Basic Electrophysiology Flashcards
normal extra-cellular ion concentrations
general: high sodium, chloride, & calcium concentrations outside the cell; low potassium concentration outside the cell
normal intra-cellular ion concentrations
general: LOW sodium, chloride, & calcium concentrations inside the cell; HIGH potassium concentration inside the cell
importance of the cardiac action potential & its phases
*foundational to the formation of various components of the ECG
*allows insight into the mechanisms of clinical arrhythmias
propagation of cardiac electrical activation - overview
SA node (sinus node) → atria → AV node → bundle of His → right and left bundle branches → Purkinje fibers → ventricles
note: the left bundle branch divides into left anterior and posterior fascicles
cardiac conduction system - overview
*specialized myocardial cells
*critical jobs:
-initiation of the heart beat (by the sinus node)
-coordinated contraction of the atria
-pause between atrial contraction and ventricular contraction to allow for atrial emptying (AV node responsible for the pause; allows time for blood to flow from atria to ventricles)
-coordinated ventricular contraction (starting with papillary muscles)
sinus node (SA node) - general principles
*pacemaking activity at the sinus node
*this region has to have some “automaticity” to start things off (automaticity = the tissue fires off on its own at a specific rate or a certain number of beats/min)
*this requires that the action potential of the SA node is different from that of atrial or ventricular myocardium
action potential of the sinus (SA) node
*“slow response” tissue action potential (“pacemaker action potential”)
*ion carrying current during upstroke: Ca2+-dependent
*conduction: SLOW conduction (decremental)
*recovery from inactivation: TIME-dependent
*has its own automaticity
phases of the pacemaker action potential in the SA and AV nodes (detailed)
*phase 0: UPSTROKE- opening of voltage-gated Ca2+ channels; fast voltage-gated Na+ channels are permanently inactivated because of the less negative resting potential of these cells; results in a slow conduction velocity, which is used by the AV node to prolong transmission from the atria to ventricles
*phase 3: REPOLARIZATION - inactivation of the Ca2+ channels and increased activation of K+ channels → increased K+ efflux
*phase 4: SLOW SPONTANEOUS DIASTOLIC DEPOLARIZATION due to If (“funny current”);
-If channels responsible for a slow, mixed Na+ inward/K+ outward current
-slope of phase 4 determines the HR
-accounts for automaticity of SA and AV nodes
factors that INCREASE automaticity
*catecholamines
*digitalis
*hypokalemia
*ischemia
*stretch
factors that DECREASE automaticity
*acetylcholine
*beta blockers
*calcium channel blockers
*hyperkalemia
AV node - general principles
*the bridge between the atrium and ventricle
*shares properties of automaticity with the sinus node
*has decremental properties (the faster you challenge, the slower the conduction)
*AV node transit time is represented by the PR interval on ECG
phases of the ventricular cardiac action potential (detailed)
*phase 4: resting state (some cells → pacemaker activity)
*phase 0: RAPID UPSTROKE & DEPOLARIZATION: rapid inward Na+ current when threshold is reached; important determinant of impulse conduction velocity
*phase 1: INITIAL REPOLARIZATION - rapid early repolarization; inactivation of voltage-gated Na+ channels; voltage-gated K+ channels begin to open (transiently outward K+ current)
*phase 2: PLATEAU PHASE - balance between Ca2+ influx and K+ efflux; Ca2+ influx triggers contraction
*phase 3: RAPID/TERMINAL REPOLARIZATION - massive K+ efflux due to opening of slow voltage-gated K+ channels and closing of Ca2+ channels
action potentials of the atria & ventricles
*“fast response” tissues
*ion carrying current during upstroke: Na+-channel dependent
*conduction: FAST conduction (“all or none”)
*recovery from inactivation: VOLTAGE-dependent
ventricular cardiac action potential phases (simple)
*phase 4: resting state
*phase 0: DEPOLARIZATION due to Na+ influx
*phase 1: early REPOLARIZATION due to K+ efflux
*phase 2: PLATEAU due to balance between Ca2+ influx & K+ efflux
*phase 3: terminal REPOLARIZATION due to continued K+ efflux with closing of Ca2+ channels
pacemaker action potential phases - simple
*phase 0: UPSTROKE due to Ca2+ influx
*phase 3: REPOLARIZATION due to K+ efflux & closing of Ca2+ channels
*phase 4: slow DEPOLARIAZATION due to funny current channels (mixed Na+ inward/K+ outward current)
