No abstract

This study investigated the changes in R-R interval in 23 patients (11 men and 12 women; mean age 61 yr) with persistent atrial fibrillation in response to several provocative maneuvers including active postural change, Valsalva maneuver, handgrip, and rhythm-controlled respiration. Averaged R-R intervals were shortened immediately after postural change (from 797 +/- 35 ms supine to 677 +/- 27 ms standing; P < 0.01) and recovered to the 90% level within 100 s. During Valsalva strain and handgrip, mean R-R intervals were significantly shortened (from 737 +/- 37 ms sitting to 697 +/- 38 ms in Valsalva and from 773 +/- 68 ms sitting to 701 +/- 58 ms in handgrip; both P < 0.01). During rhythm-controlled respiration, only two cases (10.5%) showed power peaks in spectrograms of moving-window-averaged R-R intervals at the frequency corresponding to respiration rhythm. The ventricular response to atrial fibrillation is influenced by an increase in sympathetic tone and a decrease in parasympathetic tone but is not necessarily influenced by the increase in parasympathetic dominance. These results suggest that even in atrial fibrillation patients, the autonomic nervous system modulates the ventricular rate via the atrioventricular node and atrial tissue.

Microvolt-level T-wave alternans (alternating morphology from beat to beat) during atrial pacing and exercise may predict ventricular tachycardia (VT) and fibrillation (VF) in ischemic heart disease. We tested whether such alternans during exercise could identify high-risk patients with hypertrophic cardiomyopathy (HCM). We studied 14 HCM patients and 9 normal control subjects for T-wave alternans u sing the CH2000 system with 7 multisegment electrodes in a Frank orthogonal (XYZ) configuration. Bicycle ergometer exercise was used to increase the heart rate (HR) to 95-110 beats/min. Seven patients were at high risk for ventricular arrhythmias (1 with sustained VT, 3 with abnormal paced ventricular electrograms as seen in VF survivors, and 3 with nonsustained VT and/or an adverse family history), and the other 7 were at low risk. T-wave alternans was present if alternans > 1.9 microV was consistently present with the HR in excess of a patient-specific HR threshold. Alternans was found in 5 of 7 high-risk patients (71%) vs none of 7 low-risk patients or 9 control subjects (p < 0.025 and p < 0.01, respectively). Notably, all 4 patients with sustained VT or abnormal ventricular electrograms showed alternans. Thus, high-risk patients with HCM often show T-wave alternans. Microvolt-level alternans during exercise may be a useful marker for ventricular arrhythmic risk in patients with HCM.

Background:The study evaluated interobserver differences in the classification of the T-U wave repolarization pattern, and their influence on the numerical values of manual measurements of QT interval duration and dispersion in standard predischarge 12-lead ECGs recorded in survivors after acute myocardial infarction.Methods: Thirty ECGs recorded at 25 mm/s were measured by six independent observers. The observers used an adopted scheme to classify the repolarization pattern into 1 of 7 categories, based on the appearance of the T wave, and/or the presence of the U wave, and the various extent of fusion between these. In each lead with measurable QRST(U) pattern, the RR, QJ, QT-end, QTnadir (i.e., interval between Q onset and the nadir or transition between T and U wave) and QU interval were measured, when applicable. Based on these measurements, the mean RR interval, the maximum, minimum, and mean QJ interval, QT-end and/or QT-nadir interval, and QU interval, the difference between the maximum and minimum QT interval (QT dispersion [QTD]), and the coefficient of variation of QT intervals was derived for each recording. The agreement of an individual observer with other observers in the selection of a given repolarization pattern were investigated by an agreement index, and the general reproducibility of repolarization pattern classification was evaluated by the reproducibility index. The interobserver agreement of numerical measurements was assessed by relative errors. To assess the general interobserver reproducibility of a given numerical measurement, the coefficient of variance of the values provided by all observers was computed for each ECG. Statistical comparison of these coefficients was performed using a standard sign test.Results: The results demonstrated the existence of remarkable differences in the selection of classification patterns of repolarization among the observers. More importantly, these differences were mainly related to the presence of more complex patterns of repolarization and contributed to poor interobserver reproducibility of QTD parameters in all 12 leads and in the precordial leads (relative error of 3 1 %-35% and 34%-43%, respectively) as compared with the interobserver reproducibility of both QT and QU interval duration measurements (relative error of 3%-6%, P < 0.01).This observation was not explained by differences in the numerical order between QT interval duration and QTD, as the reproducibility of the QJ interval (i.e., interval of the same numerical order as QTD was significantly better (relative error of 7.5%-13%, P < 0.01) than that of QTD. Conclusions:Poor interobserver reproducibility of QT dispersion related to the presence of complex repolarization patterns may explain, to some extent, a spectrum of QT dispersion values reported in different clinical studies and may limit the clinical utility in this parameter.

Background: Heart rate ( HR ) variability has been recognized as an important noninvasive index of autonomic nervous activities. However, the relationship between HR variability and cardiac circulating norepinephrine (NE), especially with respect to coronary ischemia, remains unclear.Hypothesis: This study was undertaken to determine whether HR variability indices can reflect cardiac NE levels during handgrip exercise. Methods: We simultaneously measured HR variability and cardiac NE overflow rate in 32 patients (30 men, 2 women) during a 6-min isometric handgrip exercise. Among the 32 subjects, 20 (19 men, 1 woman) had coronary artery disease (CAD) and 12 ( control group; 1 1 men, 1 woman) did not.Results: Hemodynamics and cardiac NE overflow rates among subjects at rest were not significantly different between the two groups. In the normal subjects, low-frequency (LF) spectra and LF/HF (high-frequency) ratios were not significantly changed during handgrip exercise, but HF spectra significantly increased from 10.1 f 4.5 to 12.2 -+ 7.0 ms (p< 0.05). In the subjects with CAD, LF and LF/HF spectra were significantly (p < 0.05 and 0.01, respectively) increased by handgrip exercise. High-frequency spectra were not significantly changed by handgrip exercise. In the normal subjects, a sigtuficant negative relation (r = -0.76, p < 0.01) was obtained between HF change and cardiac NE overflow rate, whereas this relationship was not significant in the subjects with CAD.The correlation between changes of LF/HF and cardiac NE overflow rate was significant in the normal (r = 0.56, p < 0.05) but not in subjects with CAD. Conclusion:These results suggest that vagal modulation of HR variability is more prominent in normal coronary artery subjects than in CAD subjects during handgrip exercise. Heart rate variability indices may thus serve as adequate indicators of autonomic nerve activity in subjects with normal coronary arteries but not in those with CAD, probably due to decreased adaptation to physical stress during handgnp exercise.

No abstract

Although different computerized systems have been developed to localize specific patterns in electrocardiographic (ECG) signals, it is still difficult to detect T waves and measure QT intervals during atrial fibrillation. This article demonstrates the use of an auto-correlation (ECG) based system that was used to investigate the dynamicity of QT intervals related to active postural change in patients with chronic atrial fibrillation. Twenty patients (9 male, mean age 63 years) with chronic atrial fibrillation (8 idiopathic, 12 organic heart disease) were examined. Seventeen of these patients were on digoxin, but patients with other conditions potentially affecting the autonomic nervous system were not included. A 3-channel ECG was recorded digitally during active postural change from supine to standing. Data were first analyzed by the Burdick Altair system and subsequently processed using an in-house software package evaluating auto-correlations of ECG signals. An ECG channel with suitable repolarization patterns was found in 15 patients. The mean QT interval of 409.8 +/- 11.1 ms (mean +/- SE) recorded during supine position shortened to 401.9 +/- 9.89 ms during the first minute of active standing (P < 0.05) and to 394.8 +/- 10.0 ms during the second minute of active standing (P < 0.005). It did not further change during the subsequent minutes of active standing. The study shows that automatic detection of QT intervals during atrial fibrillation is possible. Although the effect of position change of the heart cannot be completely excluded, the study suggests that QT interval is changed directly by autonomic nervous mechanisms rather than indirectly via the mean heart rate.

Autonomic nerve functions under severe hyperbaric pressure were evaluated by measuring heart rate variability (HRV) and catecholamine excretion rate in 16 normal volunteers in submarine experimental facilities simulating conditions 330 m below sea level. HRV and urinary catecholamine levels were evaluated to assess sympathetic and parasympathetic tone. High-frequency HRV increased from 5.6 +/- 1.3 to 6.3 +/- 1.4 ms2 (p < 0.05), and SD of the average normal R to R intervals for 5-minute index (SDNNI) (time domain HRV parameter) increased from 77.2 +/- 32.7 to 93 +/- 33.8 ms (p < 0.05) after 3 days. Adrenaline/creatinine increased by 18% from a basal value of 4.04 +/- 0.44 ng/dL/h. Also, there was significant negative correlation between high-frequency and urinary cathecholamine levels. Evaluation of autonomic nerve functions under hyperbaric conditions by measuring HRV was shown to be a useful method. Thus, the present results indicate that the autonomic nerve functions of people who work under deep-sea conditions can be evaluated adequately by measuring HRV.