The role of respiration in the genesis of heart rate variability (HRV) has been the subject matter of many experimental and modeling studies. It is widely accepted that the high frequency (HF) peak of a HRV power spectrum, which is centered at the average respiratory frequency, is caused by mechanisms activated by respiration. On the other hand, there is a debate on the possible role of respiration in the genesis of the low frequency (LF) peak which is usually centered around 0.1 Hz. In this study, a comprehensive cardiorespiratory interaction model is used to test various hypotheses regarding the role of respiration in the LF peak of HRV. In this model, chest and abdomen circumference signals and lung volume signal are used as respiratory inputs. Simulations are made for periodic, spontaneous and slightly irregular respiratory patterns, and it is observed that the more low frequency (LF) power there in the respiratory signals, the more LF power there in the model-predicted HRV. Experiments on nine volunteers are also performed for the same respiratory patterns and similar results are observed. Furthermore, the actual measured respiratory signals are input to the model and the model predicted and the actual HRVs are compared both in time domain and also with respect to their power spectra. It is concluded in general that respiration not only is the major contributor to the genesis of the HF peak in the HRV power spectrum, but also plays an important role in the genesis of its LF peak. Thus, the LF/HF ratio, which is used to assess sympathovagal balance, cannot be correctly utilized in the absence of simultaneous monitoring of respiration during an HRV test.

In this study, the effects of electromagnetic fields (EMFs) emitted by GSM900 based mobile phones (MPs) on the heart rate variability (HRV) were examined by using nonlinear analysis methods. The largest Lyapunov exponent (LLE) calculation was used to evaluate the effect of MP under various real exposure conditions. Sixteen healthy young volunteers were exposed to EMFs emitted by GSM900 based MP at two levels from a very low EMF (MP at stand-by) to a higher EMF (MP at pre-ring handshaking and ringing). A blind experimental protocol was designed and utilized with consideration to the physiological and psychological factors that may affect HRV. The results showed that the LLE values increased slightly with higher EMF produced by MP (P < 0.05). This change indicates that the degree of chaos in the HRV signals increased at higher EMF compared to low level EMF. Consequently, we have concluded that high level EMF changed the complexity of cardiac system behavior, significantly.

In this prospective study, the effects of mediastinal radiation therapy (RT) on autonomic nervous system (ANS) were investigated by heart rate variability (HRV) analysis that is accepted as a non-invasive indicator of ANS. Study was performed with the eligible patients had a histopathologically confirmed diagnosis of malignant disease with no known congestive heart failure, coronary heart disease, hypertension, valvular cardiac disease or arrhythmia history. Electrocardiograms of 14 voluntary patients were recorded for duration of 5 min just before and after irradiation for the first and the 15th fractions. ANS-related HRV analysis parameters were calculated as which were recommended by Task Force of ESC/NASPE (Circulation 93:1043-1065, 1996). HRV parameters that belong to pre- and post-RT treatment of patients were compared statistically. We found that there is not effect of single-dose radiation on HRV parameter. The mean RRI (782.29 ± 115.65-738.93 ± 111.01, P < 0.014) and HF power of HRVs PSD (156.94 ± 229.37-60.71 ± 77.99, P < 0.045) decreased, and LF/HF ratio (1.38 ± 0.79-2.03 ± 1.25, P < 0.039) increased significantly with 28-Gy external radiation dose. As the effect of cumulative dose was investigated on HRV parameters, the above changes were continued to increase with 30-Gy external radiation dose. We found that mediastinal RT involving heart directly in the radiation field decreased vagal and sympathetic ANS activities, and autonomic balance shifted toward sympathetic dominance.

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