Autonomic nervous system status depends on cumulated physical fatigue due to increased training loads. Therefore, heart rate variability analysis appears to be an appropriate tool to monitor the effects of physical training loads on performance and fitness, and could eventually be used to prevent overtraining states.

Intensive endurance training in elderly men enhanced parasympathetic parameters of HRV and, interestingly, of SBR. Physiological mechanisms and long-term clinical effects on health status should be further investigated.

Performance is correlated with nocturnal ANS activity at an individual level. The decrease in ANS activity during intensive training is correlated with the loss in performance, and the rebound in ANS activity during tapering tracks with the gain in performance. Interestingly, the speed of the rebound during the tapering period was quite different between swimmers. ANS activity measurement may be useful to design and control individual training periods and to optimize the duration of tapering.

Heart rate fluctuations are a typical finding during obstructive sleep apnoea, characterised by bradycardia during the apnoeic phase and tachycardia at the restoration of ventilation. In this study, a time-frequency domain analysis of the nocturnal heart rate variability (HRV) was evaluated as the single diagnostic marker for obstructive sleep apnoea syndrome (OSAS).The predictive accuracy of time-frequency HRV variables (wavelet (Wv) decomposition parameters from level 2 (Wv2) to level 256 (Wv256)) obtained from nocturnal electrocardiogram Holter monitoring were analysed in 147 consecutive patients aged 53.8¡11.2 yrs referred for possible OSAS.OSAS was diagnosed in 66 patients (44.9%) according to an apnoea/hypopnoea index o10. Using receiver-operating characteristic curves analysis, the most powerful predictor variable was Wv32 (W 0.758, pv0.0001), followed by Wv16 (W 0.729, pv0.0001) and Wv64 (W 0.700, pv0.0001). Classification and Regression Trees methodology generated a decision tree for OSAS prediction including all levels of Wv coefficients, from Wv2 to Wv256 with a sensitivity reaching 92.4% and a specificity of 90.1% (percentage of agreement 91.2%) with this nonparametric analysis.Time-frequency parameters calculated using wavelet transform and extracted from the nocturnal heart period analysis appeared as powerful tools for obstructive sleep apnoea syndrome diagnosis. Obstructive sleep apnoea syndrome (OSAS) is a common prevalent problem [1] (prevalence of 4% in middle-aged males) with major health implications ranging from traffic accident [2] to serious cardiac arrhythmias. OSAS is associated with increased risks of hypertension, myocardial infarction and stroke, and with increased mortality rates [3,4].Diagnosis of OSAS is usually performed by polysomnography in a sleep laboratory, consisting of the measurement and recording of several signals used to analyse sleep and breathing. Whereas polysomnography represented the "gold standard" for the diagnosis, it is an expensive and timeconsuming procedure with important resources invested in patients with mild-to-moderate disease. Moreover, the laboratory environment often disturbs or interferes with the patient9s sleep. Therefore, several strategies have been developed to decrease the number of the sleep recordings, including sleep questionnaires, ambulatory recordings, simplified multichannel systems and nocturnal oximetry, all showing a high specificity but a low sensitivity [5][6][7].It is known that all through the night, recurrence of apnoeas elicits a typical and cyclic heart rate pattern consisting of cyclical brady/tachycardia [8], contrasting with an altered diurnal control of the sinus node, and is related to cyclic changes in vagal and sympathetic activity. To quantify the unique heart rate rhythm induced by a successive alternance of vagal stimulation and sympathetic discharge [9,10], spectral analysis of heart rate variability (HRV) has been applied using short-term night-time recording. However, such a technique encounters major d...

To examine the baroreflex response in humans during and immediately after acute hypoxia exposure, the cardiac baroreflex sensitivity (BRS) was studied using adaptation of RR intervals in response to spontaneous systolic blood pressure fluctuations (sequences methodology) in 11 unacclimatized subjects. All measurements were made under fixed breathing rate, and realized consecutively at baseline level (20 min), at an inspired oxygen concentration of 11% (15 min) and again under normoxic conditions (20 min; recovery period). The spontaneous baroreflex response decreases progressively during hypoxic exposure, causing a tachycardic response at this FiO2 without any significant alteration of the systolic or diastolic blood pressure. The magnitude of decrease for this variable at the end of exposure averaged 42.9 +/- 15.6%. The simultaneous spectral analysis of heart rate (HR) variability in hypoxic condition confirmed an alteration in the parasympathetic activity (HFnu: -17.8 +/- 30.9% versus basal conditions, P < 0.01) counterbalanced by an exaggerated sympathetic activity (LFnu: +33 +/- 42.4%, P < 0.05) at the sinus node. Interestingly, we could observe an enhanced cardiac baroreflex response during the period following the inhalation of the hypoxic mixture (+130.6 +/- 15.6% of basal conditions, P < 0.001). There is a relationship with a significant and abrupt increase in the parasympathetic control of HR (mean HR reached 111 +/- 8.1% of the mean basal HR, P < 0.01). These results suggest that brief exposure to hypoxia under rate-controlled ventilation is associated with a significant alteration in the spontaneous cardiac baroreflex. This important cardiac autonomic imbalance is followed by a significant increase in the cardiac parasympathetic drive even after the disappearance of the hypoxic stimulus.