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How should this patient be monitored intraopera-tively?
The most important modality for monitoring this patient intraoperatively is a multiple-lead electrocardio-gram (ECG) system. Up to 89% of the ECG changes of myocardial ischemia that are present on a standard 12-lead ECG will be detected by a V5 precordial ECG lead alone. Since the late 1970s, it has been recommended that limb lead II and precordial lead V5 be monitored simultaneously for the detection of intraoperative myocardial ischemia. This combination should enable >90% of ischemic episodes to be detected. In addition, this combination also monitors the distribution of both the right and left coro-nary arteries.
Operating room ECG systems nowadays are usually capa-ble of continuous ST-segment monitoring. Generally, these determine the relationship of the ST-segment 60–80 msec after the J-point (junction between the QRS complex and the ST-segment) to the baseline (during the P-Q interval). Ischemia may be defined as >0.1 mV of horizontal or downsloping ST-segment depression or >0.2 mV of ST-segment elevation. These systems are rendered less effective by left ventricular hypertrophy and frequent electro-cautery, and are not useful in left bundle branch block or ventricular pacing.
If only a three-lead ECG system is available it is still possible to intermittently monitor both the inferior (lead II) and the lateral (V5) walls of the heart. The left arm lead is placed over the precordial V5 position and the other leads are placed in their usual positions: the right shoulder and left leg. The modified V5 lead is monitored by setting the ECG device to lead I. The monitor will display a modified V5 lead known as the CS5 (chest-shoulder 5). If the monitor is inter-mittently switched to lead II, the true lead II will be seen on the monitor. Thus, it is possible to intermittently use a multiple-lead ECG system even with a three-lead ECG system.
Transesophageal echocardiography (TEE), if available, is an extremely sensitive method of detecting myocardial ischemia. This is done by continuously imaging the trans-gastric short-axis view of the left ventricle. This images the distributions of the three major coronary vessels. The disadvantages are that it is difficult to pay continuous attention to the echo image and that changes in regional wall motion may not be specific for myocardial ischemia even if they are highly sensitive. Additionally, the cost of the equipment and need for specialized training are limiting factors in the use of TEE.
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