Interpreting an EKG

aka…things to never miss on a 12 lead EKG

Ischemic Changes

Although non-specific ST changes are common following CPB and cardioplegic arrest, the presence of ischemic EKG changes should prompt evaluation for coronary insufficiency.  The occurrence of postoperative STEMI changes or VF is considered specific for coronary obstruction until proven otherwise, and urgent coronary angiography should be considered.

Examination of the ST segment

For each of the 12 leads, identify the baseline (a horizontal line through the TP interval).

Identify the J point, where the QRS transitions to the ST segment.  Examining QRS duration in other leads may help in challenging cases.

Measure the number of mV (standard EKG 1 small box = 0.1 mV) between the baseline and the J point: above baseline = ST elevation, below baseline = ST depression.

Examine for patterns of ST and T wave changes in leads that are contiguous (e.g. V1-V4, or II, III and avF – see next page).  

Examine the contour of the ST segment between the J point and T wave peak.  A convex or flat ST segment is more common in ischemia, and a concave ST morphology is more commonly non-ischemic (e.g. pericarditis or early repolarization, though this should be considered a hint rather than a negative diagnostic criterion).

Definition of STEMI

  • New ST elevation at the J point in two contiguous leads of >0.1 mV in all leads other than leads V2-V3
  • For leads V2-V3 the following cut points apply: ≥0.2 mV in men ≥40 years, ≥0.25 mV in men <40 years, or ≥0.15 mV in women
  • New or presumed new LBBB
  • Isolated posterior AMI
  • If EKG is at all concerning, serial EKGs allow for examination of typical progression of findings.

Localizing ischemic changes

Location ST elevation Affected vessel*
Septal V1, V2 LAD
Anterior V3, V4 LAD
Lateral V5, V6 Circumflex
Anteroseptal V1-V4 Proximal LAD
Anterolateral V3-V6 Circumflex or diagonal
Inferior II, III, avF RCA
High lateral I, avL Circumflex
Posterior Tall R wave and ST depression in V1-V2 Posterior descending

‘Reciprocal’ ST depression that occurs in territories remote from ST elevation increases the specificity of findings for AMI; anterior territory ST changes are often reciprocated in the inferior leads, and vice versa.

*Location of possibly affected vessel is generally true in adults with structurally normal hearts and is included here as a reference.  However, the location of ST changes and corresponding affected myocardial territory is likely to vary in congenital heart disease, particularly in patients with single coronary arteries and those with dextrocardia.

Progression of ST changes

Non-ischemic causes of ST elevation and clues 

Pericarditis.  Common in the days-weeks following heart surgery and a/w positional pain.  The ST segment elevation in this case is commonly present in all leads and the morphology of the ST segment is concave rather than convex.

Hyperkalemia and early repolarization may cause hyperacute T wave abnormalities.  However, evolution of such T wave changes to ST elevation is specific to AMI.

LVH and LBBB confounds analysis of ST segments.  Several rule sets exist that offer guidance in these circumstances.


AHA powerpoint re: 12 lead interpretation

University of Utah tutorial

Identifying STEMI on EKG review

AHA STEMI treatment guidelines