Cardio-Vascular System Quiz

Introduction to the Cardiovascular System

• Cardiac muscle cells interconnect via intercalated discs (gap junctions).
• Myocardium refers to the contracting area from one stimulation event.
• Electrical isolation of atria and ventricles is due to the fibrous skeleton.

Automatic Rhythmicity and Pacemaker Function

• The sinoatrial (SA) node is the primary pacemaker, located in the right atrium.
• It has the highest rate of self-excitation and intrinsic discharge.
• Atrioventricular (AV) node and Purkinje fibers serve as secondary or ectopic pacemakers.

Electrical Conduction Pathway

• Action potentials propagate through intercalated discs from the SA node to the AV node, facilitating atrial contraction.
• AV node is positioned at the base of the right atrium and connects to the bundle of His for ventricular stimulation.
• The bundle of His splits into right and left bundle branches and eventually into Purkinje fibers, which initiate ventricular contraction.

Cardiac Muscle Contraction

• Sodium ions (Na+) leak into sinus nodal fibers, raising membrane potential to a threshold of -40 millivolts.
• Opening of Ca2+ and Na+ channels triggers the action potential.
• Ca2+ diffuses into myofibrils, leading to contraction through actin-myosin filament sliding.

Repolarization Process

• Ca2+ levels decrease in the cytoplasm through transport back to the sarcoplasmic reticulum (SR) and Na+-Ca2+ exchanger.
• Myocardium transitions to a relaxed state after repolarization.

Differences: Cardiac vs. Skeletal Muscle

• Cardiac muscle relies on substantial Ca2+ influx from T-tubules for contraction.
• Cardiac muscle's sarcoplasmic reticulum is less developed than in skeletal muscle, but T-tubules are significantly larger.

Electrocardiogram (ECG or EKG)

• ECG measures the heart’s electrical activity via ion movement, not direct action potentials.
• It represents electrical events leading to heart contraction and relaxation through depolarization waves.

ECG Waves and Intervals

• P wave corresponds to atrial depolarization.
• P-Q interval indicates atrial systole.
• QRS wave represents ventricular depolarization.
• S-T segment indicates plateau phase during ventricular systole.
• T wave signifies ventricular repolarization.

Implications of Electrolyte Abnormalities on ECG

• Hyperkalaemia (serum potassium > 5.2 mmol/L) results in notable ECG changes; T wave amplitude increases, P waves flatten, and QRS widens.
• Hypokalaemia yields T wave flattening or inversion, ST depression, and prominent U waves.

ECG Leads

• Bipolar limb leads measure voltage between electrodes on wrists and legs.
• Unipolar leads involve a single electrode on the body connected to ECG machinery.
• Includes multiple limb leads (AVR, AVL, AVF) and six chest leads.

Heart Sounds and ECG Relationship

• "Lub" sound coincides with the QRS wave when AV valves close.
• "Dub" sound occurs at the beginning of the T wave when semilunar valves close.

Heart Arrhythmias Detected by ECG

• Bradycardia: heart rate below 60 bpm; possible normal in active individuals.
• Tachycardia: heart rate exceeding 100 bpm; can be caused by drugs or ectopic pacemakers.
• Ventricular tachycardia: dangerous condition leading to ineffective pumping and potential sudden death.

Flutter and Fibrillation

• Flutter involves rapid yet coordinated contractions (200-300 bpm).
• Fibrillation signifies uncoordinated contractions between atria and ventricles.

Specific Types of Fibrillation

• Atrial fibrillation reduces cardiac output by 15% and increases thrombi risk.
• Ventricular fibrillation leads to mortality without intervention by CPR or defibrillation.

AV Node Block

• Changes in P-R interval indicate AV node damage.
• First-degree block shows prolonged impulse conduction (>0.2 sec).
• Second-degree block prevents every electrical wave from reaching ventricles.
• Third-degree block results in no stimulation to the ventricles, necessitating a slower pacemaker from Purkinje fibers (20-40 bpm).

Introduction to Cardiac Electrical Activity

• Cardiac muscle cells interconnected by intercalated discs allow for rapid impulse propagation.
• Myocardium refers to the area of the heart contracting from a single stimulation event.
• Atria and ventricles are electrically insulated by the fibrous skeleton.

Automatic Rhythmicity of the Heart

• Sinoatrial (SA) node is the primary pacemaker with the highest discharge rate.
• Automaticity refers to the heart's inherent ability to beat without external stimulation.
• Atrioventricular (AV) node and Purkinje fibers can act as secondary pacemakers with a slower discharge rate.

Conducting Tissues of the Heart

• Action potentials propagate through intercalated discs, from the SA node to the AV node, initiating atrial contraction.
• AV node is located at the right atrium's base; it conducts impulses to the ventricles via the bundle of His.
• Bundle of His branches into right and left bundle branches, which convert to Purkinje fibers for ventricular contraction.

Self-Excitation of Nodal Fibers

• Sodium ions leak into SA nodal fibers, raising membrane potential to a threshold of -40 mV, opening Ca²⁺ and Na⁺ channels.
• Inherent leakiness to Na⁺ in SA node contributes to its self-excitation.

Mechanism of SA Node Rhythmicity

• Sympathetic activity involves epinephrine and norepinephrine, increasing heart rate and contractility.
• Parasympathetic activity releases acetylcholine, opening K⁺ channels, slowing heart rate.
• Membrane potential of SA node fibers is between -55 mV to -60 mV.

Action Potentials in Cardiac Muscle

• Resting membrane potential for cardiac muscle is around -85 mV to -90 mV.
• Ventricular membrane potential can rise to +20 mV during depolarization.
• Action potential plateau phase in cardiac muscle is significantly longer than in skeletal muscle.

Prolonged Action Potential

• In cardiac muscle, fast Na⁺ and slow Ca²⁺ channels open, creating a longer depolarization phase.
• Calcium influx sustains muscle contractility, while delayed K⁺ permeability contributes to repolarization.

Excitation-Contraction Coupling

• Calcium-initiated calcium release occurs; action potentials open voltage-gated Ca²⁺ channels.
• Ca²⁺ stimulates contractile mechanisms via binding to troponin.

Electrocardiogram (ECG) and Heart Sounds

• "Lub" sound occurs after the QRS wave as AV valves close.
• "Dub" sound occurs at the start of the T wave as semilunar valves close.

Heart Arrhythmias

• Bradycardia indicates a heart rate below 60 bpm; tachycardia indicates a rate above 100 bpm, potentially abnormal at rest.
• Ventricular tachycardia involves ventricles contracting independently of the atria, posing a significant danger.

Flutter and Fibrillation

• Atrial flutter consists of fast but coordinated contractions (200-300 bpm).
• Fibrillation indicates uncoordinated contractions between atria and ventricles.

Types of Atrial Fibrillation

• Can result from atrial flutter, resulting in ineffective atrial contractions while maintaining a 15% reduction in cardiac output.
• Associated with increased risks of stroke and heart failure.

Types of Ventricular Fibrillation

• It leads to the ventricles failing to pump blood, requiring immediate CPR or defibrillation.
• Caused by continuous cycling of electrical waves with a risk of sudden death from progressing tachycardia to fibrillation.

AV Node Block

• Damage to the AV node reflects changes in the P-R interval on an ECG.
• First-degree block shows prolonged impulse conduction; second-degree block results in missed impulses; third-degree block results in complete failure of conduction requiring a slow pacemaker substitute.