Electrophysiology and Cardiac Arrhythmias

Cardiac Rhythm

• Normal rate: 60-100 beats per minute

• Impulse Propagation: sinoatrial node to the atrioventricular (AV node) to the His-Purkinje followed by distribution throughout the ventricle

• Normal AV nodal delay (0.15 seconds) -- sufficient to allow atrial ejection of blood into the ventricles

• Definition: arrhythmia -- cardiac depolarization different from above sequence --

  • abnormal origination (not SA nodal)

  • abnormal rate/regularity

  • abnormal conduction characteristics

  
  

  Cardiac Electrophysiology

• Transmembrane potential -- determined primarily by three ionic gradients:

  • Na⁺, K⁺, Ca ²⁺

    • water-soluble, -- not free to diffuse through the membrane in response to concentration or electrical gradients: depended upon membrane channels (proteins)

  • Movement through channels depend on controlling "molecular gates"

    • Gate-status controlled by:

      • Ionic conditions

      • Metabolic conditions

      • Transmembrane voltage

  • Maintenance of ionic gradients:

    • Na⁺/K⁺ ATPase pump

    • termed "electrogenic" when net current flows as a result of transport (e.g., three Na⁺ exchange for two K⁺ ions)

  • Initial permeability state -- resting membrane potential

    • sodium -- relatively impermeable

    • potassium -- relatively permeable

  • Cardiac cell permeability and conductance:

    • conductance: determined by characteristics of ion channel protein

    • current flow = voltage X conductance

    • voltage = (actual membrane potential - membrane potential at which no current would flow, even with channels open)

  • Sodium

    • Concentration gradient: 140 mmol/L Na⁺ outside: 10 mmol/L Na⁺ inside;

    • Electrical gradient: 0 mV outside; -90 mV inside

    • Driving force -- both electrical and concentration -- tending to move Na⁺ into the cell.

    • In the resting state: sodium ion channels are closed therefore no Na⁺ flow through the membrane

    • In the active state: channels open causing a large influx of sodium which accounts for phase 0 depolarization

    
    

    • Potassium:

      • Concentration gradient (140 mmol/L K⁺ inside; 4 mmol/L K⁺outside)

      • Concentration gradient -- tends to drive potassium out

      • Electrical gradient tends to hold K⁺ in.

      • Some K⁺ channels ("inward rectifier") are open in the resting state -- however, little K⁺ current flows because of the balance between the K⁺ concentration and membrane electrical gradients

      • Cardiac resting membrane potential: mainly determined

        1. By the extracellular potassium concentration and

        2. Inward rectifier channel state

    • Spontaneous Depolarization (pacemaker cells)-- phase 4 depolarization

      • Spontaneous Depolarization occurs because:

        • Gradual increase in depolarizing currents (increasing membrane permeability to sodium or calcium)

        • Decrease in repolarizing potassium currents (decreasing membrane potassium permeability)

        • Both

      • Ectopic pacemaker: (not normal SA nodal pacemakers) --

        • Facilitated by hypokalemic states

        • Increasing potassium: tends to slow or stop ectopic pacemaker activity

    • Channel Activation Sequence:

      • Depolarization to threshold voltage--Na⁺

        • m gate activation (activation gate); assuming inactivation (h) gates are not closed then

        • sodium permeability dramatically increased; intense sodium current

        • depolarization

        • h gate closure; Na⁺ current inactivation

      • Ca²⁺ --

    Ca^2+: Channel Activation Sequence similar to sodium; but occurring at more positive membrane potentials

Category: Physiology Notes