Systematic ECG Interpretation
The 12-lead ECG is one of the most important diagnostic tools in paramedicine. The NREMT Paramedic exam heavily tests ECG interpretation, including rhythm recognition, STEMI identification, axis determination, and clinical correlation. Developing a systematic approach ensures you do not miss critical findings.
The Five-Step Approach
Step 1: Rate
Calculate the ventricular rate. For regular rhythms, count the number of large boxes between two consecutive R waves and divide into 300 (300 / number of large boxes = rate). For irregular rhythms, count the number of QRS complexes in a 6-second strip and multiply by 10.
Step 2: Rhythm
Assess regularity by comparing R-R intervals. Regular rhythms have consistent spacing. Regularly irregular rhythms have a repeating pattern. Irregularly irregular rhythms (like atrial fibrillation) have no pattern.
Step 3: P Waves
Look for P waves before each QRS complex. Normal P waves are upright in leads I, II, and aVF. One P wave before each QRS with consistent PR intervals indicates normal sinus conduction. Multiple P waves before a QRS suggest heart block. No identifiable P waves suggest atrial fibrillation or a junctional rhythm.
Step 4: Intervals
PR interval: 0.12 to 0.20 seconds (3-5 small boxes). Prolonged PR indicates first-degree AV block. Progressively lengthening PR before a dropped QRS indicates second-degree Type I (Wenckebach). Consistent PR with intermittent dropped QRS indicates second-degree Type II.
QRS duration: Less than 0.12 seconds is normal (narrow complex). 0.12 seconds or greater indicates a wide complex rhythm, which may be ventricular in origin or supraventricular with aberrant conduction (bundle branch block).
QT interval: Should be less than half the R-R interval. Prolonged QT increases the risk of torsades de pointes.
Step 5: ST Segment and T Wave
The ST segment should be at the isoelectric line (flat). ST elevation in two or more contiguous leads indicates STEMI. ST depression may indicate ischemia, reciprocal changes, or other conditions. T wave inversion in leads where T waves should be upright suggests ischemia.
STEMI Recognition by Territory
- Anterior: ST elevation in V1-V4 (LAD occlusion)
- Lateral: ST elevation in I, aVL, V5-V6 (circumflex occlusion)
- Inferior: ST elevation in II, III, aVF (RCA occlusion, usually). When you see inferior STEMI, obtain a right-sided 12-lead (V4R) to assess for right ventricular infarction
- Posterior: ST depression in V1-V3 (may be the only finding). Obtain posterior leads (V7-V9) for confirmation
Bundle Branch Blocks
Right Bundle Branch Block (RBBB): QRS greater than 0.12 seconds with rSR' pattern in V1 (rabbit ears) and wide S wave in lead I and V6.
Left Bundle Branch Block (LBBB): QRS greater than 0.12 seconds with broad, notched R waves in I, aVL, V5-V6 and deep S waves in V1-V3. New LBBB in the setting of chest pain may be treated as a STEMI equivalent.