Week 1 - Heart

Question 1

Cardiac Electrophysiology

Normal Sinus rhythm

Answer 1

3 criteria: (1) action potential must originate in the SA node. (2) the SA nodal impulses must occur regularly at 60-100 pm. (3) activation of myocardium must occur with correct sequence and timing.

Question 2

Cardiac Electrophysiology

Action potentials in Ventricles, Atria and purkinje system: What are 3 commonalities?

Answer 2

1. long duration of AP: 150-300ms (1-ms for nerve or skeletal muscle). This results in long refractory period. 2.Stable resting membrane potential (normally for purkinje fibers). 3. Plateau: APs characterized by this sustained period of depolarization (thus long AP and refractory period)

Question 3

Cardiac Electrophysiology

dV/dT and responsiveness relationship?

Answer 3

rate of rise of the upstroke (Volts/sec). dV/dT depends on size of inward curren AND value of resting membrane potential. Where the resting membrane potential is most negative dV/dT is highest b/c inactivation gates on Na+ channels are open. At lower resting MP, more of the inactivation gates are closed, thus fewer Na+ channels are availabe for upstroke

Question 4

Cardiac Electrophysiology

SA node AP: (3) differences from atria, ventricles and purkinje fibers

Answer 4

1. automaticity- can spontaneously generate an AP. 2. Unstable resting membrane potential. 3. no sustained plateau

Question 5

Cardiac Electrophysiology

Describe Phases of AP for atrium, ventricle and purkinje fibers

Answer 5

PHASE 0, upstroke: rapid depolarization due to increased gNa+. Reaches about +20 mV. PHASE 1, initial repolarization: net outward current due to closing of inactivation gates on Na+ channels (decreased gNa+) and outward K+ current due to both chemical and electrical forces (also high gK+). PHASE 2, plateau: period (~150-200ms) of stable, depolarized membrane potential (shorter in atrium). Results from increased gCa2+ and inward Ca2+ current (slow inward current) due to L-type channels (L=long-lasting). Balance by outward K+ current. (Ca induced Ca release occurs here). PHASE 3, repolarization: decrease in gCa and increase in gK+ even past resting gK+. PHASE 4, resting membrane potential or electical diastole: ~ -85mV. Balance of K+ current and Na+/Ca influx. Phase 4 K+ conductance due to different channels –> gK1 and IK1.

Question 6

Cardiac Electrophysiology

AP phases for SA node

Answer 6

PHASE 0, upstroke: less steep and due to increase in gCa (not Na+) and resulting inward Ica through T-type Ca2+ channels (T=transient, not inhibited by L-type blockers like verapamil). NO PHASE 1 or 2. PHASE 3, repolarization: due to increase in gK. PHASE 4, spontaneous depolarization or pacemaker potential: automaticity. maximum diastolic potential (most negative) is -65 mV. Doesn’t stay there, slow depolarization produced by opening of Na+ channels and inward Na+ current called I-f (f=funny, differs from fast Na current of atrial upstroke). I-f turned on by repolarization from proceeding AP. Slow depolarization until threshold hit (T-Ca channes for upstroke) Thus RATE of phase 4 depolarization sets heart rate!!

Question 7

Cardiac Electrophysiology

Latent Pacemakers and inactivation rule?

Answer 7

cells with the capacity of spontaneous phase 4 depolarization like SA node. AV node, bundle of His, and Purkinje fibers (normally automaticity normally not expressed though). Rule: pacemaker with fasterst rate of phase 4 depolarization controls the heart rate. Referred to as “overdrive suppression”

Question 8

Cardiac Electrophysiology

ectopic pacemaker/ectopic focus (3) ways it occurs

Answer 8

latent pacemaker overcomes overdrive suppression and becomes THE pacemaker. Can occur if (1) SA node firing rate decreases/stops (2)intrinsic rate of firing for latent pacemakers becomes faster than SA node (3) if conduction of APs from SA node are blocked.