Conduction System of the Heart and Electrocardiography: Study Notes

Conduction System of the Heart

Overview of the Intrinsic Conduction System

The heart's intrinsic conduction system coordinates the rhythmic contractions necessary for effective blood circulation. Specialized cardiac muscle cells generate and transmit electrical impulses, ensuring the heart beats in a synchronized manner.

• Sinoatrial (SA) node: The primary pacemaker located in the right atrium; initiates each heartbeat by generating electrical impulses.

• Internodal pathways: Conduct impulses from the SA node through the atria, causing atrial contraction.

• Atrioventricular (AV) node: Located at the junction between atria and ventricles; delays the impulse to allow atria to contract before ventricles.

• Bundle of His (AV bundle): Transmits impulses from the AV node to the ventricles.

• Left and right bundle branches: Carry impulses down the interventricular septum toward the apex of the heart.

• Purkinje fibers: Distribute the impulse throughout the ventricular myocardium, triggering ventricular contraction.

• Moderator band: Found in the right ventricle; part of the conduction pathway.

Example: Damage to the SA node can result in arrhythmias, requiring artificial pacemaker intervention.

Electrocardiography (ECG/EKG)

Principles of ECG

An electrocardiogram (ECG) records the electrical activity of the heart over time, providing valuable information about heart rhythm and conduction.

• P wave: Represents atrial depolarization.

• QRS complex: Represents ventricular depolarization; Q wave is the depolarization of the interventricular septum, S wave is the final depolarization of the ventricles (left ventricle has more muscle and Purkinje fibers).

• T wave: Represents ventricular repolarization.

Example: A normal ECG tracing shows a regular sequence of P, QRS, and T waves.

ECG Intervals and Segments

Intervals and segments on the ECG correspond to specific phases of cardiac electrical activity and are used to diagnose conduction abnormalities.

• PR Interval: Time from the beginning of atrial depolarization to the beginning of ventricular depolarization (normal: 0.12–0.20 sec).

• QT Interval: Duration of ventricular action potential; includes both depolarization and repolarization.

• ST Segment: End of ventricular depolarization to start of repolarization.

Example: Prolonged PR interval may indicate first-degree heart block.

Heart Blocks and Clinical Significance

Abnormalities in ECG intervals can indicate various degrees of heart block, affecting the transmission of electrical impulses.

• First-degree heart block: Electrical signal slowed; prolonged PR interval.

• Second-degree heart block: Significant delay; some impulses fail to reach ventricles, may require treatment.

• Third-degree heart block: No communication between atria and ventricles; requires immediate intervention.

• Reduced PR Interval: Impulse not generated in SA node.

Example: Third-degree heart block can result in severe bradycardia and requires a pacemaker.

Refractory Periods in Cardiac Muscle

The refractory period is the time during which cardiac cells cannot initiate another action potential, preventing tetany and ensuring rhythmic contractions.

• Relative refractory period: A second action potential (AP) is possible if the stimulus is stronger than normal.

• Absolute refractory period: No second AP is possible; voltage-gated sodium channels are closed.

Example: The absolute refractory period prevents the heart from sustaining a contraction, which would be fatal.

Boundaries of ECG Components

Table: Boundaries of Each ECG Component

Feature	Boundaries
P wave	Start of P deflection to return to baseline
P-R interval	Start of P deflection to start of Q deflection
QRS complex	Start of Q deflection to S return to baseline
S-T segment	End of S deflection to start of T wave
Q-T interval	Start of Q deflection to end of T wave
T wave	Start of T deflection to return to baseline
T-P segment	End of T wave to start of next P wave
R-R interval	Peak of R wave to peak of next R wave

Abnormal ECGs and Clinical Significance

Table: Examples of Abnormal ECGs

Finding	Possible Clinical Significance
Enlarged R wave	Enlarged ventricles
Prolonged P-R interval	First-degree heart block; delayed signal from SA node to AV node
Prolonged Q-T interval	Increased risk of ventricular arrhythmias; delayed ventricular repolarization
S-T segment elevated from baseline	Myocardial infarction (heart attack)

Cardiac Output and Starling's Law

Cardiac Output Formula

Cardiac output (CO) is the volume of blood pumped by the heart per minute. It is determined by stroke volume (SV) and heart rate (HR).

• Formula:

$CO = SV \times HR$

• Starling's Law of the Heart: The force of contraction is proportional to the initial length of cardiac muscle fibers (preload); increased venous return increases stroke volume.

Example: During exercise, both SV and HR increase, leading to higher cardiac output.

ECG Recording and Electrode Placement

Standard Limb Leads and Electrode Placement

Proper placement of electrodes is essential for accurate ECG recording. Standard limb leads measure electrical activity from different angles.

• Lead I: Right arm to left arm

• Lead II: Right arm to left leg

• Lead III: Left arm to left leg

• Electrode colors: White (right forearm), Black (right leg, ground), Red (left leg)

Example: Incorrect electrode placement can result in misleading ECG readings.

ECG Data Collection and Interpretation

ECG data is collected using specialized equipment such as the BIOPAC® unit. Calibration and proper setup are necessary for reliable results.

• Calibration: Ensures accurate measurement of electrical signals.

• Data interpretation: Changes in ECG patterns can indicate physiological responses (e.g., exercise, posture changes) or pathological conditions.

Example: After exercise, ECG shows increased heart rate and changes in waveform amplitude.

Sequence of Depolarization and Repolarization

Relation to ECG Deflection Waves

The sequence of depolarization and repolarization in the heart is reflected in the deflection waves of the ECG tracing. Each phase corresponds to specific electrical and mechanical events in the cardiac cycle.

• Atrial depolarization: P wave

• Ventricular depolarization: QRS complex

• Ventricular repolarization: T wave

Example: Abnormalities in the sequence can indicate conduction defects or myocardial damage.

Sites of Auscultation of Heart Valves

Valve Locations and Clinical Relevance

Auscultation sites are used to listen to heart sounds and assess valve function. Each valve has a specific location on the chest for optimal sound detection.

• Aortic valve: Second intercostal space, right sternal border

• Pulmonic valve: Second intercostal space, left sternal border

• Tricuspid valve: Fourth intercostal space, left sternal border

• Mitral valve: Fifth intercostal space, midclavicular line

Example: Murmurs detected at these sites can indicate valvular disease.