Chapter 33. Electrocardiography

Introduction

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.

Definitions and Terms

• ▪  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 depolarization
  • —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 the ventricles
• ▪  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
    • • Limb leads
• ▪  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 arm.
• ▪  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 (Figure 33-4).
• ▪  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.
• ▪  V4 is placed in the fifth intercostals space in the midclavicular line.
• ▪  V5 is placed next to V4 but in the anterior axillary line.
• ▪  V6 is placed next to V5 but in the midaxillary line.
  • • Modified chest leads closely approximate true precordial leads, so MCL1 (modified chest lead 1) approximates V1 etc.
• ▪  Ground lead—the green lead often placed by convention of the right leg, but can be anywhere on the body ...