HF Norm Research Articles

Experimental study on physiological responses and thermal comfort under various ambient temperatures

This study mainly explored the thermal comfort from the perspective of physiology. Three physiological parameters, including skin temperature (local and mean), electrocardiograph (ECG) and electroencephalogram (EEG), were investigated to see how they responded to the ambient temperature and how they were related to the thermal comfort sensation. A total of four ambient temperatures (21 °C, 24 °C, 26 °C and 29 °C) were created, while the other thermal conditions including the air velocity (about 0.05 ± 0.01 m/s) and the air humidity (about 60 ± 5 m/s) were kept as stable as possible throughout the experiments. Twenty healthy students were tested with questionnaire investigation under those thermal environments. The statistical analysis shows that the skin temperature (local and mean), the ratio of LF norm to HF norm of ECG and the global relative power of the different EEG frequency bands will be sensitive to the ambient temperatures and the thermal sensations of the subjects. It is suggested that the three physiological parameters should be considered all together in the future study of thermal comfort.

Changes in Heart Rate Variability in Elderly Hypertensive Patients during Combined Spinal-Epidural Anesthesia

Background: We evaluated the changes in heart rate variability (HRV) in elderly patients with or without hypertension before and after combined spinal-epidural anesthesia (CSE). Methods: Elderly hypertensive (group H, n = 28) and non-hypertensive patients (group C, n = 32) were recruited. We analyzed the spectral components of HRV, total power (≤ 0.4 Hz), low and high frequency power expressed as natural logarithm (LnTP, LnLF, and LnHF), normalized HF (HFnorm), normalized LF (LFnorm), and LF/HF ratio, from 5-minute electrocardiography recordings. We also measured blood pressures (systolic; SBP, diastolic; DBP, mean; MAP), heart rate (HR) and analgesia level before and after CSE. Each group was divided into two subgroups (≤ T7 [L] or ≥ T6 [H]) according to fixed analgesia level. Results: Before CSE, group H showed significantly lower LnLF than that of group C (P ＜ 0.05). Fifteen and 10 min after CSE, decrease of SBP and increase of HR in group HH were significantly greater compared with group CL and lesser with group CH, respectively. LnTP, LnLF and LnHF in both control groups (CL and CH) were significantly decreased 15-20 min after CSE compared with pre-CSE values. LnLF and LnHF in group HH were significantly lower than those in group HL 10-15 min after CSE. But there were no significant changes of LFnorm, HFnorm and LF/HF in all four groups. Conclusions: The group H showed significantly lower LnLF than that of group C before CSE. In group HH, LnLF and LnHF were significantly lower than those in group HL 10-15 min after CSE, but the balance of the autonomic nervous system was not changed significantly. (Korean J Anesthesiol 2006; 50: 663~73)

Effects of aqua-treadmill exercise on selected blood parameters and on heart-rate variability of horses.

The objectives of the present study were to investigate the effects of Aquatraining of horses (aqua-treadmill exercise; treadmill manufactured by Equitech - L.u.S. Equipment, Warendorf, Germany) on selected blood parameters [lactic acid concentration (mmol/l), haemoglobin content (g/l)] and on heart-rate variability (HRV) [heart rate (beats per min; b.p.m.), standard deviation of all NN-intervals (SDNN; ms), normalized power of the low and high frequency band (LFnorm, Hfnorm; au), % recurrence, % determinism and ratio(corr)]. Seven horses performed six exercise tests with different work loads (walking (x = 1.56 +/- 0.08 m/s) and trotting (x = 2.9 +/- 0.13 m/s): dry, water above the carpus and water above the elbow). The standardized test-protocol was: 5 min warm-up at walk while the water was pumped in, followed by the 20-min exercise period at walk or trot, followed by a 5-min walk while pumping out the water. Blood samples were taken prior to each test at rest in the stable, as well as exactly 5 min after the end of the 20-min exercise period. Electrocardiograms were recorded during rest and the 20-min exercise period. Compared to rest, neither the chosen velocities, the two water levels, nor the dry tests led to a significant increase of the lactic acid concentration in any horse. The haemoglobin content showed a significant increase as a result of exercise. Significant differences could be found between the heart rates at rest and the six exercise tests and between the mean of the levels 'walking' and the mean of the levels 'trotting'. An exercise-induced change of HRV was characterized by a decreasing SDNN, a significantly higher LFnorm (sympathetic influence) combined with a significantly lower HF(norm) power (parasympathetic activity) and a rising degree of order (significantly higher % determinism and nearly unchanged % recurrence) and stability (significantly rising ratio(corr)) of the recurrence plot. In conclusion, the used training-protocol for aqua-treadmill exercises only represents a medium-sized aerobic work load for horses, but the different levels of burden were indicated especially by changes in HRV.

Entropy measures of heart rate variation in conscious dogs.

Our goal was to determine the contributions of sympathetic and parasympathetic activity to entropy measures of heart rate variability (HRV). We compared our results with two commonly used methods to analyze HRV: standard deviation (SDNN) and power spectral analysis (HF norm). Beat-by-beat analysis of R-R intervals was performed in conscious dogs. The R-R intervals were analyzed with approximate entropy (ApEn) and entropy of symbolic dynamics (SymDyn) to assess the effects of reducing system complexity. This was achieved by pharmacologically inhibiting sympathetic, parasympathetic, and total autonomic nervous system regulation of heart rate. Three conditions were examined: rest, standing, and systemic hypotension. At rest or standing, sympathetic inhibition (propranolol) had no effect on ApEn or SymDyn, whereas parasympathetic (atropine) and combined (propranolol + atropine) inhibition reduced both entropy measures to near zero. Systemic hypotension reduced both entropy measures in intact dogs. When hypotension was induced after sympathetic inhibition, ApEn was increased compared with hypotension alone, whereas parasympathetic inhibition with hypotension resulted in near-zero ApEn. Changes in the entropy measures of HRV were directionally similar to changes in SDNN and HF norm. These results indicate that the entropy of R-R intervals reflects parasympathetic modulation of heart rate.

Power Spectral Analysis of Heart Rate Variability in Hypertensive Male

Background: Essential hypertension is associated with altered cardiovascular autonomic nerve function. Heart Rate Variability (HRV) analysis is an important tool for quantitative measurement of autonomic nerve activity. Objective: To assess the cardiac autonomic nerve function status in essential hypertension by analyzing power spectral measures of heart rate variability. Methods: This cross sectional study was carried out in the Department of Physiology, Bangabandhu Sheikh Mujib Medical University between July 2008 and June 2009. Sixty male hypertensive patients with age range 40-60 years (group B) were enrolled from the Out Patient Department of Cardiology, BSMMU, Dhaka. Thirty age and sex matched apparently healthy normotensive subjects (group A) were studied as control, Hypertensive patients were further divided into group B1 consisting of 30 untreated patients on their 1st day of diagnosis and group B2 consisting of 30 patients under antihypertensive therapy. Power Spectral measures of Heart Rate Variability (HRV) such as total power, low frequency (LF) and high frequency power (HF) LH/HF ratio were assessed by a digital Polygraph. For statistical analysis, Independent sample t-test, One-way ANOVA test and Pearson’s correlation coefficient test were done as applicable. Results: Mean TP, LF, HF and HF norm was significantly (p<0.001) lower and LF/HF ratio and LF norm were significantly (p<0.001) higher in both untreated and treated hypertensive patients compared to those of normotensive subjects. Again, significant differences of LF norm and HF norm and LF/HF ratio were found between the two hypertensive groups. The LF/HF ratio and LF norm were positively and HF norm was negatively correlated with SBP and DBP in both the hypertensive groups. However, all these findings were more marked in untreated hypertensive patients than those of treated group. Conclusion: Sympathovagal balance in hypertensive patients is towards higher sympathetic and lower vagal modulation and effective treatment of the patients can return the balance towards normal. Key words: LF norm; HF norm; LF/HF ratio; hypertension DOI: http://dx.doi.org/10.3329/jbsp.v6i1.8082 J Bangladesh Soc Physiol. 2011 June; 6(1): 32-38