Physiology11 min readAI-Generated6 views

EKG Interpretation & The Cardiac Cycle: A Complete Guide

The EKG is one of the most powerful diagnostic tools in medicine — and it looks terrifying to most students. But the EKG is just a graphical record of the heart's electrical activity. Once you understand what each wave represents physiologically, you can interpret any tracing systematically. This guide walks you through everything: from the cardiac cycle to full EKG interpretation.

AI-generated content. This guide was written by MedAI's AI and is intended as a study aid. Always cross-reference with your official course materials, textbooks, and instructor guidance before your exam.

The Cardiac Conduction System

The heart generates and propagates its own electrical impulses through a specialized conduction system. The sequence of activation determines the waveform pattern on the EKG.

Structure	Function	Intrinsic Rate	EKG Correlate
SA Node (sinoatrial)	Primary pacemaker of the heart; right atrium near SVC	60–100 bpm	Sets the heart rate; precedes P wave
AV Node (atrioventricular)	Delays signal transmission to ventricles (~100ms); allows atrial filling	40–60 bpm	PR interval (0.12–0.20 sec)
Bundle of His	Conducts signal from AV node to bundle branches	40–60 bpm	Part of PR interval
Bundle Branches (R & L)	Rapidly conduct signal down interventricular septum	~40 bpm	Part of QRS complex
Purkinje Fibers	Final rapid distribution to ventricular myocardium	20–40 bpm	Completes QRS complex

EKG Waveforms: What Each One Means

Wave/Interval	Duration (Normal)	Represents	Abnormality
P wave	<120 ms	Atrial depolarization	Absent: atrial fibrillation; Peaked (>2.5mm): right atrial enlargement
PR interval	120–200 ms	Conduction through AV node	>200 ms: 1st-degree AV block; progressively longer then dropped: 2nd-degree Mobitz I (Wenckebach); fixed dropped beats: Mobitz II; no relationship: 3rd-degree
QRS complex	<120 ms	Ventricular depolarization	>120 ms: bundle branch block, ventricular tachycardia, hyperkalemia
ST segment	Isoelectric (flat)	Plateau phase of ventricular AP (phase 2)	Elevation: STEMI, pericarditis, Brugada. Depression: NSTEMI, ischemia, digitalis effect
T wave	Upright in most leads	Ventricular repolarization	Inverted: ischemia, RBBB, LVH. Peaked: hyperkalemia. Flattened: hypokalemia
QT interval	<440 ms (corrected)	Total ventricular depolarization + repolarization	Prolonged QTc: risk of Torsades de Pointes (VTach). Drugs: quinidine, haloperidol, macrolides, fluoroquinolones
U wave	Small, after T wave	Late repolarization (Purkinje fibers?)	Prominent U wave: hypokalemia

The 5-Step EKG Interpretation

1) Rate (count large boxes: 300/# boxes between R waves). 2) Rhythm (regular? P before every QRS?). 3) Axis (quadrant method: QRS polarity in leads I and aVF). 4) Intervals (PR, QRS, QTc). 5) Waveform changes (ST elevation/depression, T-wave inversion, Q waves).

The Cardiac Cycle: Pressure, Volume & Valves

The cardiac cycle is the sequence of mechanical events that accompany each heartbeat. Heart sounds, murmurs, and the EKG all map directly onto specific events in the cycle.

Phase	Valve Status	Pressure Change	EKG	Sound
Late diastole (filling)	Mitral + tricuspid OPEN; Aortic + pulmonic CLOSED	LV pressure low, filling from LA	After T wave; P wave begins	S4 if present (atrial kick)
Isovolumetric contraction	ALL VALVES CLOSED	LV pressure rises rapidly; volume constant	QRS to start of ejection	S1 (mitral + tricuspid close) — beginning of systole
Ejection (systole)	Aortic + pulmonic OPEN; Mitral + tricuspid CLOSED	LV pressure > aortic pressure; SV ejected; volume falls	ST segment	Ejection click (stenotic valve) if present
Isovolumetric relaxation	ALL VALVES CLOSED	LV pressure falls rapidly; volume constant	T wave	S2 (aortic + pulmonic close) — beginning of diastole; A2 before P2
Rapid filling (early diastole)	Mitral + tricuspid OPEN	Blood rushes into ventricle from pressure gradient	After T wave	S3 (if present: volume overload, heart failure, mitral regurgitation)

High-Yield Arrhythmias

Arrhythmia	EKG Findings	Cause	Treatment
Atrial Fibrillation	No P waves; irregularly irregular rhythm; fibrillatory baseline	Hypertension, valvular disease, thyrotoxicosis, alcohol ("holiday heart")	Rate control (β-blockers, CCBs); rhythm control (cardioversion); anticoagulation (stroke prevention)
Atrial Flutter	"Sawtooth" flutter waves at 300 bpm; regular ventricular rate (150 bpm with 2:1 block)	Right atrial macro-reentry	Cardioversion; ablation; anticoagulation
Ventricular Tachycardia (VT)	Wide QRS (>120ms) ≥3 beats at >100 bpm; AV dissociation; fusion beats	MI scar reentry; cardiomyopathy	Stable: amiodarone. Unstable: synchronized cardioversion
Ventricular Fibrillation (VF)	Chaotic, no organized complexes	Ischemia, electrolyte disturbance	Immediate DEFIBRILLATION (unsynchronized). No pulse = VF or pulseless VT
Torsades de Pointes	VT with rotating axis ("twisting of points"); prolonged QTc	Prolonged QT (drugs, hypokalemia, hypomagnesemia)	Magnesium sulfate IV; correct electrolytes; remove offending drug
1st-degree AV block	PR > 200ms; every P followed by QRS	Increased vagal tone, digoxin, myocarditis	Usually none required
3rd-degree AV block	Complete AV dissociation; P and QRS independent	Inferior MI (right coronary artery), Lyme disease, structural	Permanent pacemaker

STEMI Localization by EKG Leads

Affected Territory	Occluded Artery	EKG Leads with ST Elevation	Reciprocal Changes
Inferior wall	RCA (90%) or LCx	II, III, aVF	I, aVL (lateral)
Lateral wall	LCx or diagonal branch of LAD	I, aVL, V5-V6	II, III, aVF
Anterior wall	LAD (diagonal branches)	V1-V4	II, III, aVF
Posterior wall	RCA or LCx	Tall R in V1-V2 + ST depression V1-V3	ST elevation in V7-V9 (posterior leads)
RV infarct	Proximal RCA	ST elevation in V1, V4R	Classic: inferior STEMI + hypotension + JVD + clear lungs

Hyperkalemia EKG Changes (Know in Order)

As K⁺ rises: (1) Peaked T waves (K 5.5–6.5) → (2) Widened QRS (K 6.5–7.5) → (3) Sine wave pattern (K >7.5) → (4) VF / asystole. TREATMENT: Calcium gluconate (stabilizes membrane), then insulin+glucose (shifts K into cells), then kayexalate/dialysis (removes K).