What are all components of an ECG tracing—including waves, intervals, and segments—their normal durations and amplitudes, and what each component represents?

ECG Components: Comprehensive Overview

An ECG tracing consists of waves (P, Q, R, S, T, U), intervals (PR, QRS, QT), and segments (PR, ST, TP) that represent specific electrical events in the cardiac cycle, each with defined normal durations and amplitudes that vary by age, sex, and lead position.

Waves

P Wave

• Represents: Atrial depolarization
• Normal duration: <120 ms (0.12 seconds)
• Normal amplitude:
  • <2.5 mm (0.25 mV) in limb leads
  • Usually upright in leads I, II, aVF
  • Biphasic or inverted in aVR

QRS Complex

• Represents: Ventricular depolarization

• Components:

  • Q wave: Initial negative deflection
    • Normal duration: <30 ms in most leads
    • May exceed 30 ms in aVL and aVF (men only) 1
    • Amplitude varies by lead and heart position
  • R wave: First positive deflection
    • Amplitude varies significantly with age, sex, and heart position
    • Higher in young and middle-aged males than females 2
    • Greater in blacks than whites 2
    • Can show up to 40% variation throughout the day 3
  • S wave: Negative deflection following R wave

• Total QRS duration:

  • Normal: 80-120 ms
  • Onset occurs up to 20 ms earlier in V2-V3 than limb leads 2

T Wave

• Represents: Ventricular repolarization (phase 3 of action potential) 2
• Normal characteristics:
  • Usually upright in leads I, II, V3-V6
  • Amplitude varies considerably (up to 40% diurnal variation) 3
  • Reflects transmural dispersion of repolarization
  • Direction generally concordant with QRS in normal axis

U Wave

• Represents: Delayed Purkinje system repolarization or electrical interaction among ventricular layers during late repolarization 4
• Normal characteristics:
  • Small positive deflection after T wave
  • Best seen in V2-V5
  • Amplitude normally less than T wave
  • Abnormal when: Inverted in V2-V5, merged with T wave, or amplitude exceeds T wave 2

Intervals

PR Interval

• Represents: Time from atrial depolarization onset to ventricular depolarization onset (includes AV nodal conduction delay)
• Normal duration: 120-200 ms
• Measurement: From P wave onset to QRS onset
• Note: Shows positive correlation with R-R interval normally; becomes negative with stress/catecholamines 5

QRS Duration

• Represents: Total ventricular depolarization time
• Normal duration: 80-120 ms
• Measurement: From earliest QRS onset to latest QRS offset across all leads when measured globally 6
• Clinical note: Global measurement from simultaneous leads yields systematically longer values than single-lead measurements 6

QT Interval

• Represents: Total ventricular depolarization and repolarization time
• Normal duration (rate-corrected):
  • Men: <450 ms
  • Women: <460 ms (longer than men)
• Measurement:
  • From QRS onset to T wave end
  • Measure in lead showing longest QT (usually V2 or V3) 2
  • If difference >40 ms from other leads, verify with adjacent leads 2
  • When T and U waves merge, measure in leads without U waves (often aVR, aVL) 2
• Critical point: Always visually validate computer-measured QT prolongation 2

Segments

PR Segment

• Represents: Isoelectric period during AV nodal delay
• Location: From P wave end to QRS onset
• Normal: Flat, isoelectric baseline

ST Segment

• Represents: Plateau phase (phase 2) of ventricular action potential 2
• Location: From J point (QRS end) to T wave onset
• Normal characteristics:
  • Nearly isoelectric, at same level as TP segment
  • Measured at J point and 40-80 ms after J point (exercise testing) 2
  • Elevation in V1-V3: Greater in young/middle-aged males than females, greater in blacks than whites 2
  • Can be described as: elevated, depressed, upsloping, horizontal, downsloping
• Clinical distinction:
  • Primary abnormalities: Changes in myocyte repolarization (ischemia, drugs, electrolytes) 2
  • Secondary abnormalities: Due to altered depolarization sequence (bundle branch blocks, pacing) 2

TP Segment

• Represents: Electrical diastole (resting membrane potential)
• Location: From T wave end to next P wave onset
• Normal: Flat, serves as isoelectric baseline reference

Key Measurement Principles

Global vs. Single-Lead Measurements:

• Modern simultaneous 12-lead recording allows detection of earliest onset and latest offset across all leads 6
• Global measurements are systematically longer than single-lead measurements 6
• QT intervals can differ by 50-65 ms between leads normally 2

Common Pitfalls:

• Low-frequency filtering can distort ST segment reference points 2
• QRS amplitude affects ST segment deviation magnitude 2
• Rate correction formulas (Bazett, Fridericia) have significant limitations at extreme heart rates 2
• T wave offset determination remains challenging even with automated systems 6