Cardiac Conduction System Overview

Cardiac Conduction System

• Bundle branches end at Purkinje fibers for ventricular conduction, causing contraction and cardiac output.
• Three primary pacemaker cells possess automaticity to generate action potentials for heart depolarization.
• Sequential and synchronous functioning of pacemaker cells is essential for normal cardiac rhythm and adequate cardiac output.

Pacemaker Cell Details

• SA Node: Primary pacemaker with an intrinsic rate of 60-100 bpm, located in the right atrium.
• AV Node: Acts as a secondary pacemaker and triage station, intrinsic rate of 40-60 bpm, located in the intraatrial septum.
• Purkinje Fibers: Serve as the final conduction pathway, becoming dominant if both SA and AV nodes fail, with an intrinsic rate of 20-40 bpm.

ECG Waveforms and Segments

• P Wave: Indicates atrial contraction upon SA node firing.
• Atrial Kick: Refers to the left atrium's ability to eject blood into the left ventricle effectively.
• QRS Complex: Represents ventricular contraction; any delay indicates time taken for depolarization of both ventricles.
• ST Segment: Flat segment between S wave and beginning of T wave; reflects time of ventricular depolarization to repolarization, compared to the isoelectric line.
• J Point: Marks the transition from QRS complex to ST segment; ST elevation measured from this point.
• T Wave: Indicates ventricular repolarization; it should present as upright.
• QT Interval (0.33-0.43 seconds): Measures time for complete depolarization and repolarization; variations in length indicate electrolyte imbalances.

Heart Rate Calculation and Rhythm Assessment

• Heart rate calculated using R-R intervals from ECG strips.
• Conditions affecting heart rhythm: CAD, HF, cardiomyopathy, electrolyte imbalances, various stimulants (caffeine, nicotine).
• Low cardiac output can lead to hypotension.

Treatments for Abnormal Heart Rates

• Atropine: Administered for symptomatic bradycardia (<60 bpm).
• Beta-blockers and Calcium Channel Blockers: Used for tachycardia (>100 bpm).
• Amiodarone: Used to control high heart rates.

First-Degree AV Block

• All impulses reach ventricles with delayed AV conduction.
• Characterized by prolonged PR interval (>0.20 seconds) while QRS remains normal.
• Common etiologies include MI, CAD, and electrolyte imbalances.
• Monitoring with no immediate treatment required.

Second-Degree AV Block Type I (Mobitz I)

• Occurs with progressive lengthening of the PR interval until a QRS complex is dropped.
• Atrial rate is normal, but ventricular rate is irregular due to blocked conduction.
• Treatment focuses on underlying causes; beta-blockers or anti-arrhythmic agents may be used to prevent progression.

Ventricular Tachycardia (Vtach)

• Defined as more than three consecutive PVCs with a ventricular rate of 150-250 bpm.
• PR interval is not measurable; QRS is wide and distorted (>12 seconds).
• Torsades de pointes, a form of polymorphic Vtach, can be caused by low magnesium levels.

Coronary Artery Disease (CAD)

• Women have a higher rate overall, with African Americans experiencing earlier onset and severity.
• Risk factors include high lipid levels, tobacco use, obesity, hypertension, diabetes, and stress.
• Atherosclerosis, characterized by endothelial damage, is a primary cause leading to potential MI.
• Aspirin is used to reduce the risk of thrombus formation associated with plaque rupture.

Acute Coronary Syndrome (ACS)

• Encompasses conditions related to cardiac ischemia or infarction, including stable and unstable angina.
• Chronic Stable Angina: Intermittent chest pain relieved by rest or nitroglycerin.
• Unstable Angina/NSTEMI: Represents partial vessel occlusion and persistent chest pain unresponsive to treatment.
• ST-segment elevation MI (STEMI): Total occlusion resulting in irreversible myocardial damage.

Diagnostic Tests for Cardiac Events

• Troponin: Specific marker for acute MI, peaks in 4-6 hours and indicates cardiac muscle injury.
• 12-lead ECG: Essential for diagnosing ischemia and infarction; ST segment changes provide critical information.
• Other Tests: CK-MB for muscle injury, chest X-ray to rule out other causes, D-dimer for possible embolism.
• Echocardiogram and Stress Test: Evaluate heart function and ischemic response during exertion.

ECG Representations of Ischemia and Infarction

• Ischemia shows decreased oxygenation.
• Injury manifests as current ST elevation.
• Infarction indicates tissue necrosis with potential changes in the Q wave.
• NSTEMI: Characterized by ST depression with inverted T waves.
• STEMI: Defined by significant ST elevation in multiple leads, indicating severe myocardial damage.

This quiz covers the cardiac conduction system, focusing on the roles of bundle branches, Purkinje fibers, and pacemaker cells. It explains how these components work together to initiate contraction and ensure adequate cardiac output. Test your understanding of the automaticity and coordination necessary for normal cardiac rhythm.