Adjustment of magnesium sulfate infusion rate in patients with preterm labor

OBJECTIVE: Our purpose was to investigate factors that might influence serum magnesium levels during intravenous magnesium sulfate tocolytic therapy. STUDY DESIGN: Thirty-three women receiving magnesium sulfate for preterm labor participated in this prospective, observational study. Gestational ages were 24 to 34 weeks. Four groups of women were identified according to the maintenance magnesium infusion rate required for arresting preterm labor after 5 g of therapy induction: 1.5, 2, 2.5, and 3 g/h. Serum magnesium samples were drawn after a predefined period of at least 18 hours of arrested preterm labor, at a minimum of every 6 hours. Variables examined included serum albumin; serum protein; serum ionized calcium; serum creatinine; creatinine clearance; 24-hour urine output; maternal height, weight, body surface area; and body mass index. RESULTS: By use of a multivariate stepwise regression model we identified four variables that independently and significantly contributed to the model: magnesium infusion rate (P < .001); total serum protein level (P < .001); serum creatinine level (P = .009); and maternal weight squared (P = .026). Seventy-two percent of the variance was accounted for by use of these parameters. A predictive linear model, developed to relate these factors, produced the following formula: Suggested magnesium infusion rate = 0.89 x Serum magnesium concentration (mg/dL) - 3.16 x Serum creatinine (mg/dL) - 0.66 x Serum total proteins (g/dL) + 0.0001 x (maternal weight)2 (kg) + 2.30. CONCLUSIONS: Serum creatinine, serum protein, and maternal weight can be used to adjust the dose of magnesium sulfate in patients with premature labor to achieve therapeutic serum levels of magnesium more rapidly and safely.     

Effect of dexamethasone and betamethasone on fetal heart rate variability in preterm labour: a randomised study

OBJECTIVE: To compare the effects of betamethasone and dexamethasone on fetal heart rate in appropriately grown fetuses. METHODS: Eighty-two pregnant women (97 fetuses) with preterm labour were randomly allocated to receive betamethasone (n=42) or dexamethasone (n=40) for fetal lung maturation in a nonblinded fashion. Computerised cardiotocogram (CTG) parameters were compared before, during and after treatment. RESULTS: A decrease in fetal heart rate variability was found with betamethasone but no significant changes were found with dexamethasone. Fetal heart rate variability returned to pre-treatment values within a week after cessation of treatment with betamethasone. Neonatal outcome was similar in the two groups. CONCLUSIONS: These findings might prove useful in the management of compromised fetuses with decreased fetal heart rate variability in which the CTG should be used together with other parameters to assess fetal wellbeing during corticosteroid treatment. Dexamethasone may be preferable as the drug of choice since it was associated with significantly less alteration in fetal heart rate variability compared with betamethasone.     

Effects of intravenous magnesium sulfate on arrhythmias in patients with congestive heart failure

Intravenous magnesium is an effective treatment for ventricular tachycardia of some etiologies, and in patients with congestive heart failure low serum magnesium concentrations are associated with frequent arrhythmias and high mortality. This suggests that magnesium administration may decrease the frequency of ventricular arrhythmias in patients with heart failure. We therefore assessed the impact of an intravenous magnesium infusion upon the frequency of ventricular premature depolarizations in 40 patients with New York Heart Association (NYHA) class II to IV heart failure and serum magnesium < or = 2.0 mg/dl. Within 1 week of a baseline 6-hour ambulatory electrocardiographic recording, an infusion of 0.2 mEq/kg of MgSO4 was given over 1 hour and a repeat 6-hour recording was obtained. There was an inverse relationship between the change in magnesium concentration and the change in frequency of premature ventricular depolarizations; premature ventricular depolarizations declined by 134 +/- 207 hr-1 in patients in whom serum magnesium concentration increased > or = 0.75 mg/dl, but increased by 72 +/- 393 hr-1 in patients with a change < 0.75 mg/dl (p < 0.05). For all patients, the frequency of premature ventricular depolarizations was 283 +/- 340 hr-1 pretreatment and 220 +/- 269 hr-1 following magnesium infusion (p = 0.21). Patients with > or = 300 premature ventricular depolarizations hr-1 demonstrated a decrease from 794 +/- 309 to 369 +/- 223 hr-1 (p < 0.001). Intravenous magnesium administration decreased the frequency of couplets from 233 +/- 505 to 84 +/- 140 (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of antenatal betamethasone and dexamethasone administration on fetal heart rate variability in growth-retarded fetuses

The results of the operative treatment of 27 humeral shaft fractures treated at the University of Louisville during a 2-year period were reviewed. The aim of this study was to analyze 1) the indications and results of surgical treatment, 2) the indications for nailing versus plating, and 3) the failures and their treatment (especially surgical nonunions). Indications for surgery were polytrauma patients (including open fractures, associated neurovascular injuries, associated ipsilateral forearm injuries) and isolated unstable fractures in which closed reduction failed. Plate and screw osteosynthesis was used in patients with proximal and distal fractures, in the presence of neurovascular injuries, progressive radial nerve palsy and failure of closed reduction due to interposition of soft tissue. Intramedullary antegrade nailing was preferentially used in polytrauma patients. Seven patients (25%) needed further surgery because of nonunion. The frequency was higher after plating (30%) than after nailing (20%), it was more common in comminuted fractures, middle third fractures and after insufficient distal locking. Exchange nailing resulted in union in 5 of the 7 cases. Although excellent results with low complication rates are reported in the recent literature following plate and screw osteosynthesis or locked intramedullary nailing, we found that operative treatment of difficult humeral shaft fractures is still fraught with a high complication rate.     

Effects on hypoxaemia on foetal heart rate, variability and cardiac rhythm

1. Experiments were carried out in 30 chronically catheterized foetal sheep (128-144 days; term 150 days) and in seven of these foetuses before, during and after acute hypoxaemia. The extent to which changes in sympathoadrenal activity and cardiac vagal activity affected the foetal cardiac response to hypoxaemia was measured. Three measurements were used: foetal heart rate (FHR), heart rate variability (HRV; measured as the coefficient of variation in pulse interval) and power spectral density (PSD; measured over the frequency ranges of 0.04-1.3 Hz). Cardiac vagal activity was blocked by atropine, beta-adrenoceptor activity was blocked by propranolol. 2. Under normoxaemic conditions, cardiac vagal blockade caused a rise in mean arterial pressure (MAP; P < 0.001), an increase in FHR (P < 0.001), a decrease in HRV (P < 0.001) and a decrease in PSD (P < 0.001). beta-adrenoceptor blockade caused a rise in MAP (P < 0.001), a fall in FHR (P < 0.01), a decrease in HRV (P < 0.001) but no change in PSD. 3. During mild hypoxaemia (PO2 = 12-14.5 mmHg) and moderate hypoxaemia (PO2 = 10-11.9 mmHg), foetal MAP (P < 0.001, P< 0.001), HRV (P < 0.01, P < 0.001) and PSD in the frequency range 0.04-0.45 Hz increased (P < 0.05-P < 0.001). Foetal heart rate decreased when foetuses became moderately hypoxaemic (P < 0.001). 4. After cardiac vagal blockade, hypoxaemia was associated with an increase in FHR compared with non-blocked hypoxaemic foetuses (P < 0.01, P < 0.001). The increase in HRV was abolished (P < 0.001, P < 0.001) as was the increase in PSD (P < 0.01-P < 0.001). 5. After beta-adrenoceptor blockade, the bradycardia that occurred during hypoxaemia was enhanced (P < 0.01, P < 0.05), the increase in HRV was not affected and neither was the increase in PSD. 6. As FHR and HRV of normoxaemic foetal sheep were affected both by atropine and propranolol, it would seem that both cardiac vagal and sympathoadrenal activity modulate the foetal heart under resting conditions. The lack of any effect of beta-adrenoceptor blockade on PSD under these conditions suggests that power spectral analysis (PSA) is not as sensitive as the other two methods in detecting sympathetically mediated modulation of the heart. 7. Because the hypoxaemia induced bradycardia and increase in HRV and in PSD were abolished by atropine (P < 0.01-P < 0.001), it is concluded that during hypoxaemia foetal HRV is mainly modulated by changes in cardiac vagal tone. Propranolol had no effect on foetal HRV, although it reduced it under normoxaemic conditions; therefore, it is concluded that cardiac sympathetic neural activity was not increased in acute hypoxaemia uncomplicated by acidosis. However, there was strong evidence of increased sympathoadrenal tone on the foetal heart in hypoxaemia, that is, there was a rise in FHR in hypoxaemic atropinized foetuses and a greater fall in FHR in beta-adrenoceptor blocked hypoxaemic foetuses. Therefore, this increased sympathetic influence on the foetal heart during hypoxaemia must be predominantly the result of increased adrenomedullary secretion of catecholamines. 8. Maintenance of foetal cardiac output depends on the chronotropic and ionotropic effects of catecholamines. Therefore, this adrenomedullary influence on the foetal heart during hypoxaemia is important to offset the opposing effects of increased cardiac vagal tone.     

Metaiodobenzylguanidine and heart rate variability in heart failure

It is assumed that the low-frequency power (LF) of heart rate variability (HRV) increases with progress of congestive heart failure (CHF), therefore positively correlating with cardiac 123I-metaiodobenzylguanidine (MIBG) washout. It is demonstrated here that HRV, including normalized LF, correlated inversely with MIBG washout and positively with the ratio of heart-to-mediastinum MIBG activity in controls and CHF patients, whereas these correlations were not observed within CHF patients. Thus MIBG washout may increase and HRV including normalized LF may decrease with CHF, although the HRV and MIBG measures may not similarly change in proportion to the severity of the cardiac autonomic dysfunction in CHF.     

Effects of heart rate on vulnerability to fibrillation in a computer model

The SET-2400W is a newly designed whole-body PET scanner with a large axial field of view (20 cm). Its physical performance was investigated and evaluated. The scanner consists of four rings of 112 BGO detector units (22.8 mm in-plane x 50 mm axial x 30 mm depth). Each detector unit has a 6 (in-plane) x 8 (axial) matrix of BGO crystals coupled to two dual photomultiplier tubes. They are arranged in 32 rings giving 63 two-dimensional image planes. Sensitivity for a 20-cm cylindrical phantom was 6.1 kcps/kBq/ml (224 kcps/microCi/ml) in the 2D clinical mode, and to 48.6 kcps/kBq/ml (1.8 Mcps/microCi/ml) in the 3D mode after scatter correction. In-plane spatial resolution was 3.9 mm FWHM at the center of the field-of-view, and 4.4 mm FWHM tangentially, and 5.4 mm FWHM radially at 100 mm from the center. Average axial resolution was 4.5 mm FWHM at the center and 5.8 mm FWHM at a radial position 100 mm from the center. Average scatter fraction was 8% for the 2D mode and 40% for the 3D mode. The maximum count rate was 230 kcps in the 2D mode and 350 kcps in the 3D mode. Clinical images demonstrate the utility of an enlarged axial field-of-view scanner in brain study and whole-body PET imaging.     

Effects of isoproterenol infusions on heart rate variability in patients with panic disorder

Some evidence suggests that patients with panic disorder have a decreased cardiac vagal and a relatively higher sympathetic activity. In this study, spectral analysis of the time series of heart rate before and after isoproterenol infusions was used to study heart rate variability in six panic disorder patients and 11 normal control subjects. These preliminary data reveal a significant increase of sympathovagal ratios only in the patient group after isoproterenol administration. The findings suggest a relative increase in cardiac sympathetic and a relative decrease in cardiac vagal function in patients with panic disorder during isoproterenol infusions.     

Effects of mental state on heart rate and blood pressure variability in men and women

The effect of different mental states on autonomic modulation of the cardiovascular system was assessed in healthy, normotensive men (n = 18) and women (n = 12). Heart rate variability (HRV), systolic blood pressure variability (BPV) and arterial baroreflex function were assessed during 4 tests at rest ((10 min + 5 min recovery) x 4): (1) Control (spontaneous breathing, (SB) (2) Mental distraction (SB + word puzzle) (3) Conscious control of breathing (paced at SB rate) and (4) Mental stress (SB + computer quiz). There were no significant gender differences in the responses to the interventions in terms of arterial (spontaneous) baroreflex (SPBX) control of HR, and indices of time and frequency domains of HRV and BPV, with the exception of the sympathetic indicator of HRV (low frequency power/total power; P < 0.01) which was lower in women during control and mental stress tests. Conscious control of breathing at SB did not alter HRV, BPV or SPBX in either men or women. Mental distraction and mental stress led to decreases in indices of time and frequency domains of HRV and BPV in all subjects, as well as increases in HR during distraction and in systolic BP during stress. These findings suggest that in studies of cardiovascular control: (1) Paced breathing at SB can be used for individuals with irregular breathing patterns (2) The extent of mental stress achieved is intervention-specific and for the most part, independent of gender and (3) Resting assessment of HRV, BPV and SPBX can be made by having subjects sit quietly without interventions in a controlled laboratory setting.     

Signs of vagal reinnervation 4 years after heart transplantation in spectra of heart rate variability

OBJECTIVES: Transurethral injection of glutaraldehyde cross-linked bovine collagen has recently been advocated as a potentially useful treatment modality for management of urinary incontinence. The reported clinical experience with urethral collagen injection in adult males has been limited. METHODS: This study summarizes the current literature and reviews the clinical results of collagen injection in a group of 25 men with incontinence after either transurethral or radical prostatectomy. RESULTS: The overall results in this series were disappointing. Only 2 patients (8%) achieved significant improvement with this treatment. Eight patients (32%) experienced minimal improvement in symptoms, and 15 (60%) remained incontinent with no improvement in symptoms after collagen injection. The number of injection procedures and volume of collagen material implanted did not correlate with clinical outcome. Five patients (20%) have subsequently required placement of an artificial urinary sphincter to control their incontinence. CONCLUSIONS: We conclude that transurethral injection of glutaraldehyde cross-linked bovine collagen has a limited role in the management of urinary incontinence in adult men after prostatectomy.     

Effects of acute exposure to simulated altitude on heart rate variability during exercise

It has been shown that fluctuation of human heartbeat intervals [heart rate variability (HRV)] reflects variations in cardiac autonomic nervous system activity. The present study was designed to investigate whether the acute exposure to moderate levels of simulated altitude and the resultant hypoxia could modify HRV during exercise. Seven healthy men completed one resting measurement in the upright sitting position and two submaximal steady-state cycle ergometer exercises at intensities equivalent to 25 and 50% of their estimated maximal work rate. Experiments were conducted in random order at altitude equivalents of 500, 1,500, 2,500, and 3,500 m within 2 h of exposure to that altitude. Beat-to-beat HRV was measured continuously during the tests. HRV data were analyzed by "coarse-graining spectral analysis" (Y. Yamamoto and R.L. Hughson, Physica 68D: 250-264, 1993) to break down their total power (PT) into harmonic and nonharmonic (fractal) components. The harmonic component was further divided into low (0.0- to 0.15-Hz; PL)- and high (> 0.15-Hz; PH)-frequency components, and the indicators of relative sympathetic (SNS) and parasympathetic (PNS) nervous system activities were calculated by PL/PH and PH/PT, respectively. The fractal component was used to calculate the spectral exponent (beta) to evaluate the overall "irregularity" of HRV. The effects of exercise intensity (increase in heart rate, SNS indicator, and beta and decrease in PNS indicator) were significant (P < 0.05) at all altitude levels. The altitude effects (increase in heart rate and SNS indicator and decrease in PNS indicator) were found only during exercise at 3,500 m (P < 0.05). There was no significant effect of altitude on beta (P > 0.05). These data indicate that acute effects of altitude exposure on HRV were found 1) during exercise at moderate altitude (> 2,500 m) and 2) mainly for the harmonic components of HRV.     

The influence of premedication on heart rate variability

Analysis of heart rate variability has been used to study the effects of midazolam, morphine and clonidine on the autonomic nervous system, when administered to patients for premedication. Ninety-five patients were studied 60 min before and 60 min after premedication. Normal saline (n = 25), midazolam 0.08 mg.kg-1 (n = 24), morphine 0.15 mg.kg-1 (n = 23), or clonidine 2 micrograms.kg-1 (n = 23) were administered intramuscularly by random allocation. A Holter device was connected to the patient during the study period. Using power spectral analysis the low-frequency and high-frequency components were calculated from the Holter recordings. These are markers for sympathetic and parasympathetic activity respectively; the low- to high-frequency ratio was also calculated, a ratio of > 1 signifying sympathetic dominance. A significant reduction was noticed in both low-frequency and high-frequency power in the three premedicated groups, whereas no changes were observed in the normal saline group. In the case of midazolam, both the low and high frequencies were decreased but the low- to high-frequency ratio did not change significantly. Morphine and clonidine depressed the low-frequency component more than the high-frequency component and the low- to high-frequency ratio was decreased, suggesting parasympathetic dominance. We conclude that heart rate variability may be a useful tool for investigating the effect of drugs on the autonomic nervous system.     

Effects of clonidine on power spectral analysis of heart rate variability in mild essential hypertension

Patients with essential hypertension often show alterations of the autonomic nervous system. We evaluated the sympathetic and parasympathetic drive to the heart in 12 mildly hypertensive patients and 9 healthy subjects by power spectral analysis of heart rate variability. All subjects underwent measurements of RR interval, low (LF) and high frequency (HF) components of heart rate variability, LF/HF ratio and blood pressure in the resting and sitting positions, both before and after oral clonidine (300 microg), a central sympatholytic agent. In the supine position before clonidine, hypertensive patients had higher blood pressure and lower HF values than healthy subjects. Clonidine induced increases in RR interval and HF in both groups, while LF and LF/HF ratio decreased in healthy subjects, but not in hypertensive patients. On assuming the sitting position, both groups showed reductions in RR and HF and increments in LF and LF/HF. In healthy subjects, the response to the postural challenge was unaffected by clonidine. In contrast, hypertensive patients showed no changes in LF and LF/HF ratio, and a significantly lower decrease in HF. These differences were probably due to the existence of two subsets of patients, one exhibiting similar responses to clonidine as healthy subjects, and the other showing no appreciable response to the drug. These results suggest that hypertensive patients have an altered sympatho-vagal balance to the heart, which can be unmasked by clonidine. This phenomenon should be taken into account to achieve a better control of the overall cardiovascular risk of hypertensive patients.     

Circadian pattern of heart rate variability in chronic heart failure patients. Effects of physical training

The effect of physical training on the circadian pattern of heart rate variability (recorded over 24 h in relation to both time and frequency) was assessed in 12 chronic heart failure patients randomized, in a cross-over design, to 8 weeks training or detraining, and compared with 12 age-matched normals. Training improved heart rate variability indices: all R-R interval 5 min standard deviations increased by 17.6%, the root mean square of the differences of successive R-R intervals by 34.9%, the percentage difference between adjacent normal R-R intervals > 50 ms by 112.5%, total power by 58.3%, high frequency by 128.5% and low frequency by 65.0%. Compared with controls, circadian variations in autonomic parameters were maintained in chronic heart failure. Training-induced changes were observed at different time intervals throughout the day: the highest values were at 0100 h-0700 h (detraining: low frequency 361 +/- 83 ms2, high frequency 126 +/- 47 ms2; training: low frequency 535 +/- 202 ms2, high frequency 227 +/- 115 ms2, P < 0.01) and the lowest at 1300 h-1900 h (detraining: low frequency 91 +/- 23 ms2, high frequency 39 +/- 14 ms2; training: low frequency 154 +/- 42 ms2, high frequency 133 +/- 67 ms2, P < 0.05). In chronic heart failure, training maintains and improves circadian variations in heart rate variability measures.     

Effects of two yoga based relaxation techniques on heart rate variability (HRV).

Heart rate variability (HRV) was studied in cyclic meditation (CM) and supine rest (SR). CM included yoga postures followed by guided relaxation. Forty-two male volunteers were assessed in CM and SR sessions of 35 minutes, where CM or SR practice was preceded and followed by 5 minutes of SR. During the yoga postures of CM and after CM, low frequency power and the low frequency to high frequency power ratio decreased, whereas high frequency power increased. Heart rate increased during the yoga postures and decreased in guided relaxation and after CM. There was no change in SR. Hence, it appeared that predominantly sympathetic activation occurred in the yoga posture phases of CM while parasympathetic dominance increased after CM. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Age-dependent influence on heart rate variability in salt-sensitive hypertensive subjects

OBJECTIVE: The known association between systemic arterial hypertension in its initial stages and increased sympathetic nervous system drive prompted us to evaluate the influence of age on autonomic nervous system function in subjects with salt-sensitive arterial hypertension. DESIGN: In a randomized study, autonomic nervous system function was assessed by power spectral analysis of heart-rate variability calculated with an autoregressive algorithm in salt-sensitive hypertensives and controls at baseline and under sympathetic stress (passive head-up tilt). For 1 week before the study, all subjects kept to a diet supplying 120 mEq sodium. Sodium sensitivity was assessed by measuring and comparing arterial pressures after a 7-day controlled dietary sodium intake of 20 mEq per day after a 7-day period on 220 mEq sodium/day. SETTING: Geriatric division at the I Medical Clinic of the University of Rome "La Sapienza". PARTICIPANTS: Sixty-five patients with salt-sensitive hypertension (age range 19 to 89 years) and 64 age-matched normotensive controls, divided for data comparison into three age-groups: < 44 years; 44 to 64 years; and > or = 65 years. MEASUREMENTS: With an autoregressive algorithm in a power spectral analysis of heart rate variability, we detected four spectral frequency-domains: total power (0.0033 to 0.40 Hz), high-frequency power (0.16 to 0.40 Hz), low-frequency power (0.04 to 0.15 HZ) and very-low-frequency power (0.0033 to 0.04 Hz). To determine sodium sensitivity, for 1 week before the study all subjects kept to a diet supplying 120 mEq sodium. Sodium sensitivity was assessed by measuring and comparing arterial pressures after a 7-day controlled dietary intake of 20 mEq per day and after a 7-day period of 220 mEq sodium/day. RESULTS: Results were expressed as natural logarithms of power and normalized units. The hypertensive patients of all ages had significantly lower total power of heart rate variability than the normotensive controls (P < .05). At baseline, the youngest hypertensives had lower natural logarithms and low-frequency normalized units than controls (P < .001). After tilt, only their low-frequency normalized units exceeded those of controls (P < .001). The middle-aged hypertensive group had higher low-frequency normalized units than controls at baseline (P < .05) and after tilt (P < .001). At baseline and after tilt, the oldest hypertensives had lower low-frequency natural logarithms than controls (P < .05) and normalized units equal to those of controls. But the hypertensives of all ages were less able than controls (P < .001) to increase low-frequency power after head-up tilt. In the less than 44-year-old hypertensives, diastolic pressure correlated significantly with low-frequency power of heart rate variability, expressed in normalized units, at baseline (P < .05) and after head-tilt (P < .05). A significant inverse correlation was found between age and the natural logarithm of low-frequency power at baseline (r = -.682, P < .001) and after tilt (r = -.800; P < .001). Also, a significant inverse correlation was found to exist in normotensive subjects between the natural logarithm of low-frequency at baseline (r = -.595; P < .001) and after tilt (r = -.391; P < .001). The two regression line coefficients for age correlated significantly (P < .001) with the natural logarithm of low-power frequency after tilt. CONCLUSION: Whereas sodium chloride-sensitive hypertension appears to be associated with sympathetic hyperactivity in young and middle-aged subjects, in older people it is not. Sympathetic activity diminishes with age, declining faster in hypertensive subjects.     

Effects of melatonin and diazepam on the heart rate variability in normal rats and rats subjected to stress

1. Using story telling with the older resident provides an opportunity for the nurse to demonstrate caring, and a common reality between them is developed. 2. Story telling by older residents, far from merely "living in the past," can promote their health and well being. 3. In the narrational relationship between nurse and older resident, story telling is encouraged and supported; for the nurse, this means valuing oneself, older adults, and story telling.     

Effects of passive tilt and submaximal exercise on spectral heart rate variability in ventricular fibrillation patients without significant structural heart disease

It has been shown that tilt and exercise elicit significant changes in autonomic activity in normal subjects and that submaximal exercise causes a greater reduction in heart rate variability (HRV) in animals susceptible to ventricular fibrillation (VF). Whether there is an abnormal HRV response to tilt and exercise in patients at risk of sudden cardiac death (SCD) remains unknown. Short-term HRV before and during passive tilt and exercise was studied in 12 survivors of out-of-hospital cardiac arrest with documented VF and compared with 12 age- and sex-matched normal controls. No patient had significant structural heart disease or left ventricular dysfunction. HRV was computed as total-frequency (TF, 0.01 to 1.00 Hz), low-frequency (LF, 0.04 to 0.15 Hz) and high-frequency (HF, 0.15 to 0.40 Hz) components. There was no significant difference between normal controls and SCD survivors in HRV before or during tilt or submaximal exercise testing. The HF component was significantly decreased during tilt compared with that in the supine position in both normal controls (5.85 +/- 0.61 vs 5.08 +/- 0.95 In(msec2), p = 0.005) and patients (5.58 +/- 1.49 versus 4.74 +/- 1.18 In(msec2), p = 0.003). There was again no significant change in the TF or LF components during tilt in either patients or controls. All frequency components were significantly decreased during submaximal exercise testing in both patients and controls. However, there was no significant difference in any of these tilt- and exercise-induced changes in HRV between normal controls and SCD survivors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Autonomic control of heart rate during physical exercise and fractal dimension of heart rate variability

The objectives of the present study were to investigate autonomic nervous system influence on heart rate during physical exercise and to examine the relationship between the fractal component in heart rate variability (HRV) and the system's response. Ten subjects performed incremental exercise on a cycle ergometer, consisting of a 5-min warm-up period followed by a ramp protocol, with work rate increasing at a rate of 2.0 W/min until exhaustion. During exercise, alveolar gas exchange, plasma norepinephrine (NE) and epinephrine (E) responses, and beat-to-beat HRV were monitored. HRV data were analyzed by "coarse-graining spectral analysis" (Y. Yamamoto and R. L. Hughson. J. Appl. Physiol. 71: 1143-1150, 1991) to break down their total power (Pt) into harmonic and nonharmonic (fractal) components. The harmonic component was further divided into low-frequency (0.0-0.15 Hz) and high-frequency (0.15-0.8 Hz) components, from which low-frequency and high-frequency power (Pl and Ph, respectively) were calculated. Parasympathetic (PNS) and sympathetic (SNS) nervous system activity indicators were evaluated by Ph/Pt and Pl/Ph, respectively. From the fractal component, the fractal dimension (DF) and the spectral exponent (beta) were calculated. The PNS indicator decreased significantly (P < 0.05) when exercise intensity exceeded 50% of peak oxygen uptake (VO2 peak). Conversely, the SNS indicator initially increased at 50-60% VO2peak (P < 0.05) and further increased significantly (P < 0.05) at > 60% VO2peak when there were also more pronounced increases in NE and E.(ABSTRACT TRUNCATED AT 250 WORDS)