Comprehensive Guide to EKG Interpretation and Cardiac Pathophysiology

ECG/EKG

A recording of the electrical activity of the heart at the body surface. ECG and EKG refer to the same test.

Normal cardiac conduction sequence

SA node → atrial cells → AV node → Bundle of His → bundle branches → Purkinje fibers.

Narrow QRS association with normal ventricular conduction

Conduction slows through the AV node but travels rapidly through the bundle branches and Purkinje fibers, producing rapid coordinated ventricular depolarization.

Leads I, II, and III

Bipolar limb leads with positive and negative poles that view the heart in the frontal plane.

Leads aVR, aVL, and aVF

Augmented unipolar limb leads that view the heart in the frontal plane.

Leads V1–V6

Unipolar precordial/chest leads that view the heart in the horizontal plane.

10 electrodes producing a 12-lead ECG

4 limb electrodes and 6 precordial electrodes that generate 12 lead views.

Positive electrode of an ECG lead

The viewpoint from which the heart’s electrical activity is being recorded.

One small horizontal ECG box

0.04 seconds.

One large horizontal ECG box

0.20 seconds.

One small vertical ECG box

0.1 mV.

One large vertical ECG box

0.5 mV.

Standard ECG paper speed

25 mm/sec.

Large ECG boxes that equal 3 seconds

15 large boxes.

Difference between ECG interval and segment

An interval includes waves, whereas a segment is the time between waves.

P wave

Represents atrial depolarization.

Normal P-wave duration

Approximately 0.08–0.10 sec.

QRS complex

Represents ventricular depolarization.

Normal QRS duration

0.04–0.12 sec.

T wave

Represents ventricular repolarization.

PR interval

Represents atrial depolarization plus AV nodal/conduction delay.

Normal PR interval

0.12–0.20 sec = 3–5 small boxes.

ST segment

The isoelectric period during which the ventricles are depolarized.

QT interval

Represents ventricular depolarization + ventricular repolarization; corresponds approximately to ventricular action-potential duration.

QT cutoff for basic ECG interpretation

QT

Clinical importance of prolonged QT

It can occur from hypocalcemia or medications and predisposes to torsades de pointes.

Q wave

The first downward deflection of the QRS complex.

R wave

The first upward deflection of the QRS complex.

S wave

A downward deflection following an R wave.

R-prime (R′) wave

A subsequent upward deflection following the initial R wave.

Systematic approach on EVERY ECG

Rate, Rhythm, Axis, Examine P, PR, QRS, ST, T, QT.

5-step approach for rhythm strip

Rate, Regularity, P waves, PR interval, QRS duration/morphology.

Where to evaluate rate/rhythm, axis, structural/ischemic abnormalities

Rate and rhythm from the rhythm strip; axis from limb leads; all 12 leads for hypertrophy, ischemia, and infarction.

Heart-rate ranges in the lecture

Normal = 60–99 bpm; bradycardia = <60; tachycardia = ≥100.

Calculate heart rate from a 6-second rhythm strip

Count the R waves and multiply by 10.

Calculate heart rate from a 10-second rhythm strip

Count the R waves and multiply by 6.

Large-box method for calculating heart rate

Find an R wave on a bold line and count large boxes until the next R: 300 → 150 → 100 → 75 → 60 → 50.

Preferred method for very slow heart rates or irregular rhythms

The 6-second rate method.

Regular rhythm definition

Consistent R-R intervals.

Irregular rhythm definition

Variable R-R intervals.

P waves presence favoring rhythm origins

Sinus or atrial.

P waves absence favoring rhythm origins

Junctional or ventricular.

Identical P waves suggest what

One pacemaker focus.

Varying P-wave morphology suggests what

Multiple atrial automaticity foci.

Every P wave is followed by a QRS suggests what

No second- or third-degree AV dissociation/block.

Some P waves not followed by QRS suggests what

AV block.

Consistently prolonged PR interval indicates what

First-degree AV block.

Progressively increasing PR duration indicates what

Second-degree AV block Mobitz I/Wenckebach.

Completely variable PR interval suggests what

Third-degree AV block.

QRS <0.12 sec indicates what

A supraventricular impulse with normal ventricular conduction.

QRS >0.12 sec indicates what

Ventricular origin or delayed intraventricular conduction such as bundle branch block.

ECG features of normal sinus rhythm

Rate 60–100; regular; normal P waves; P before every QRS; PR 0.12–0.20; QRS 0.04–0.12 sec.

ECG features of sinus arrhythmia

Rate 60–100; irregular; normal P waves; normal PR; narrow QRS.

Physiology of sinus bradycardia

The SA node depolarizes more slowly than normal, but conduction through the rest of the system remains normal.

ECG features of sinus tachycardia

Rate >100; regular; normal P waves; normal PR; narrow QRS.

Notable aspect of sinus tachycardia

It is generally a response to physical or psychological stress rather than a primary arrhythmia.

Sinus arrest definition

The SA node stops firing.

Appearance of sinus arrest on ECG

An irregular pause interrupts an otherwise sinus rhythm.

Occurrence after sinus arrest

A lower automaticity focus can produce an escape beat or escape rhythm.

Prolonged electrical inactivity can lead to

Asystole.

Escape beat definition

A backup automaticity focus fires after a pause in normal pacing activity.

Escape rhythm definition

A lower automaticity focus takes over when the dominant pacemaker stops entirely.

Consecutive new beats required to call it an escape rhythm

At least 3 beats.

Intrinsic pacemaker hierarchy

SA node: 60–100, Atrial cells: 60–80, AV junction: ~40–80, Ventricular cells: 20–40 bpm.

Atrial escape beat appearance

Occurs after a pause; the P wave looks different; QRS remains narrow.

Atrial escape rhythm features

Rate 60–80; regular; P waves present but different morphology from sinus P waves; normal PR; narrow QRS.

Junctional escape beat appearance

Occurs after a pause; P wave absent or inverted; PR absent; QRS narrow.

Junctional escape rhythm features

Rate ~40–60; regular; P waves absent or inverted; PR absent; QRS narrow.

Ventricular escape beat appearance

Occurs after a pause; no P wave; no PR interval; wide QRS >0.12 sec.

Ventricular escape rhythm features

Rate 20–40; regular; absent P waves; absent PR; wide QRS.

Key difference between premature beat and escape beat

Premature: abnormal beat comes early; Escape: pause occurs first, then the backup beat appears.

Causes of irritable ectopic focus

Caffeine, cocaine, methamphetamines, and medications.

PAC features

Premature altered P′ wave; altered timing/morphology; generally normal PR; narrow QRS.

Reason for QRS being narrow during a PAC

The premature impulse originates in the atrium but travels normally through the AV node.

Notable fact about PACs

PACs are the most common premature beat.

PJC features

Premature beat; P absent or inverted; PR absent; QRS narrow.

PVC features

Premature beat; no preceding P; absent PR; wide/bizarre QRS >0.12 sec.

Reason for PVC being wide and bizarre

The impulse begins within ventricular myocardium.

Monomorphic/uniform PVCs

PVCs with the same morphology, suggesting one ventricular focus.

Polymorphic/multiform PVCs

PVCs with different morphologies, suggesting multiple ventricular foci.

Wandering atrial pacemaker definition

The atrial pacemaker focus changes from beat to beat.

Wandering atrial pacemaker ECG features

Rate 60–100; irregular; P′ morphology varies; PR generally normal; narrow QRS.

Multifocal atrial tachycardia distinction from wandering atrial pacemaker

Rate >100 bpm.

MAT ECG features

Rate >100; irregular; varying P′ morphologies; PR may vary/shorten; narrow QRS.

Disease commonly associated with MAT

COPD.

Reason MAT is associated with COPD

Right atrial dilation, hypercarbia, and increased sympathetic activity can contribute.

Atrial flutter ECG features

Rapid identical flutter waves producing a sawtooth baseline, with predictable AV conduction.

Mechanism of atrial flutter

Reentry.

Reason not all atrial flutter waves produce QRS complexes

The AV node blocks many of the rapid impulses.

Atrial fibrillation ECG features

No discernible P waves; fuzzy/chaotic baseline; narrow QRS; irregularly irregular rhythm.

Reason atrial fibrillation is irregularly irregular

Chaotic atrial impulses reach the AV node.

First-degree AV block features

PR >0.20 sec consistently; every P produces a QRS; regular rhythm.

Core concept of first-degree AV block

Every impulse conducts, but conduction is delayed.

Mobitz I/Wenckebach features

PR progressively lengthens until a P wave is not followed by a QRS.

Mnemonic for identifying Wenckebach

“Long, longer, drop = Wen, kee, bach.”

Mobitz II features

Constant PR interval; some P waves fail to conduct and QRS complexes are dropped.

Mobitz II QRS width

May be narrow or wide depending on the level of the block.

Distinction between Mobitz I and Mobitz II

Mobitz I = PR progressively lengthens. Mobitz II = PR stays constant.

Third-degree AV block features

P waves march independently of QRS; completely variable PR intervals.

Another name for third-degree AV block

Complete heart block.