Electrocardiography (ECG) is a standard intensive care unit monitoring
modality both in the continuous and twelve lead formats. It is used
both for the detection of cardiac ischemia and arrhythmias.
- ▪ ECG waves (Figure 33-1)
- —P wave—initial electrocardiographic
deflection from the baseline in a cardiac contraction—corresponds
to the electrical activation of the atria following sinoatrial node
- —PR interval—corresponds to the period
of atrial conduction through the atrioventricular node
- —QRS complex—corresponds to electrical
activation of the ventricles
- —ST segment—segment connecting the QRS
complex to the T wave, and may indicate coronary ischemia (if depressed
below baseline) or myocardial infarction (if elevated above baseline)
- —T wave—corresponds to repolarization of
- ▪ ECG lead—used to mean both the electrical
wire connecting the patient to the ECG machine, and one of the axes
of cardiac electrical conduction commonly evaluated by ECG.
- —Limb leads show conduction between any two
of the three limbs comprising Einthoven’s triangle (right
arm, left arm, left leg) as shown in Figure 33-2. Green is traditionally
used as a ground lead.
- —Lead vector—shows the direction
of conduction between the negative and positive terminals of a given
lead as shown in 12-lead ECG in Figure 33-3
- ▪ Lead I shows the axis of conduction from the right
arm (white lead) to the left arm (black lead), with an upward deflection
indicating conduction from right to left.
- ▪ Lead II shows the axis of conduction from the right arm
toward the left leg (red lead).
- ▪ Lead III shows the axis of conduction from the left shoulder
toward the left leg.
- —Augmented limb leads show the axis of conduction
from an imaginary central location (comprised of the center point
between the three reference limbs) toward those limbs.
- ▪ aVR shows electrical conduction from the central lead
toward the right arm.
- ▪ aVL shows conduction from the central lead to the left
- ▪ aVF shows conduction from the central lead to the left
leg (or foot).
- • Precordial leads show conduction from the
imaginary central point toward various locations on the chest wall
- ▪ V1 is placed in the fourth intercostals space
on the right side of the sternum.
- ▪ V2 is placed in the fourth intercostals space
to the left of the sternum.
- ▪ V3 is placed midway between leads V2 and
- ▪ V4 is placed in the fifth intercostals space in
the midclavicular line.
- ▪ V5 is placed next to V4 but in the anterior
- ▪ V6 is placed next to V5 but in the midaxillary
- • Modified chest leads closely approximate
true precordial leads, so MCL1 (modified chest lead 1) approximates
- ▪ Ground lead—the green lead often placed by
convention of the right leg, but can be anywhere on the body and