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.