Chapter 111: Electrocardiography

STANDARD APPROACH TO THE ECG

Normally, voltage standardization is 1.0 mV per 10 mm, and paper speed is 25 mm/s (each horizontal small box = 0.04 s).

Heart Rate

Beats/min = 300 divided by the number of large boxes (each 5 mm apart) between consecutive QRS complexes. For faster heart rates, divide 1500 by number of small boxes (1 mm apart) between each QRS.

Rhythm

Sinus rhythm is present if every P wave is followed by a QRS, PR interval ≥0.12 s, every QRS is preceded by a P wave, and the P wave is upright in leads I, II, and III. Arrhythmias are discussed in Chaps. 122 and 123.

Mean Axis

If QRS is primarily positive in limb leads I and II, then axis is normal. Otherwise, find limb lead in which QRS is most isoelectric (R = S). The mean axis is perpendicular to that lead (Fig. 111-1). If the QRS complex is positive in that perpendicular lead, then mean axis is in the direction of that lead; if negative, then mean axis points directly away from that lead.

Left-axis deviation (more negative than –30°) occurs in diffuse left ventricular disease, inferior MI, and in left anterior hemiblock (small R, deep S in leads II, III, and aVF).

Right-axis deviation (>90°) occurs in right ventricular hypertrophy (R > S in V₁) and left posterior hemiblock (small Q and tall R in leads II, III, and aVF). Mild right-axis deviation is common in thin, healthy individuals (up to 110°).

INTERVALS (NORMAL VALUES IN PARENTHESES)

PR (0.12–0.20 s)

• Short: (1) preexcitation syndrome (look for slurred QRS upstroke due to “delta” wave), (2) nodal rhythm (inverted P in aVF).

• Long: first-degree atrioventricular (AV) block (Chap. 122).

QRS (0.06–0.10 s)

Widened: (1) ventricular premature beats, (2) bundle branch blocks: right (RsR′ in V₁, deep S in V₆) and left (RR′ in V₆ [Fig. 111-2]), (3) toxic levels of certain drugs (e.g., flecainide, propafenone, quinidine), (4) severe hypokalemia.