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).

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.

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.

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.

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.

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.