Asian Cardiovasc Thorac Ann 2006;14:294-299
© 2006 Asia Publishing EXchange Ltd
Electrocardiographic Changes after Coronary Artery Surgery
Kerim Cagli, MD,
Cemal Ozbakir, MD1,
Kumral Ergun, MD1,
Vedat Bakuy, MD,
Renda Circi, MD,
Pinar Circi, MD
Cardiovascular Surgery Clinic
1 Cardiology Clinic, Turkiye Yuksek Ihtisas Hospital, Ankara, Turkey
For reprint information contact: Vedat Bakuy, MD Tel: 90 532 626 5104 Fax: 90 312 291 2703 Email: vedatbakuy{at}yahoo.com, Abay Kunanbay Cad. No: 36 D: 2, 06700 Kavaklidere, Ankara, Turkey.
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ABSTRACT
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The electrocardiographic changes early after uncomplicated coronary artery bypass with complete revascularization were examined preoperatively and on the 1st and 3rd postoperative days in 53 patients. Heart rate, PR index, corrected PR interval, corrected P dispersion, corrected duration of QRS complex, corrected QT dispersion, corrected QT interval, rhythm, QRS axis, ST-segment changes, and blocks were determined. Changes in new parameters obtained by different combinations of R, S, and T waves were also studied. On the 1st postoperative day, atrial fibrillation was significantly less prevalent, right bundle branch block increased significantly, and QRS axis was significantly more positive but returned to baseline on the 3rd postoperative day. Postoperative heart rate and PR index were significantly higher than preoperative values. In the postoperative period, corrected PR interval was significantly lower, corrected QRS complex duration was significantly shorter, corrected QT interval was significantly longer, and corrected QT dispersion showed a significant increase on the 1st postoperative day. This study defines electrocardiographic changes in uncomplicated patients with complete revascularization. Any deviations from these findings may alert us to the need for further evaluation of an undesired event.
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INTRODUCTION
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In addition to underlying coronary artery disease, the nonphysiological procedures performed in coronary artery bypass grafting (CABG), including cardiopulmonary bypass (CPB), crossclamping, hypothermia, hemodilution, cardioplegia, as well as postoperative complications (hemorrhage, graft occlusion, arrhythmias) have detrimental effects on the electrical and mechanical activity of the heart.1 However, restoration of flow in a significantly stenosed or occluded coronary artery has a beneficial effect.2 The combination of these factors determines the postoperative clinical course and prognosis.3 Methods are needed to identify whether recovery from disease and operation-related undesirable effects is achieved during the postoperative period. An electrocardiogram (EKG) is the first choice of test in many cardiovascular diseases; however, interpretation of a postoperative EKG with differentiation of normal and abnormal findings is not easy.4,5 In addition, several postoperative factors including blood pressure alterations, electrolyte disturbances, degree of ischemia, tissue oxygenation, use of antiarrhythmic and postanesthetic drugs, pain control, and duration of surgery make the analysis of postoperative EKG changes more difficult. As identification of the EKG changes expected to occur after uncomplicated CABG with complete revascularization may help to us to understand which changes are abnormal, we compared the preoperative and postoperative surface EKG of patients who underwent uncomplicated CABG with complete revascularization. In addition to conventional analysis, we included new EKG parameters derived from R-, S-, and T-wave amplitudes that may indicate recovery from CPB along with the benefits of revascularization.
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PATIENTS AND METHODS
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Sixty-one consecutive patients who underwent elective on-pump CABG between January 2001 and January 2002 were enrolled in the study. Patients with left ventricular (LV) aneurysm in whom interpretation of the EKG is difficult, those with LV systolic dysfunction (LV end-diastolic pressure > 18 mm Hg or LV performance score 
12) who might need postoperative support, and those with a history of myocardial infarction (MI) within the previous 3 months were excluded from the study. A permanent pacemaker, chronic renal failure, collagen disorders, malignancies, or metabolic diseases other than diabetes mellitus, and use of medical or mechanical support after surgery were also exclusion criteria. All study subjects gave their written informed consent, and the study was approved by the Medical Ethics Committee of our institution. Two men and 3 women in whom complete revascularization could not be achieved were excluded. Three patients who needed postoperative inotropic support were also excluded. The other 53 patients were evaluated.
During surgery, the same anesthetic protocol including intravenous fentanyl administration was applied in each patient. Anticoagulation was achieved with heparin. Hemodynamic stabilization was maintained throughout surgery (mean arterial pressure > 70 mm Hg, heart rate 60–100 beats·min–1). Postoperative pain control was with nonsteroidal antiinflammatory agents or morphine, if needed. Intravenous amiodarone infusion was only used in patients with new-onset postoperative atrial fibrillation (AF) for therapeutic reasons. No medication was used prophylactically to prevent postoperative AF. A standard 12-lead surface EKG of each patient was obtained preoperatively and on the 1st and 3rd postoperative days. Each EKG was recorded at a speed of 50 mm·sec–1. All measurements were performed by the same cardiologist blinded as to when the EKG was obtained.
Age, sex, obesity, family history of coronary artery disease, smoking status, hypertension, diabetes mellitus, LV end-diastolic pressure and performance score, number of grafts, crossclamp and CPB times were recorded. Electrocardiographic measurements included heart rate, PR interval, P dispersion, duration of QRS complex, QT interval, and QT dispersion, which were performed manually using a ruler. QT intervals were measured by two different methods: in the conventional method, it was measured from the start of the Q wave to the end of the T wave, and these results were used to calculate the QT dispersion; additionally, it was measured from the start of the Q wave to the start of the T wave to represent only ventricular depolarization, and this parameter was termed the QT interval. To calculate PR index, the PR interval was multiplied by heart rate. PR interval, P dispersion, QRS interval, QT interval, and QT dispersion were corrected for heart rate using Bazett’s Formula (Corrected value = observed value/
[R-R']).6 The sum of the V1–V6 R-wave amplitudes, V1–V6 S-wave amplitudes, and the ratio of V1–V6 R to S for the widespread anterior wall were calculated. The sum and ratio of R- and S-wave amplitudes were also calculated for left-sided (D1, D2, aVF, aVL) and right-sided (D3, aVR) extremity derivations. The most accurate method based on estimation of the QRS area in each of the limb leads and a plot of these as vectors on the respective lead axis of a triaxial reference system was used for determination of the mean QRS axis in the frontal plane. Axis measurements were performed and repeated only for patients with a normal axis. Patients with bundle branch block were excluded from axis measurements. A QRS axis lying between –30° and +90° was accepted as normal.7 The presence of any type of block and ST-segment depression or elevation less than 1 mm was also noted in each patient.
The data are presented as mean ± standard deviation. For parametric data, repeated sample variance analysis was used. The paired t test was used to test the time periods for significant changes. Due to the time-variant nature of variables obtained by counting, the McNemar test was used. For non-normally distributed parameters, the Friedman test, which is the nonparametric equivalent of the repeated sample variance analysis, was used. When there was a difference, the Wilcoxon test, which is the nonparametric alternative to the paired t test, was used to detect between which periods the difference existed. A p-value of less than 0.05 was considered significant. All statistical analyses were performed using SPSS version 11.0 software (SPSS, Inc., Chicago, IL, USA).
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RESULTS
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Demographic characteristics of the 53 patients are shown in Table 1
. Their ages ranged from 34 to 78 years. Perioperative findings are presented in Table 2. A left internal thoracic artery graft was anastomosed to the left anterior descending artery in 51 (96%) patients. Grafts other than the left internal thoracic artery and saphenous vein were not used in any patient. All patients showed an uneventful postoperative course without severe respiratory failure, severe electrolyte disturbance, perioperative MI, or hemodynamic instability. Comparison of the EKG parameters in the 3 different periods are shown in Tables 3, 4, and 5. On the 3rd postoperative day, any new onset of AF was successfully converted to sinus rhythm by amiodarone. The number of patients with ST-segment elevation did not differ significantly throughout the observation period (Table 3). Postoperative plasma creatine kinase elevation of more than 2 times normal did not occur in any patient. Comparison of new parameters composed of summations and ratios of R-, S-, and T-wave amplitudes in different combinations are shown in Table 5.
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Table 5. Comparison of New Parameters Obtained from Combination of R, S and T Wave Amplitudes in Different Derivations
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DISCUSSION
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Analysis and comparison of the preoperative and early postoperative EKG with conventional and new parameters showed that many of them changed significantly after surgery. As uncomplicated patients with complete revascularization represent a group negatively affected by surgical procedures but recovering from disease in a positive way, EKG changes should be accepted as ‘expected findings’ of these combined processes. While operative procedures may be predominantly responsible for the changes occurring on the 1st postoperative day, changes on day 3 may result from relief of ischemia. In this study, postoperative AF was found to be lower than the preoperative incidence and that of previous reports.8 This may be partly due to the complete relief of ischemia, shorter operating time, uncomplicated postoperative course without severe respiratory failure or electrolyte imbalance, perioperative MI, or hemodynamic instability, or to a low-risk patient profile for postoperative AF.8 However, because only one EKG was evaluated on each day, intermittent AF attacks could not be included in the analysis. Therefore, these data cannot define the real incidence of postoperative AF.
Although left bundle branch block did not change, right bundle branch block showed a significant increase in the postoperative period. This may be explained by the longer refractory period of the right bundle branch, resulting in a rate-dependent functional block. These blocks did not cause any significant hemodynamic problems. Previously, Klein and colleagues5 reported that increased plasma creatine kinase and MB isoenzyme levels and conduction defects did not relate to MI in the postoperative period. As our patient population was free of all postoperative complications, including enzyme elevation, we suggest that these blocks are innocent. The QRS axis became more positive on postoperative day 1, but it remained within the normal range throughout the study period. Possible explanations for these axis deviations are opening of the pericardium, resulting in a change of position of the heart in the mediastinum, enlargement of the heart due to CPB-related myocardial edema, and reperfusion of hibernating areas of myocardium. As CPB-related myocardial edema partially regresses on postoperative day 3, return of the axis to the preoperative value on day 3 may reflect myocardial edema as a cause of axis deviation.
Despite improved intraoperative myocardial protection and surgical techniques, some degree of ischemia occurs during CABG, but only a minority of patients experience an actual perioperative MI. The diagnosis of MI after cardiac surgery is more difficult than at other times because of the nonspecific ST-T wave abnormalities and nearly universal elevation of cardiac enzymes.4 Although a combined approach and integrated diagnostic findings are needed for definite diagnosis, an EKG showing new and persistent Q waves accompanied by new, persistent, and evolutionary ST-T wave abnormalities is most helpful. In addition to reversible ischemia, the techniques involved in open heart surgery, including hypothermia and use of potassium-rich cardioplegic agents, electrolyte imbalances such as hypocalcemia, and axis shifts are all responsible not only for ST-T wave changes but also for changes in other parameters such as PR indices and cQT.1,9,10 Therefore, these EKG criteria are diagnostic only if they are seen on serial electrocardiograms. In this study, no patient met the criteria for diagnosis of perioperative MI. ST-segment depression < 1 mm was observed more commonly on postoperative days 1 and 3, but ST-segment elevation showed a more stable course. We suspect that ST-segment depression is more likely to develop as a nonspecific finding after CABG.
Heart rate showed a significant increase postoperatively. A parallel increment was also observed in PR index. We think that postoperative pain and catabolism, altered cardiac autonomic activity, CPB-related systemic inflammation, pericardial irritation, hemodilution, and postoperative fluid and electrolyte imbalances are possible mechanisms of this relative tachycardia.11,12 The PR interval shortened significantly on the 1st postoperative day as the heart rate increased. However, PR interval corrected for heart rate was observed to shorten significantly on postoperative day 1 as well. We assume this decrease may be related to improvements in atrial ischemia and the conduction system by revascularization. cPd, a marker of the heterogeneity of atrial depolarization, did not show a significant change throughout the study period.13 Although it decreased on the 3rd postoperative day it did not reach a statistically significant level. There is controversy surrounding the effect of open heart surgery on P-wave dispersion and the association of P-wave dispersion with postoperative AF.14,15 As continuous monitoring for AF was not performed, it is difficult to draw a conclusion about the relationship between P-wave dispersion and AF incidence; however, lack of a correlation between P-wave dispersion and AF makes this unlikely. cQRS produced by ventricular depolarization showed a significant decrease on the 3rd postoperative day; this means ventricular activation became faster. We assume that this results from relief of ischemia which causes conduction delays similar to peri-infarction block. cQTd increased significantly on the 1st postoperative day but decreased to the preoperative level on the 3rd day. We assume that the early increase was due to organic changes in the myocardium (e.g., edema) which had resolved by the 3rd postoperative day. A significant decrease in QTd at 6 months and 2 years after CABG has been reported.16 Our study showed that this decrement can occur as early as the 3rd postoperative day. As no ventricular tachycardia was observed in this patient population, the predictive value and clinical significance of this finding is questionable.
We used a modified QT interval from the start of the Q wave to the start of the T wave. Normally the QRS complex represents the ventricular activation time and the T wave represents ventricular repolarization. The QT interval represents the electrical systole of the ventricle. We planned to evaluate pure ventricular depolarization so we did not include the T wave-to-QT interval measurement. We also planned to observe the differences between the phases of depolarizing activity by examining the corrected duration of the QRS complex. The QRS complex represents phase 0 of the activation potential curve, and the ST segment represents phases 1–2.17 The results showed that phase 0, reflecting the sodium-dependent fast inward current, became faster, and phases 1–2, reflecting the entry of calcium into the cells, became slower after the operation. Without biochemical and molecular evaluation, speculation on these different findings is impossible.
Open heart surgery affects the amplitudes of R, S, and T waves in different ways. Decreases in QRS voltage because of large effusions and formation of a QS complex (pseudoinfarction pattern) are well-known examples.18 In this study, we compared different combinations of the R-, S-, and T-wave amplitudes. Although the sum of the S-wave amplitudes of V1–V6 did not change throughout the study period, the sum of the R-wave amplitudes of V1–V6 decreased on the 1st postoperative day ( p > 0.05) and returned to the baseline value on the 3rd day ( p < 0.05). The ratio of V1-V6 R waves to the V1–V6 S waves also did not change. In past reports, the QRS changes in V5 were found to be especially useful for evaluating recovery from CPB.19 We propose that the sum of the V1–V6 R waves may also be a marker of CPB-related detrimental effects. In addition, this increment may be partly dependent on resolution of the minimal postoperative pericardial effusion; echocardiography might be used for a prompt conclusion. The sum of the T waves on lateral derivations (V3–V6 T) decreased significantly on the 1st postoperative day and returned to the baseline value on the 3rd day. The ratio of V3–V6 R waves to V3–V6 T waves increased on the 1st postoperative day ( p < 0.05) and remained unchanged on the 3rd day.
Pinsky and colleagues20 showed that temporary changes in the R/T ratio represent LV preload alterations; permanent changes represent alterations in contractile function. In our study, the persistence of a high V3–V6 R/T value on the 3rd postoperative day may also represent improved contractility gained by grafting, and electrophysiologic recovery of the LV free wall. In extremity derivations, parallel changes with precordial derivations were observed. Both the sum of the R waves and the ratio of R to S waves in the left extremity derivations (D1, D2, aVF, aVL) were increased on the 3rd postoperative day; whereas the sum of the S waves in the same derivations decreased. Although the combination of right derivations (D3, aVR) showed the opposite numerical changes, we think that it was consistent with other findings because of the nature of aVR.
This study defines the expected EKG changes in a patient population with complete revascularization and without postoperative complications. These findings may be used as markers of the natural postoperative course, including beneficial effects of both revascularization and the recovery from CPB. Any deviation from these findings may be an early sign of an unfavorable event, and may be helpful in avoiding delay in further evaluation.
Presented at the 12th Annual Meeting of the Asian Society for Cardiovascular Surgery, Istanbul, Turkey, April 18–22, 2004.
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ABSTRACT
INTRODUCTION
