- ST segment: interval b/t end of QRS complex (J point, ST junction) & beginning of T wave
- Junction (J) point = where QRS complex ends & ST segment begins
- Cardiac events: electrically neutral period between ventricular depolarization (QRS complex) & repolarization (T wave); myocardium maintains contraction in order to expel blood from ventricles
- Normal location: at level of baseline (isoelectric); in general, normal variation of up to 1 mm from baseline in limb leads & 3 mm in right precordial leads (V1-V2)
ST Segment in Limb leads
- Isoelectric in ~75% normal adults 
- ST elevation or ST depression up to 0.1 mV (1 mm) generally considered within normal limits 
- ST elevation more common & often present in inferior leads (II, III, aVF)*
- ST depreesion rarely in I, II, aVF*
- *Because ST vector in frontal plane (if present) is often directed leftward & inferiorly
ST Segment in Precordial leads (V1-V6)
- ST elevation:
- >90% normal adults 
- Magnitude often proportional to QRS amplitude
- Greater in men than women (esp. in young individuals) 
- >0.2 mV rare if >40 y/o
- Body surface maps with 150 torso electrodes => maximum ST elevation average at 40 ms into ST segment = 0.2 mV (nearly 2x ST segment voltage sensed by 6-standard precordial electrodes) 
- V2-V3: most prominent (up to 0.3 mV or more)
- V5-V6: rarely >0.1 mV
- ST depression: always abnormal
- Because ST vector in horizontal plane is directed anteriorly & leftward
ST Segment in Posterior leads (V7-V9)
- 0.5-1.0 mm ST elevation at 80 ms after J point
- Prevalence in normal young men: V7 = 8.9%, V8 = 5.8%, V9 = 3.1% 
1. Hiss RG, Lamb LE, Allen MF. Electrocardiographic findings in 67,375 asymptomatic patients. Am J Cardiol 6:200, 1960.
2. Kossman CE. The normal electrocardiogram. Circulation 8:920, 1953.
3. Green LS, Lux RL, Haws CW, et al. Effect of age, sex, and body habitus on QRS and ST-T potential maps of 1100 normal subjects. Circulation 71:244, 1985.
4. Mirvis DM. Evaluation of normal variations in S-T segment patterns by body surface isopotential mapping: S-T segment elevation in the absence of heart disease. Am J Cardiol 50:122, 1982.
5. Chia B-L, Tan H-C, Yip JWL. Electrocardiographic patterns in posterior chest leads (V7, V8, V9) in normal subjects. Am J Cardiol 85:911, 2000.