Heart rate variability and altitude: Implications for training

Heart rate variability and altitude: Implications for training

Studying heart rate (HR) modulation by way of heart rate variability (HRV) is a convenient method to assess the sympathetic-parasympathetic balance of the autonomic nervous system. The autonomic nervous system is divided into the sympathetic and parasympathetic systems and many organs are innervated by both sympathetic and parasympathetic nerve fibers. The outflow of neural information in both systems is via a two-neuron pathway (i.e. pre- and post-glanglionic neuron). Their respective action differs; the sympathetic component being generally excitatory while the parasympathetic component is considered relaxing. The sympathetic system is involved in energy production and its action is generally catabolic. It also stimulates the production of ATP through muscular glycolysis. Oppositely, the parasympathetic system is highly involved in the control and regulation of homeostasis, in the re-synthesis of carbohydrate and in both muscular and hepatic glycolysis. Thus the primary function of the parasympathetic nervous system is to conserve the body’s resources and maintain organ functions during periods of physical inactivity. Both systems are always active and this baseline activity is known as resting tone. The tone of an organ can be changed in either direction (i.e. increase or decrease) and is usually regulated by a change in the outflow of either one of the two arms of the autonomic nervous system.

The pacemaker cells in the heart provide it with its automatic rhythm. Nevertheless, the heart is continuously under control since its natural pace is around 100 bpm, while the observed HR among healthy subjects is always slower and a HR of 50 bpm is not unusual in endurance athletes. This means that, at rest, the heart is permanently slowed by parasympathetic activity. Since HR also increases when physical activity increases, this implies that the heart is innervated by both sympathetic and parasympathetic nerve fibers. The sympathetic action on the heart induces an increase both in HR and in the strength of cardiac contractions, and thus an increase in cardiac output. It also controls the arterial pressure and the dilation of the arterioles and of the peripheral vessels. The total peripheral resistance, determined by the ratio between cardiac output and arterial pressure, is increased when the adrenergic (i.e. sympathetic) activity is increased due to the augmentation of the cardiac output and arteriolar vasoconstriction. Akselrod et al. (1981) has shown that it is possible to have indirect indicators for both the sympathetic and parasympathetic tones, based on the analysis of the regulation of rhythmic heart activity. The beat by beat observation of HR or of the intervals (in milliseconds) between two beats, namely R-R intervals, has shown a wide variability of the inter-beat times. The spectral analysis of HRV is the most useful tools to assess the sympatheticparasympathetic balance. In this method, both sympathetic and parasympathetic tones are described by power, i.e. the magnitude and the frequency. Four levels of frequencies are commonly determined:

• The ultra low frequency (ULF) is characterized by very slow spontaneous rhythms and is calculated based on at least 24 hours of recording.

• The very low frequency (VLF) represents the long term mechanisms of regulation probably related to thermoregulation or vasomotricity.

• The low frequency (LF); its physiological interpretation remains controversial.

• The high frequency (HF) reflects the parasympathetic influence on the heart and is determined using breathing frequency (Pomeranz et al. 1985).

Because the LF represents either the sympathetic influence alone or the balance between sympathetic and parasympathetic systems, the ratio LF/HF is usually used to assess the sympathovagal balance (Malliani 1999). An elevated LH/HF ratio represents a predominance of the sympathetic modulation. Acute hypoxia induces an increase in HR due to higher sympathetic activity and epinephrine/norepinephrine concentration coupled with a lower parasympathetic tone (Mazzeo et al. 1994). As a consequence the total spectrum power of HRV is decreased. The HF component is also strongly decreased while the LF increases. Thus, the LF/HF ratio is largely increased (Bernardi et al. 2001). With acclimatization sympathetic stimulation decreases but without reaching the normoxic baseline level. It ensues that a decrease in HR during exercise is particularly marked at maximal exercise. This is explained by a downregulation of the ?-adrenergic receptors (Favret et al. 2001). The total spectral energy of HRV increases due to a rise in HF, thus inducing a drop in the LF/HF ratio without reaching the normoxic level (i.e. sympathetic activity remains higher than parasympathetic) (Bernardi et al. 1998). The use of HRV in training follow up is a recent and original approach. Low intensity (i.e. below lactic threshold) endurance training develops parasympathetic activity responsible for the decreased resting and exercising HR observed in these athletes (Mourot et al. 2004b). Concomitantly, endurance training decreases the sympathetic activity (Hedelin et al. 2001). Intensive training stimulates the sympathetic component, inducing an increase in resting HR and aerobic performance (Iellamo et al. 2002). Based on this relationship between exercise/training and autonomic activity, it has been proposed that baseline autonomic function is an important determinant of the response to aerobic training in both sedentary people (Hautala et al. 2003)and athletes (Hedelin et al. 2001).

Similarly, HRV is a good tool to correctly prescribe training loads in athletes (Kiviniemi et al. 2007). Practical experience from coaches suggests that this is even more important with elite athletes in whom training loads need to be very carefully adjusted. When training loads are too strenuous and the recovery period of time between bouts is not long enough, the athlete can reach a physiological state, known as “overreaching”. The borderline between overreaching and overtraining states is thin. The clinical symptoms associated with both of these states are individual and specific (Kuipers 1998). The analysis of HRV might represent an indication for both overreaching and overtraining. There are discrepancies in the literature about the physiological manifestation of these two types of fatigue. The overreaching state has been characterized by a progressive sympathetic predominance (Pichot et al. 2002), as well as by a parasympathetic modulation (Portier et al. 2001), whereas the overtraining state is characterized by either an increase in the HRV and HF frequency (Hedelin et al. 2000a), or no alteration in resting or orthostatic HRV (Hedelin et al. 2000b). It has also been proposed that an over-trained subject presents a sympathetic modulation and a lesser neurovegetative response to a tilt test (Mourot et al. 2004a). Aerobic training and hypoxia can induce adverse autonomic and cardiovascular responses (Schmitt et al. 2006). Exposure to hypoxia induces autonomic and cardiovascular effects similar to acute exercise (Bernardi et al. 1998; Cornolo et al. 2004), which could persist post-exercise or post-hypoxia (Cornolo et al. 2006). Thus, during high intensity training, the effects of the hypoxic and training stresses can accumulate (Cornolo et al. 2006; Povea et al. 2005), even possibly leading to a state of over-reaching fatigue.

Oppositely, moderate intensity aerobic training in hypoxia may protect the neurovegetative activity from hypoxic effects and allow adaptations complementary of endurance training alone. Indeed, it appears that during continuous hypoxia the altitude is a key determinant of the cardiovascular and autonomic adaptations, while the alternation between hypoxic and normoxic exposure of intermittent hypoxia, such as “living high – training low”, limits the cardiovascular and autonomic adaptations. These adaptations therefore depend on the ratio between hypoxia and normoxia. Thereby, remnant effects of every daily hypoxic exposure may persist when the normoxic training starts, thus adding on top of regular sympathetic stimulation of exercise and then, in turn, limiting the decrease in the sympathetic activity usually observed at rest following aerobic training. Finally, following-up training by using HRV appears to be very relevant and is a very helpful tool to detect early overtraining fatigue then allowing readjustment of training loads. This is of particular interest during altitude training where the effects of aerobic training differ from those of hypoxic exposure. The case of “living high –training low” and more generally of intermittent hypoxic exposure/training, is even more specific because the repeated daily acute exposures present remnant effects, in turn affecting normoxic training. Nevertheless, these adverse effects of hypoxia and training if do not induce overtraining fatigue do not necessarily affect the final outcome, namely the performance.