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Ann Thorac Surg 2001;72:1546-1551
© 2001 The Society of Thoracic Surgeons
a Division of Cardiology, University Hospital, Zurich, Switzerland
b Division of Cardiovascular Surgery, University Hospital, Zurich, Switzerland
c Division of Cardiology, Stadtspital Triemli, Zurich, Switzerland
Accepted for publication July 30, 2001.
* Address reprint requests to Dr Scharf, Division of Cardiology, Department of Medicine, University Hospital, CH-8091 Zurich, Switzerland
e-mail: christoph.scharf{at}dim.usz.ch
| Abstract |
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Methods. The SAECG was recorded in 561 patients (mean age: 60 ± 8.8 years) within 10 days of coronary artery bypass grafting. Signal-averaged ECG was performed with a bandpass filtering of 40 to 250 Hz for more than 250 beats until a noise level of 0.6 µV was achieved. All patients were followed for 5.5 ± 1.2 years after the procedure.
Results. Preoperative angiographic ejection fraction was at least 60% in 393 patients (72%), 40% to 60% in 126 patients (23%), and 40% or less in 28 patients (5%). There were 34 deaths, 10 of which were SCD. Late potentials were found in a total of 150 patients (27%) and were equally frequent preoperatively and postoperatively and among patients with (30%) and without (27%) SCD. The only predictors for overall mortality were age and a reduced ejection fraction.
Conclusions. Signal-averaged ECG did not predict prognosis in low-risk patients undergoing coronary artery bypass grafting.
| Introduction |
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| Patients and methods |
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According to the standards for time domain analysis [2], real-time recording was performed using a Marquette MAC 15 HiRes ECG recorder (Milwaukee, WI), as previously described [8]. The QRS signal in the three orthogonal Frank leads was filtered (40 to 250 Hz) and averaged for 250 beats or more, until a noise level of 0.6 µV or less was achieved. Three parameters were calculated: filtered QRS duration (dQRS, ms), duration of the terminal low-amplitude signal lower than 40 µV (LAS40, ms), and the root mean square of the terminal 40 ms of the QRS complex (RMS40, µV). The presence of LPs was defined when two of the following three criteria were met: dQRS longer than 120 ms, LAS40 at least 38 msec, and RMS40 lower than 20 µV.
Medical records of all patients were reviewed for clinical and angiographic findings. Follow-up of patients was achieved by telephone calls to patients, their relatives, or family physicians. In the case of death, the exact circumstances for each case were obtained from relatives, family physicians, review of medical records in the case of hospitalization, or state death chart registry. The primary end point of the study was SCD, which was defined as "witnessed instant death," "death occurring after cardiac arrest due to ventricular fibrillation," or "unwitnessed death without preceding symptoms of any underlying disease or specific etiology." Secondary end points were all-cause mortality and cardiac death including heart failure and myocardial infarction.
Predictors for survival were assessed using Cox regression for continuous variables and Kaplan-Meier analysis for discontinuous variables. Differences in patient characteristics and SAECG were assessed using the paired t-test and the Wilcoxon rank-sum test as appropriate. Values are expressed as mean ± standard deviation. Calculations were made using a commercially available statistical package (SPSS 9.0 for Windows; SPSS, Inc, Chicago, IL). A p-value less than 0.05 was considered statistically significant.
| Results |
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Long-term follow-up
Follow-up was available for all 561 patients for a mean of 5.5 ± 1.26 years, resulting in a total of 3,085 patient-years of follow-up. Thirty-four patients (6.5%) died during follow-up, for an annual mortality of 1.2% after hospital discharge. Sudden cardiac death occurred in ten patients (29% of total mortality). The frequency of SCD was constant during follow-up, with two to three cases per year. The earliest SCD occurred 2 days after hospital discharge, after an uneventful postoperative course. Preoperatively, this patient (with LPs) had exercise-induced ventricular tachycardia lasting for 15 seconds, which means that ventricular arrhythmia was the likely cause of death.
Clinical and angiographic characteristics of patients with SCD are summarized in Table 1. Perioperative myocardial infarction occurred in one patient. One patient underwent concurrent CABG and aortic valve replacement, and SCD occurred after 5 years. In three other patients not classified as SCD, ventricular arrhythmia could have been responsible for subsequent death: one patient died of worsening heart failure and pneumonia 3 months after documented ventricular fibrillation and successful cardiopulmonary resuscitation, and two other patients died in accidents. Acute myocardial infarction was responsible for five deaths (15%). Thus, fifteen of thirty-four patients died from cardiovascular causes (44%). Cancer accounted for ten deaths (29%), stroke for three, pneumonia for two, accidents for two, hepatic cirrhosis for one, and suicide for one.
Predictors of SCD and total mortality
The clinical, angiographic, perioperative, and electrocardiographic characteristics of survivors, nonsurvivors, and patients with SCD are outlined in Table 1. Individual values in SAECG of all patients with SCD are summarized in Table 4. Importantly, LPs were equally frequent in patients with and without SCD (30% vs 27%) and identified only three of ten patients with SCD (sensitivity 30%). Because the prevalence of SCD was low (1.8%), the negative predictive value of a normal SAECG was 98% (7 false-negative and 406 true-negative). However, 98% of LPs were false-positive (specificity 73%). Survival analysis did not show any significant relationship between LPs and SCD, cardiac death, or all-cause mortality. Neither preoperative nor postoperative LPs alone predicted SCD. There was an insignificant trend in the relationship of mean duration of filtered QRS complex to overall survival (p = 0.078), but not with SCD. Of 80 patients with postoperative LPs, only four died and only one of those died suddenly during follow-up.
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| Comment |
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We studied unselected patients at low risk, only 5% of whom had an ejection fraction below 40%. Nevertheless, a reduced ejection fraction was the only predictor of SCD (p < 0.05), which agrees with other large-scale studies like MADIT [12], MUSTT [13], and CIDS [14], in which patients with a severely reduced ejection fraction profited most from ICD implantation.
The effect of prophylactic ICD implantation in patients undergoing CABG was tested in a large multicenter CABG Patch Trial [11]. Patients with an abnormal SAECG and a reduced left ventricular ejection fraction less than 36% were randomly assigned to an ICD implantation group during CABG [11]. No benefit from ICD implantation was seen, and this was explained by the low incidence of arrhythmic death [15]. Our data suggest, however, that most patients with subsequent SCD presented with a normal SAECG and presumably were missed by the inclusion criteria. To date only one study including 100 patients with short follow-up assessed the prognostic role of SAECG in patients undergoing CABG, which confirms our results [9].
Our data appear to be in contrast to many previous reports demonstrating a prognostic role for SAECG in CAD patients. There is no ready explanation for this discrepancy. However, since the landmark study by Simson in 1981 [16] and numerous other studies [5, 17], remarkable progress in treatment of acute and chronic CAD has lowered cardiac mortality substantially. It is conceivable that thrombolysis and early revascularization after myocardial infarction, as well as widespread use of beta blockers, statins, and ACE inhibitors have changed patient characteristics and might render findings on SAECG less important for predicting survival and SCD, as compared in patients studied before such therapies became widespread.
Although surgical revascularization did not influence the occurrence of LPs, there was a discrete but significant nonlinear effect on SAECG parameters. The clinical significance of these minor changes remains unknown. The changes in the occurrence of LPs between pre- and postoperative SAECG recordings are randomly occurring, without prognostic implication and the analysis of these subgroups should not lead to any conclusions in our opinion. To date, the effect of revascularization on SAECG has been demonstrated mainly in acute coronary occlusion, and reperfusion [18]. Results indicating abolition of LPs after CABG have been obtained in only a few selected patients and have not been replicated [7]. Another finding of our study is that the duration of extracorporeal circulation and aortic cross-clamping does not influence SAECG parameters or long-term prognosis. The longer extracorporeal circulation time in nonsurvivors was due to prolonged warming of a subgroup of significantly older patients; thus, it was not an independent predictor of prognosis. This finding may reassure thoracic surgeons and anesthesiologists in the current practice of extracorporeal circulation.
Limitations of this study
A major limitation of clinical follow-up studies is the diagnosis of SCD. However, all but one of our SCD cases were witnessed, and no specific symptoms were noted before the event in any case. Acute myocardial infarction seems to be less frequent in survivors of out-of-hospital ventricular fibrillation [19, 20]. We did not include three patients, in whom death could be attributed to arrhythmia: two accidents and one death from pulmonary complications 3 months after initially successful cardiopulmonary resuscitation from ventricular fibrillation. Two of these patients had LPs. Even if these cases were included in the survival analysis, the presence or absence of LPs would not predict SCD (p < 0.78 on Cox regression), and the sensitivity would rise from 30% (3 of 10) to 38% (5 of 13) only.
Another limitation of our study is the lack of information about concomitant medication during long term follow-up, which is known to influence prognosis. The definition of an abnormal SAECG was prospectively defined [2]. Even when applying the criteria of previous studies by Simson [16], Kuchar and associates [5], or Gomes and colleagues [17], only 4 of 10 patients would have positive LPs, increasing the sensitivity to 40% (Table 3).
As we followed a low-risk population undergoing routine CABG, a low number of end points was reached which may lead to a type II error and could explain the discrepancy with other reports. Because most SCD cases occur in low-risk patients not identified by traditional risk factors, however, it was the aim of our study to assess the predictive value of SAECG in unselected patients undergoing revascularization at our institution.
In conclusion, the SAECG was not useful to predict prognosis in unselected patients undergoing CABG, and it does not appear to be helpful in a screening strategy for ICD implantation.
| References |
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