Basics of Electrocardiography

Terminology and Literal Meaning

• Word breakdown
  • "Electro" → electricity
  • "Cardio" → heart
  • "-graphy" → process of recording
• Literal translation = “process of recording the electricity of the heart.”
• Product of the recording process = electrocardiogram (the printed/onscreen trace).

The Electrocardiograph (ECG/EKG) Device

• Non-invasive; uses wires + adhesive electrodes that stick to the skin—no surgical incision required.
  • “ECG” (English) and “EKG” (German “Kardia”) are interchangeable.
• Detects only the heart’s electrical activity.
  • Cannot directly measure pressure, valve defects, chamber volumes, etc.
  • Other modalities fill those gaps: blood-pressure cuffs/catheters, ultrasound/echocardiography, etc.
• Harnesses the fact that body tissues are good conductors, letting surface electrodes pick up deep cardiac currents.

Biophysical Principles & Signal Source

• Multiple organs make electricity (brain, skeletal muscle, nerves) ➜ concern: how to isolate the heart’s signal.
  • Heart outcompetes others because of large net amplitude created by the thick, synchronously firing ventricular myocardium.
• Comparison: brain recording = electro-encephalograph ("encephalo" = brain) which requires scalp electrodes and different amplification.

Electrodes, Leads & “Camera Angles”

• Electrode = the physical sticky sensor.
• Lead = the difference in electrical potential detected between two electrodes.
  • 3 electrodes (RA, LA, LL) ⇒ 3 bipolar leads (I, II, III).
• Analogy: each lead is a camera shot of a 3-D object. More angles ⇒ fuller picture.
  • Clinical standard = 12-lead ECG for comprehensive views.
• In this introductory lesson we analyze Lead II (RA → LL) because it shows the clearest classic waveforms.

Coordinate System of a Lead-II Trace

• x-axis: time (seconds).
• y-axis: voltage (millivolts, mV).
• Baseline (isoelectric line): no net change in cardiac electrical state.
  • NOT necessarily "resting potential"—could also be plateau phases where voltage is static.
• Deflection above or below baseline = “waveform” created by a significant, synchronous change in many cells.

Contractile vs Conductive Cells & Why Only 3 Waves Appear

• Myocardial composition: ~99 % contractile myocytes, <1 % specialized conductive cells (SA node, AV node, bundle, Purkinje).
• Conductive-cell APs are numerically tiny ➜ below detection threshold.
• Contractile-cell depolarization/repolarization creates large, rapid, coordinated currents ➜ detectable.
• Consequently only THREE major electrical events appear:
  1. P wave – atrial depolarization
  2. QRS complex – ventricular depolarization (appears as Q, R, S peaks but treated as one event)
  3. T wave – ventricular repolarization
• Atrial repolarization exists but is buried within the large QRS signal.

Sequence of a Normal Cardiac Cycle on the ECG (Lead II)

• SA node fires (not directly visible).
• P wave: atrial contractile cells depolarize → atria begin systole.
• P–R Segment (baseline):