BMS 302: Electrocardiography

What makes the inside of a resting cell membrane negative?

Uneven ion distribution — more Na⁺ outside, more K⁺ inside.

What maintains resting membrane potential?

Leakage channels (mainly K⁺ leak out) and Na⁺/K⁺ pump.

What channels are involved in action potentials?

Voltage-gated Na⁺, K⁺, and Ca²⁺ channels.

What ion movement causes depolarization?

Na⁺ (or Ca²⁺ in slow response) enters the cell.

What ion movement causes repolarization?

K⁺ leaves the cell.

What is a functional syncytium?

Heart cells acting as a single coordinated unit via gap junctions.

What is a pacemaker cell?

A cell that spontaneously generates action potentials.

What is an ectopic pacemaker?

An abnormal pacemaker outside the SA or AV node.

What is the isoelectric line?

The flat baseline on an ECG when no current flows.

List the order of depolarization through the heart.

1. SA node

2. AV node

3. Bundle of His

4. Right & Left bundle branches

5. Purkinje fibers.

What allows depolarization to spread cell-to-cell?

Gap junctions in intercalated discs.

What prevents direct spread from atria to ventricles?

Fibrous skeleton of the heart (connective tissue barrier).

SA node action potential type?

Slow response.

AV node action potential type?

Slow response.

Bundle of His, bundle branches, Purkinje fibers action potential type?

Fast response.

Atrial muscle fibers action potential type?

Fast response.

Ventricular muscle fibers action potential type?

Fast response.

Ectopic pacemaker cells (non-SA/AV) action potential type?

Usually slow response.

Which tissues can act as pacemakers?

SA node, AV node, Purkinje fibers, and ectopic pacemaker cells.

Which ions are higher inside the cell?

K⁺.

Which ions are higher outside the cell?

Na⁺ and Ca²⁺.

During resting potential (fast response), how do ions move?

Na⁺ – not much (1)
K⁺ – diffuses out (2)
Ca²⁺ – limited permeability (3)

During prepotential (slow response):

Na⁺ – slowly diffuses in (3)
K⁺ – reduced permeability out (4)
Ca²⁺ – slowly diffuses in (3)

The membrane potential becomes ______ during prepotential.

Less negative.

Increasing prepotential slope does what?

Increases rate of spontaneous depolarization (faster HR).

Decreasing prepotential slope does what?

Decreases rate of spontaneous depolarization (slower HR).

Which node has a steeper prepotential slope?

SA node (faster rate).

Slow Response (SA/AV nodes) depolarization ions?

Ca²⁺ – enters rapidly

Slow Response (SA/AV nodes) repolarization ions?

K⁺ – exits rapidly

Fast Response (Myocardium) depolarization ions?

Na⁺ – enters rapidly

Fast Response (Myocardium) plateau phase ions?

K⁺ – exits slowly; Ca²⁺ – enters slowly

Fast Response (Myocardium) repolarization ions?

K⁺ – exits rapidly (2)

What is electrocardiography?

Recording of the heart’s electrical activity over time.

P wave:

Atrial depolarization

QRS complex:

Ventricular depolarization (atrial repolarization hidden)

T wave:

Ventricular repolarization

Lead I measures between which limbs?

Right arm (-) and Left arm (+)

Lead II measures between which limbs?

Right arm (-) and Left leg (+)

Lead III measures between which limbs?

Left arm (-) and Left leg (+)

What is Einthoven’s Law?

Lead I + Lead III = Lead II

Increased P wave duration means?

Atrial enlargement.

Increased PR interval means?

AV node conduction delay (1° heart block).

Increased QRS duration means?

Bundle branch block or ventricular conduction delay.

Increasing heart rate does what to cycle length and QT interval?

Decreases both.

Decreasing heart rate does what?

Increases both cycle length and QT interval.

Right axis deviation:

Right ventricular hypertrophy, pulmonary hypertension.

Left axis deviation:

Left ventricular hypertrophy, systemic hypertension, obesity, pregnancy.

Cause of AV block?

Impaired conduction through AV node. ECG change: Prolonged PR interval or dropped QRS.

Cause of bundle branch block?

Blocked conduction in one bundle branch.
ECG change: Widened QRS complex.

Given Data Example:
Lead I = +0.9 mV, –0.1 mV → +0.8 mV
Lead II = +1.2 mV, –0.2 mV → +1.0 mV
Lead III = +0.5 mV, –0.3 mV → +0.2 mV

Mean electrical axis = 40°
Einthoven’s Law check: Lead I + Lead III = Lead II → 0.8 + 0.2 = 1.0 ✅