Morio Ito

Circadian Variation of QT Interval Dispersion Correlation with Heart Rate Variability

The circadian variation of QT interval dispersion and its correlation with heart rate variability (HRV) was examined in 17 normal subjects by using 24-hour recordings of three-lead electrocardiograms. Measurements of HRV, R-R intervals, and QT intervals were made for the first 6 minutes of each hour over a 24-hour period. Spectral analysis of HRV yielded low-frequency power (LF) (0.04-0.15 Hz), high-frequency power (HF) (0.15-0.40 Hz), and the ratio of LF to HF (LF/HF). A rate-corrected QT interval (QTc) was calculated by Bazetts formula, and QT and QTc dispersion was defined as the difference between the maximum and minimum values in any two leads. High-frequency power and QT interval were greater at night than during the day: conversely, LF/HF and dispersion of QT and QTc were greater during the day. The QTc interval remained virtually unchanged throughout the 24-hour period. The dispersion of QTc showed a significant negative correlation with HF and a significant positive correlation with LF/HF. The results suggest that an increased sympathetic tone or a decreased vagal tone increases QT dispersion in healthy subjects.

Circadian rhythm of the signal averaged electrocardiogram and its relation to heart rate variability in healthy subjects

Objective To examine the circadian variation in the signal averaged electrocardiogram (saECG) and heart rate variability and investigate their relations in healthy subjects. Methods 24 hour ECGs were obtained with a three channel recorder using bipolar X, Y, and Z leads in 20 healthy subjects. The following variables were determined hourly: heart rate, filtered QRS (f-QRS) duration, low and high frequency components of heart rate variability (LF and HF), and the LF/HF ratio. Results Heart rate, f-QRS duration, HF, and the LF/HF ratio showed significant circadian rhythms, as determined by the single cosinor method. Heart rate and the LF/HF ratio increased during daytime, and f-QRS duration and HF increased at night. f-QRS duration was negatively correlated with heart rate (r = 0.95, p < 0.001) and the LF/HF ratio (r = 0.94, p < 0.001) and positively with HF (r = 0.93, p < 0.001). Conclusions f-QRS duration has a significant circadian rhythm in healthy subjects and is closely related to the circadian rhythm of autonomic tone.

Evaluation of Autonomic Influences on QT Dispersion Using the Head-Up Tilt Test in Healthy Subjects

Our objective was to examine the autonomic influence on QT interval dispersion using the head‐up tilt test in healthy subjects. RR and QT intervals, heart rate variability, and plasma norepinephrine concentration were measured in the supine position and tilting to 70± for 20 minutes using a footboard support in 15 healthy male volunteers (mean age ± SD: 28.0 ± 4.5 years). The rate‐corrected QT interval (QTc) was calculated using Bazetts formula, and QT and QTc dispersions were defined as the maximum minus minimum values for the QT and QTc, respectively, from the 12‐lead ECG. Spectral analysis of the heart rate variability generated values for the low‐ and high‐frequency powers (LF and HF) and their ratio (LF/HF). Compared with values obtained in the supine position, tilting significantly increased QT (P < 0.05) and QTc dispersion (P < 0.01), the LF/HF ratio (P < 0.0001), and plasma norepinephrine concentration (P < 0.0001), and significantly decreased HF (P < 0.0001). QTc dispersion was positively correlated with the LF/HF ratio and plasma norepinephrine concentration, and negatively correlated with HF. These results suggest that head‐up tilt testing increases QT dispersion by increasing sympathetic tone and/or decreasing vagal tone in healthy subjects.

Mechanism of β-Adrenergic Receptor Upregulation Induced by ACE Inhibition in Cultured Neonatal Rat Cardiac Myocytes Roles of Bradykinin and Protein Kinase C

Background—Although bradykinin is thought to contribute to the effects of ACE inhibitors on the cardiovascular system, its precise role remains to be elucidated. Evidence suggests that bradykinin might be important in the upregulation of β-adrenergic receptors (β-ARs) induced by ACE inhibitors, and the role of bradykinin in this effect has now been investigated with cultured neonatal rat cardiac myocytes. Methods and Results—The density of β-ARs on the myocyte surface was determined with a binding assay with [3H]CGP-12177. Incubation of cultured myocytes for 24 hours with the ACE inhibitor captopril (1 μmol/L) increased β-AR density by 35% and enhanced the response of cells to isoproterenol but not to forskolin. Neither an angiotensin-II type 1 (AT1) receptor antagonist, CV-11974, nor angiotensin-I affected β-AR density. However, the bradykinin B2 receptor antagonist Hoe 140 abolished the effect of captopril on β-AR upregulation in a dose-dependent manner. The protein kinase C inhibitor staurosporine (20 ...

Gender differences in ventricular repolarization: Terminal T wave interval was shorter in women than in men

NAKAGAWA, M., et al. : Gender Difference in Ventricular Repolarization: Terminal T Wave Interval was Shorter in Woman than in Men. The incidence of sudden death is lower in women than in men, although women have a longer QT interval and are more prone to develop torsades de points than men. It has been recently proposed that the time interval between the apex and end of the T wave (Ta‐e) represents the transmural dispersion of ventricular repolarization. Gender and age differences in Ta‐e interval have not been fully assessed previously. Standard surface 12‐lead ECGs recorded in 760 healthy subjects (382 women, 0–88 years of age) were studied. The intervals from j‐point to the apex of the T wave (JaT) and to the end of the T wave (JeT) were measured in lead V5 in each ECG and corrected by preceding RR intervals using the formula of Bazett (JaTc and JeTc). The Ta‐e and Ta‐e/JeT ratio were also evaluated. Both JaTc and JeTc intervals were significantly longer in women aged > 20 years than in men of the same age(P < 0.0001). The difference was due to shortening of these intervals after puberty in men. However, the Ta‐e interval was significantly shorter in women than in men(P < 0.05)and subsequently the Ta‐e/JeT ratio was significantly smaller in women than in men (P < 0.0001). The results showed gender differences in the Ta‐e interval and JaTc and JeTc intervals in healthy adults, and suggest that the small transmural dispersion of repolarization in women, in spite of the long JaTc and JeTc intervals, might be a beneficial antiarrhythmic property. (PACE 2003; 26[Pt. I]:59–64)

Paradoxically Shortened QT Interval after a Prolonged Pause

We analyzed Holter ECG recordings in 15 patients with episodes of prolonged RR intervals > 2.5 seconds. In 13 patients, the QT interval showed a linear prolongation when RR interval was < 1.5 seconds and became relatively flat at longer RR intervals. In the remaining two patients, the QT and RR intervals were correlated within physiological range of RR intervals. However, at longer RR intervals, the QT interval was unexpectedly shortened and constant. The paradoxically shortened QT interval observed in the present 2 cases may indicate an abnormal adaptation of repolarization time to an abrupt increase in the preceding RR intervals.

Effects of oral diltiazem on ventricular premature contractions.

The effects of oral diltiazem (90-180 mg/day for four weeks) on ventricular premature contractions (VPCs) were studied in 16 patients with frequent VPCs using 24-hour ambulatory ECG recordings. VPC frequency was evaluated as a function of underlying heart rate. Plots of VPC frequency vs. heart rate were made at 1-beat/min intervals for all heart rates recorded for at least five minutes during 24 hours. Patterns of correlation between VPC frequency and heart rate observed before diltiazem therapy included: 1) a relatively linear increase in VPCs with heart rate (positive correlation) in ten patients, 2) a linear decrease (negative correlation) in one patient, and 3) an increase at low heart rates and a decrease at high heart rates (bidirectional correlation) in five patients. Diltiazem significantly reduced the mean VPC frequency per 24 hours for patients with a positive correlation, but induced no significant change for patients with a negative or a bidirectional correlation. At the 65% level of VPC reduction, diltiazem was effective in eight of ten patients with a positive correlation but was not effective in the six patients with other correlations (p less than 0.01). These results suggest that an evaluation of VPC frequency as a function of heart rate predicts the response of VPCs to diltiazem.

Relation between QT and RR intervals in patients with bradyarrhythmias.

OBJECTIVE--To investigate the relation between QT and RR intervals in the sick sinus syndrome or high degree atrioventricular block. PATIENTS--32 patients with episodes of prolonged RR intervals (> or = 2.6 s) on Holter electrocardiographic recordings. DESIGN--QT and RR intervals were measured manually every 100 to 150 beats on electrocardiographic strips reprinted from the Holter tape over 24 hours. The slope of the QT/RR relation was determined by the linear regression equation for RR intervals < or = 1.4 s (slope 1) and > 1.4 s (slope 2). RESULTS--Slope 2 (0.0068 (0.0030)) was significantly lower than slope 1 (0.0824 (0.0059), P < 0.0001) in the overall patient population. Slopes 1 and 2 were significantly lower (P < 0.001) in the 23 patients with QT intervals at the preceding RR interval of 1 s (QT1s) of < 0.44 s (0.0692 (0.0053) and 0.0019 (0.0030), respectively) than in the nine patients with QT1s intervals > or = 0.44 s (0.1159 (0.0091) and 0.0194 (0.0055), respectively). Slopes 1 and 2 correlated positively with QT1s interval in all patients. CONCLUSIONS--The QT/RR relation was comparatively flat when the RR interval was prolonged. Patients with prolonged QT intervals showed exaggerated prolongation of the QT interval with prolonged cycle lengths when compared with patients with normal QT intervals.

Dynamics of T-U wave in patients with idiopathic ventricular tachycardia originating from the right ventricular outflow tract

Postextrasystolic U wave augmentation is observed in patients with long QT syndrome and those with organic heart disease. This phenomenon is considered a marker of increased risk of arrhythmia. However, the characteristics of the U wave have not been evaluated in patients with idiopathic VT originating from the right ventricular outflow tract (RVOT‐VT). The present study evaluated the dynamic change in the T‐U wave in patients with RVOT‐VT. Holter ECGs obtained from 14 patients with RVOT‐VT and 11 healthy control subjects were analyzed. The amplitude of T and U waves (Tamp and Uamp) and preceding RR intervals were measured during stable sinus rhythm (rate dependent change) and in the postextrasystolic sinus complex (pause dependent change). Uamp correlated negatively and significantly with the preceding RR interval in 13 (93%) RVOT‐VT patients but in only 2 (18%) control subjects. The average value of the slope of the Uamp/RR relationship was negative (−0.22 ± 0.10 mV/s) in the RVOT‐VT group, but was positive (0.04 ± 0.07 mV/s, P < 0.001) in the control group. Pause dependent U wave augmentation was observed in 12 (86%) of 14 patients. Increased frequency of consecutive preceding premature ventricular contractions (PVCs) was associated with a larger postextrasystolic Uamp. PVC or the first ventricular beat of VT arose from near the peak of augmented U waves. The dynamic changes in the T‐U wave were observed in patients with RVOT‐VT. Further investigations are required to elucidate the precise role of the U wave in arrhythmogenesis in those patients. (PACE 2004; 27:148–155)

Noninvasive assessment of the cardiac baroreflex: Response to downward tilting and comparison with the phenylephrine method

OBJECTIVES We studied the relation between changes in systolic blood pressure and RR interval during downward tilting in comparison with assessment of baroreflex sensitivity (BRS) measured by the phenylephrine method (Phe-BRS) and with measures of heart rate variability (HRV). BACKGROUND The method most extensively used for assessing BRS involves bolus injections of phenylephrine. Several noninvasive methods proposed to assess BRS have not been widely applied in the clinical setting. METHODS Sixteen healthy male volunteers were studied (mean age +/- SD 27.5+/-4.6 years). Arterial blood pressure using tonometry and electrocardiogram was simultaneously recorded. After 20 min of 70 degrees upright tilting, the table was returned to supine position at a speed of 3.2 degrees/s. Subsequently, BRS was assessed using an intravenous bolus injection of phenylephrine (2 to 3 microg/kg). Heart rate variability under resting conditions also was analyzed. RESULTS In all subjects, a beat to beat systolic blood pressure increase associated with corresponding RR interval lengthening was observed during downward tilting as well as during phenylephrine administration. During both testing procedures, these two variables showed linear correlation, and the slope of regression line during downward tilting (DT-BRS) correlated significantly with Phe-BRS (r = 0.79, p = 0.0003). The DT- and Phe-BRS also correlated significantly with the high frequency component of resting HRV (r = 0.70, p = 0.0023 for DT-BRS; r = 0.58, p = 0.0185 for Phe-BRS). CONCLUSIONS We conclude that in a small homogeneous group DT-BRS provided an assessment of reflex cardiac vagal function comparable to that obtained by the phenylephrine method.