Review Question - QID 109638

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Electrocardiogram (ECG) ECG And Conduction Pathway QID: 109638 (M1.CV.17.4867)

QID 109638 (Type "109638" in App Search)

A 75-year-old male presents to the emergency room complaining of chest pain. He reports a pressure-like “crushing” sensation over his sternum. His past medical history is notable for diabetes, hyperlipidemia, and hypertension. He takes aspirin, metformin, glyburide, rosuvastatin, and enalapril. The patient drinks socially and has a 40 pack-year smoking history. His temperature is 99°F (37.2°C), blood pressure is 150/85 mmHg, pulse is 130/min, and respirations are 24/min. On exam, the patient is diaphoretic and has notably increased work of breathing. An electrocardiogram is shown in Figure A which demonstrates abnormalities in the isoelectric portion of the QT interval in several leads. Which ion channels are normally open during this phase of the cardiac action potential?

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Hyperpolarization-activated, cyclic nucleotide-gated channels

10%

32/327

T-type channels and slow delayed rectifier channels

30/327

Inward rectifier channels

13%

43/327

Slow delayed rectifier channels and rapid delayed rectifier channels

16%

51/327

L-type channels and slow delayed rectifier channels

45%

147/327

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This patient presents with ischemic chest pain and electrocardiogram findings suggestive of a lateral myocardial infarction. The ST segment represents the isoelectric portion of the QT interval and correlates with phase 2 (plateau) of the ventricular action potential in which the L-type calcium channels and slow delayed rectifier potassium channels are both open.

The QT interval represents the time from the first ventricular depolarization to the last ventricular repolarization. The ST segment occurs within the QT interval and is isoelectric to the baseline because the ventricles are completely depolarized. The ST segment corresponds to phase 2 (plateau) of the normal ventricular action potential. In this phase, the membrane potential is constant as the inward calcium current via L-type calcium channels balances the outward potassium current via slow delayed rectifier potassium channels.

Figure A demonstrates the characteristic appearance of an acute lateral myocardial infarction. Note ST elevations in leads I, aVL, and V2-V6.

Incorrect Answers:
Answer 1: Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are open during phase 4 (diastolic depolarization) of the pacemaker action potential. In this phase, HCN channels generate “funny” current which eventually activates T-type voltage-gated calcium channels.

Answer 2: T-type calcium channels open in phase 4 (diastolic depolarization) of the pacemaker action potential after sufficient repolarization via the HCN channels. The continued depolarization from T-type channels subsequently activates L-type calcium channel opening which leads to phase 0 (upstroke).

Answer 3: Inward rectifier channels are open in phase 4 (rest) of the cardiac myocyte action potential. They close upon depolarization, and reopen upon repolarization.

Answer 4: Slow delayed rectifier channels and rapid delayed rectifier channels are open during phase 3 (repolarization). In this phase, outward potassium current repolarizes the cardiac myocyte cell membrane until it reaches resting membrane potential.

Bullet Summary:
The ST segment corresponds to phase 2 of the ventricular action potential in which L-type calcium channels and slow delayed rectifier potassium channels are open.

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