What are the key components of an electrocardiogram (EKG) reading?

Key Components of an EKG Reading

A systematic EKG interpretation must evaluate five critical components in sequence: rate and rhythm, intervals (PR, QRS, QT), axis, waveform morphology (P waves, QRS complexes, ST segments, T waves), and any abnormalities in repolarization patterns.

Essential Waveforms and Intervals

P Wave and PR Interval

• The P wave represents atrial depolarization and should be assessed for presence, morphology, and relationship to QRS complexes to determine rhythm origin 1.
• The PR interval measures atrioventricular conduction time and helps identify conduction abnormalities 1.

QRS Complex

• The QRS complex represents ventricular depolarization and must be evaluated for duration, amplitude, and morphology 1.
• QRS duration and morphology changes directly affect ST-T wave interpretation through secondary repolarization abnormalities 1.
• In bundle branch blocks, ST-segment and T-wave vectors are directed opposite to the mean QRS vector (left bundle branch block) or opposite to the slow terminal QRS component (right bundle branch block) 1.

ST Segment

• The ST segment reflects the plateau phase of ventricular repolarization and is measured at the J point (junction of QRS and ST segment) 1.
• ST-segment displacement must be referenced against the TP or PR segment as the baseline 1.
• Normal ST elevation varies by age, sex, and race: in lead V2, the upper 98th percentile is approximately 0.3 mV in white men <40 years, 0.25 mV in white men ≥40 years, and 0.15 mV in white women 1.
• Distinguish primary from secondary repolarization abnormalities: primary abnormalities indicate actual changes in ventricular myocyte repolarization (ischemia, electrolyte abnormalities, drugs), while secondary changes result from altered depolarization sequence without cellular repolarization changes 1.

T Wave

• The T wave represents ventricular repolarization (phase 3 of the action potential) and should be assessed for amplitude, morphology, and axis 1.
• When QRS axis is normal, an abnormal T-wave axis direction indicates primary repolarization abnormalities 1.
• T-wave changes may be primary (from ischemia, myocarditis, drugs, electrolyte abnormalities, hyperventilation, position changes, catecholamines) or secondary (from conduction abnormalities) 1.

U Wave

• The U wave is a low-amplitude deflection after the T wave, most evident in leads V2 and V3 with normal amplitude approximately 0.33 mV or 11% of T-wave amplitude 1, 2.
• U waves are heart-rate dependent: rarely present at rates >95 bpm, present in 90% of cases at rates <65 bpm 1, 2.
• Report U waves when inverted, merged with T wave, or amplitude exceeds T wave 1, 2.
• Inverted U waves in V2-V5 are abnormal and may indicate acute ischemia or hypertension 1, 2.
• Increased U-wave amplitude or U-T fusion occurs with hypokalemia, quinidine-like drugs, increased sympathetic tone, or long QT syndrome 1, 2.

QT Interval

• The QT interval extends from QRS onset to T-wave end, representing total ventricular depolarization and repolarization time 1.
• Measure QT in the lead showing the longest interval, usually V2 or V3; if this differs by >40 ms from other leads, the measurement may be erroneous 1.
• When T and U waves are superimposed, measure QT in leads without U waves (often aVR, aVL) or extend the T-wave downslope tangent to the TP segment 1.
• Visually validate all computer-generated QT measurements due to their critical importance for identifying arrhythmia risk 1.
• QT prolongation requires clinical evaluation for causes including drugs, hypokalemia, hypocalcemia, and congenital long QT syndrome 1.

Critical Measurement Techniques

Lead Selection

• QRS onset occurs up to 20 ms earlier in V2 and V3 than in limb leads, affecting interval measurements 1.
• Modern digital ECG machines record all leads simultaneously, allowing temporal alignment and superimposition for more accurate measurement of QRS onset and T-wave end 1.

Common Pitfalls

• Automated QT measurements may be longer than single-lead measurements because simultaneous recording captures the earliest QRS onset and latest T-wave end across all leads 1.
• Low-frequency filtering removes baseline drift but prevents determination of absolute DC voltage levels, so ST elevation may reflect PR/TP depression, true ST elevation, or both 1.
• Secondary repolarization changes from transient conduction abnormalities (ectopic beats, transient bundle branch blocks) usually revert promptly, but changes from prolonged ventricular pacing take hours to days to resolve 1.