Cardiac electrophysiology and Cardiac Arrhythmias Flashcards
Draw and describe the phases of cardiac action potential. Which channels areopen and which currents are responisble for each phase?
Phase 0: depolarization; Na+ current (open fast Na+ channels)
Phase 1: partial repolarization; Ito (outward K+ current that drives Vm to be more negative)
Phase 2: plateau (sustained depolarization); Ca2+ current
Phase 3: repolarization; Ik (delayed action)
Phase 4: rest
Which cells undergo the fast response? What about the slow response? Why don’t the slow response cells have a Phase 1? Explain the difference between slow and fast response. What are ERP and RRP?
Non-pace maker cells undergo the fast response b/c they have fast Na+ channels
Pacemaker cells undergo the slow response b/c they don’t have fast Na+ channels. Instead, the Ca2+ current is responsible for their depolarization (initiated first by If current at Phase 4), and the Ca2+ are slow channels.
ERP = effective refractory period (same as absolute refractory period)
RRP = relative refractory period
What is responsible for maintaining resting Vm in cardiac cells? Do we consider Na+ current when setting resting Vm in cardiac cells? If not, how is Na+ current involved in membrane potential?
At phase 4 = resting Vm set by the Inward Rectifying K+ current
Don’t consider Na+ current for resting Vm in cardiac cells b/c K+ current = high; external Na+ concentration determines peak membrane potential
Which currents are involved in AP propagation in pacemakers and at which phases are they active?
If
Ica
Ik
What is the effect of Ina in Ventricular Ap propagation? What happens when you add TTX?
What are the effects of Ica and Ik on ventricular AP?
Ina = fast Aps; when Ina is reduced, fast APs get converted to slow APs(in which the upstroke will then be due to Ca2+ channels)
TTX blocks Na channels and reduces Na conductance
Ica effect: responsible for Ap plateau
Ik: repolarizes membrane; blocking K+ channels prolongs Aps
Describe pacemaker depolarization. What is the role of I funny channels? During which phase are these channels active?
Due to inward current via I funny channels. Channels are slowly activated and drive Vm towards threshold.
Start activating at phase 4 of SA nodal action potentials
In which situations would the cardiac AP be slower?
What is the order of the speed on conduction in the heart and why?
Fewer Na+ channels (e.g. using TTX or another Na+ channel blocker)
Fewer Ca2+ channels activated in SA/Av node
If threshold is more positive
Purkinje fibers>Atria + Ventricles>SA/AV nodes
What is the effect of cycle length on the propagation speed of an AP? How are cycle lengthand Heart Rate related?
The faster the heart beat, i.e. the shorter the cycle, the faster the AP
HR and cycle length: shorter cycle length = faster HR
On an EKG, what do a P wave, QRS complex, and T wave represent?
P wave = atrial depolarization
QRS complex = ventricular depolarization
T wave = ventricular repolarization
Describe the role of NE/E (sympathetic system) on pacemaker AP
NE/E = activate Adenylyl cyclase >> increases cAMP >> stimulates If and Ca channels >> increase in inwards If and Ica current >> faster depol of SA/AV nodal cells >> faster AP= faster pacemaker
How do catecholamines like isopterenol increase contractility?
They increase Ca2+ conductance (increase in cAMP >> PKA activated >> add P to L type Ca2+ channels >> increase Ca2+ conductance)
Describe the effect of the parasympathetic system (cholinergic agonists i.e. ACh) on pacemaker APs
2 ways: increased ACh >> activates Gs beta and gamma subunits >> activates Ik(ach) channels >> increased outward current (less inward current = hyperpolarization in SA/AV nodal cells) >> slower depolarization b/c threshold is now more positive (closer to reversal potential for Ik)
Define an arrhythmia. What are the categories of arrhythmias discussed?
Arrhythmia: any rhythm that’s not a normal sinus rhythm
Conduction abnormalities + Abnormal automaticity
Types of Conduction abnormalities discussed
1st degree heart block
2nd degree heart block (Mobitz Type I and II)
3rd degree heart block
Bypass conduction
1st degree heart block. How do you id it on an EKG and what does that mean in terms of AP propagation?
Prolonged PR interval. Slowing of action potential.