Q
Q wave, Initial downward deflection of the QRS complex of the ECG (see Fig. 59b; Electrocardiography). Small (q) waves are normal in leads aVL and I when left axis deviation is present, and in leads II, III and aVF with right axis deviation. They may be large in aVR. Pathological (Q) waves are wide (e.g. > 30–40 ms) and deep (e.g. > 2–4 mm, or more than a quarter of the height of the R wave in the same lead). In the absence of left bundle branch block they suggest MI, which may be old or of new onset; in acute S-T elevation MI, Q waves are associated with poorer outcomes.
QRS complex, Represents ventricular depolarisation; normally follows the P wave of the ECG (see Fig. 59b; Electrocardiography). Upper-case letters are used if a particular wave is considered large, lower-case if small. The initial deflection is termed the q (Q) wave if downward, and R wave if upward. The downward S wave follows an R wave. The QRS complex may be used to calculate the electrical axis. Normally has rS pattern in V1, qR pattern in V6. The initial small deflection represents left-to-right septal depolarisation; the larger subsequent deflection represents (mainly left) ventricular depolarisation. Normal duration: < 0.12 s. Abnormalities may represent arrhythmias, heart block, bundle branch block or MI.
Q–T interval, Represents the duration of ventricular systole; varies with age, sex and heart rate. Measured from the beginning of the QRS complex to the end of the T wave of the ECG (see Fig. 59b; Electrocardiography). Corrected for heart rate by dividing by the square root of the preceding R–R interval in seconds (Bazett’s formula). Normal range is 0.35–0.43.
Shortened in hypercalcaemia, hyperkalaemia and digoxin therapy. Prolonged Q–T syndromes may be caused by hypocalcaemia, hypothyroidism and hypothermia, and are associated with recurrent syncope or sudden death due to ventricular arrhythmias, including VT and torsade de pointes.
[H Cuthbert Bazett (1885–1950), English-born US physiologist]
Q–Tc dispersion
