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 Presented in part at the 12th World Congress of Anesthesiologists, Montréal, Quebec, Canada, June 49, 2000 (Abstract P2.2.23).
Accepted for publication: September 17, 2002
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Abstract |
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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: 1427
Keywords: heart, ischaemia; monitoring, vectorcardiography; surgery, carotid endarterectomy
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Introduction |
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Methods |
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Results |
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Discussion |
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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 patients 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 (35 µg kg1 min1) 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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References |
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