Differences in Heart Rate Variability in the Frequency Domain between Different Groups of Patients

Background and Objectives: Heart rate variability (HRV) is defined as a physiological variation in duration between sinus beats. The aim of this study was to research and analyze the HRV between various groups of patients. Materials and Methods: A retrospective study was conducted in an outpatient setting. Patients who had undergone a tilt-table test were selected for this study and were divided into three groups based on their self-reported health anamnesis: group 1 (n = 84, mean age 45.8 ± 17.8) consisted of patients with no known orthostatic intolerance or neurodegenerative disease, group 2 consisted of patients with a known or suspected orthostatic intolerance (n = 50, mean age 46.5 ± 18.6), and group 3 consisted of patients with a known or suspected neurodegenerative disorder (n = 29, mean age 55.6 ± 20.4). During the tilt-table test, HRV frequency-domain parameters-normalized low frequency (LFnu) and high frequency (HFnu), absolute powers-absolute low frequency (LF-RRI), absolute high frequency (HF-RRI), and LF/HF ratio-were recorded during 5 min rest in the supine position. Results: Group 1 had a reduced LFnu at 52.93% (SD: 18.00) compared to group 2 at 58.57% (18.06) and group 3 at 61.80% (SD: 17.74), and group 1 had increased HFnu: group 1-47.08% (SD: 17.97), group 2-41.41% (SD: 18.03), and group 3-38.16% (SD: 14.7). LFnu and HFnu differences were statistically significant (p < 0.05). LF-RRI was reported as follows: group 1-531.32 ms² (SD: 578.57), group 2-346.2 ms² (SD: 447.96), and group 3-143.21 ms² (SD: 166.96). HF-RRI was reported as follows: group 1-835.87 ms² (SD: 1625.42), group 2-297.46 ms² (SD: 507.15), and group 3-70.83 ms² (SD: 75.67). LF-RRI and HF-RRI comparisons between groups were statistically significant (p < 0.001). LF/HF ratios were reported as follows: group 1-1.91 (SD: 2.29), group 2-2.43 (SD: 2.33), and group 3-2.54 (SD: 2.17). LF/HF ratio comparisons between groups were statistically significant at p < 0.05. Conclusions: This study shows that patients with known or suspected orthostatic intolerance and neurodegenerative disorders have reduced HRV, possibly caused by reduced parasympathetic modulation. HRV in patients with known or suspected neurodegenerative disorders is reduced more severely than in patients with orthostatic disorders. Other studies in HRV have indicated a possible increase of risk in cardiovascular disorders in patients with reduced HRV, and therefore, HRV analysis could be a potential clinical diagnostic tool. However, the lack of universally agreed upon methodology, reference values, and possible external and internal factor influence hinders the introduction of HRV examinations into wider clinical practice.