivity to stimulations of the arterial (carotid) baroreceptors on the heart and the vessels. For this purpose, we used a non-invasive method [5, 6]which enabled us to distinguish between the effects of sympathetic and vagal efferent activity of the arterial baroreceptors, based on the response of the heart and arterial vessels to different frequencies of stimulations.
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2 Methods |
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TopAbstract1 Introduction2 Methods3 Results4 DiscussionReferences |
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2.1 Subjects and protocol
Seventeen runners (4 women and 13 men) were studied. They were selected to reflect the wide age range of competitors from 26 runners who took part in the 1995 Sandia Wilderness Crossing Research Run. The race follows rocky trails in wilderness for a distance of 46 km. There are no aid stations. The mean altitude of the race is 2500 m above sea level, the highest point being at 3300 m. Twelve kilometres are at or above 2700 m altitude. All participants in the study lived at an altitude of 1500–1880 m above sea level and frequently trained on the wilderness trail used during the race. They were experienced mountain runners who had completed at least one previous Sandia Wilderness Crossing Research Run. The characteristics of the subjects and athletic histories are given in Table 1. The protocol was approved by the University of New Mexico Institutional Committee for Human Experimentation, and all subjects gave informed consent. The US Forest Service issued a special use permit for this event.
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In all subjects the recordings were repeated with the same technique during 4 consecutive days: the day before the run (from 2 p.m. to 7 p.m.), next day after the run (with an average time delay of 30 min after finishing (from 11 a.m. to 6 p.m.), then 24 h (from 2 p.m. to 7 p.m.) and 48 h (from 2 p.m. to 7 p.m.) after the race. Recordings before, 24 h and 48 h after the run were made in the laboratory, whereas the recordings at the end of the run were obtained in a tent placed in front of the finish line, using the same equipment powered by a generator.
Cardiovascular signals were recorded during 4 consecutive conditions: (1) supine position (4 min); (2) supine position during neck suction cycled at 6 cycles/min (i.e. 0.10 Hz, for 2 min, with a pressure swing of 0 to –30 mmHg during each sinusoidal cycle); (3) during neck suction cycled at 12 cycles/min (i.e., 0.20 Hz, with the same pressure swing, for 2 min); and (4) during upright posture (for 4 min). During all recordings the breathing of the subjects was maintained at a constant frequency (15 breaths/min) using visual instructions, taking care to avoid hyperpnoea. Breathing was maintained at a constant frequency to obtain respiratory fluctuations in cardiovascular signals different from that induced by neck suction. In each condition, we recorded the electrocardiogram (by chest leads), the respiratory signal (by electrical impedance pneumography), continuous non-invasive blood pressure (by Finapres, mod. 2300, Ohmeda, Englewood), and the pressure within the neck collar (by a pressure transducer part n. 286-692, RS, Corby, UK).
2.2 Data acquisition
The signals were digitised on line by a special 12-bit analogue to digital converter at a sampling rate of 300 samples/s for the EKG, at a sampling rate of 90 samples/s for blood and neck pressures, and 45 samples/s for respiration. The converter was connected to a Macintosh Powerbook 170 portable computer (Apple Inc., Coupertino, CA, USA) via RS-232 serial interface. A ‘C’ language program identified all QRS complexes in each sequence and then located the peak of each R-wave. The RR interval, systolic and diastolic pressure, neck suction and respiration time series were obtained from these data. The very few premature beats observed were interactively identified and corrected by linear interpolation with the previous and following beats. The original signals and time series were then stored for further analysis of each signal, including mean signal, signal variability (evaluated as the standard deviation), and autoregressive power spectrum analysis.
2.3 Power spectrum analysis
We applied power spectrum analysis to RR interval, respiratory, systolic and diastolic blood pressure and neck suction signals, using an autoregressive method, as previously described [5]. Two orders of spontaneous oscillations were considered: the so-called low-frequency rhythm (LF, from 0.03 to 0.15 Hz, normally observed at a frequency close to 0.10 Hz) and the respiratory rhythm (the so-called high-frequency component, HF, which in the present study was maintained at 0.25 Hz equivalent to 15 breath/min).
These fluctuations are known to reflect, at the level of the heart, the sympatho-vagal balance, as the LF of the RR interval are sensitive to both vagal and sympathetic influences, whereas the HF are sensitive to vagal influences only [5–7]. At the blood pressure level, the LF are considered as an index of sympathetic modulation, whereas the HF are considered as a mechanical effect of changes in stroke volume, due to changes in left ventricular venous return induced by respiration [3, 4, 6].
In addition, we also considered the increase in oscillation induced by neck suction during slow (0.10 Hz, hence superimposed to the spontaneous LF) or fast stimulation (0.20 Hz, hence close to but distinct from that of respiration; see example, Fig. 5).
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2.4 Orthostatic test
The orthostatic challenge is known to increase sympathetic activity. In terms of spectral analysis of cardiovascular fluctuations, this is reflected in a relative increase in LF and a relative decrease in HF of RR interval spectrum and by an increase in LF spectrum of the blood pressure [3, 4, 7]. Hence, any increase in LF dominance in the upright posture after the race, compared to the recording obtained 24 h before the run, would have indicated an increased sympathetic activation after exercise.
2.5 Neck suction
This protocol allows the observation of the effects of baroreceptor activity on the heart separate from that on blood vessels, through modulation of both parasympathetic and sympathetic activities [5].
The sinusoidal change in (negative) pressure imposed at the neck activates and de-activates the carotid baroreceptors: in a normal subject the same sinusoidal change is then evident in the RR interval and in the blood pressure and can be recognized by spectral analysis (see Fig. 5): when the stimulus rate is slow (in the range of 0.1 Hz, or 6 cycles/min) it can be transmitted to both the RR interval and the blood pressure, whereas when the stimulus is faster (i.e., close to the respiratory rate, 0.20 Hz or 12 cycles/min) it appears only in the RR interval. Considering that: (a) the RR interval is under both vagal and sympathetic control, whereas blood pressure is under sympathetic control only; (b) the vagus is able to modulate both fast and slow changes in RR interval, whereas the sympathetic is able to modulate only slow cardiovascular fluctuations [5–7], we can assume that: (1) a response to fast neck suction at the RR interval level is an index of vagal baroreceptor activation; (2) a response to the slow neck suction at the blood pressure level is an index of sympathetic baroreceptor activation; (3) a response to slow neck suction at the RR interval level is an index of baroreceptor stimulation on the heart, via either sympathetic or vagus or both. Since the stimulus is standardized at 30 mmHg sinusoidal swings (for both frequencies), the amount of response can be quantified by the increase in the oscillatory component induced by the neck suction in the target signal (i.e., RR interval or blood pressure) with respect to the control condition.
Hence, any increase in the response to low-frequency neck suction of blood pressure after the race, with respect to the control day, would indicate an increased sensitivity to sympathetic stimulation after exercise, whereas any increase in the response to high-frequency neck suction in the RR interval would signify an increased sensitivity to vagal stimulation.
2.6 Statistical analysis
The results are given as means±s.e.m. Due to the skewed distribution of low- and high-frequency oscillations they were analyzed statistically only after natural logarithmic transformation. Data were analyzed by repeated measures analysis of variance, to assess the effects of posture or neck suction versus recordings in supine position without stimulation, and to assess changes versus the day before the race, on each variable. If significant (i.e., P
0.05) results were obtained by the analysis of variance, comparisons were obtained by Scheffé's test.
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3 Results |
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TopAbstract1 Introduction2 Methods3 Results4 DiscussionReferences |
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3.1 Baseline
Parasympathetic predominance 24 h before the run was evident by: low mean RR interval (Fig. 1), predominance of respiratory sinus arrhythmia (HF) over non-respiratory (LF) fluctuations in RR interval (Fig. 2, first column). Increases in indices of sympathetic activity were found with either orthostatic test (i.e., relative increase in LF vs. HF in the RR interval, and absolute increase in LF in blood pressure, Fig. 2, first column) and slow neck suction (Fig. 3, first column), and a response to parasympathetic stimulation by fast neck suction (Fig. 4, first column).
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3.2 Early post-race effects
Thirty minutes after completion of the run a significant decrease in mean RR interval (P<0.0001), systolic (P<0.0003) and diastolic (P<0.05) blood pressures was found (Fig. 1, second column). There was also a significant drop in overall RR interval variability (RR interval standard deviation: from 44.3±4.7 to 19.0±3.6 ms, P<0.001) without changes in variability in systolic and diastolic blood pressures. Both the LF and HF components of RR interval variability decreased significantly, but the decrease in HF was greater than the decrease in LF, so that there was a relative predominance in LF, indicating relative sympathetic predominance (Fig. 2, second column). This was confirmed by the increase in LF/HF ratio with respect to baseline (1.63±0.37 vs 0.65±0.15, P<0.05).
Sympathetic stimulation induced by the orthostatic test increased the LF in systolic and diastolic blood pressures, but this further reduced the RR interval LF and HF compared to the supine position, although in relative terms a predominance in LF components of RR interval (Fig. 2, second column) was still discernible. Overall, the LF and HF fluctuations in systolic and diastolic blood pressure did not show major differences in the supine or upright position compared to baseline.
Neck suction stimulation at low frequency (0.10 Hz) induced significant changes in all signals, similar to or greater than (for diastolic pressure) those observed 24 h before the run (Fig. 3, second column). This confirmed a persistent ability of the sympathetic to modulate the RR interval and blood pressure. Conversely, neck suction stimulation at high frequency (0.20 Hz) was significantly less effective in changing RR interval compared to baseline (3.55±0.37 vs 5.33±0.34 ln-ms2, P<0.005, Fig. 4, second column). This stimulation had no effect on blood pressure at any time. Therefore, only RR interval data are shown in Fig. 4.
3.3 Twenty-four hours after the race (prolonged effects)
Mean RR interval was similar to baseline, but systolic and diastolic blood pressures were lower (P<0.035 and P<0.025, respectively, Fig. 1 third column). The LF and HF components of variability in all signals and the changes induced by orthostatic stimulation remained similar to baseline, (Fig. 2 third column). This was confirmed by the return of LF/HF ratio to baseline values (0.64±0.18). Neck suction stimulation at low frequency (0.10 Hz), however, became ineffective at this time, in modulating systolic and diastolic blood pressure, (Fig. 3, third column), indicating reduced sensitivity of the arterial vessels to sympathetic modulation, but this stimulation remained effective on RR interval similar to baseline (Fig. 3, third column).
Neck suction stimulation at high frequency (0.20 Hz) was significantly more effective in modulating the RR interval than at baseline (6.44±0.32 vs 5.33±0.34 ln-ms2, P<0.0005, Fig. 4, third column), indicating an increased sensitivity to the vagal stimulation. Data from one subject at baseline and 24 h after the race during fast (0.20 Hz) neck suction stimulation are shown in Fig. 5.
Although the response to fast neck suction stimulation was inversely related to age of the subjects (P<0.05 and P<0.005 at baseline and 24 h after the race respectively), the increase in response from baseline to 24 h after the race was independent of age, but correlated with years of running corrected by age of the subject (P<0.05). This was the only significant association between the increase in sensitivity to vagal stimulation 24 h after the run and training levels, unrelated to or corrected for age.
3.4 Forty-eight hours after the race (prolonged effects)
All tests gave results similar to those obtained at baseline. However, slow neck suction remained incapable of increasing the LF in systolic and diastolic blood pressures (Fig. 4, fourth column).
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4 Discussion |
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TopAbstract1 Introduction2 Methods3 Results4 DiscussionReferences |
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Numerous long-distance races at altitude are staged annually. Participants number in the hundreds and for some events entry is controlled by lottery. The Sandia Wilderness Crossing Research Run was in its twelfth year when the study reported here was carried out. Each year the runners volunteer for many tests and the race continues to serve as a field laboratory for the examination of human adaptation to prolonged exertion at altitude.
In recent years the role of the autonomic nervous system in adaptation has become apparent. Many animal and a few human studies attest to the crucial importance of sympathetic activation that occurs on acute altitude exposure. The hypoxia of altitude has clinical significance at sea level. A number of human diseases have hypoxia of tissues as their basic pathogenic mechanism. Thus lessons from the mountain have relevance in the clinic and may contribute to an understanding of the compensatory autonomic mechanisms that allow adaptation to aging and disease [8]. Thus our study has implications that extend beyond our group of athletes and the archival value of the results.
Strenuous exercise induces immediate and prolonged effects on cardiovascular autonomic modulation. Orthostatic and baroreceptor stimulation via sympathetic and vagal challenges have, however, not previously been employed in a prolonged observational period in relation to long-lasting exertion at altitude.
Short-term exercise in the laboratory, regardless of the level of training, is followed by a persistent sympathetic predominance [2, 9–11]. We found similar effects of prolonged exertion in the field at altitude in trained mountain runners. In addition, a reduction in vagal efferent baroreflex activity was also discernible in our athletes.
Thirty minutes after the race the blood pressure was lower than at baseline, and no significant changes in autonomic modulation were observed in the supine position and after sympathetic activation by orthostasis or slow neck suction stimulation. This indicated that the effect of sympathetic modulation of vessels was, at least partially, preserved after the race. However, in view of the decreased blood pressure and our previous observation of a persistent vasodilation after a single bout of submaximal exercise [11], this modulation was insufficient to counteract the post-exertional hypotension. Sinusoidal neck suction showed relative preservation of sympathetic modulation to the vessels, perhaps limiting the effects of vasodilating substances and other vaso-paralytic mechanisms considered important in post-exercise hypotension [11].
One day after completion of the Sandia Wilderness Crossing Research Run the autonomic pattern after neck suction stimulation indicated an increased effect of baroreflex-induced vagal modulation, and a decreased effect of baroreflex-induced sympathetic modulation. This, together with the persistent reduction in systolic and diastolic blood pressures, suggested that a change in sympatho-vagal balance persisted for at least 24 h after the race. The reduced blood pressure cannot be fully explained by the data collected in the present study, although an increased baroreflex sensitivity with or without resetting, reduced neural sympathetic drive and exhaustion of circulating vasoactive substances may all play a role.
An increase in circulating opioids has been demonstrated in runners in the Sandia Wilderness Crossing Research Run. This increase was only evident after 25 km of running and disappeared 2 h after finishing the race [12]. Opioids in the circulation are unlikely to pass the blood–brain barrier and at any rate could not have affected baroreflexes through central mechanisms 24 h after the race. Atrial natriuretic peptide and arginine vasopressin levels also increased after the Sandia Wilderness Crossing Research Run [13]. These peptides could have had an effect on vasomotion, but it is unlikely that they continued in the circulation for much longer than the opioids. Similarly catecholamines are unlikely candidates to explain the persistent hypotension. They have been measured in the circulation after a standard marathon at 1800 m altitude. Though very large increases in circulating levels occur towards the end of the race, these levels return to baseline within 1 h of finishing [14]. A partial failure of autonomic control of pupillary function after a 161 km altitude race was evidenced by significant delays in pupil cycle times to levels seen in patients with autonomic failure [15]. Thus dysfunction of autonomic control of different organ systems after prolonged exertion at altitude may contribute to the post-exertional state of athletes.
We intentionally allowed a wide range of ages in participants in the Sandia Crossing Run. Altitude is associated with transient ‘aging’ of nervous system function and endurance training tends to prevent this altitude-associated decay in function [8, 16, 17]. These previous studies gain support from our findings that an age-independent increase in responsiveness to fast neck suction stimulation occurred 24 h after the Sandia Wilderness Crossing Research Run. Though the training levels of our runners as judged by distance of running and previous races completed varied widely, their fitness level was nevertheless homogeneous as judged by their ability to finish the race within the time limits given by the organizers. Of interest was that the number of years of training correlated, independently of age, with the increased vagal responsiveness elicited by the race, suggesting that longer exercise training and experience in altitude trail running does not only modify the somatic motor system but also affects autonomic motor control of the cardiovascular system.
The Sandia Wilderness Crossing Research Run characteristically attracts competitors who have successfully completed previous races. Thus longitudinal studies of perennial participants are now possible, offering an opportunity to assess the effects of years of training at altitude on health and disease.
4.1 Conclusions
We found that the autonomic effects of long-distance running at altitude are biphasic: immediately after completion of the exercise, moderate sympathetic predominance and baroreceptor efferent activity via the sympathetic branch are evident; 24 h later there is a reversal in baroreceptor efferent activity with enhancement of the parasympathetic branch, whereas the sympathetic component of the response is attenuated.
Time for primary review 21 days
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