Autonomic nervous control of the heart in exercising man

Electrocardiographic criteria for vagotonia-validation with pharmacological parasympathetic blockade in healthy subjects

BACKGROUND

The importance of vagal tone on cardiac function and cardiovascular mortality is well established. Although the presence of an enhanced cardiac vagal tone (CVT) is frequently diagnosed using the 12-lead resting electrocardiogram (ECG) in daily practice, most of the proposed criteria have been determined on an empirical basis. Our objective was to evaluate the effects of pharmacological blockade of the parasympathetic component of the autonomic nervous system on resting ECG tracings.

METHODS

Nine healthy young adults (24+/-5 year-old) underwent parasympathetic blockade with atropine sulfate i.v. (0.04 mg kg(-1)) and resting ECGs were obtained before and 15 min thereafter. CVT was assessed by a dimensionless index, which measures the RR interval reduction caused by the vagal withdrawal induced by a 4-s exercise test performed on a cycle ergometer where the subjects pedal as fast as possible with no added resistance.

RESULTS

This index was 1.63+/-0.24 and 1.03+/-0.03, before and after atropine, respectively (P<0.0001). Atropine reduced the R-R intervals (P<0.0001), and the amplitude of T-waves in several leads (DII: P=0.03; V4: P=0.04; V5: P=0.03; V6: P=0.01), and abolished the appiculation of T-waves, J-point and ST-segment elevations (P<0.05), and U-waves (P<0.05), which were present in baseline ECG in all subjects in at least two leads. The R-wave amplitude in leads V4, V5, and V6 (all P>/=0.10) was not modified by atropine infusion.

CONCLUSION

The duration of the R-R intervals and the amplitude of T-waves in leads DII, V4, V5, and V6, and the presence of T-wave appiculation, U-waves, and elevation of J-point and ST-segment should be used to detect enhanced cardiac vagal tone in healthy subjects.

Effect of static and dynamic exercise on heart rate and blood pressure variabilities

PURPOSE

This study examines the effect of static and dynamic leg exercises on heart rate variability (HRV) and blood pressure variability (BPV) in humans.

METHODS

10 healthy male subjects were studied at rest, during static exercise performed at 30% of maximal voluntary contraction (SX30), and during dynamic cycling exercises done at 30% of VO2max (DX30) and at 60% of VO2max (DX60). Respiration, heart rate, and blood pressure signals were digitized to analyze temporal and spectral parameters involving short and overall indexes (SD, deltaRANGE, RMSSD, Total power), power of the low (LF), middle (MF), and high (HF) frequency components, and the baroreceptor sensitivity by the alphaMF index.

RESULTS

During SX30, indexes of HRV as SD, deltaRANGE, Total power, and MF in absolute units increased in relation with rest values and were significantly higher (P < 0.001) than during DX30 and DX60; HF during SX30, in normalized and absolute units, was not different of the rest condition but was higher (P < 0.001) than HF during DX30 and DX60. Parameters of BPV as SD and deltaRANGE increased (P < 0.001) during both type of exercises, and significant (P < 0.01) increments were observed on MF during SX30 and DX30; systolic HF was attenuated during DX30 (P < 0.05), whereas diastolic HF was augmented during DX60 (P < 0.001). Compared with rest condition, the alphaMF index decreased (P < 0.01) only during dynamic exercises.

CONCLUSION

Because HRV and BPV response is different when induced by static or dynamic exercise, differences in the autonomic activity can be advised. Instead of the vagal withdrawal and sympathetic augmentation observed during dynamic exercise, the increase in the overall HRV and the MF component during static exercise suggest an increased activity of both autonomic branches.

Autonomic nervous control of the heart rate response to dynamic incremental exercise: evaluation of the Rosenblueth-Simeone model

To evaluate the validity of the Rosenblueth-Simeone model for the heart rate response to incremental dynamic exercise, 11 healthy men performed maximal exercise tests on a cycle ergometer after administration of placebo, propranolol, atropine or both propranolol and atropine. The model showed that the influence of sympathetic activity on heart rate increased at intensities up to those which resulted in a heart rate 70% maximal heart rate on placebo, and levelled off at higher intensities, while there was a progressive withdrawal of the parasympathetic activity. The ratio between heart rate predicted by the model and the recorded heart rate following placebo treatment tended to be less than 1.0 at lower exercise intensities, and approached the unit at intensities above those which resulted in a heart rate higher than 70% of maximal heart rate on placebo. There was a strong correlation (r = 0.94, P less than 0.01) between the heart rate on placebo and the heart rate estimated by the model. Nevertheless, there was some scattering of the data around the identity line, with a standard error of the estimate for the regression line of 11 beats.min-1. Thus, during incremental exercise, the influence of sympathetic activity on heart rate does not become progressively more important at higher exercise intensities. The application of the Rosenblueth-Simeone model shows limitations during incremental exercise, particularly at low exercise intensities.

Abnormal heart rate control in vasospastic angina: effects of calcium antagonists

We examined the effects of administration of calcium antagonists on the heart rate response to treadmill exercise in 11 patients with vasospastic angina and 8 healthy young volunteers. The exercise test was performed by walking on a treadmill at a constant speed and grade according to a scheme of pseudo-randomized sequence for 19 min. The dynamic property of heart rate response to exercise was evaluated by using a frequency analytic procedure. The exercise test was also studied in 21 age-matched normal controls without drug administration. Administration of calcium antagonists revealed no significant effects on heart rate and blood pressure at rest in young healthy subjects or in patients with vasospastic angina. Young volunteers showed the same normal properties of heart rate response to exercise before and after calcium antagonists. Vasospastic angina showed abnormal heart rate response to exercise and revealed characteristically different transfer function from that in normal controls. These characteristics were not affected by treatment with calcium antagonists except for a slight, uniform decrease of gain of the system over the whole frequency range. Accordingly, the present exercise test can feasibly be used in the diagnosis and management of vasospastic angina even when calcium antagonists are administered to the patients.

Diagnosis of vasospastic angina by analysis of heart rate response to exercise: effects of different levels of work load

To examine alterations in control functions of the heart, which may account for the pathophysiologic conditions precipitating coronary arterial spasm, heart rate response to exercise in vasospastic angina was evaluated by using our previously developed method of frequency analysis. We also examined the effects of three different levels of work load on the heart rate response to treadmill exercise in 9 patients with vasospastic angina and 7 normal controls: stage 1 (2.5 Km/h, 10%), stage 3 (4.5 Km/h, 10%), and stage 5 (5.5 Km/h, 14%). The transfer function of heart rate control in vasospastic angina was characteristically different from that in normal controls: lower gain and more delayed phase angle of the system. Although this abnormality was observed in every test at 3 different levels of work load, the abnormality was more striking in tests at lower levels of work load: stage 1 or 3. The moderately light exercise test at stage 3 is most suitable as a test for detecting abnormal heart rate response to exercise in vasospastic angina because the exercise test at stage 1 had a poor S/N ratio.

Abnormal heart rate response to exercise in vasospastic angina: pathophysiologic mechanism in the provocation of coronary spasm

To examine the alteration in control function of the heart, which may account for the pathophysiologic condition precipitating coronary arterial spasm, the dynamic property of heart rate response to exercise in vasospastic angina was evaluated by using our previously developed frequency analytic procedure. We studied 21 patients with vasospastic angina, divided into two groups (active angina and inactive angina) and 12 normal control subjects. When compared with the transfer function of the heart rate control system in normal control subjects, the transfer function in patients with active vasospastic angina showed moderately lower gain, especially in the middle frequency range, and significantly delayed phase angle over the whole frequency range, especially in the middle and high frequency ranges. These abnormalities were not observed in inactive vasospastic angina. The present exercise test to detect abnormal heart rate control can feasibly be used in the detection and management of vasospastic angina.

Does aging differentially reduce heart rate variability related to respiration?

Segments of resting heart rate were submitted to spectral analysis in order to assess changes in spectral power with age. Different frequencies of heart rate variability have been related to vagal and sympathetic cardiac control. Data were from 56 male volunteers ranging in age between 18 and 65 years of age. Spectral results revealed that respiratory sinus arrhythmia declined less with age than lower (vasomotor) frequencies. Assessment of total (rather than relative) power at respiratory frequencies revealed the expected drop in power with age. Thus, the decline in heart rate variability with age may not be due predominantly to a decline in frequencies related to respiration.