Cardiology Notes

Blood Circulation

  • Function: Provide adequate blood flow to every cell of the body.
  • Pumps: Two pumps (right ventricle for pulmonary circulation and left ventricle for systemic circulation) work in series.
  • Blood Flow: Blood pumped per unit time by both ventricles is equivalent.
  • Organ Blood Supply:
    • Most organs are arranged in parallel allowing for adjustment in blood supply without affecting others.
    • Exception: Liver receives blood from both the aorta and GI tract.

Cardiovascular System

  • Pulmonary Circulation: Blood flows within the lungs; gas exchange occurs between blood and alveolar air.
  • Systemic Circulation: Blood flows to organs excluding the lungs; involved in material exchange with interstitial fluid (ISF).

Resting Membrane Potential (RMP)

  • Definition: The electrical potential across a cell membrane when the cell is not actively conducting an impulse.
  • Determined by:
    1. Concentration gradients of ions (e.g., K+, Na+).
    2. Relative permeability/conductance of the membrane.
    3. Ion pumps (e.g., Na+-K+ ATPase).
  • Value: Approximately -90 mV due to:
    • Efflux of K+ through inward rectifying K+ channels (equilibrium potential: -94 mV).
    • Minimal inflow of Na+ and Ca++ (equilibrium potentials: +61 mV for Na+).
    • Activity of Na+-K+ ATPase (3 Na+ out for 2 K+ in).

Ionic Concentration Maintenance

  • During RMP:
    1. Continuous K+ efflux to maintain equilibrium.
    2. Slight Na+ influx.
  • During Action Potential:
    1. Elevated intracellular Na+ and Ca++ relative to K+ outflux.
  • Pumps emphasize the importance of ionic gradients including Na+-K+ ATPase and Ca++ ATPase.

Cardiac Automaticity and Rhythmicity

  • Automaticity: Heart's ability to initiate contraction without external stimuli due to pacemaker cells.
  • Rhythmicity: Heart's capability to beat consistently, largely controlled by SA node (90-105/min) and AV node (60/min).
  • Vagal Tone: Parasympathetic activity decreases heart rate versus sympathetic activity increases it.

Action Potential Phases in Pacemaker Cells

  • Phases of depolarization in SA and AV nodes:
    • Phase 4: Gradual depolarization caused by Na+ influx (funny channels) and inward Na+-Ca++ exchanger action.
    • Phase 0: Depolarization occurs quickly once the threshold (-40 mV) is reached through the closure of funny channels and opening of L-type Ca++ channels.
    • Phase 3: Repolarization occurs through delayed rectifying K+ channels, lowering the membrane potential back to resting levels.

Influence of Autonomic Activity

  • Sympathetic activity influences heart rate positively (chronotropic effect).
  • Parasympathetic activity has a negative chronotropic effect.
  • Other factors influencing heart rate include catecholamines, body temperature, and extracellular K+ levels.

Electrical Conduction System of the Heart

  • Sino-atrial Node (SA Node): Located in the right atrium, initiates action potential, conducts through internodal pathways to AV Node.
  • Atrioventricular Node (AV Node): Responsible for delaying conduction to allow ventricular filling before contraction.
  • Bundle of His and Purkinje Fibers: Transmit impulses rapidly to ensure synchronized contraction of ventricles.

Cardiac Cycle Phases

  • Phases:
    1. Atrial Systole - contraction of atria.
    2. Isovolumetric Contraction - ventricles contract, pressure rises but volume remains constant.
    3. Rapid Ejection - blood ejected from ventricles to aorta and pulmonary arteries.
    4. Isovolumetric Relaxation - cessation of ejection, chambers relax but no change in volume.
    5. Filling Phases - blood returns to the heart and fills the ventricles.
  • Duration: Cardiac cycle duration is approximately 0.8 seconds at a normal heart rate of 75 bpm.

Cardiac Output & Regulation Factors

  • Cardiac Output (CO): Volume pumped by each ventricle per minute, calculated as Stroke Volume (SV) × Heart Rate (HR).
  • Stroke Volume Influencers:
    • Preload: Degree of stretch of cardiac muscle before contraction; increased by factors leading to increased venous return.
    • Afterload: Pressure against which the heart must work; increased afterload decreases SV.
    • Inotropy: Relates to contractility, where increases in inotropic state lead to larger stroke volumes.

Cardiac Reserve

  • Definition: Maximum percentage increase in CO above normal in response to body demands.
  • Factors Affecting Cardiac Reserve:
    • Heart rate reserve and stroke volume reserve, with physical condition, age, and training affecting effectiveness.