Heart Rate Variability Parameters and the PNNX Family as Discriminators between Athletes and Sedentary People

IntroductionChronotropic activity is determined by intrinsic characteristics of cardiac pacemaker cells and the Autonomic Nervous System (ANS) activity through reflex circuits involving their sympathetic and parasympathetic subsystems. These subsystems stimulate or inhibit the sinus node depending on the regulated frequencies and the kinetics of the neurotransmitters involved (Aubert, Seps and Beckers, 2003), which results in variations of the heart rate (HR). The Parasympathetic System (PS) slows down the heart rate by releasing acetylcholine, a neurotransmitter with a very short latency activation period and fast metabolism. Meanwhile, the Sympathetic System (SS) increases the HR, the conduction speed and the inotropismo by using as neurotransmitters reabsorbed and slowly metabolised noradrenaline and adrenaline. Thus, the quick response of the parasympathetic system regulates beat to beat the cardiac function (Pumprla, Howorka and Groves, 2002) .The dynamic interaction of the above listed components produces variations in the periodicity of the cardiac cycles. Some effects might be identified soon in the cycle to cycle variations, and some others may be associated to modulations with longer lasting effects (tonics) and probably more stable. In this context, the temporal evolution of the R-R intervals, whose representation is known as tachogram, might be analysed numerically from different mathematical approaches, two of the most frequently used are a) time domain and b) frequency domain analysis. The first methods are based on statistical indicators of the time variation of consecutive cycles (differences between two consecutives R-R intervals, RMSSD, pNNx etc.) or measuring features of the cycles, analysed as a set, (such as mean, standard deviation, frequency distributions, etc.); meanwhile, the frequency approach analyse the energy or power content of the components associated with very low (Moreover, time domain measurements exhibit quick adjustments of the ANS, which are primarily related to vagal activity (Zaza and Lombardi, 2001), as well as the High-Frequency component (HF), while Low Frequency parameters (LF) and LF / HF ratio has been interpreted as a mixture of sympathetic and parasympathetic activity. Additionally, the Very Low Frequency component (VLF) has been associated to adjustment during respiratory cycles and it usually requires extended electrocardiographic records in order to cover several phases of such oscillations (Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology,1996). From which it follows that time domain parameters can be as effective as frequency domain parameters (especially HF) when analysing control mechanisms of the RR variability in periods of short duration records.Literature has converged that when bradycardia is monitored in athletes at rest, it shows a systematic decrease in heart rate after sustained training (Aubert, Seps and Beckers, 2003), providing evidence of the influence of both subsystems of the ANS over heart chronotropism. However, a small number of these studies have focused on the analysis of the chronotropic regulatory mechanisms and very few have assessed which of the classic parameters, derived from HRV analysis, discriminate better between athletes and sedentary people at rest (Olivasse, Paulo and Jurandir N, 2014), (Shin , Minamitani and Onishi S, 1995), (Aubert, Beckers and Ramaekers, 2000); only one of these works provides new measures related to slowdowns in two consecutive RR intervals through the phase-rectified signal averaging (PSRA) in athletes and sedentary during rest, comparing these new measures against classical parameters of vagal modulation as HF and RMSSD, in order to better illustrate the behaviour of the SNA (Olivasse, Paulo and Jurandir, 2014). …