0.4 hz, low frequency (lf) from 0.04 to 0.15 hz, and the very low frequency (vlf) from 0.0033 to 0.04 hz. several methods of analysis are available. power spectral density (psd), using parametric or nonparametric methods, provides basic information on the power distribution across frequencies. one of the most commonly used psd methods is the discrete fourier transform. methods for the calculation of psd may be generally classified as nonparametric and parametric. in most instances, both methods provide comparable results. the advantages of the nonparametric methods are (1) the simplicity of the algorithm used (fast fourier transform in most of the cases) and (2) the high processing speed. the advantages of parametric methods are (1) smoother spectral components that can be distinguished independent of preselected frequency bands, (2) easy postprocessing of the spectrum with an automatic calculation of low- and high-frequency power components with an easy identification of the central frequency of each component, and (3) an accurate estimation of psd even on a small number of samples on which the signal is supposed to maintain stationarity. the basic disadvantage of parametric methods is the need of verification of the suitability of the chosen model and of its complexity (that is, the order of the model). in addition to classical fft-based methods used for the calculation of frequency parameters, a more appropriate psd estimation method is the lomb–scargle periodogram. analysis has shown that the ls periodogram can produce a more accurate estimate of the psd than fft methods for typical rr data. since the rr data is an unevenly sampled data, another advantage of the ls method is that in contrast to fft-based methods it is able to be used without the need to resample and detrend the rr data. alternatively, to avoid artefacts that are created when calculating the power of a signal that includes a single high-intensity peak (for example caused by an arrhythmic heart beat), the concept of the 'instantaneous amplitude' has been introduced, which is based on the hilbert transform of the rr data. a newly used hrv index, which depends on the wavelet entropy measures, is an alternative choice. the wavelet entropy measures are calculated using a three-step procedure defined in the literature. first, the wavelet packet algorithm is implemented using the daubechies 4 (db4) function as the mother wavelet with a scale of 7. once the wavelet coefficients are obtained, the energy for each coefficient are calculated as described in the literature. after calculating the normalized values of wavelet energies, which represent the relative wavelet energy (or the probability distribution), the wavelet entropies are obtained using the definition of entropy given by shannon. non-linear methods given the complexity of the mechanisms regulating heart rate, it is reasonable to assume that applying hrv analysis based on methods of non-linear dynamics will yield valuable information. although chaotic behavior has been assumed, more rigorous testing has shown that heart rate variability cannot be described as a low dimensional chaotic process. however, application of chaotic globals to hrv has been shown to predict diabetes status. the most commonly used non-linear method of analysing heart rate variability is the poincaré plot. each data point represents a pair of successive beats, the x-axis is the current rr interval, while the y-axis is the previous rr interval. hrv is quantified by fitting mathematically defined geometric shapes to the data. other methods used are the correlation dimension, symbolic dynamics, nonlinear predictability, pointwise correlation dimension, approximate entropy, sample entropy, multiscale entropy analysis, sample asymmetry and memory length (based on inverse statistical analysis). it is also possible to represent long range correlations geometrically. long term correlations sequences of rr intervals have been found to have long-term correlations. however, one flaw with these analyses is their lack of goodness-of-fit statistics, i.e. values are derived that may or may not have adequate statistical rigor. different types of correlations have been found during different sleep stages. heart rate dependence of hrv parameters a basic problem is that all the parameters used to characterize hrv strongly depend on heart rate, but many articles have not adjusted properly or at all for hr differences when comparing hrv in multiple circumstances. however, the exact hrv(hr) relationship is still a matter of debate. for time domain parameters (rmssd, sdnn, etc.) the results imply that, if there exists a universal function, it should be either exponential or hyperbolic in nature. the evaluation procedures used to determine hrv(hr) function have not allowed to decide between these options, so far. a new evaluation method has recently allowed to determine a hrv(hr) function with unprecedented precision: it can be described by two descending exponential components for healthy individuals, in general. duration and circumstances of ecg recording time domain methods are preferred to frequency domain methods when short-term recordings are investigated. this is due to the fact that the recording should be at least 10 times the wavelength of the lowest frequency bound of interest. thus, recording of approximately 1 minute is needed to assess the hf components of hrv (i.e., a lowest bound of 0.15 hz is a cycle of 6.6 seconds and so 10 cycles require ~60 seconds), while more than 4 minutes are needed to address the lf component (with a lower bound of 0.04 hz). although time domain methods, especially the sdnn and rmssd methods, can be used to investigate recordings of long durations, a substantial part of the long-term variability is day–night differences. thus, long-term recordings analyzed by time domain methods should contain at least 18 hours of analyzable ecg data that include the whole night. physiological correlates of hrv components autonomic influences of heart rate although cardiac automaticity is intrinsic to various pacemaker tissues, heart rate and rhythm are largely under the control of the autonomic nervous system. the parasympathetic influence on heart rate is mediated via release of acetylcholine by the vagus nerve. muscarinic acetylcholine receptors respond to this release mostly by an increase in cell membrane k+ conductance. acetylcholine also inhibits the hyperpolarization-activated "pacemaker" current. the "ik decay" hypothesis proposes that pacemaker depolarization results from slow deactivation of the delayed rectifier current, ik, which, due to a time-independent background inward current, causes diastolic depolarization. conversely, the "if activation" hypothesis suggests that after action potential termination, if provides a slowly activating inward current predominating over decaying ik, thus initiating slow diastolic depolarization. the sympathetic influence on heart rate is mediated by release of epinephrine and norepinephrine. activation of β-adrenergic receptors results in camp-mediated phosphorylation of membrane proteins and increases in ical and in if the result is an acceleration of the slow diastolic depolarization. under resting conditions, vagal tone prevails and variations in heart period are largely dependent on vagal modulation. the vagal and sympathetic activity constantly interact. because the sinus node is rich in acetylcholinesterase, the effect of any vagal impulse is brief because the acetylcholine is rapidly hydrolyzed. parasympathetic influences exceed sympathetic effects probably through two independent mechanisms: a cholinergically induced reduction of norepinephrine released in response to sympathetic activity, and a cholinergic attenuation of the response to an adrenergic stimulus. components the rr interval variations present during resting conditions represent beat-by-beat variations in cardiac autonomic inputs. however, efferent vagal (parasympathetic) activity is a major contributor to the hf component, as seen in clinical and experimental observations of autonomic maneuvers such as electrical vagal stimulation, muscarinic receptor blockade, and vagotomy. more problematic is the interpretation of the lf component, which was considered by some as a marker of sympathetic modulation (especially when expressed in normalized units) but is now known to include both sympathetic and vagal influences. for example, during sympathetic activation the resulting tachycardia is usually accompanied by a marked reduction in total power, whereas the reverse occurs during vagal activation. thus the spectral components change in the same direction and do not indicate that lf faithfully reflects sympathetic effects. hrv measures fluctuations in autonomic inputs to the heart rather than the mean level of autonomic inputs. thus, both withdrawal and saturatingly high levels of autonomic input to the heart can lead to diminished hrv. changes related to biologic states and pathologies a reduction of hrv has been reported in several cardiovascular and noncardiovascular diseases. myocardial infarction depressed hrv after mi may reflect a decrease in vagal activity directed to the heart. hrv in patients surviving an acute mi reveal a reduction in total and in the individual power of spectral components. the presence of an alteration in neural control is also reflected in a blunting of day-night variations of rr interval. in post-mi patients with a very depressed hrv, most of the residual energy is distributed in the vlf frequency range below 0.03 hz, with only a small respiration-related variations. diabetic neuropathy in neuropathy associated with diabetes mellitus characterized by alteration in small nerve fibers, a reduction in time domain parameters of hrv seems not only to carry negative prognostic value but also to precede the clinical expression of autonomic neuropathy. in diabetic patients without evidence of autonomic neuropathy, reduction of the absolute power of lf and hf during controlled conditions was also reported. similarly, diabetic patients can be differentiated from normal controls on the basis of reduction in hrv. heart transplants a very reduced hrv with no definite spectral components has been reported in patients with a recent heart transplant. the appearance of discrete spectral components in a few patients is considered to reflect cardiac reinnervation. this reinnervation may occur as early as 1 to 2 years after transplantation and is assumed to be of sympathetic origin. in addition, a correlation between respiratory rate and the hf component of hrv observed in some transplanted patients also indicates that a nonneural mechanism may generate a respiration-related rhythmic oscillation. myocardial dysfunction a reduced hrv has been observed consistently in patients with cardiac failure. in this condition characterized by signs of sympathetic activation such as faster heart rates and high levels of circulating catecholamines, a relation between changes in hrv and the extent of left ventricular dysfunction was reported. in fact, whereas the reduction in time domain measures of hrv seemed to parallel the severity of the disease, the relationship between spectral components and indices of ventricular dysfunction appears to be more complex. in particular, in most patients with a very advanced phase of the disease and with a drastic reduction in hrv, an lf component could not be detected despite the clinical signs of sympathetic activation. this reflects that, as stated above, the lf may not accurately reflect cardiac sympathetic tone. liver cirrhosis liver cirrhosis is associated with decreased hrv. decreased hrv in patients with cirrhosis has a prognostic value and predicts mortality. loss of hrv is also associated with higher plasma pro-inflammatory cytokine levels and impaired neurocognitive function in this patient population. sepsis hrv is decreased in patients with sepsis. loss of hrv has both diagnostic and prognostic value in neonates with sepsis. the pathophysiology of decreased hrv in sepsis is not well understood but there is experimental evidence to show that partial uncoupling of cardiac pacemaker cells from autonomic neural control may play a role in decreased hrv during acute systemic inflammation. tetraplegia patients with chronic complete high cervical spinal cord lesions have intact efferent vagal neural pathways directed to the sinus node. however, an lf component can be detected in hrv and arterial pressure variabilities of some tetraplegic patients. thus, the lf component of hrv in those without intact sympathetic inputs to the heart represent vagal modulation. sudden cardiac death victims of sudden cardiac death have been found to have had lower hrv than healthy individuals. hrv can be observed to be depressed prior to the development of scd, which raises questions about whether or not altered autonomic function plays a role in the development of electrical instability. hrv is also depressed in scd survivors, who are at high risk for subsequent episodes. cancer hrv correlates with the progression of disease and outcome of cancer patients, according to a systematic review of published studies. patients in the early stages of cancer have a significantly higher hrv when compared to patients in the later stages of cancer, suggesting disease severity influences hrv. different ranges of hrv can be observed between cancer types. pregnancy hrv alterations occur in healthy pregnancies as well as similar changes in pregnancies with gestational diabetes that include lower