Module 7: Cardiophysiology

• there are two types of heart cells:

• autorhythmic- self-stimulating, non-contractile cells

  • Na+ and Ca2+ depolarize

  • K+ repolarizes

  • no resting potential

  • contains the mainly the SA node and some of the AV node

• contractile-

  • Na+ depolarizes

  • K+ repolarizes

  • Ca2+ acts as a second messenger

• autorhythmic cells

  

  phase 1.) the unstable membrane potential is activated by ion channels and initiate pacemaker potential

• Na+ and Ca++ depolarize

• K+ repolarize

• Na+ first, Ca++ later

• no resting potential

phase 2.) voltage gated Ca2+ channels open at threshold

• Ca++ is being used for depolarization

• AP rises

phase 3.) K+ leak and repolarization occurs when during an increased outflow of K+ and Ca2+ decrease

phase 4.) K+ channels close and AP drifts towards threshold

phase 5.) hyperpolarization = pacemaker potential returns to lowest point

• conduction system - autorhythmic cells are organized into nodes and pathways

• intercalculated disks- connections between cells (both types)

  • desmosomes hold all cells together

  • gap junctions allow for AP flow between cells triggering depolarization to spread across the heart

• SA node- “pacemaker” of the heart because the rate of depolarization is the fastest

• AV node- AV node delay

• AV bundle (bundle of HIS)- carries signal to interventricular septum through non-conductive tissue that separates atria from ventricles

• bundle branches (left and right) - carries signal to interventricular septum through non-conductive tissue that separates atria from ventricles

• conduction pathways (purkinje fibers) - spreads signal out from the apex of the heart

• contractile cells

• AP involves voltage gated Na+ and K+ channels

• Ca++ acts as a second messenger

• initiated and stimulated by influx of ions through gap junctions, not neural stimulation

phase 0.) rapid depolarization caused by voltage-gated Na+ channels

phase 1.)

• early repolarization

• faster opening of K+ channels cause repolarization

phase 2.)

• plateau

• Ca2+ influx delays repolarization

• efflux of K+ leads to plateau making AP and refractory period last longer and nearer to 0 charge

phase 3.)

• rapid repolarization caused by slow opening of K+ channels

phase 4.) resting potential

increased cardiac output (sympathetic)

• increase in Na+ and Ca2+ permeability

• decrease in K+ permeability

• decreases AV node delay; less delay

• increased depolarization

decreased cardiac output (parasympathetic)

• K+ permeability increases

• Ca2+ permeability decreases

• increases hyperpolarization

• increases AV node delay

• shortens AP in atria

EKG

• QRS complex

• Q wave- initial depolarization interventricular septum

• R wave- depolarization from interventricular septum to apex

• S wave- depolarization from apex to ventricular myocardium (lub)

• T wave- repolarization of ventricles leading to ventricular diastole (dub)

• Intervals and segments – named sections of EKG trace

  • Segment – space between two waves, not including a wave

  • Interval – includes one or more waves

• Interbeat Interval (IBI) - Time between beats, one complete cycle, usually R wave to R wave

• P-R Interval (or P-Q Interval) - from start of P wave to start of Q wave, time for atrial depolarization

and contraction

• P-R Segment (or P-Q Segment) - From end of P wave to start of Q wave (or R if Q is not easily seen),

  • time for atrial contraction and AV node delay

• S-T Segment – From end of S wave to start of T wave

• Q-T Interval - From start of Q wave to end of T wave, time for ventricular depolarization, contraction, and repolarization cycle