What are the steps for interpreting an electrocardiogram (EKG)?

Systematic Approach to ECG Interpretation

A systematic approach to ECG interpretation is essential for accurate diagnosis of cardiac conditions, requiring careful analysis of rate, rhythm, axis, intervals, and waveform morphology. 1

Step 1: Assess Heart Rate and Rhythm

• Calculate heart rate by counting the number of large squares between consecutive R waves (300 divided by number of large squares) or by counting QRS complexes in a 6-second strip and multiplying by 10 1
• Evaluate rhythm regularity by examining R-R intervals for consistency - regular rhythms have uniform spacing between QRS complexes 1
• Identify the underlying rhythm (sinus, atrial, junctional, ventricular) based on P wave morphology and relationship to QRS complexes 2

Step 2: Analyze Intervals and Segments

• Measure PR interval (normal 120-200 ms or 3-5 small squares) to assess atrioventricular conduction 1
• Evaluate QRS duration (normal <120 ms or <3 small squares) to assess ventricular conduction 1
• Calculate QT interval and correct for heart rate (QTc, normal <450 ms for men, <460 ms for women) to assess ventricular repolarization 1
• Examine ST segments for elevation or depression that may indicate ischemia, injury, or other pathology 2

Step 3: Determine Electrical Axis

• Examine leads I and aVF to quickly determine the quadrant of the axis 1
• Normal axis is between -30° and +90° 1
• Left axis deviation: positive in lead I, negative in aVF 2
• Right axis deviation: negative in lead I, positive in aVF 2

Step 4: Analyze P Waves

• Assess P wave morphology (normal: upright in I, II, aVF; biphasic in V1) 1
• Evaluate P wave duration (<120 ms) and amplitude (<2.5 mm) 2
• Look for abnormal P wave patterns suggesting atrial enlargement or ectopic atrial rhythms 2

Step 5: Analyze QRS Complexes

• Examine QRS morphology in all leads for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave) suggesting myocardial infarction 1
• Assess R wave progression across precordial leads (R wave amplitude should increase from V1 to V4, then decrease toward V6) 2
• Look for bundle branch blocks, characterized by QRS widening (>120 ms) and specific morphology patterns 2

Step 6: Evaluate ST Segments and T Waves

• Assess ST segments for elevation or depression (measured at the J point) 2
• Evaluate T wave morphology (normally upright in leads I, II, V3-V6; inverted in aVR; variable in III, aVL, aVF, V1, V2) 2
• Look for ST-T wave patterns suggesting ischemia, injury, electrolyte abnormalities, or other pathology 2

Step 7: Look for Additional Features

• Identify any U waves (small deflection after T wave) which may indicate hypokalemia, bradycardia, or other conditions 2
• Assess for signs of chamber enlargement (P wave abnormalities, QRS voltage criteria) 2
• Look for conduction abnormalities (PR prolongation, QRS widening) 2

Special Considerations

• Age and gender affect normal ECG parameters (e.g., QT intervals are typically longer in women) 1
• Certain medications can significantly alter ECG findings (e.g., antiarrhythmics, psychotropics) 1
• Technical factors like electrode placement and filtering can affect ECG appearance and interpretation 2
• Computerized ECG interpretations should always be verified by a qualified physician, as automated algorithms have limitations in diagnostic accuracy 1, 3

Common Pitfalls to Avoid

• Relying solely on computer interpretation without physician verification can lead to diagnostic errors 4
• Failing to use a systematic approach may result in missed diagnoses 5
• Interpreting ECG findings in isolation without considering clinical context can lead to inappropriate management decisions 4
• Inadequate high-frequency characteristics in ECG machines can cause systematic underestimation of signal amplitude and smoothing of important features 4