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I now have comprehensive material from Goldman-Cecil Medicine and Costanzo Physiology to deliver a thorough answer. Let me compile the full response.

ECG - Basic Principles & How to Read One

What is an ECG?

An electrocardiogram (ECG or EKG) records tiny electrical potential differences on the body's surface that reflect the sequence of depolarization and repolarization through the myocardium. Because the atria and ventricles depolarize at different times, and in specific sequences, potential differences are established between heart regions, which electrodes placed on the skin can detect.
(Costanzo Physiology 7th Ed.; Goldman-Cecil Medicine)

The ECG Paper

Normal ECG waveform with labeled intervals
Standard ECG paper has a grid where:
  • Horizontal axis = time: each small box (1 mm) = 0.04 sec (40 ms); each large box (5 mm) = 0.2 sec (200 ms)
  • Vertical axis = voltage: 1 mm = 0.1 mV; standard calibration is 10 mm = 1 mV
  • A standard 12-lead ECG records 10 seconds total, with groups of leads (I, II, III / aVR, aVL, aVF / V1-V3 / V4-V6) each displaying 2.5 seconds, plus a single rhythm strip for the full 10 seconds at the bottom.
(Goldman-Cecil Medicine, p. 421-422)

The Waveforms - What Each One Means

ECG waveform labeled diagram
Wave / SegmentElectrical Event
P waveAtrial depolarization (SA node fires → atria contract)
PR intervalConduction delay through AV node + His-Purkinje (atria → ventricles)
QRS complexVentricular depolarization (ventricles about to contract)
ST segmentPlateau phase of ventricular action potential (isoelectric)
T waveVentricular repolarization (electrical recovery)
QT intervalTotal ventricular activity: depolarization + repolarization
U waveOccasionally seen; cause debated (possibly Purkinje repolarization)
J pointJunction between end of QRS and start of ST segment
Note: Atrial repolarization occurs during the QRS complex and is buried/invisible on the normal ECG because ventricular mass far exceeds atrial mass.
(Goldman-Cecil Medicine, p. 421; Guyton & Hall Medical Physiology)

Normal Interval Values

ParameterNormal Range
Heart rate50-100 beats/min
P wave duration< 120 ms (< 3 small boxes)
PR interval90-200 ms (2.5-5 small boxes)
QRS duration75-110 ms (< 3 small boxes)
QTc (males)390-450 ms
QTc (females)390-460 ms
QRS axis-30° to +90°
(Goldman-Cecil Medicine, Table 42-1)

The 12 Leads - What They Look At

A standard ECG uses 10 electrodes to generate 12 vantage points (leads):
Limb leads (frontal plane):
  • Bipolar: I (left arm - right arm), II (left leg - right arm), III (left leg - left arm)
  • Augmented unipolar: aVR, aVL, aVF
Precordial leads (horizontal plane):
  • V1-V6, placed across the chest wall
By convention, when a depolarization wave moves toward a lead electrode, the deflection is positive (upward); when moving away, it is negative (downward).

Systematic Approach to Reading an ECG

Follow this order every time:

1. Rate

  • Regular rhythm: HR = 300 ÷ (number of large boxes between R waves)
  • Irregular rhythm: Count QRS complexes in the 10-second strip × 6
  • Formula: HR = 60,000 ÷ RR interval (in ms)

2. Rhythm

  • Is it regular or irregular?
  • Is there a P wave before every QRS?
  • Is the P wave upright in leads I and II? (confirms sinus origin)

3. P Wave

  • Duration < 120 ms
  • Morphology: smooth, rounded, upright in I, II, aVF
  • Biphasic in V1 is normal

4. PR Interval

  • 120-200 ms (3-5 small boxes)
  • Prolonged PR = first-degree AV block
  • Short PR = pre-excitation (e.g., WPW)

5. QRS Complex

  • Duration < 120 ms
  • Wide QRS (≥ 120 ms) = bundle branch block or ventricular origin
  • Check for Q waves (pathological if > 40 ms wide or > 25% of R wave height)

6. Axis

  • Normal axis: -30° to +90°
  • Quick check: if QRS is positive (upright) in both I and aVF → normal axis
  • Positive I, negative aVF → left axis deviation
  • Negative I, positive aVF → right axis deviation

7. ST Segment & T Wave

  • ST should be isoelectric (flat at baseline)
  • ST elevation → consider MI, pericarditis, Brugada
  • ST depression → ischemia, digoxin effect, reciprocal change
  • T wave normally positive in I, II, V3-V6; inverted in aVR

8. QT Interval

  • Measure in leads II, V5, V6; use longest value
  • Correct with Bazett's formula: QTc = QT / √(RR interval in seconds)
  • Prolonged QTc → risk of torsades de pointes

Key Physiology to Remember

  • The QRS is larger than the P wave because ventricular muscle mass far exceeds atrial mass.
  • The QRS duration equals the P wave duration despite the size difference because the His-Purkinje system conducts so much faster than the atrial conduction system - this is why ventricular depolarization happens just as quickly.
  • Depolarization waves (P, QRS) move toward a lead = positive deflection. Repolarization waves (T) are positive in most leads because ventricular repolarization occurs in the opposite direction to depolarization (epicardium repolarizes first).
(Costanzo Physiology 7th Ed.; Guyton & Hall Medical Physiology)
Would you like to go deeper into any specific part - arrhythmia recognition, bundle branch blocks, ST changes, or axis calculation?
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