Chapter 18: The Heart – Conduction, Rhythm, and Cardiac Function

The Cardiac Conduction System

Anatomy and Function of the Conduction System

The heart's conduction system is responsible for initiating and propagating electrical impulses that coordinate cardiac muscle contraction. This system ensures the heart beats in a synchronized manner, allowing efficient blood flow.

• Sinoatrial (SA) node: Located in the right atrium, acts as the primary pacemaker, generating impulses at 80-100 times per minute.

• Atrioventricular (AV) node: Receives impulses from the SA node, delays transmission (~100 ms) to allow atrial systole to complete.

• AV bundle (Bundle of His): The only electrical connection between atria and ventricles.

• Bundle branches: Right and left branches travel through the interventricular septum toward the apex.

• Purkinje fibers: Radiate upward from the apex, spreading depolarization throughout the ventricles.

Example: The AV node delay ensures that the atria contract and empty blood into the ventricles before ventricular contraction begins.

Impulse Propagation and Heartbeat

Electrical impulses travel from the SA node to the AV node, then through the AV bundle, bundle branches, and Purkinje fibers. This sequence initiates atrial and ventricular systole, ensuring coordinated contraction.

• Gap junctions: Allow rapid spread of impulses across atrial muscle.

• AV node delay: Permits completion of atrial systole before ventricular contraction.

• Ventricular contraction: Begins at the apex and progresses toward the base, ejecting blood superiorly.

Cardiac Arrhythmias and Heart Blocks

Types and Effects of Arrhythmias

Arrhythmias are abnormal heart rhythms caused by damage to the conduction system. They can impair pumping efficiency and may progress to life-threatening conditions like fibrillation.

• Junctional rhythm: Occurs when the SA node is defective; AV node takes over, resulting in slower heart rate (40-60 bpm).

• Heart block: Damaged AV node leads to impaired conduction (1°, 2°, 3° block).

• Ectopic focus: Hyperexcitable area acts as an abnormal pacemaker.

Example: Artificial pacemakers are used to restore normal rhythm in cases of severe heart block.

Papillary Muscles and Conduction

Bundle branches deliver impulses directly to papillary muscles via shortcut pathways, such as the moderator band. Papillary muscles contract slightly before the rest of the ventricular myocardium, tensing the chordae tendineae and bracing the AV valves.

Nerve Innervation of the Heart

Autonomic Nervous System Regulation

The heart receives dual innervation from the autonomic nervous system (ANS), which modulates intrinsic cardiac rhythm and contractility.

• Sympathetic fibers: Increase heart rate and contractility via norepinephrine (NE).

• Parasympathetic fibers: Decrease heart rate via acetylcholine (Ach) from the vagus nerve.

• Both divisions: Innervate SA and AV nodes, affecting heart rate.

Electrocardiography (ECG/EKG)

Principles and Diagnostic Use

Electrocardiography records the electrical activity of the heart using surface electrodes. The ECG is a composite of all electrical events and is used to diagnose arrhythmias and other cardiac conditions.

• P wave: Atrial depolarization

• QRS complex: Ventricular depolarization (obscures atrial repolarization)

• T wave: Ventricular repolarization

• PR interval: 120-200 ms

• QT interval: 400-430 ms

• R-R interval: 600-1000 ms (one cycle)

• ST segment: ~80 ms

Einthoven's Triangle and Lead Placement

Einthoven's triangle is formed by three limb leads (right arm, left arm, left leg) and is used in ECG to measure electrical activity from different angles. The heart is at the center of this triangle.

Heart Sounds and Auscultation

Valve Closure and Murmurs

Heart sounds are produced by valve closure events and are detected by auscultation (listening with a stethoscope). The 'lub' sound indicates AV valve closure, while 'dup' indicates semilunar valve closure. Murmurs are abnormal sounds caused by turbulent blood flow, often due to valve problems.

The Cardiac Cycle

Phases of the Cardiac Cycle

The cardiac cycle is the period between the start of one heartbeat and the beginning of the next. It consists of alternating contraction (systole) and relaxation (diastole) phases.

• Systole: Chamber contracts, pushing blood into adjacent chamber or vessel.

• Diastole: Chamber fills with blood, preparing for the next cycle.

Example: At a resting heart rate of 75 bpm, late diastole is usually chosen as the beginning of the cycle. Ventricles passively fill to ~80%, then atrial systole completes filling.

Volume Changes During the Cardiac Cycle

• End diastolic volume (EDV): ~120 ml

• Stroke volume (SV): ~70 ml (amount ejected per beat)

• Ejection fraction: ~60% of EDV

• End systolic volume (ESV): ~50 ml (remaining after contraction)

Factors Influencing Cardiac Function

Stroke Volume and Cardiac Output

Stroke volume (SV) is the amount of blood pumped by one ventricle with each beat. Cardiac output (CO) is the volume of blood pumped by each ventricle per minute.

• Formula: $SV = EDV - ESV$

• Formula: $CO = HR \times SV$

Example: For a heart rate (HR) of 75 bpm and SV of 70 ml, $CO = 5250 \text{ ml/min} = 5.25 \text{ liters/min}$

Preload, Afterload, and Contractility

• Preload: Degree of ventricular stretching during diastole; directly proportional to EDV.

• Afterload: Tension required to eject blood; inversely proportional to SV; increased by hypertension.

• Contractility: Force produced during contraction at a given preload; influenced by ANS and hormones.

Congestive Heart Failure and Congenital Defects

Heart Failure

Congestive heart failure occurs when cardiac output is insufficient to perfuse peripheral tissues. Causes include coronary atherosclerosis, hypertension, myocardial infarcts, and cardiomyopathy.

• Left heart failure: Pulmonary congestion and edema.

• Right heart failure: Peripheral congestion and edema.

Congenital Heart Defects

Congenital heart defects are structural abnormalities present at birth, often due to environmental influences during pregnancy. Tetralogy of Fallot is a common defect with four distinct abnormalities.

• Pulmonary trunk narrowing (stenosis)

• Right ventricle hypertrophy

• Ventricular septal defect

• Overriding aorta

Most defects can be treated surgically.