Circadian variation in ventricular electrical instability associated with coronary artery disease

Circadian rhythms and cardiovascular health

The functional organization of the cardiovascular system shows clear circadian rhythmicity. These and other circadian rhythms at all levels of organization are orchestrated by a central biological clock, the suprachiasmatic nuclei of the hypothalamus. Preservation of the normal circadian time structure from the level of the cardiomyocyte to the organ system appears to be essential for cardiovascular health and cardiovascular disease prevention. Myocardial ischemia, acute myocardial infarct, and sudden cardiac death are much greater in incidence than expected in the morning. Moreover, supraventricular and ventricular cardiac arrhythmias of various types show specific day-night patterns, with atrial arrhythmias--premature beats, tachycardias, atrial fibrillation, and flutter - generally being of higher frequency during the day than night--and ventricular fibrillation and ventricular premature beats more common, respectively, in the morning and during the daytime activity than sleep span. Furthermore, different circadian patterns of blood pressure are found in arterial hypertension, in relation to different cardiovascular morbidity and mortality risk. Such temporal patterns result from circadian periodicity in pathophysiological mechanisms that give rise to predictable-in-time differences in susceptibility-resistance to cyclic environmental stressors that trigger these clinical events. Circadian rhythms also may affect the pharmacokinetics and pharmacodynamics of cardiovascular and other medications. Knowledge of 24-h patterns in the risk of cardiac arrhythmias and cardiovascular disease morbidity and mortality plus circadian rhythm-dependencies of underlying pathophysiologic mechanisms suggests the requirement for preventive and therapeutic interventions is not the same throughout the day and night, and should be tailored accordingly to improve outcomes.

Circadian rhythms in cardiac arrhythmias and opportunities for their chronotherapy

It is now well established that nearly all functions of the body, including those that influence the pharmacokinetics and pharmacodynamics of medications, exhibit significant 24-hour variation. The electrical properties of the heart as well as cardiac arrhythmias also vary as circadian rhythms, even though the suboptimal methods initially used for their investigation slowed their identification and thorough characterization. The application of continuous Holter monitoring of the electrical properties of the heart has revealed 24-hour variation in the occurrence of ventricular premature beats with the peak in events, in diurnally active persons, between 6 a.m. and noon. After the introduction of implantable cardioverter-defibrillators, ventricular tachycardia or fibrillation were also found to peak in the same period of the day. Even defibrillator energy requirements show circadian variation, thus supporting the need for a temporal awareness in the therapeutic approach to arrhythmias. Imbalanced autonomic tone, circulating levels of catecholamines, increased heart rate and blood pressure, all established determinants of cardiac arrhythmias, show circadian variations and underlie the genesis of the circadian pattern of cardiac arrhythmias. Arrhythmogenesis appears to be suppressed during nighttime sleep, and this can influence the evaluation of the efficacy of antiarrhythmic medications in relation to their administration time. Unfortunately, very few studies have been undertaken to assess the proper timing (chronotherapy) of antiarrhythmic medications as means to maximize efficacy and possibly reduce side effects. Further research in this field is warranted and could bring new insight and clinical advantage.

Holter monitoring and cardiac event recording

The detection and characterization of transient arrhythmias is possible with continuous ambulatory electrocardiography (Holter monitoring) or cardiac event recording. These techniques have significantly increased our appreciation of transient arrhythmias, often undetected with routine echocardiography and have provided highly sensitive means to assess need and efficacy of antiarrhythmic therapy. Correlation of heart rate and arrhythmia with the patient's activity or clinical signs is also possible.

Circadian variation of sustained ventricular tachycardia in patients subject to standard adrenergic blockade

Morning peaks in the circadian variation of sustained ventricular tachycardia (VT) may reflect the contribution of sympathetic activation to onset of VT. We hypothesized that adrenergic blockade would eliminate this morning peak. Fifty-four patients using a defibrillator had 1114 time-stamped episodes of VT requiring therapy with a device: 1012 episodes with and 102 episodes without antiadrenergic medications. Nine patients had episodes both with and without antiadrenergic medication and were examined separately. In patients taking antiadrenergic agents, data fitted to a harmonic regression model revealed a morning peak at 9:00 AM (R2= 0.542; p < 0.05), with a secondary peak at 4 PM. Those not receiving antiadrenergic therapy had a similar morning peak. Antiadrenergic agents as used in standard clinical practice do not prevent circadian variation in onset of VT. This variation may be mediated by systems other than adrenergic receptor-linked activation or may reflect inadequacy of adrenergic blockade in standard clinical dosing.

Relation of circadian ventricular ectopic activity to cardiac mortality. CAST Investigators

The relation between the circadian occurrence of ventricular premature depolarizations (VPD) and sudden arrhythmic death was examined in a subset of patients entered into the Cardiac Arrhythmia Suppression Trial (CAST). Ambulatory electrocardiographic recordings with hourly measurement of VPD frequency were available in 357 patients. Forty percent of the patients (142 of 357) demonstrated circadian variation in VPD frequency between 6:00 A.M. and 9:59 A.M. that was significantly higher (p < 0.05) than what could randomly be expected from an overall 24-hour average for that patient. The only baseline characteristics in patients with circadian VPDs were age (p < 0.04), history of cardiac arrest (p < 0.01), presence of higher frequency of VPDs (p < 0.002), more frequent episodes of ventricular tachycardia (p < 0.04), and more frequent episodes of slow runs (p < 0.04). There was no difference in mortality in patients with or without circadian VPD variation; drug treatment did not effect mortality. These data indicate that the presence of circadian VPDs is not a predictor of sudden arrhythmic death in patients with a high frequency of VPDs.

Circadian variation in defibrillation energy requirements

BACKGROUND

Reports have demonstrated a circadian variation in the incidence of acute myocardial infarction, ventricular arrhythmias, and sudden cardiac death. We tested the hypothesis that a similar circadian variation exists for defibrillation energy requirements in humans.

METHODS AND RESULTS

We reviewed the time of defibrillation threshold (DFT) measurements in 134 patients with implantable cardioverter-defibrillators (ICDs) who underwent 345 DFT measurements. The DFT was determined in 130 patients at implantation, in 121 at a 2 months, and in 94 at 6 months. All patients had nonthoracotomy systems. The morning DFT (8 AM to 12 noon) was 15.1 +/- 1.2 J compared with 13.1 +/- 0.9 J in the midafternoon (12 noon to 4 PM) and 13.0 +/- 0.7 J in the late afternoon (4 to 8 PM), P < .02. In a separate group of 930 patients implanted with an ICD system with date and time stamps for each therapy, we reviewed 1238 episodes of ventricular tachyarrhythmias treated with shock therapy. To corroborate the hypothesis that energy requirements for arrhythmia termination vary during the course of the day, we plotted the failed first shock frequency for all episodes per hour. There was a significant peak in failed first shocks in the morning compared with other time intervals (P = .02).

CONCLUSIONS

There is a morning peak in DFT and a corresponding morning peak in failed first shock frequency. This morning peak resembles the peaks seen in other cardiac events, specifically sudden cardiac death. These findings have important implications for appropriate ICD function, particularly in patients with marginal DFTs.

Circadian patterns and triggers of sudden cardiac death

Sudden cardiac death and other acute cardiovascular events have been demonstrated to occur in certain temporal patterns. The study of these patterns may yield important clues to the pathophysiology of the disease process. Most studies of the timing of onset of sudden cardiac death have revealed a prominent midmorning peak, thought to be related to a surge in catecholamines associated with arising and assuming the upright posture, that is blunted or eliminated by beta blockers. In addition, some studies have also shown a secondary peak in late afternoon or early evening of uncertain cause. The development of third-generation implantable cardioverter defibrillators with memory capabilities offers a unique opportunity to accurately define event chronology.

Circadian pattern of ventricular tachyarrhythmias in patients with implantable cardioverter-defibrillators

OBJECTIVES

This study examined the temporal patterns of ventricular tachycardia detections by implantable cardioverter-defibrillators for circadian variability.

BACKGROUND

Previous studies of circadian arrhythmia patterns have been methodologically limited by very brief observational periods. Late-generation implantable cardioverter-defibrillators accurately record the times of arrhythmia detections during unlimited follow-up.

METHODS

Forty-three patients with late-generation implantable cardioverter-defibrillators were followed up for 226 +/- 179 days (mean +/- SD). The times of all recorded episodes of ventricular tachyarrhythmias were retrieved from the data log of each device during follow-up.

RESULTS

The weighted distribution of 830 ventricular tachyarrhythmia episodes from the 43 patients fit a single harmonic sine curve model with a peak between 2 and 3 P.M. (95% confidence interval 1:13 to 4:13 P.M., R = 0.75, p < 0.05). The distributions of spontaneously terminating episodes, episodes receiving device therapy, episodes receiving shocks and episodes in the absence of antiarrhythmic therapy also fit the sine curve model (all R = 0.53 and 0.73, all p < 0.05), all with peak frequencies between 2:08 and 3:09 P.M. and 95% confidence intervals for peak frequencies between 11:38 A.M. and 5:07 P.M. Episodes recorded during continuous antiarrhythmic drug therapy did not fit the model (p > 0.05).

CONCLUSIONS

The distribution of ventricular tachyarrhythmias detected by late-generation implantable cardioverter-defibrillators follows a circadian pattern, with a peak tachycardia frequency between noon and 5 P.M. This pattern was not observed in patients receiving antiarrhythmic drug therapy. Knowledge of circadian periodicity for these events has implications for patient management.

Cardiac arrhythmias and the autonomic nervous system

The multiple facets of cardiac arrhythmias and their relationship with the autonomic nervous system can be investigated by studying the spontaneous heart rate behavior through ambulatory ECG recordings, an approach that complements the limitations of invasive electrophysiologic investigations. Information obtained from heart rate behavior is more reliable in the absence of structural heart disease and ventricular hypertrophy/failure, during which compensatory mechanisms involving the autonomic nervous system tend to limit reflex changes in heart rate. Thus, in such situations, less marked sinus rhythm variations preceding the arrhythmia onset do not imply a more limited influence of the autonomic nervous system, and the sensitivity of the electrophysiologic substrate may otherwise vary. These two factors may combine to form the basis of the "adrenergic paradox" that implies that the more marked the autonomic nervous system dependence of tachyarrhythmias, the less obvious its evidence. Assessment of the QT interval dynamicity may also allow one to evaluate the modulation of autonomic neural effects on the ventricular tissues. Finally, it may be difficult to distinguish clearly autonomic nervous system dependence from rate dependence: the latter frequently conditions the behavior of the trigger whereas the former mainly concerns the electrophysiologic substrate. There are many examples of the importance of the autonomic nervous system as a determinant of cardiac arrhythmias. In the atrium, either limb of the autonomic nervous system, particularly the parasympathetic limb, can generate atrial fibrillation. The absence of structural heart disease defines pure electrophysiologic substrates responsible for benign forms of ventricular tachycardia as well as potentially lethal tachyarrhythmias of the long QT syndrome and its variants. In both, the role of the autonomic nervous system is essential, although the therapeutic consequences are crucial only in the latter. In the presence of heart disease and, in particular, heart failure, the autonomic nervous system behavior is more difficult to assess than in the absence of structural heart disease. This does not mean that its role is less crucial. In this situation the beneficial effects of beta blockers may be as important as in normal hearts although physicians should be more cautious when heart failure is present.

Comparison of ventricular arrhythmia induction with use of an indwelling electrode catheter and a newly inserted catheter

Two methods of serial electrophysiologic testing are in widespread use. Most commonly, the electrode catheter is removed after each study and a new catheter reinserted through the femoral vein for every subsequent test. An alternative method employs an electrode catheter that remains in place during several days of serial testing. Little is known about differences between these two methods with respect to the likelihood of induction of arrhythmia or the frequency of complications. To determine whether inducibility of sustained arrhythmia is altered or if the frequency of complications is unacceptably high with use of an indwelling catheter, a prospective randomized study was conducted in 78 patients. Each patient underwent baseline testing, several days of electropharmacologic testing with an indwelling catheter, a 24 h drug elimination period and placement of a new electrode catheter. Ventricular stimulation studies were then performed in each patient with both the indwelling and new electrode catheters. No differences were found between the indwelling and new catheter tests with respect to induction of arrhythmia, number of extrastimuli required to induce arrhythmia, rate of arrhythmia or requirement for cardioversion. Ventricular pacing thresholds were higher and effective refractory periods were slightly longer when measured with the indwelling catheter. Complications related to the 156 catheter insertions included two that may have been related to the indwelling catheter (one episode of staphylococcal sepsis and one presumed pulmonary embolism) and four that were related to invasive procedures (pneumothorax in all). There were no long-term adverse sequelae of these complications.(ABSTRACT TRUNCATED AT 250 WORDS)