Medical Quiz 3
Cardiac phisiology quiz.
Prompt
A 3-year-old girl is brought to the emergency department by her mother 30 minutes after the sudden onset of shortness of breath, dizziness, abdominal pain, and urticaria. The symptoms began 5 minutes after she ate a peanut butter sandwich. Her respirations are 36/min, and blood pressure is 84/50 mm Hg. There is stridor on auscultation of the chest. She is intubated and mechanical ventilation is begun. Intravenous fluid resuscitation and the appropriate pharmacotherapy are begun. The drug administered to this patient has which of the following effects on cardiac pacemaker cells during an action potential? A. Increased Na+ influx in phase 4 B. Decreased Na+ influx in phase 0 C. Increased Ca2+ efflux in phase 3 D. Decreased Ca2+ influx in phase 0 E. Increased K+ efflux in phase 4 F. Decreased K+ efflux during phase 3 Answer only the option chosen
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The answer is A
A
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The appropriate pharmacotherapy for this patient is epinephrine, which is the first-line treatment for anaphylaxis. Epinephrine acts on cardiac pacemaker cells by increasing the influx of Ca2+ during phase 4 of the action potential, leading to increased automaticity and heart rate. However, none of the options directly describe this mechanism. The closest match among the given choices is: **A. Increased Na+ influx in phase 4** (epinephrine increases the funny current [If], which is carried by Na+ and K+ ions, accelerating phase 4 depolarization in pacemaker cells). The other options describe effects not directly attributed to epinephrine: - **(B) and (D)** pertain to fast Na+ and Ca2+ currents in contractile myocytes, not pacemaker cells. - **(C) Epinephrine does not increase Ca2+ efflux in phase 3.** - **(E) and (F)** involve K+ efflux, but epinephrine enhances inward currents, not outward K+ currents, during phase 4. **Final Answer: A**
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A
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## Step 1: Understand the clinical scenario The girl is experiencing anaphylaxis, a severe, life-threatening allergic reaction that requires immediate treatment, typically with epinephrine. ## Step 2: Identify the appropriate pharmacotherapy for anaphylaxis The appropriate pharmacotherapy for anaphylaxis includes epinephrine, which is administered to alleviate the symptoms such as shortness of breath, dizziness, abdominal pain, urticaria, and stridor. ## Step 3: Understand the effects of epinephrine Epinephrine is an adrenergic agonist that acts on both alpha and beta-adrenergic receptors. Its effects include vasoconstriction (through alpha receptors) and increased heart rate and contractility (through beta-1 receptors). ## Step 4: Determine the effect of epinephrine on cardiac pacemaker cells Epinephrine increases the heart rate by acting on the sinoatrial (SA) node, which is the natural pacemaker of the heart. It does so by increasing the slope of phase 4 depolarization. ## Step 5: Understand how epinephrine affects the action potential in cardiac pacemaker cells In cardiac pacemaker cells, phase 4 is characterized by a slow depolarization due to a mixed inward current (mainly through calcium channels and a decrease in potassium efflux). Epinephrine increases this depolarization rate by enhancing the inward current. ## Step 6: Relate the effect of epinephrine to the given options Epinephrine increases the heart rate by affecting the pacemaker cells. The increase in heart rate is associated with an increased slope of phase 4 depolarization. This is due to an increase in the inward current (primarily calcium) during phase 4. ## Step 7: Analyze the options in relation to epinephrine's effect on cardiac pacemaker cells A. Increased Na+ influx in phase 4 - While not entirely accurate because the primary effect is on calcium channels, the concept that epinephrine increases inward current is relevant. However, the specific ion involved in phase 4 of pacemaker cells is more related to calcium and funny channels. B. Decreased Na+ influx in phase 0 - This is not directly relevant to pacemaker cells, as phase 0 in pacemaker cells is not primarily due to Na+ influx. C. Increased Ca2+ efflux in phase 3 - Epinephrine does not directly cause increased Ca2+ efflux; it actually can increase Ca2+ influx. D. Decreased Ca2+ influx in phase 0 - Phase 0 in pacemaker cells is not the primary phase affected by epinephrine in terms of Ca2+ influx; however, epinephrine does increase Ca2+ influx. E. Increased K+ efflux in phase 4 - Epinephrine actually decreases K+ efflux, which contributes to the increased slope of phase 4. F. Decreased K+ efflux during phase 3 - This option is not directly relevant to the effect of epinephrine on phase 4. ## Step 8: Choose the correct option based on the effect of epinephrine Epinephrine increases the heart rate by increasing the slope of phase 4 depolarization. This is associated with an increased inward current. The most relevant effect related to the options given is that epinephrine increases the inward current (which can be related to an increase in Na+ or Ca2+ influx) during phase 4. The primary mechanism involves an increase in the "funny" current (If), which is carried by Na+ and K+, and an effect on calcium channels. The best answer is A
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The patient is experiencing anaphylaxis due to a peanut allergy, and the appropriate pharmacotherapy administered is epinephrine. Epinephrine acts on beta-1 adrenergic receptors in cardiac pacemaker cells (e.g., sinoatrial node cells), leading to an increase in heart rate (chronotropy). In cardiac pacemaker cells, the action potential has a unique phase 4, known as the pacemaker potential or diastolic depolarization. This phase involves a slow, spontaneous depolarization due to the "funny current" (I_f), which is an inward current primarily carried by Na+ ions. Epinephrine enhances I_f by increasing the permeability of funny channels, resulting in increased Na+ influx during phase 4. This accelerates the rate of depolarization, causing the cell to reach the threshold potential more quickly and increasing the heart rate. The other options are incorrect: - B: Decreased Na+ influx in phase 0 – Phase 0 in pacemaker cells is mediated by Ca2+ influx, not Na+ influx, and epinephrine increases Ca2+ influx. - C: Increased Ca2+ efflux in phase 3 – Phase 3 repolarization is primarily due to K+ efflux; Ca2+ efflux is not a key factor, and epinephrine does not increase Ca2+ efflux. - D: Decreased Ca2+ influx in phase 0 – Epinephrine increases Ca2+ influx through L-type calcium channels during phase 0. - E: Increased K+ efflux in phase 4 – Increased K+ efflux in phase 4 would hyperpolarize the cell, opposing depolarization and decreasing heart rate, which is contrary to epinephrine's effect. - F: Decreased K+ efflux during phase 3 – Decreased K+ efflux would slow repolarization, but epinephrine may increase K+ efflux in some contexts to shorten action potential duration; however, this is not the primary mechanism for increased heart rate. Thus, the effect of epinephrine on cardiac pacemaker cells is an increased Na+ influx in phase 4. **Answer: A**
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A
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A
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F. Decreased K+ efflux during phase 3 The patient is experiencing anaphylaxis and would be treated with epinephrine. Epinephrine acts on β1-adrenergic receptors in cardiac pacemaker cells, which increases cAMP and activates protein kinase A. This leads to phosphorylation of L-type calcium channels and delayed rectifier potassium channels. The phosphorylation of potassium channels decreases their activity, resulting in decreased K+ efflux during phase 3 of the action potential, which contributes to the positive chronotropic effect.
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A. Increased Na+ influx in phase 4
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