(Circulation. 1996;94:1607-1612.)
© 1996 American Heart Association, Inc.
Articles |
Circadian Variation in Defibrillation Energy Requirements
the Section of Cardiovascular Medicine, Lahey-Hitchcock Medical Center, Burlington, Mass; the Department of Cardiology, New England Deaconess Hospital, Boston, Mass; and the Clinical Department, Cardiac Pacemakers Inc/Guidant, St. Paul, Minn.
Correspondence to Ferdinand J. Venditti, MD, Chief, Cardiovascular Medicine, Lahey-Hitchcock Medical Center, Burlington, MA 01805.
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Abstract |
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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.
Key Words: circadian rhythm • defibrillation • tachyarrhythmias
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Introduction |
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Many physiological variables have peaks and troughs in the course of a 24-hour day. Several reports have demonstrated a circadian variation in the incidence of cardiovascular events such as acute myocardial infarction, thrombotic stroke, transient myocardial ischemia, and sudden cardiac death.1 2 3 4 In all these conditions, there appears to be a prominent peak in the morning hours soon after the patient awakens. This pattern resembles the circadian variation in other metabolic factors such as renin activity and catecholamine levels.5 6 7
Other reports have demonstrated a circadian variation in the onset of sustained ventricular tachyarrhythmias and the incidence of implantable cardioverter-defibrillator (ICD) therapy for ventricular arrhythmias.8 9 Once again, a prominent morning peak between 10 AM and 12 noon has been observed in onset of ventricular tachycardia.8 Likewise, there is a reported peak in ICD therapies for spontaneous ventricular arrhythmias between 6 AM and 12 noon as assessed by ICD data logs.9
In this report, we sought to determine whether other electrical properties of the heart varied in a fashion similar to arrhythmia frequency. Thus, we evaluated the defibrillation threshold (DFT) measured in a standard fashion throughout the day to determine whether a circadian variation in measured DFT exists. Also, to determine whether changes in DFT corresponded to changes in ICD efficacy, we evaluated the first shock failure rate in a second cohort of patients with ICDs that accurately recorded date and time stamps for each arrhythmia episode. Because defibrillation is a stochastic process and efficacy is best represented as a probability density function,10 11 even high energies will not always result in defibrillation. Therefore, first shock efficacy will seldom be 100% regardless of the programmed shock energy in large populations. First shock efficacy rates vary from 80% to 95%, depending on programmed energy and other factors. We hypothesized that if energy requirements for defibrillation vary during the course of the day, first shock efficacy rates should parallel this change.
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Methods |
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DFT Measurement Group
We reviewed the records of 134 consecutive patients with nonthoracotomy ICDs implanted in our institution from May 1991 through April 1994. All patients underwent DFT testing on at least one occasion during that interval. All operating room and electrophysiology laboratory procedures were documented by final reports and procedure logs that documented the chronology of events during each case. The latter document was used to determine the time of DFT measurement. All testing was performed as part of the implantation procedure or the clinical evaluation of investigational devices in protocols approved by the Human Subjects Committee of the Lahey-Hitchcock Clinic Medical Center (Burlington, Mass). All patients gave written informed consent for device implantation and testing.
Population
The patient group was a consecutive series of patients with sustained arrhythmia referred to the Arrhythmia Service of the Lahey Clinic for nonthoracotomy ICD implantation. All patients underwent invasive electrophysiological testing. Cardiac catheterization or echocardiography was used to assess left ventricular function. Exercise testing and ambulatory Holter monitoring were performed as necessary for the care of the individual patient.
Of the 134 patients studied, 109 (81.3%) were men. The mean age (±SD) was 64±11 years. The underlying disease process was coronary artery disease in 105 patients (81%), with 95 (73%) having had a prior myocardial infarction. No patients had a recurrent ischemic event during the chronic phase of this study. The presenting arrhythmia was ventricular fibrillation in 37%; it was sustained monomorphic ventricular tachycardia in 63%. The mean left ventricular ejection fraction was 31±11%. During the study, 56 of the 134 patients (48%) received antiarrhythmic agents or ß-blockers (Table 1
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All patients received a nonthoracotomy system. The transvenous leads used were the ENDOTAK 60 or 70 series leads with or without a subcutaneous patch or array (Cardiac Pacemakers Inc, St Paul, Minn). Sixty-seven percent of the systems were implanted with a transvenous lead only. All ICD generators were Cardiac Pacemakers Inc model 1600, 1625, or 1705. Thirty patients (22%) received a biphasic waveform system.
At the time of implantation, the DFT was determined for a median of 2 different lead configurations (range, 1 to 10) in each patient. Fifty patients were implanted after only 1 lead configuration was tested because of an acceptable DFT (DFT 
15 J) with the first lead configuration tested. In those patients in whom multiple lead configurations were tested, the configuration with the lowest DFT was selected for the final implantation scheme and was the DFT used in this analysis. Because this implant DFT optimization was not uniform, it may have introduced bias into the implantation testing data. Therefore, a second analysis was done without the "optimized" DFTs obtained during implantation in some patients.
DFT Testing
All DFTs were determined by use of a standard step-down technique described in detail previously.12 The individual energy steps used were 2 to 5 J. All testing was started at the 20-J energy level. Ventricular fibrillation was induced with AC current, ramp pacing, or device-based fibrillation induction. Test shocks were delivered after 
10 seconds of ventricular fibrillation. The DFT was defined as the lowest programmed energy level with successful conversion to normal sinus rhythm.
The time of DFT was defined as the time at which the lowest-energy successful shock was delivered. DFT testing took on average 15 to 45 minutes from first to last delivered shock. All implant DFT testing was performed with patients under general anesthesia with enthane, halothane, methohexital, propafol, or a combination of two or more of these agents. All long-term DFT testing was performed with only propafol as the anesthetic agent.
Spontaneous First Shock Efficacy Group
Patients enrolled in the multicenter clinical evaluation of the VENTAK P2 ICD system (Cardiac Pacemakers Inc) make up the second cohort studied. There were 930 patients implanted with this ICD system through May 1994. Informed consent was obtained from all patients before implantation. Therapy history, including date and time stamps, was used to time events. This ICD also records electrograms during the arrhythmias that are treated. Therapy history, electrograms, and external telemetry were used to eliminate supraventricular arrhythmias from the group analyzed.
Baseline Characteristics
This group consisted of 930 patients with a mean age of 62±12 years; 727 (78%) were men. The presenting arrhythmia was sustained monomorphic ventricular tachycardia in 464 patients (50%), ventricular fibrillation in 384 (41%), and ventricular tachycardia–ventricular fibrillation in 82 (9%). The underlying disease process was coronary artery disease in 673 patients (73%), dilated cardiomyopathy in 178 (19%), and miscellaneous in 76 (8%). Of the 930 patients, 583 (62.7%) were not receiving class I or III antiarrhythmic drugs or ß-blockers (Table 2).
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Efficacy Assessment
In total, 1686 spontaneous episodes occurred in 327 patients; 1628 (96.6%) were evaluated by recorded electrograms, therapy history, telemetry, or a combination of the three to determine whether therapy was appropriate (ie, if therapy was for ventricular tachyarrhythmias). A total of 1238 episodes in 259 patients were determined to be ventricular tachyarrhythmias. The mean cycle length of treated arrhythmias was 332±84 ms (range, 156 to 600 ms). The distributions over 24 hours of slow (cycle length 
250 ms) and fast (cycle length <250 ms) tachycardias were similar.
All ventricular arrhythmia episodes were then categorized as first shock successful or first shock unsuccessful on the basis of therapy history. The hourly failed first shock rate for ventricular events was then determined and plotted.
Drug-Free Group
There were 583 patients without antiarrhythmic or ß-blocker drug use during the study. There were 635 spontaneous episodes in 155 patients. The distribution of first shock failure was evaluated for this group separately to eliminate any effect that drug use might have on shock efficacy.
Statistical Analysis
Data were analyzed by use of ANOVA for the effect of time on DFT. Post hoc comparison was performed by use of Scheffe's method. Unpaired t testing was used when appropriate to assess differences between groups. We used 
2 testing to determine whether there was a nonuniform distribution of timing of failed first shocks during the 24-hour period. Miettiner modification of Fisher's exact test was used to determine differences between time periods.
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Results |
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DFT Group
A total of 345 DFT measurements were determined between 7:55 AM and 8:05 PM in this group of patients. There were 130 measurements during implantation, 121 at the time of the 2-month testing, and 94 at the 6-month testing. Measured DFTs ranged from 3 to 30 J.
DFTs measured in the morning (8 AM to 12 noon) were higher than those measured in the early (12 noon to 4 PM) or late (4 PM to 8 PM) afternoon (Fig 1). The measured DFT in the morning was 16% higher than those measured after 12 noon (P<.05). When only single-determination DFTs were analyzed, with any "optimization" bias from testing of multiple lead configurations to find the one with the lowest DFT during implantation eliminated, the morning peak in DFT was even more pronounced (Fig 2). In this second analysis, the DFT was 30% higher in the morning compared with measurements made in the afternoon (P<.01).
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When implant DFTs were analyzed separately (Table 3), there were no significant differences between the three time intervals. However, this initial analysis included measurements in which optimization of the DFT was performed at the time of implantation. When these measurements were censored, the trend toward a morning peak in DFT was restored, with a 13% difference seen between the morning and the afternoon.
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Several previous investigators have demonstrated an increase in the measured DFT some time after implantation.12 13 14 Therefore, to eliminate the effect of maturation of the lead system, we analyzed the long-term DFT measurements by pooling the 2- and 6-month testing data. Again, a prominent morning peak in DFT was observed, with the measured DFT 19% higher than those measured after 12 noon (P<.05).
To eliminate the effect of multiple determinations for a given patient, the DFTs from the 2-month period were analyzed separately. This group would then represent one DFT measurement for each patient at a time when DFTs should be stable. As Table 3
shows, there still was a significant variation in DFT, with the morning DFT being 25% higher than the afternoon measurements (P<.05). When the 6-month data were analyzed, a similar trend that was not statistically significant existed.
Antiarrhythmic drugs can affect DFTs; therefore, a final analysis of measurements done off all electrophysiologically active agents was performed (antiarrhythmics and ß-blockers). In this drug-free group, the morning DFT was still significantly higher than the mean DFT measured in the early and late afternoon (Fig 3).
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Spontaneous First Shock Efficacy Group
In the 1238 spontaneous therapy episodes analyzed, the failed first shock rate was 15% (186 shocks). However, the failed first shock rate (Fig 4) was not uniformly distributed over time; a greater proportion of first shocks was unsuccessful in the morning compared with other times in the day (P<.002). When analyzed by 3-hour intervals (Table 4), a prominent peak was noted between 6 and 9 AM (24.9% failure rate) and a nadir between 12 midnight and 3 AM (7.0% failure rate).
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To determine whether patients with therapy in the morning hours might be a sicker group, we determined the mean implant DFTs and left ventricular ejection fractions for those with therapy in the morning (5 to 10 AM) compared with other times. The mean left ventricular ejection fraction was 28.1±12.6% for the morning shock group and 30.0±11.1% for the remaining patients (P=NS). The implant DFTs also were not different at 10.2±5.7 and 9.3±6.0 J, respectively (P=NS).
When only spontaneous episodes occurring in patients who were off all antiarrhythmic drugs and ß-blocker drugs were evaluated, there was still a trend toward a nadir in failed shocks in the early morning hours (12 midnight to 3 AM) and a peak between 6 and 9 AM. This was not statistically significant (P=.12); however, there was no difference in time distribution of failed first shocks between the drug-free subgroup and the group taking electrophysiologically active drugs (Fig 5).
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Discussion |
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Many factors affect defibrillation energy requirements, including the shock waveform,15 16 electrode size and configuration,17 chronicity of the electrode system,12 13 14 cardiac mass encompassed in the defibrillation field,18 duration of ventricular fibrillation,19 20 concomitant antiarrhythmic drug use,21 22 23 24 25 and underlying cardiac disease.26 In the present study, we have demonstrated a prominent morning peak in the measured DFT, suggesting that there is a circadian variation in defibrillation energy requirements. Furthermore, our data show a decreased first shock efficacy rate during that same time period, supporting the notion that morning energy requirements for arrhythmia termination are higher than those at other times of the day.
A number of electrophysiological parameters have been shown to demonstrate a circadian variation in the human heart. Energy requirements for cardiac stimulation can increase by as much as 30% to 40% during sleep,27 and other investigators have shown a marked prolongation in the QT interval during sleep.28 Electrophysiological testing has demonstrated a 7% to 8% variation in myocardial effective refractory periods, with peaks occurring during early morning hours.29 The relationship between awakening and refractory period was even more significant. Sinus node recovery times and AV node conduction times likewise can increase by 6% to 13% in the early morning hours. However, the susceptibility to induction of sustained ventricular tachyarrhythmias by programmed stimulation does not appear to vary.30
Possible Mechanisms of Circadian Variation in Energy Requirement
The factor or factors responsible for the observed increase in energy requirements are unclear. Available data suggest that the likely mechanism for a shock to be successful in defibrillating the myocardium requires at least 90% of the fibrillating mass to be exposed to a >6 V/cm potential gradient when a monophasic waveform is used.31 Shock potential gradients that are higher than this critical level prolong action potential duration (and thus refractoriness), preventing further propagation of fibrillation wave fronts and the initiation of new wave fronts. Although electrophysiological properties such as effective refractoriness appear to follow a circadian pattern, it is not clear how these parameters would affect the above-described mechanism of defibrillation.
Myocardial ischemia could result in higher energy requirements. Investigators have demonstrated a higher incidence of transient myocardial ischemia in the morning hours.4 Certainly, episodes of tachyarrhythmias that occur concomitantly with ischemia might require higher energy levels for conversion. However, if there was a similar distribution in first shock efficacy in patients without coronary artery disease, then ischemia could not be the sole explanation for the observed pattern. Subgroup analysis could not be performed in this study because of the preponderance of patients with coronary artery disease.
When patients awaken in the morning, there is a marked change in posture. Assumption of the upright position could result in changes in orientation of the electrodes relative to the bulk of myocardium. Additionally, relative volume depletion from the overnight fast of sleeping could contribute to myocardial-electrode orientation differences. These changes may result in higher energy requirements. Preliminary results suggest that although pacing techniques for termination of ventricular tachycardia may be affected by the upright posture, cardioversion energy requirements do not change.32 Even if the lower first shock efficacy rate in the morning were related to posture and blood volume changes, this would not explain the higher measured DFTs, all of which were determined in patients in the supine position.
Many of the circadian hormonal changes result in peaking of plasma levels during the morning hours. Epinephrine and norepinephrine peak in the 6 AM to 12 noon period.5 6 7 In one small study, first shock efficacy was significantly reduced when epinephrine was infused33 ; infusion rates were used so that plasma concentrations of epinephrine were twice the baseline levels and therefore equivalent to levels achieved with mild to moderate exercise. With this modest elevation in levels, the first shock efficacy rate dropped from 100% to 75% (P<.01). Therefore, it is possible that the catecholamine levels that follow a circadian variation may influence defibrillation energy requirements.
Study Limitations
The major limitations of this study are related to the method of DFT measurement. Defibrillation threshold is a misnomer because a true "threshold" does not exist. Defibrillation efficacy is best defined as a probability density function with low, intermediate, and high probabilities of success at various energy levels. A measured DFT is just a point on the sigmoid-shaped probability curve of efficacy. The reliability of the location on the curve of any given measurement depends on the methods used to determine the DFT. Standard step-down techniques, as used in this study, tend to determine the E50 point (energy with 50% efficacy) on the probability curve.34 35 36 Although not precise, the reproducibility of this technique has been demonstrated to be similar to that of other, more involved techniques for DFT testing.36
The reported energy levels in this study are the programmed energy levels, not the actual delivered energy. Although the delivered energy may be slightly different, this group of ICDs uses a variable pulse width to deliver close to the full programmed energy regardless of changes in impedance. Even if there are small differences between programmed and delivered energy, the expected range would be <1 J.
The time of DFT measurements was not selected randomly but rather was based on operating room and anesthesiologist availability, which were not under our control. In most instances, individual patients contributed more than one DFT measurement to the total group data. However, when only the 2-month testing data were analyzed, which would result in a single measurement per patient, there still were significant differences between morning DFTs and those measured in the afternoon.
The DFTs reported in this study were determined during 12 hours of a 24-hour day. Therefore, these data suggest but do not prove an early morning peak in DFT because no data are presented for the 8 PM to 8 AM time period. However, the efficacy data from the second cohort of patients contain 24-hour data, thus strongly supporting a morning peak in energy requirements for ventricular arrhythmia termination.
The spontaneous first shock efficacy group includes episodes of ventricular tachyarrhythmias that probably were monomorphic ventricular tachycardia. Because the energy requirements for the conversion of ventricular tachycardia to sinus rhythm are usually lower than defibrillation energy requirements, the inclusion of these episodes could affect the results; however, the incidences over time of ventricular tachycardia and ventricular fibrillation were similar, so any error introduced should be constant. In addition, this portion of the data is presented to support the DFT observation in the initial group of patients studied and is not intended to stand alone.
Clinical Implications
Our data confirm a circadian variation in energy requirement for successful defibrillation with higher energy requirements in the morning hours after awakening. As reported by others, this is the time when patients are more likely to have spontaneous ventricular arrhythmias and sudden cardiac death. As a result, this higher energy requirement occurs during the time when patients are very likely to need effective ICD therapy. Therefore, this variability needs to be taken into account when implant decisions are made in the operating room during DFT testing. To avoid underestimating the DFT, testing to determine efficacy of an ICD system should ideally be performed in the morning (6 AM to 12 noon) when the highest energy requirement is likely to be found for a given patient. First shock energy programming decisions need to incorporate the possibility of a 15% to 20% change in energy requirements during the course of the day. Otherwise, patients with low safety margins may be at risk for a higher failure rate of first shocks and the deleterious effects of delay in the termination of their tachyarrhythmias.
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Acknowledgments |
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We gratefully acknowledge the efforts of all the VENTAK P2 investigators.
Received January 9, 1996; revision received April 17, 1999; accepted April 23, 1996.
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