Dynamic QRS-complex and ST-segment monitoring by continuous vectorcardiography during carotid endarterectomy

S. Kawahito*, H. Kitahata, K. Tanaka, J. Nozaki and S. Oshita

Department of Anesthesiology, Tokushima University School of Medicine, 3-18-15 Kuramoto, Tokushima 770-8503, Japan

Corresponding author. E-mail: kawahito@clin.med.tokushima-u.ac.jp
{dagger}Presented in part at the 12th World Congress of Anesthesiologists, Montréal, Quebec, Canada, June 4–9, 2000 (Abstract P2.2.23).

Accepted for publication: September 17, 2002


    Abstract
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Background. Many authors report a high incidence of cardiac events during carotid endarterectomy. The aim of the present study was to evaluate the usefulness of dynamic continuous on-line vectorcardiography for monitoring the occurrence of myocardial ischaemia during carotid endarterectomy.

Methods. We studied 21 patients undergoing carotid endarterectomy. Patients underwent general anaesthesia with isoflurane or sevoflurane. The vectorcardiogram was monitored continuously during carotid endarterectomy. Electrodes were placed according to the previously described lead system and connected to a computerized system for on-line vectorcardiography. Two trend variables were recorded: the QRS vector difference, which reflects changes in the shape of the QRS complex; and the ST vector magnitude, which represents deflection of the ST segment from the isoelectric level. The ST segment deflection was measured 60 ms after termination of the QRS complex.

Results. Vectorcardiography was successfully recorded in all 21 patients. Three patients showed intraoperative vectorcardiogram abnormalities. In one of these three patients, both ST vector magnitude and QRS vector difference increased after induction of anaesthesia and ST vector magnitude returned to baseline after administration of nitroglycerin. In the other two patients, both ST vector magnitude and QRS vector difference gradually increased after cross-clamping of the internal carotid artery and ST vector magnitude returned to baseline after unclamping. QRS vector difference remained elevated for several hours in all three patients.

Conclusions. Monitoring ST vector magnitude and QRS vector difference by vectorcardiography may be useful for identifying myocardial ischaemia during carotid endarterectomy.

Br J Anaesth 2003; 90: 142–7

Keywords: heart, ischaemia; monitoring, vectorcardiography; surgery, carotid endarterectomy


    Introduction
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Carotid endarterectomy is a commonly performed vascular procedure. The efficacy of carotid endarterectomy for asymptomatic patients with atherosclerosis of the carotid bifurcation has been established in several studies.1 2 These studies also documented a high incidence of medical comorbidity in this population. Arteriosclerotic involvement of the extracranial cerebral arteries is known to occur in a significant number of patients with coronary artery disease. Stroke and myocardial infarction are the two main complications. In contrast to the decline in neurological morbidity and mortality rates subsequent to the development of several effective cerebral monitoring techniques, the incidence of cardiac complications has not declined.3 Many researchers still report a high incidence of myocardial ischaemia during and after carotid endarterectomy.19 To fully realize the benefits of carotid endarterectomy, complications must be kept to a minimum. Thus, accurate non-invasive detection of myocardial ischaemia would be very useful during carotid endarterectomy. The only method available currently for detection of myocardial ischaemia is continuous ST-segment trend monitoring of the electrocardiogram (ECG).10 11 Although transoesophageal echocardiography is most accurate for early detection of ischaemia,12 it is not applicable during carotid endarterectomy. Dynamic, continuous on-line vectorcardiography allows real-time assessment of ST-segment and QRS-complex changes. In this paper, we describe our initial experience with the system in patients undergoing carotid endarterectomy. The aim of the present study was to evaluate the usefulness of dynamic continuous on-line vectorcardiography for monitoring the occurrence of myocardial ischaemia during carotid endarterectomy.


    Methods
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
The study protocol was approved by the Ethics Committee on Human Study of Tokushima University School of Medicine, and written informed consent was obtained from all patients. Twenty-one patients (14 men and 7 women; mean age 66 yr, range 55–76 yr) undergoing carotid endarterectomy were included in this study. Patient characteristics are shown in Table 1. Anaesthesia was induced with i.v. fentanyl (2–6 µg kg–1) and thiamylal (3–4 mg kg–1) on inspired oxygen 100%. After induction of anaesthesia, tracheal intubation was facilitated with vecuronium bromide (0.15 mg kg–1). Anaesthesia was maintained with isoflurane or sevoflurane and nitrous oxide in oxygen. The same surgeon performed carotid endarterectomy in all patients, using a standard technique with a patch. For patients in whom an EEG abnormality was observed or stump pressure was <50 mm Hg on test clamping of the carotid artery, a shunt circuit was applied.


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Table 1 Patient characteristics
 
The vectorcardiogram was monitored continuously during carotid endarterectomy. The monitoring system (MIDA, Myocardial Ischaemia Dynamic Analysis, Hewlett Packard; Andover, MA, USA) consists of a microprocessor-controlled data acquisition module, an IBM-compatible personal computer, a vectorcardiogram monitor, and a laser printer. Vectorcardiography signals are based on an eight body-surface electrode model originally described by Frank13 (Fig. 1A). The sensitivity of this system is 1 µV, and the sampling rate of each lead is 500 samples s–1. ECG complexes are detected, acquired, and categorized according to their shape, into one of five classes. The most dominant beat type is automatically determined by computer during the first 10 s of recording and termed the zero class, but the operator is able to change this zero class to any other class detected. All acquired ECG signals, along with their beat classifications, are transferred through a high-speed communication line from the acquisition module to the computer for averaging and analysis.



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Fig 1 (A) Placement of the eight surface electrodes for vectorcardiography recording as described by Frank. Five electrodes (A, C, E, I, M) are located at the same transverse level (approximately the fifth interspace), H is on the back of the neck, and F is the standard left leg electrode. The eighth electrode is for the ground and usually placed on the right leg or hip. The signals from these electrodes are converted via an acquisition module into three orthogonal components projected in the three perpendicular axes x, y, and z, and then sent to a microcomputer system. (B) The reference complex is the average of the first 2 min of data recorded for each patient. The QRS vector difference is the shaded area between the two average complexes. The ST vector magnitude is the deviation of the ST-segment from the baseline, measured 60 ms after termination of the QRS complex.

 
On the vectorcardiogram, the complex distributions of surface potentials in the electrical field are reduced to three axes (x, y, and z) perpendicular to one another. The X, Y, and Z leads are computed from sampled unipolar Frank leads,13 continuously displayed on the colour monitor, and averaged to form mean vectorcardiography complexes. In the system used, averaging may be performed for consecutive periods, each 10 s to 4 min long. For this study, periods of 2 min were used for averaging. Only zero class beats were used for analysis. For each 2-min period, the resulting mean vectorcardiography complex was analysed and compared to the mean reference vectorcardiography complex, which was collected during the second averaging period. A set of several trend variables was calculated at the end of every averaging period, and two trend variables were closely studied: the QRS vector difference, which reflects changes in the shape of the QRS complex; and the ST vector magnitude, which represents the deflection of the ST segment from the isoelectric line. The ST-segment deflection was measured 60 ms after termination of the QRS complex. These variables are illustrated in Figure 1B. The computer automatically stores all mean complexes and trend curves on the hard disk and keeps them available for on-line review at any time during the recording period. The data can be transferred from the hard disk to a floppy disk and stored. In the last four patients studied, ST trend monitoring was used alongside vectorcardiography.


    Results
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Vectorcardiography was successfully recorded in all 21 patients. Three patients (patients 4, 9, and 13) showed intraoperative vectorcardiogram abnormalities. In one of these three (patient 4), both ST vector magnitude and QRS vector difference increased after induction of anaesthesia. ST vector magnitude returned to baseline after administration of nitroglycerin, but QRS vector difference remained elevated for >7 h. Figure 2 shows the QRS vector difference and ST vector magnitude trend curves during carotid endarterectomy for patient 4. This patient showed rapid QRS vector difference change (increase in QRS vector difference by more than 0.5 µV s min–1 to plateau) and ST vector magnitude change (decrease in ST vector magnitude by more than 20 µV min–1 to the baseline level and >50% recovery within 20 min), suggestive of reperfusion.



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Fig 2 Trend curves of QRS vector difference and ST vector magnitude during carotid endarterectomy for patient 4. With continuous dynamic monitoring of vectorcardiography, sudden changes in QRS vector difference and ST vector magnitude status after induction of anaesthesia can be seen.

 
In the other two patients (9 and 13), both ST vector magnitude and QRS vector difference gradually increased after cross-clamping of the internal carotid artery. ST vector magnitude returned to baseline after unclamping, but QRS vector difference remained elevated for a further 2 h. Figure 3 shows the trend curves for patient 9. This patient showed a gradual QRS vector difference change (increase in QRS vector difference by more than 0.25 µV s min–1 to plateau) and a gradual ST vector magnitude change (increase in ST vector magnitude by more than 3 µV min–1 to peak). For patients 18–21 inclusive, in whom ST trend monitoring was used in addition to vectorcardiography, no ischaemia was detected with either technique.



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Fig 3 Trend curves of QRS vector difference and ST vector magnitude during carotid endarterectomy for patient 9. This patient’s trend curves display gradual changes in QRS vector difference and ST vector magnitude after cross-clamping of the carotid artery.

 

    Discussion
 Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 
Several reports have demonstrated the potential utility of automated ECG ST-segment trending monitors for early detection of perioperative myocardial ischaemia.10 11 However, in comparison to Holter ECG recordings, ST trend monitors have only moderate sensitivity and specificity (<75% overall) for detecting ECG ST-segment changes,10 and they offer no information on the development of necrosis. Therefore, sole reliance on ST trending monitors for the detection of myocardial ischaemia may be insufficient. Ideally, this study should be repeated with the ECG ST-segment trending monitor in addition to the vectorcardiography in a larger series of patients. For the last four patients, although we used both monitoring systems and compared the incidence of ischaemic episodes, no ischaemic episodes were detected with either system. Transoesophageal echocardiography is also a very useful tool for early detection of ischaemia;12 however, it is not applicable during carotid endarterectomy.

Dynamic QRS-complex and ST-segment monitoring via continuous on-line vectorcardiography is a new method for monitoring patients with coronary disease. Use of continuous vectorcardiography monitoring was first described in 1974 by Hodges and colleagues.14 The system was not an on-line system, and results were not available until hours or even days later. Further progress towards on-line monitoring was made by Sederholm15 and Grøttum.16 The lead system for vectorcardiography registration used today is based on the system presented in 1956 by Ernest Frank.13 This method allows for the evaluation of dynamic vectorcardiography changes in patients with ongoing myocardial ischaemia. Dynamic vectorcardiography (i.e. continuous computerized vectorcardiography with full real-time capacity), has been used in studies of patients with acute myocardial infarction or unstable angina during angioplasty or surgery.1722 Because dynamic vectorcardiography calculates and presents all data in real time, it is uniquely suited for on-line monitoring of patients with acute myocardial ischaemia and ongoing infarction. Use of the Frank leads rather than the conventional 12 leads has been shown to be highly sensitive in the detection of myocardial ischaemia17 20 and may facilitate the detection of myocardial damage.1925 Data regarding the sensitivity and specificity of this method when applied intraoperatively, will be important. Gustafsson and colleagues23 used only vectorcardiography after 2 h of monitoring as discriminator for acute myocardial infarction and reported a sensitivity of 44% and a specificity of 90%. However, vectorcardiography and myoglobin monitoring in combination had a sensitivity of 100% and a specificity of 69%.

There are several reasons for the superiority of the continuous vectorcardiography technique. First, in contrast to the standard 12-lead ECG, continuous vectorcardiography is based on signal averaging, which allows better detection of small myocardial potentials. Second, the standard 12-lead ECG has no electrodes that reflect the posterolateral regions, which in part explains its lower sensitivity for ischaemia in these regions.2628 The Frank lead system used in continuous vectorcardiography may be more sensitive to ischaemia in these locations. Third, the standard 12-lead ECG, in contrast to continuous vectorcardiography (ST vector magnitude), does not immediately display directional changes. Hence, ischaemia resulting in predominantly directional ST-segment changes will not be detected easily with a 12-lead ECG. Thus, continuous vectorcardiography might be better suited to monitoring all infarct sites during carotid endarterectomy.

We selected two variables (QRS vector difference and ST vector magnitude) as indicators of myocardial ischaemia. Several studies show a strong relationship between the degree of ST vector change and the myocardium at risk.18 2022 24 Change in the QRS-complex is also the ECG expression of irreversible injury to the myocardium.18 19 2123 Thus, these two variables are essential and sufficient criteria. Findings from previous studies23 24 indicate that an ischaemic episode is suggested if there is an increase in QRS vector difference >15 µV s and/or an increase in ST vector magnitude >0.05 mV from the patient’s baseline curve.

Of the 21 patients in our study, three revealed vectorcardiogram abnormalities during carotid endarterectomy. In one of the three, both ST vector magnitude and QRS vector difference increased after the induction of anaesthesia and returned to baseline after administration of nitroglycerin. This patient was suffering from unstable angina and had undergone coronary artery bypass grafting 2 months previously. The myocardial ischaemia was associated with severe haemodynamic changes after induction of anaesthesia and tracheal intubation, but his postoperative course was uneventful. In the other two patients, both QRS vector difference and ST vector magnitude gradually increased after cross-clamping of the internal carotid artery. In all three patients, although ST vector magnitude returned to baseline after reperfusion, QRS vector difference remained elevated for several hours, indicating myocardial damage. These patients did not meet our criteria for the use of a shunt. However, we routinely use dopamine or dobutamine to maintain a high blood pressure during cross-clamping; dopamine (3–5 µg kg–1 min–1) was used for these two patients. The haemodynamic change after cross-clamping of the internal carotid artery and/or induced hypertension by dopamine may have caused the myocardial ischaemia.

Our clinical study was limited in several ways. The sample size is small and there was a low incidence of myocardial ischaemia. Testing in a large sample is needed. In addition, there is no gold standard for the determination of myocardial ischaemia. Biochemical markers such as myoglobin, troponins, and myocardial creatine kinase need to be studied in relation to vectorcardiogram findings. The vectorcardiogram system requires accuracy and a noise-free ECG signal. Even minimal noise from electromagnetic devices in the operating room may produce major distortion, the changes on vectorcardiography have not been correlated with either routine ST-segment monitoring or some other standard measurement techniques for ischaemia. However, we conclude that vectorcardiography monitoring of ST vector magnitude and QRS vector difference may be useful in identifying myocardial ischaemia during carotid endarterectomy.


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 Introduction
 Methods
 Results
 Discussion
 References
 
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