A non-invasive method for testing neural circulatory control in man

Physical activity and heart rate variability measured simultaneously during waking hours

Heart rate (HR) variability (HRV) during ambulatory recordings may be affected by individual differences in daily physical activity (PA). However, the influence of various levels of PA on different measures of HRV is not exactly known. We examined the association between simultaneously measured HRV and objective PA data obtained with an accelerometer during waking hours among 45 healthy adults. Bouts of PA were identified from minute-by-minute accelerometer data as metabolic equivalent (METs) values and calculated as mean METs for 30 min. HRV was analyzed concurrently. Within-individual correlation analyses and sign tests were performed to study the relationships between various HRV indexes and PA. The mean PA time was 15:44 +/- 1:01 h, and the mean MET was 1.91 +/- 0.14. HR and sample entropy, but not the other measures of HRV, had a significant relationship with PA, as shown by both correlation analyses (r = 0.64, P = 0.021, and r = -0.55, P = 0.022, respectively) and sign tests (P < 0.0001 for both). Beat-to-beat R-R interval fluctuation expressed as SD1 also demonstrated a significant relation to PA according to the sign test (P = 0.037) and a trend of association according to the correlation analysis (r = -0.40, P = 0.129). The complexity measure of HRV, in addition to average HR and the short-term index of HRV (SD1), is significantly influenced by the level of PA during ambulatory conditions. Long-term HRV indexes remained relatively stable at various activity levels, making them the most robust indexes for the assessment of cardiac autonomic function during free-running ambulatory conditions.

Incipient cardiovascular autonomic imbalance revealed by wavelet analysis of heart rate variability in Type 2 diabetic patients

AIM

Incipient cardiovascular autonomic imbalance is not readily diagnosed by conventional methods. Spectral analysis of heart rate variability (HRV) by wavelet transform (WT) was used to measure cardiovascular autonomic function in patients with Type 2 diabetes.

METHODS

Thirty-two diabetic patients without (D), 26 with cardiovascular autonomic neuropathy (DAN) and 72 control subjects (C) participated. A 30-min HRV time series was analysed by wavelet transformation and four characteristic frequency intervals were defined: I (0.0095-0.021 Hz), II (0.021-0.052 Hz), III (0.052-0.145 Hz) and IV (0.145-0.6 Hz).

RESULTS

When compared with C, in both D and DAN the normalized power and amplitude of interval II were increased and of interval IV decreased, resulting in a significantly higher II/IV ratio. Furthermore, in DAN the normalized power and amplitude of interval I were increased and of interval III decreased when compared with the D and C groups. The diabetic patients were divided in two equal subgroups according to HbA(1c) < 8.0% and >or= 8.0%. In the subgroup with HbA(1c) >or= 8.0%, normalized power in interval II was significantly higher and in interval IV significantly lower than in the subgroup with HbA(1c) < 8.0%. In D, but not in DAN patients prescribed ACE inhibitors, the absolute amplitude and power of oscillations were significantly higher than in patients not taking ACE inhibitor therapy.

CONCLUSIONS

Patients with diabetes have increased sympathetic and decreased parasympathetic cardiac activity regardless of the presence of autonomic neuropathy. Glycaemic control and treatment with ACE inhibitors may favourably influence HRV in diabetic patients without autonomic neuropathy.

Effects of endurance training on heart rate and blood pressure variability

PURPOSE

To study the influences of a 1-year controlled, randomized endurance exercise training period on heart rate (HR) and blood pressure variability in a representative sample of Finnish men in their late middle age.

METHODS AND RESULTS

Subjects were 140 sedentary men aged 53-63 years. The men were randomized into two identical groups: an intervention (EX) and a reference (CO) group. One hundred and twelve of them remained in the final analysis (EX: n=59, CO: n=53). EX trained for 30-60 min three to five times a week with the intensity of 40-60% of maximal oxygen consumption. In EX, 1 year of regular exercise training increased oxygen consumption at respiratory compensation threshold by 11% (P < or = 0.001) in a maximal cardiorespiratory test. Total power and very low frequency power of R-R interval variability (ms2) tended to increase in the EX group by 26 and 42% and to decrease in the CO group by 13 and 10% (interaction P<0.05 and P<0.01), respectively. There were no significant changes in blood pressure variability.

CONCLUSION

Regular low- to moderate-intensity exercise training could retard the declining tendency in cardiac autonomic nervous function in older men during 1 year.

The effects of midazolam and ephedrine on post-exercise autonomic chronotropic control of the heart in normal subjects

Benzodiazepines may induce hypotension by inhibiting the pressor response. Ephedrine has adrenergic effects on the circulation. After exercise, changes in cardiovascular control impair orthostatic tolerance. The impaired pressure response can be compensated for by chronotropic control of the heart. We studied the effect of midazolam and ephedrine on post-exercise cardiac autonomic chronotropic control in six 21-year-old female volunteers, who received single doses of 15 mg midazolam, 50 mg ephedrine, or placebo orally according to a placebo-controlled, double-blind, crossover design. After exercise, the subjects assumed the supine position for rest, then a -10 degrees head-down position followed by a 70 degrees head-up position. Power spectral analysis of heart rate variability for 7 min and steady-state brachial arterial blood pressure were measured in each position. After administration of midazolam, three subjects had an abnormal fall in their arterial blood pressure (with one presyncope) as a response to head-up tilt. Changes in heart rate variability exceeded those seen during placebo treatment (p < 0.01) and involved oscillations, suggesting activation of both sympathetic and parasympathetic dynamics. After ephedrine administration, arterial blood pressure increased during head-down tilt, but parasympathetic dynamics to the heart were dampened. Head-up tilt induced increased sympathetic stimulation of the heart and a sympathicotonic cardiovascular response (p < 0.01). In conclusion, midazolam induced unexpectedly great changes in dynamic cardiac control during cardiovascular stimulation. Ephedrine increased tonic sympathetic activity and stabilized the neural circulatory control of the heart by immobilizing dynamic parasympathetic activation.

Heart rate variability in coronary artery disease

Heart rate variability (HRV) reflects the modulation of cardiac function by autonomic and other physiological systems, and its measurement from ambulatory electrocardiograph (ECG) recordings is a useful method for both clinical and scientific purposes. Heart rate variability can be measured by several linear and non-linear methods, and various methods can give different information on neural and other physiological influences on the heart. Heart rate variability is abnormal in various settings of ischaemic heart disease, and the most important current application of HRV analysis in clinical cardiology is its measurement in postinfarction patients, in whom abnormal HRV indicates an increased risk of cardiac mortality. Future research may expand the clinical utility of HRV measurement to other clinical situations.

Dynamics of vasomotor thermoregulation of the skin in term and preterm neonates

Eighteen fullterm infants and 17 preterm infants were studied on their 3rd day of life to investigate the reactivity of skin blood flow to thermal stimulation. The infants were studied during quiet sleep. After a 10-min control period a constant air current was used to synchronise the external cutaneous stimulus to the distal lower extremity of each infant: the heating element of an air blower was automatically switched on and off to generate successive warm and cool periods of equal duration (5 cycles/min). Heart rate (HR), skin blood flow (SBF) and respiratory waveform signals were recorded and their variability was analysed using the fast Fourier transform and spectral analysis. Fullterm infants showed a clear response to external thermal stimulation: both HR and SBF were synchronised to the stimulation frequency. A response of preterm infants was present but it was markedly attenuated in comparison to term infants. The effect of stimulation did not seem to be dependent on postnatal age. The results suggest that the vasomotor control is immature in preterm infants.

Non-invasive assessment of autonomic nervous function in Gilles de la Tourette syndrome

Autonomic nervous function was investigated in 18 Gilles de la Tourette (GTS) patients and in 23 controls, who matched the patients in age, sex, baseline blood pressure and baseline heart rate. Four heart rate tests were used (variation at rest, during deep breathing, following standing up and during a Valsalva manoeuvre), and two blood pressure tests (standing up and sustained handgrip). The only significant difference between the groups was found in the Valsalva test. This was due to the initial heart rate increase which was higher in the GTS group; the subsequent decrease did not differ between the groups. Increased sympathetic activity is a possible explanation for this finding, but no additional evidence in its favour was found. No signs of autonomic failure were found.