What are the basic findings and interpretations when reading an electrocardiogram (EKG)?

Basic EKG Interpretation: A Systematic Approach

When reading an EKG, follow a structured sequence: assess rate and rhythm first, then measure intervals (PR, QRS, QT), determine the electrical axis, and finally analyze waveform morphology (P waves, QRS complexes, ST segments, and T waves) for abnormalities. 1

Step 1: Rate and Rhythm Assessment

Calculate heart rate by counting the number of large squares between consecutive R waves or by counting QRS complexes in a 6-second strip and multiplying by 10. 1 Normal heart rate is 60-100 beats per minute. 2

Evaluate rhythm regularity by examining R-R intervals for consistency, which is crucial for identifying arrhythmias. 1 Identify the underlying rhythm (sinus, atrial, junctional, or ventricular) based on P wave morphology and its relationship to QRS complexes. 1

Step 2: Interval Measurements

Measure the PR interval (normal 120-200 ms or 3-5 small squares) to assess atrioventricular conduction. 1, 2 Prolongation suggests AV block, while shortening may indicate pre-excitation syndromes.

Assess QRS duration (normal <120 ms or <3 small squares) to evaluate ventricular conduction. 1, 2 Widening indicates bundle branch blocks or ventricular rhythms.

Calculate the QT interval and correct for heart rate (QTc). Normal values are <450 ms for men and <460 ms for women. 1, 2 This is critical for assessing repolarization abnormalities and risk of arrhythmias. 3

Step 3: Electrical Axis Determination

Examine leads I and aVF to quickly determine the axis quadrant. 1 Normal axis is between -30° and +90°. 1

• Normal axis: Positive deflection in both lead I and aVF
• Left axis deviation: Positive in lead I, negative in aVF 1
• Right axis deviation: Negative in lead I, positive in aVF 1

Axis deviation provides essential information about chamber enlargement, myocardial hypertrophy, and conduction defects that directly impact morbidity and mortality. 1

Step 4: Waveform Morphology Analysis

P Wave Assessment

Examine P wave morphology (normally upright in leads I, II, aVF; biphasic in V1) to assess atrial conduction. 1 P wave duration should be <120 ms and amplitude <2.5 mm. 1

QRS Complex Analysis

Analyze QRS morphology for pathologic Q waves (>1 mm wide and >1/3 the height of the R wave), which suggest myocardial infarction. 1 Assess R wave progression across precordial leads, with amplitude normally increasing from V1 to V4, then decreasing toward V6. 1

ST Segment Evaluation

Examine ST segments for elevation or depression measured at the J point, which may indicate ischemia, injury, or infarction. 1 ST elevation is a critical finding requiring immediate clinical correlation. 4, 1

T Wave Analysis

Assess T wave morphology, which is normally upright in leads I, II, V3-V6, inverted in aVR, and variable in III, aVL, aVF, V1, and V2. 1 T wave inversions or abnormalities may indicate ischemia or electrolyte disturbances.

Critical Pitfalls to Avoid

Never rely solely on computerized interpretations without physician verification, as computer-generated readings remain prone to errors. 1, 2 All automated interpretations must be verified by a qualified physician who integrates clinical data and compares with previous tracings. 1

Avoid electrode misplacement, particularly of precordial leads, which can significantly alter interpretation and lead to false diagnoses. 2 Ensure proper high-frequency response (minimum 150 Hz for adults, 250 Hz for children) to prevent systematic underestimation of signal amplitude and loss of diagnostic features like Q waves. 2

Always interpret the EKG in clinical context. The same EKG pattern may occur in different structural and pathophysiologic states, explaining the frequent low specificity for determining disease etiology. 1 Never interpret findings in isolation without considering the patient's symptoms, medications, and prior tracings. 2

Special Considerations

Account for age and gender differences in normal parameters, as QT intervals are typically longer in women. 1 Recognize that certain medications (antiarrhythmics, psychotropics) can significantly affect EKG findings. 1, 3

Look for additional features including U waves (which may indicate hypokalemia or bradycardia), signs of chamber enlargement, and conduction abnormalities. 1 The EKG provides both electrical information (rhythm, rate, axis) and biomechanical information (ventricular hypertrophy, repolarization changes associated with ischemia). 3