Ann Thorac Surg 2004;77:1293-1297
© 2004 The Society of Thoracic Surgeons
Original article: cardiovascular
High-frequency QRS potentials as a marker of myocardial dysfunction after cardiac surgery
Shonosuke Matsushita, MD, PhDa*,
Yuzuru Sakakibara, MD, PhDa,
Tomohiro Imazuru, MDa,
Mio Noma, MD, PhDa,
Yuji Hiramatsu, MD, PhDa,
Osamu Shigeta, MD, PhDa,
Tomoaki Jikuya, MD, PhDa,
Toshio Mitsui, MD, PhDa
a Department of Surgery, University of Tsukuba, Institute of Clinical Medicine, Tsukuba, Japan
Accepted for publication September 11, 2003.
* Address reprint requests to Dr Matsushita, Department of Surgery, Institute of Clinical Medicine, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8575, Japan
e-mail: shomatsu{at}md.tsukuba.ac.jp
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Abstract
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BACKGROUND: High-frequency QRS potentials are sensitive to myocardial ischemia. The aim of this study was to evaluate the usefulness of high-frequency QRS potentials as a marker of myocardial dysfunction after cardiac surgery.
METHODS: Seventy patients undergoing coronary artery bypass grafting or heart valve surgery were involved. High-frequency QRS potentials were measured by signal-averaged electrocardiogram, and calculated as the root-mean-square voltage of the total QRS duration (RMST). The postoperative RMST was expressed as a percentage of the preoperative RMST. The mean RMST at 1 to 2 hours after removing the aortic cross-clamp was compared with the cardiac index, inotropic agents, and aortic cross-clamping time. The occurrence of ventricular tachycardia within 24 hours and the RMST at 2 postoperative days were also evaluated. Patients were divided into quartile groups from highest to lowest at postoperative RMST (groups 1, 2, 3, and 4, respectively, from maximum to minimum).
RESULTS: In postoperative states, cardiac index significantly decreased in accordance with the RMST decrease in a stepwise manner, although there were no differences in cardiac index among the four groups preoperatively. Inotropic agents and aortic cross-clamping time increased as RMST decreased. A high rate of ventricular tachycardia within 24 hours and delayed RMST recovery at 2 postoperative days were seen in group 4. The curve of sensitivity and specificity showed that severe reduction (threshold, 35%) of RMST indicated low-output syndrome.
CONCLUSIONS: The severe reduction of filtered high-frequency QRS potentials was related to myocardial dysfunction. Measurement of filtered high-frequency QRS potentials could become a useful, noninvasive, real-time monitor of myocardial dysfunction after surgery.
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Introduction
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Numerous methods that focus on changes in a patient's hemodynamic condition and on the detection of ongoing myocardial ischemia have been evaluated for perioperative monitoring of cardiac surgery [1]. Continuous monitoring of cardiac output by Swan-Ganz catheter and electrocardiogram are the most conventional methods. In addition, transesophageal echocardiography can be used to assess perioperative cardiac function in critically ill patients [2]. Signal-averaged electrocardiogram (SAECG) has been established as a tool for detecting ventricular late potentials, which are applied for the prediction of ventricular tachycardia [3]. There have been reports that the high-frequency QRS potentials obtained from SAECG have also been useful for the detection of acute myocardial ischemia in patients undergoing percutaneous transluminal coronary angioplasty [4, 5]. In this study, we investigated SAECG as a monitor of postoperative patient care in cardiac surgery.
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Patients and methods
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Patients and measurements
Measurements of SAECG were taken in nonsequential patients undergoing open heart surgery during a period of 5 years. Seventy adult patients (mean age, 57 ± 11 years) were used as subjects in this study. All patients gave informed consent based on the Helsinki Declaration regarding ethical principles of medical research involving human subjects. The types of cardiac surgery involved were coronary artery bypass grafting and prosthetic valve replacement. Data on the high-frequency QRS potentials (root-mean-square voltage of the total QRS duration: RMST) are expressed as a percentage of preoperative values. Patients were divided into four groups (quartiles) from highest to lowest according to values of RMST. The RMST values were derived from the average RMST from 1 to 2 hours after removing the aortic cross-clamp. Group 1 was defined as the first upper quartile. Group 2 was defined as the second, group 3, the third, and group 4 was the fourth upper quartile. In our cardioplegia protocol, 1,000 mL of crystalloid cardioplegic solution (sodium, 147 mmol/L; potassium, 20 mmol/L) was infused as an initial dose followed by 500 mL every 30 minutes.
Signal-averaged electrocardiogram
An SAECG system (model ART-101; Arrhythmia Research Technology, Fitchburg, MA) was used for the analyses in this study. The surface electrodes were placed according to the three orthogonal directions, and midsternal incisions were avoided (Fig 1).
Bipolar orthogonal x, y, and z leads were recorded, and each signal-averaged potential was filtered with the use of a bidirectional Butterworth filter with cutoff frequencies of 40 Hz and 250 Hz. The three filtered signals were then combined into one vector magnitude defined as (x2 + y2 + z2)/2. The onsets and offsets of the high-frequency QRS potentials were determined automatically. Among several variables collected from the SAECG, we used the filtered QRS potentials of the total QRS duration (RMST) as a marker of high-frequency QRS potentials in accordance with previous reports [4–8].
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Fig 1. Detection of filtered QRS potentials. Bipolar surface electrodes are placed to obtain orthogonal x, y, and z axis potentials. Each signal is filtered at a bandpass with 40 to 250Hz. Then, the x, y, and z vectors are combined into one vector magnitude in a root-mean-square value. The filtered root-mean-square voltage of the total QRS duration (RMST) reflects the average amplitude of the filtered QRS signals. The RMST was used as a variable for high-frequency QRS signal in this study.
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Hemodynamic and other clinical variables
The mean cardiac index (liters per minute per square meter) at 1 to 2 hours after removing the aortic cross-clamp was determined as the hemodynamic variable. The cardiac index was measured using the thermodilution method with a Swan-Ganz catheter (Edwards Lifesciences, Irvine, CA). The aortic cross-clamping time was also evaluated. As with the other cardiac function–related factors, the maximum dose of inotropic agents at 1 to 2 hours after removing the aortic cross-clamp was evaluated. The occurrence of ventricular tachycardia (at least three sequential ventricular ectopic beats exceeding 100 beats/min) within 24 hours after removing the aortic cross-clamp was recorded. As preoperative variables, ejection fraction and cardiac index were evaluated by echocardiography and Swan-Ganz catheter, respectively. Age and types of surgery were also evaluated.
Statistics
Data are presented as mean ± standard deviation. Statistical differences among the groups were compared using analysis of variance with Bonferroni correction for multiple comparisons. The 
2 test and correlation analysis were also used for evaluation. A p value less than 0.05 was considered statistically significant.
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Results
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The mean RMST (expressed as a percentage of preoperative measurement) was 67% (range, 13% to 150%) at 1 to 2 hours after removal of the aortic cross-clamp. According to the quartile classification, preoperative and postoperative data were compared in each group (Table 1).
There were no differences among the four groups in preoperative cardiac index and ejection fraction. The absolute values of RMST were significantly higher in group 4 than in the other groups. Group 4 was significantly higher than group 1 in age. In postoperative states, cardiac index was significantly decreased according to the RMST decrease (Fig 2).
There was a significant correlation between them (r2 = 0.537, p < 0.001; Fig 3).
The curves of sensitivity and specificity of RMST to low-output syndrome (<2.20 L · min−1 · m−2) are shown in Figure 4.
The series of sensitivity and specificity of RMST to low-output syndrome is shown in Table 2. The threshold of RMST to low-output syndrome was 35% as determined by the highest concordance rate (
statistic). Aortic cross-clamping time tended to increase as RMST decreased. The doses of inotropic agents (dopamine or dobutamine and norepinephrine) tended to increase as RMST decreased. The correlation of RMST and the dose of dopamine or dobutamine was significant (r2 = −0.429, p < 0.001). The occurrence of ventricular tachycardia was higher in group 4 than in group 1. The RMST at the second postoperative day showed a significantly lower RMST in group 4 than in other groups. An example of an actual filtered QRS configuration is shown in Figure 5.
The RMST decreased from 55.9 µV to 11.1 µV (20%) because of inadequate myocardial protection during surgery (Fig 5).
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Table 1. Preoperative and Postoperative Variables According to Root-Mean-Square Voltage of Total QRS Duration Quartiles at 1 to 2 Hours After Removal of Cross-Aortic Clamp
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Fig 2. Changes in the cardiac index in the root-mean-square voltage of the total QRS duration quartile groups. Patients were divided into four quartile groups according to the root-mean-square voltage of the total QRS duration at 1 to 2 hours after removal of the aortic cross-clamp. There was a stepwise decrease in cardiac index according to the root-mean-square voltage of the total QRS duration reduction.
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Fig 3. Correlation between root-mean-square voltage of the total QRS duration (RMST) and the cardiac index. The root-mean-square voltage of the total QRS duration and the cardiac index were significantly correlated with r2 = 0.537, p < 0.001.
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Fig 4. The curve for sensitivity ( ) and specificity (<) of root-mean-square voltage of the total QRS duration (RMST) to cardiac index. Severe reduction of root-mean-square voltage of the total QRS duration increased the sensitivity to low-output syndrome.
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Fig 5. A change of the actual root-mean-square (RMS) configuration from a patient in group 4 who underwent mitral valve replacement. The root-mean-square voltage of the total QRS duration (RMST) changed from (left) 55.9 µV to (right) 11.1 µV (20%).
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Comment
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High-frequency potentials were obtained by filtering QRS signals. Simson [3] reported a new method of obtaining high-frequency QRS potentials in 1981. The QRS signals were averaged and processed by a bidirectional digital filter that allows low-amplitude signals to be detected. With this method, ventricular late potentials were measured and evaluated for the prediction of ventricular tachyarrhythmias. In the present study we applied this method to monitoring cardiac function after cardiac surgery. Because the value of absolute high-frequency RMST (microvolts) differs in each patient, it has been recommended to use a ratio (postoperative RMST-to-preoperative RMST) for evaluating the changes of RMST in interindividual variation [5].
Apart from tachyarrhythmias, high-frequency QRS potentials have been investigated for detection of myocardial ischemia in an animal model [6] and in patients undergoing percutaneous transluminal coronary angioplasty [4, 5]. In the animal experiment high-frequency QRS potentials decreased at the site of the epicardial electrodes in the region of the myocardial ischemia [6]. In the patients undergoing percutaneous transluminal coronary angioplasty, the high-frequency QRS potentials were superior to conventional ST changes in helping to detect ischemia. The sensitivity for detecting acute coronary artery occlusion was 88% using the high-frequency QRS potentials, whereas the rates for both ST elevation and depression were 71% [5].
In the present study, we found that the reduction of high-frequency QRS potentials (RMST) was related to cardiac dysfunction after cardiac surgery. Cardiac index showed a stepwise reduction as RMST decreased (Fig 2). From the investigation of sensitivity and specificity, severe reduction of RMST (<35%) indicated low-output syndrome. The dose of inotropic agents increased as RMST decreased. The occurrence of ventricular tachycardia was higher in patients with reduced RMST (group 4), and the ischemic period represented as the aortic cross-clamping time was longer in the patients with lower RMST. As for preoperative variables, age and absolute RMST values were higher in group 4 than in other groups. These preoperative factors could contribute in part to postoperative cardiac dysfunction and may affect the reduction of postoperative RMST. The reduction of RMST in group 4 persisted to at least the second postoperative day, showing a slow recovery of RMST.
The pathophysiologic background underlying the decrease of the high-frequency QRS potentials has been incompletely understood. However, one possible explanation for the decrease in high-frequency QRS potentials during acute ischemia is the slowing of conduction velocity in the region of the ischemia [7]. The slowing of conduction velocity has been reported to bring about a shift from a high-frequency activity to a lower frequency in a QRS complex [7]. A possible common factor between both the slowing of conduction velocity and the deterioration of cardiac function is elevation of the intracellular calcium concentration. An excessive concentration of intracellular calcium induces closure of the gap junction, which causes delayed conduction [9]. From this we speculate that the degree of reduction in high-frequency QRS potentials measured by SAECG could indicate myocardial dysfunction after cardiac surgery.
There have been other papers regarding coronary artery bypass grafting and SAECG. Scharf and colleagues [10] reported no relationship between preoperative SAECG variables and long-term sudden death. Cook and associates [11] demonstrated that preoperative abnormal SAECG in patients with low ejection fraction (<0.36) showed no recovery of ejection fraction at the time of discharge. Terada and coworkers [12] indicated that preoperative late potentials were eliminated after successful revascularization at 1 week after coronary artery bypass grafting. Postoperative measurements of SAECG in these studies were conducted at least 1 week after surgery with the intention of avoiding perioperative ischemic insults. In contrast, our study aimed to estimate cardiac dysfunction immediately after ischemic insults during cardioplegic arrest using high-frequency potentials of SAECG.
Signal-averaged electrocardiogram has some advantages over conventional methods of cardiac monitoring for cardiac surgery. First, it is a noninvasive method with just a few surface electrodes being enough to obtain sufficient information for analysis. Second, it has a near real-time nature. Although 1 or 2 minutes are needed to average the QRS configuration, the RMST calculation can be performed and displayed instantly. The significance of RMST is that it makes it possible to evaluate the cardiac function through electrocardiography, which is essentially electrical information. In this respect, it can be seen that SAECG is a useful means to monitor cardiac function after cardiac surgery.
In conclusion, we suggest that the severe reduction of high-frequency QRS potentials may become a marker of myocardial dysfunction after cardiac surgery. Signal-averaged electrocardiogram is a convenient, noninvasive, real-time method to obtain the high-frequency QRS potentials and can be applicable to perioperative monitoring in cardiac surgery.
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Acknowledgments
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The authors thank Avi Landau for assistance with the language.
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References
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Abstract
Introduction
