Normally, voltage standardization is 1.0 mV per 10 mm, and paper speed is 25 mm/s (each horizontal small box = 0.04 s).
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.
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. Bradyarrhythmias and 123. Tachyarrhythmias.
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°).
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