Electrocardiogram: Decoding the Heart's Secret Language

Comprehensive practice flashcards covering the basic principles, components, intervals, and common abnormalities of the electrocardiogram (ECG) based on Internal Medicine II lecture notes.

What is an Electrocardiogram (ECG/EKG)?

A graphical representation of electrical activity generated by the heart, utilized as a noninvasive, inexpensive, and highly versatile test.

Which three components produce the electric currents that spread through the heart?

Cardiac pacemaker cells, specialized conduction tissue, and the heart muscle itself.

What are the two specific types of electrical potentials recorded by an ECG?

Depolarization (stimulation) and repolarization (recovery) potentials.

Which structure in the conduction system possesses spontaneous automaticity and provides normal control of the heart rate?

The sinoatrial (SA) node.

What is the function of the atrioventricular (AV) node in the conduction system?

It slows the conduction time and is responsible for the normal delay between atrial and ventricular conduction.

What is the correct sequence of the ventrical conduction system after the AV Node?

Bundle of HIS $\rightarrow$ Bundle Branches $\rightarrow$ Purkinje Fibers.

What does the P wave represent on an ECG?

Atrial depolarization.

What does the QRS complex represent on an ECG?

Ventricular depolarization and contraction.

What do the ST segment and T wave represent on an ECG?

The plateau phase and repolarization phase, which facilitate ventricular relaxation and filling.

At a standard recording speed of $25\,mm/s$, how much time do the smallest ($1\,mm$) horizontal divisions represent?

$40\,ms$ ($0.04\,s$).

What time interval is represented by the heavier lines (5 mm) on ECG graph paper?

$200\,ms$ ($0.20\,s$).

What is the voltage calibration for a standard ECG record?

$1\,mV = 10\,mm$.

What is the normal duration of the PR interval?

$120-200\,ms$ ($0.12-0.20\,s$), which corresponds to $3$ to $5$ small squares.

What is the normal duration of the QRS interval?

$100-110\,ms$ or less (less than $3$ small squares).

What is the formula for the "300 method" of determining heart rate?

$300 / \text{number of big squares between consecutive R waves}$.

What is the formula for the "1500 method" of determining heart rate?

$1500 / \text{number of small squares between consecutive R waves}$.

What are the four major sets of ECG intervals mentioned in the lecture?

$RR$, $PR$, $QRS$, and $QT/QTc$.

What are the characteristics of a normal P wave in Lead II?

It is always positive, with a duration of $< 120\,msec$ and an amplitude of $< 2.5\,mm$ (small squares).

How are the 12 conventional ECG leads divided?

Six limb (extremity) leads that record the frontal plane and six chest (precordial) leads that record the horizontal plane.

What is the normal range for a QRS axis?

$-30^\circ$ to $+90^\circ$.

What is indicated by an axis more positive than $+90^\circ$ to $+100^\circ$?

Right axis deviation.

What is indicated by an axis more negative than $-30^\circ$?

Left axis deviation.

What clinical abnormality is associated with a tall upright P wave in Lead II (greater than $2.5\,mm$)?

Right Atrial Abnormality (P pulmonale).

What clinical abnormality is characterized by a P wave duration $> 120\,msec$ and prominent notching in Lead II?

Left Atrial Abnormality (P mitrale).

What is the Sokolow-Lyon voltage criterion for Left Ventricular Hypertrophy (LVH)?

$SV_1 + RV_5 > 3.5\,mV$ (or $RaVL > 1.1\,mV$).

What are the Cornell voltage criteria for LVH in men and women?

$SV_3 + RaVL > 2.8\,mV$ for men and $SV_3 + RaVL > 2.0\,mV$ for women.

What is the QRS duration threshold for a complete bundle branch block?

$\ge 120\,ms$.

What characterizes a First-degree heart block?

The conduction time is prolonged ($PR > 0.20\,s$), but all impulses are conducted.

What is the difference between Mobitz Type I and Mobitz Type II second-degree AV blocks?

Mobitz Type I (Wenckebach) involves progressive PR lengthening until a drop; Mobitz Type II involves sudden conduction blocks without prior PR lengthening.

Which ECG changes are associated with transmural vs. subendocardial acute ischemia?

Transmural ischemia causes ST Segment elevation; Subendocardial ischemia causes ST segment depression.

What is Electrical Alternans and what condition is it typically associated with?

Beat-to-beat alternation of QRS amplitude, often associated with pericardial effusion.

How is Low Voltage QRS defined on an ECG?

$< 5\,mm$ in limb leads and $< 10\,mm$ in chest leads.