12-Lead ECG Interpretation Lecture Review

Vocabulary flashcards covering 12-lead ECG fundamentals, lead placement, vector axes, hypertrophy criteria, bundle branch blocks, Sgarbossa criteria, and key clinical syndromes.

Lead I

A bipolar frontal plane limb lead formed between the right arm and left arm electrodes, with the left arm acting as the positive terminal.

Lead II

A bipolar frontal plane limb lead formed between the right arm and left leg electrodes, with the left leg acting as the positive terminal.

Lead III

A bipolar frontal plane limb lead formed between the left arm and left leg electrodes, with the left leg acting as the positive terminal.

Augmented Voltage Leads

The unipolar leads aVR, aVL, and aVF, which use mathematical formulas to calculate a central negative reference between two limb electrodes and amplify the recorded electrical voltage by approximately $50\%$.

Wilson Central Terminal

A reference point calculated by combining the three limb lead inputs to create a central negative reference within the heart for the horizontal precordial leads ($V_1$ through $V_6$).

Standard ECG Calibration Settings

Standard monitor and printer operating parameters calibrated to a frequency response of $0.05\text{ to }150\,\text{Hz}$ and a paper print speed of $25\,\text{mm/s}$.

Pathological Q Wave

A Q wave with a width greater than one small box, present in two or more contiguous leads, and with an amplitude greater than $25\%$ of the following R wave.

Mean Electrical Axis

The average overall direction of ventricular depolarization traveling through the heart, measured in degrees across the frontal plane hexaxial system.

Normal Axis

A mean electrical frontal plane axis falling between $-30^\circ$ and $+90^\circ$.

Right Axis Deviation

A mean electrical frontal plane axis falling between $+90^\circ$ and $+180^\circ$, commonly associated with pulmonary hypertension, pulmonary emboli, emphysema, or tall, thin anatomy.

Left Axis Deviation

A mean electrical frontal plane axis falling between $-30^\circ$ and $-90^\circ$, often associated with obesity, pregnancy, or left ventricular hypertrophy.

Extreme Axis Deviation

A mean electrical frontal plane axis falling between $-90^\circ$ and $-180^\circ$, occasionally observed in conditions such as dextrocardia.

Sokolow-Lyon Criteria

A method to evaluate left ventricular hypertrophy, satisfied when the depth of the S wave in lead $V_1$ plus the amplitude of the tallest R wave in lead $V_5$ or $V_6$ is greater than $35$ small boxes ($\text{mm}$).

Right Ventricular Hypertrophy (RVH)

A ventricular abnormality recognized on an ECG by tall R waves measuring $7\,\text{mm}$ or greater in lead $V_1$, commonly accompanied by right axis deviation.

Bundle Branch Block

An intraventricular conduction defect where an impulse arrives supraventricularly but conduction is delayed in one of the branches, resulting in a widened QRS duration of greater than $120\,\text{ms}$.

William Marrow Mnemonic

A memory aid for distinguishing bundle branch blocks: left bundle branch block features a "W" shape in lead $V_1$ and an "M" shape in lead $V_6$, while right bundle branch block features an "M" shape (rabbit ears) in lead $V_1$ and a "W" shape in lead $V_6$.

Concordance

An ECG finding where the ST segment deviates in the same direction as the primary deflection of the QRS complex.

Discordance

An ECG finding where the ST segment deviates in the opposite direction from the primary deflection of the QRS complex.

Modified Sgarbossa Criteria

A non-point-based diagnostic tool to detect acute MI in the presence of a left bundle branch block or paced rhythm, assessing for concordant ST elevation $\ge 1\,\text{mm}$, concordant ST depression in $V_1$-$V_3$ $\ge 1\,\text{mm}$, or discordant ST elevation $\ge 25\%$ of the depth of the preceding S wave.

Monofascicular Block

A conduction block occurring in a single fascicle of the cardiac bundle branch system.

Bifascicular Block

A conduction delay involving two divisions of the ventricular conduction system, most commonly a right bundle branch block combined with either a left anterior or left posterior fascicular block.

Trifascicular Block

A conduction delay involving the right bundle branch in combination with either the main left bundle branch or both the left anterior and left posterior fascicles.

Pericarditis

An inflammatory process affecting the pericardium that produces widespread, global ST segment elevation across nearly all leads with sparing/depression in lead aVR.

Delta Wave

A slurred upstroke at the beginning of the QRS complex caused by ventricular pre-excitation across an accessory conduction pathway in Wolff-Parkinson-White syndrome.

Brugada Syndrome

A congenital condition linked to a cardiac sodium channel gene mutation, characterized by ST elevation patterns (such as coved, saddleback, or downsloping) in the precordial leads and high risk of sudden cardiac events.

Wellens' Syndrome

A clinical presentation characterized by biphasic (Type A) or deeply inverted (Type B) T waves in leads $V_2$ and $V_3$, indicating critical stenosis of the proximal left anterior descending coronary artery.

"Widowmaker" Infarction

An acute occlusion of the left anterior descending (LAD) coronary artery identified by significant ST segment elevation across all septal ($V_1$, $V_2$) and anterior ($V_3$, $V_4$) leads.

Common Causes of Right Axis Deviation

• Pulmonary hypertension

• Acute or chronic pulmonary embolism

• Emphysema (hyperinflation causing a low diaphragm)

• Tall, thin body habitus (anatomically vertical heart)

Common Causes of Left Axis Deviation

• Left ventricular hypertrophy (LVH)

• Obesity or pregnancy (elevated diaphragm shifting heart horizontally)

• Large abdominal tumors or ascites

• Impostor morphology from hyperkalemia

Etiologies of Poor R-Wave Progression

• Left bundle branch block

• Prior or acute anterior myocardial infarction

• Left ventricular hypertrophy

• Severe emphysema

• Incorrect precordial lead placement

Primary Etiologies of Left Ventricular Hypertrophy (LVH)

• Chronic systemic hypertension (most common)

• Aortic stenosis

• Aortic regurgitation

• Cardiomyopathy

• Exogenous anabolic steroid use

Common Etiologies of Right Ventricular Hypertrophy (RVH)

• Pulmonary hypertension

• Recurrent or chronic pulmonary emboli

• Chronic pulmonary disease

• Congenital heart lesions

Underlying Cause of Wolff-Parkinson-White (WPW) Pre-excitation

An anomalous accessory pathway that bypasses the AV node, conducting impulses directly to ventricular myocardium.

Genetic Etiology of Brugada Syndrome

A congenital mutation affecting the cardiac sodium channel gene.

Underlying Pathology of Wellens' Syndrome

Critical stenosis of the proximal left anterior descending (LAD) coronary artery.

Pathophysiological Cause of QRS Widening in Bundle Branch Blocks

Interruption of rapid conduction through specialized bundle branches, forcing depolarization to spread slowly via cell-to-cell myocardial conduction.

Common Causes of Right Axis Deviation

• Pulmonary hypertension

• Acute or chronic pulmonary embolism

• Emphysema (hyperinflation causing a low diaphragm)

• Tall, thin body habitus (anatomically vertical heart)

Common Causes of Left Axis Deviation

• Left ventricular hypertrophy (LVH)

• Obesity or pregnancy (elevated diaphragm shifting heart horizontally)

• Large abdominal tumors or ascites

• Impostor morphology from hyperkalemia

Etiologies of Poor R-Wave Progression

• Left bundle branch block

• Prior or acute anterior myocardial infarction

• Left ventricular hypertrophy

• Severe emphysema

• Incorrect precordial lead placement

Primary Etiologies of Left Ventricular Hypertrophy (LVH)

• Chronic systemic hypertension (most common)

• Aortic stenosis

• Aortic regurgitation

• Cardiomyopathy

• Exogenous anabolic steroid use

Common Etiologies of Right Ventricular Hypertrophy (RVH)

• Pulmonary hypertension

• Recurrent or chronic pulmonary emboli

• Chronic pulmonary disease

• Congenital heart lesions

Underlying Cause of Wolff-Parkinson-White (WPW) Pre-excitation

An anomalous accessory pathway that bypasses the AV node, conducting impulses directly to ventricular myocardium.

Genetic Etiology of Brugada Syndrome

A congenital mutation affecting the cardiac sodium channel gene.

Underlying Pathology of Wellens' Syndrome

Critical stenosis of the proximal left anterior descending (LAD) coronary artery.

Pathophysiological Cause of QRS Widening in Bundle Branch Blocks

Interruption of rapid conduction through specialized bundle branches, forcing depolarization to spread slowly via cell-to-cell myocardial conduction.

Culprit Coronary Artery in Inferior Wall Infarction (Leads II, III, and aVF)

Right coronary artery (RCA).

Culprit Coronary Artery in Anterior Wall Infarction (Leads $V_3$ and $V_4$)

Left anterior descending (LAD) coronary artery.

Culprit Coronary Artery in Septal Wall Infarction (Leads $V_1$ and $V_2$)

Left anterior descending (LAD) coronary artery (septal branches).

Culprit Coronary Artery in Lateral Wall Infarction (Leads I, aVL, $V_5$, and $V_6$)

Left circumflex (LCx) coronary artery.

Culprit Coronary Artery in Posterior Wall Infarction (Leads $V_7$, $V_8$, and $V_9$)

Left circumflex (LCx) coronary artery or posterior descending artery (PDA, typically originating from the right coronary artery).

Culprit Coronary Artery in Right Ventricular Infarction (Leads $V_1\text{R}$ through $V_6\text{R}$)

Right coronary artery (RCA), particularly a proximal occlusion.

Culprit Vessel in Extensive Anteroseptal ("Widowmaker") Infarction

Proximal left anterior descending (LAD) coronary artery.

Left Main Coronary Artery (LMCA)

The major coronary artery trunk originating directly from the aorta that bifurcates into the left anterior descending (LAD) and left circumflex (LCx) arteries.