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Flashcards in ECG Deck (47):

P wave

Atrial depolarization (sum of R + L)

Smaller amplitude due to less total muscle mass


QRS Complex

Ventricular depolarization; occurs simultaneously as atrial repolarization, obscuring the signal

Upward deflection of R wave corresponds to phase 0 of action potential; isoelectric ST segment links QRS to T wave and corresponds to phase 2 of the action potential (long plateau)

Normal duration is 0.06-0.10s


T wave

Repolarization of the ventricles, corresponding to phase 3 of the action potential

T wave (hyperpolarization moving away from the lead) should deflect in the same direction as QRS (depolarization moving toward the lead)


QT Interval

Total duration of ventricular depolarization and repolarization


PR interval

Spans from the onset of the P wave to the onset of the QRS complex

Represents AV node conduction time

Normal duration is 0.12-0.20s


ECG paper speed

25 mm/s


ECG chronology - small vs. large squares

Thin lines (small squares) are spaced 0.04s apart

Thick lines (large squares) are spaced 0.2s apart


Unipolar leads

Measure the difference in electrical potential between one point on the body and a virtual reference point set at 0 electrical potential, located in the center of the heart

aVR, aVL, aVF


Bipolar leads

Measure the difference in electrical potential between two different points on the body

Standard Limb Leads I, II, and III


ST depression - Interpretation

Caused by ischemia due to sudden high oxygen demand in the presence of a fixed coronary obstruction

Resting ST segment is normal but during exercise there is a ST depression due to transient ischemia


T wave inversion - Interpretation

Caused by ischemia due to acute coronary artery obstruction during low oxygen demand; ischemia may be transient or result in tissue injury

Recall that normally T waves deflect in the same direction as QRS complex


ST elevation - Interpretation

Sign of transmural cardiac injury in an acute coronary syndrome, usually acute myocardial infarction


Significant Q wave - Definition & Interpretation

Defined as a Q wave that is:

1. Greater than or equal to 1/4 the amplitude of the R wave
2. Greater than or equal to one small box (0.04s) wide
3. Seen in at least 2 leads reflecting the same region of the ventricle

Absence of normal transmural vector produces a negative deflection in leads over infarcted tissue


Lateral Leads

I and AVL


Inferior Leads

II, III, and AVF


Right Chest Leads

V1 and V2

Monitor the RV


Left Chest Leads

V5 and V6

Monitor the LV


ECG features of transmural myocardial infarction

ST elevation with Q waves


ECG features of subendocardial myocardial infarction

ST depression without Q waves


QT Interval - Definition & Causes

Defined as a QT interval that is more than half of the RR interval

Caused by:
Hypocalcemia, hypokalemia, hypomagnesemia
Class 1A or 3 anti-arrhythmic drugs
Congenital Long QT Syndrome


ECG features of Left Ventricular Hypertrophy

Big R waves in L-sided leads:

I, aVL, V5, V6


ECG features of Right Ventricular Hypertrophy

Big R waves in R-sided leads:

V1, V2


ECG features of Hypercalcemia

Shortened QT interval


ECG features of Hypocalcemia

Prolonged QT interval


U wave

A small, variable wave following the T wave; thought to represent repolarization of the Purkinje fibers


Sinus Tachycardia

Regular, fast heart rate > 100bpm with normal P and QRS features

Commonly occurs during exercise or emotional stress; increased cardiac oxygen demand may precipitate angina in patients with coronary artery disease

No treatment usually needed; may be treated with beta blockers, if severe


Sinus bradycardia

Regular, slow heart rate ( seen in the elderly

Treatment with atropine or cardiac pacemaker


1st degree AV block

Defined as a PR interval > 0.2 seconds; interpreted as delayed conduction through the AV node

Caused by drugs (B-blockers, digitalis) and conduction system disease


2nd degree AV block

Some P waves conduct normally to the ventricles but others do not and therefore are not followed by R waves

Rate may be too slow to support adequate CO, resulting in syncope or confusion requiring the use of a pacemaker

Caused by conduction system disease and high vagal tone


3rd degree AV block

P waves and QRS show regular rhythm but occur at different rates with P rate > QRS rate

Caused by AV node failure secondary to severe conduction system disease; may cause syncope or sudden death, usually requires a pacemaker


Atrial Flutter

P waves flutter at rate of 240-320 bpm; pulse may be regular or irregular and ventricular rates vary but are typically rapid

Some risk of embolic stroke due to clot in the left atrium; treated with Warfarin for anti-coagulation and B-blockers, Ca2+ channel blockers, or Digoxin for rate control


Atrial Fibrillation

Irregular ventricular rhythm without P waves

Risk of embolic stroke due to atrial thrombi, heart failure due to rapid heart rate and loss of atrial kick

Treatment: Anti-coagulation, cardioversion (electrical or medical), and rate control with drugs


Atrial Tachycardia

Characterized by rapid HR (>180 bpm) with narrow QRS complexes and abnormal P waves

Terminated by adenosine infusion; recurrence prevented by ablation of re-entry pathway


Junctional Rhythm

Regular rhythms arising from the AV node; usually characterized by a narrow QRS without P waves, which are buried within the QRS signal

When P waves are present they are usually inverted because the wave is conducted upward from the AV node rather than downward from the SA node


Ventricular Tachycardia

Ectopic ventricular focus conducted by slow myocardium; caused by fibrosis, infiltrate, dilation - long path allows re-entry

Characterized by repetitive wide-abnormal QRS without preceeding P wave


Lead I

Standard (Bipolar) Limb Lead

L. arm (+)
R. arm (-)


Lead II

Standard (Bipolar) Limb Lead

L. foot (+)
R. arm (-)


Lead III

Standard (Bipolar) Limb Lead

L. foot (+)
L. arm (-)



Augmented (Unipolar) Limb Lead

L. foot (+)



Augmented (Unipolar) Limb Lead

R. arm (+)



Augmented (Unipolar) Limb Lead

L. arm (+)


V1 & V2

Precordial Chest Leads

R. ventricle + septum


V3 and V4

Precordial Chest Leads



V5 and V6

Precordial Chest Leads

L. Ventricle


Evolution of ECG in transmural myocardial infarct

1. Peaked T wave ("hyperacute T wave")

2. T-wave inversion - sign of ischemia

3. ST elevation - sign of transmural, ischemic injury occurring within 1-2 hours of insult

4. Q wave + ST elevation + T inversion - typical of transmural infarcts


5. ST depression with no Q wave - typical of subendocardial infarcts


Atrial Premature Beats

P wave occurs too early following the last T wave and looks slightly different from other P waves; QRS is normal

Mostly benign


Premature Ventricular Contraction (PVC)

Depolarization originates in the ventricle and contraction occurs via contractile myocytes

Characterized by wide QRS signal without P waves

Common in normal subjects; treatment usually not required