The electrocardiogram - Anesthesia Clinical management

The electrocardiogram

Intraoperative electrocardiography does not draw on the full power of this sophisticated monitor. Instead of 12 leads, we usually settle for just three or five leads. A little ditty helps with remembering where to put the leads:

White on right, red to ribs, and what is left over to the left shoulder.

With five leads, we add a brown lead for the V 5 position (over the fifth rib in the anterior axillary line) and a green lead that goes to the right side and serves as a ground.

The ECG leads can either be positive or negative, and the lead selector switch changes the polarity of the leads. Think of the negative lead as the exploring sensor.

·           Lead I looks across the chest:

White on right shoulder is negative

Black on left shoulder is positive

Red on ribs is ground.

·           Lead II looks along the axis of the heart:

White on right shoulder is negative

Black on left shoulder is ground

Red on ribs is positive.

With the five-lead ECG, lead V5 serves as the exploring, negative electrode overlying the left ventricle; the others (both shoulders and right side) become background.

Many ECG monitors for the operating room offer a “monitoring mode,” which is heavily filtered in order to reduce the distortions produced by artifacts induced by motion or electrical noise, e.g., the infamous electrocautery system. While the monitoring mode usually provides clean and stable tracings, the filtering can obscure diagnostic changes or it can mimic changes that will not be seen in a diagnostic 12-lead ECG. Thus, when detecting ST segment depression in the ECG in the monitoring mode, consider the clinical context and switch to diagnostic mode for confirmation before treating the patient. Similarly, when anesthetizing a patient at high risk for myocardial ischemia, use diagnostic mode at least intermittently to evaluate the ST-segment trends.

In the operating room, we are primarily concerned with rhythm and ST segment elevation (impending infarct?) or depression (ischemia?). The best leads to detect such changes are leads II and V5. Lead II shows the best P waves and thus enables us to observe the cardiac rhythm, such as the nodal rhythm frequently observed in the anesthetized patient. Cardiac output and arterial pressure fall a little when the ventricle is deprived of the “atrial kick.” Lead V5 looks at the left ventricle, the part of the myocardium most likely to suffer ischemia.

In healthy patients, the information about SpO₂, arterial pressure, and heart rate are more helpful than ECG data. The ECG earns its keep in patients with heart disease and in the rare event of a cardiac arrest and resuscitation. When premature ventricular contractions arise in a patient who did not have them before, we are alerted and begin to search for an explanation. Hypercarbia is a common culprit. Think of ventricular hypoxia when ST segments begin to change (>1.5 mm ST depression or elevation; most ominous is a downward sloping, depressed ST segment), T waves flip, and particularly when the rhythm switches to ventricular tachycardia.