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USMLE - Pharm

Kaplan Section 3 Chapter 1 Cardiac Renal Fundamentals

QuestionAnswer
In cardiac cells, in which direction is the Na [ ] gradient? Na+ outside > Na+ inside --> Na+ flows down gradient INTO cell
What are the fast-response fibers in cardiac cells? In cardiac muscle and in the His-Purkinje system
In Phase 0 of the cardiac action potential of FAST-repsonse fibers, which channels open first? fast I-Na+ channels
In Phase 0 of the cardiac action potential of FAST-repsonse fibers, what does the rate of depolarization depend on? Resting membrane potential of the cell ---(determines)---> # of fast I-Na+ channels open ---(determines)---> rate of depolarization. The more negative the resting potential, the faster the response. As membrane voltage increases, the number of Na channe
Which class of drugs blocks phase 0 of the cardiac action potential of FAST-repsonse fibers? Class I anti-arrhythmic drugs
What is Phase I of the cardiac action potential of FAST-repsonse fibers? Talk about the movement of ions. Overshoot. Na+ channels inactivated. K+ channels open transiently --> K+ flow OUT. Cl- channels open transiently --> Cl- flows IN
Which class of anti-arrhythmic drugs blocks phase 1 of the cardiac action potential of FAST-repsonse fibers? None
What is Phase 2 of the cardiac action potential of FAST-repsonse fibers? Talk about the movement of ions. Plateau. Slow Ca2+ IN. Delayed K+ OUT (I-K channel).
Which class of anti-arrhythmic drugs blocks phase 2 of the cardiac action potential of FAST-repsonse fibers? None
What is Phase 3 of the cardiac action potential of FAST-repsonse fibers? Talk about the movement of ions. Repolarization. Delayed K+ rectifier OUT current increases rapidly as the slow Ca2+ IN current dies out.
Which class of anti-arrhythmic drugs blocks phase 2 of the cardiac action potential of FAST-repsonse fibers? Class III anti-arrhythmic drugs slows the repolarization (makes the slope less steep)
What is the slow Na+ current in the cardiac action potential of FAST-repsonse fibers? Window current (Na+ IN) that lasts from phase 0 through phase 3. Can help prolong the duration of the action potential.
What is Phase 4 of the cardiac action potential of FAST-repsonse fibers? Talk about the movement of ions. Return to resting membrane potential
What is normal cardiac resting membrane potential of FAST-repsonse fibers? -85mV
How is the cardiac resting membrane potential maintained in FAST-repsonse fibers? Na+/K+ ATPase (pumps Na+ OUT and pumps K+ IN)
What are the slow-response fibers in cardiac cells? SA and AV Nodes
In slow-response fibers, what does depolarization depend on? Activation of Ca2+ channels (I-Ca-L, I-Ca-T)
Which class of drugs blocks phase 0 of the cardiac action potential in SLOW-repsonse fibers? Class IV anti-arrhythmic drugs
Which channels cause repolarization of the cardiac action potential in SLOW-response fibers? Delayed rectifier K+ current (OUT)
What is phase 4 of the cardiac action potential in SLOW-repsonse fibers? Talk about the movement of ions. Spontaneous depolarization. Na+ IN (I-f channel), Ca2+ IN (I-Ca-T channel), K+ OUT (I-K channel)
Which class of drugs blocks phase 4 of the cardiac action potential in SLOW-repsonse fibers? Class II and IV anti-arrhythmic drugs
What is automaticity in a heart cell? The ability to depolarize spontaneously.
What determines the pace of the heart rate? The fastest phase 4 slope (steepest slope) of a slow-repsonse fiber, usually, the SA node.
What is the effective refractory period? no stimulus of any magnitude can elicit a response. (i.e. from phase 0 to phase 3 of the cardiac AP because Na+ channels are effectively inactivated because the potential is too "positive" for them to be open)
Which types of drugs prolong the effective refractory period? K+ channel blockers (class II antiarrhythmics)
What is the relative refractory period? a strong stimulus can elicit a repsonse, but the timing will be out of sync with the rest of the heart --L can lead to arrhythmias
What do decreases in the effective refractory period of an action potential allow? longer relative refractory period --> greater chance of premature impulse being generated --> arrhythmia
At which membrane potential does the activating (M) Na gate close? -50mV
At which membrane potential does the inactivating (h) Na gate open? -85mV
Why is the rate of Na channel recovery slower in ischemic tissue? Some cells in ischemic tissue may be depolarized at rest --> dec number of channels able to participate in the next depolarization --> dec conduction rate
Which receptors innervate the SA node? PANS - M2 and SANS B1
What is the difference between depolarization in fast vs. slow response cardiac cells? fast (cardiac muscle/His/Purkinje) -- Na+ influx; slow (SA/AV nodes) -- Ca2+ influx
When are Class I anti-arrhythmics least effective? When Na+ channels are in the resting state (M gates closed, h gates open) -- state dependent
Created by: Missy Kratz Missy Kratz on 2008-02-05



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