ECG Basics and Core Concepts (Objectives 12-13)

ECG (ECG or EKG) is a graphical recording of the heart's electrical activity during a single cardiac cycle.
Cardiac cycle: atrial contraction → atrial relaxation → ventricular contraction → ventricular relaxation; typical heart rate ≈ $75$ bpm.

ECG is recorded with electrodes connected to a device called an electrocardiograph.
Leads: 12 standard leads for a full picture; 3 leads in many labs (less complete but educational).

Repeating waveforms: P wave, QRS complex, T wave; intervals include PR and ST.
P wave: atrial depolarization; precedes atrial contraction.
PR interval: time for the impulse to travel through the atria and AV node; atrial contraction occurs during this interval.
QRS complex: ventricular depolarization; leads to ventricular contraction; large, sharp deflection.
ST segment: interval after ventricular depolarization ends and before ventricular repolarization begins.
T wave: ventricular repolarization.
Atrial repolarization occurs during the QRS complex and is masked by the large ventricular depolarization signal; there is no separate name for it.
PQRST sequence: orderly labeling of the waves in an ECG trace.

P wave: atrial depolarization (slightly positive).
QRS complex: ventricular depolarization (large amplitude; rapid).
T wave: ventricular repolarization.
Between QRS and return to baseline, the atria repolarize but this is masked by the QRS.

The trace is read on a moving baseline; upward deflections are positive, downward are negative.
R peaks (QRS) are sharp and easy to identify for rhythm and rate analysis.
Normal pattern: one P wave, one QRS complex, one T wave per cycle.

Heart rate can be estimated by counting the number of squares between consecutive QRS peaks or calculating from the R–R interval.
Formula: $\text{HR} = \dfrac{60}{T<em>{RR}}$ where $T{RR}$ is the time between consecutive R peaks (in seconds).
In practice, use the R peaks for rate; measure multiple intervals and average.

Sinus bradycardia: heart rate $< 60$ bpm.
Sinus tachycardia: heart rate $> 100$ bpm.
Junctional rhythm: SA node nonfunctional; AV node paces at $40$–$60$ bpm; P waves may be absent.
Second-degree AV block: AV node fails to conduct some SA impulses; more P waves than QRS complexes.
Ventricular fibrillation: disorganized, chaotic electrical activity in the ventricles; no effective contraction; no pulse; life-threatening.
Atrial fibrillation: ectopic pacemaker activity or reentry causes rapid, irregular atrial activity; P waves absent with irregular rhythm.
Atrial flutter: rapid atrial contractions; often due to ectopic pacemaker or reentry; characteristic flutter waves.
Ectopic pacemaker and reentry: additional automatic foci or abnormal circuit causing irregular rhythm.
Bundle branch block (BBB): block in right or left bundle branches; QRS becomes wide/notched and ventricles contract asynchronously.

Focus on rate (from R–R), rhythm regularity, and morphology of P, QRS, and T waves.
Narrow QRS suggests supraventricular origin; wide QRS suggests ventricular origin or BBB.
Note deviations from normal P, QRS, and T patterns to identify potential pathologies.

P wave = atrial depolarization; QRS = ventricular depolarization; T wave = ventricular repolarization.
Atrial repolarization occurs during the QRS and is not separately visible.
PR interval reflects atrial conduction time and AV delay; ST segment is the interval between depolarization and repolarization of the ventricles.
Heart rate is best assessed from the QRS peaks using the R–R interval; normal range is around $60$–$100$ bpm; outside this range indicates bradycardia or tachycardia.
Common arrhythmias include sinus bradycardia, sinus tachycardia, junctional rhythm, AV block, ventricular fibrillation, atrial fibrillation, atrial flutter, ectopic pacemaker and reentry, and bundle branch blocks.