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USMLE - Pharm
Kaplan Section 3 Chapter 1 Cardiac Renal Fundamentals
| Question | Answer |
|---|---|
| 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 |
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