Class IV antiarrhythmics: Calcium channel blockers and others

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Class IV antiarrhythmics: Calcium channel blockers and others

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Class IV antiarrhythmics: Calcium channel blockers and others

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A 62-year-old man comes to the emergency department for evaluation of confusion. He is accompanied by his spouse, who states that he has appeared confused and lethargic since this morning. Since then, he has had three episodes of vomiting. He has also reported ýellow tinted vision. His medical history includes chronic systolic heart failure and hypertension. His current medications include hydrochlorothiazide, lisinopril, spironolactone, digoxin and metoprolol.  He was hospitalized two days ago for an episode of atrial fibrillation with a rapid ventricular response, which was treated with metoprolol and an amiodarone drip. He does not smoke or use recreational drugs. He consumes 2-3 beers on the weekends. His temperature is 37.0°C (98.6°F), pulse is 45/min, respirations are 20/min, and blood pressure is 95/85 mmHg. Mucous membranes are moist. Fingerstick glucose is 110 mg/dL. ECG is shown below. Which of the following additional findings would you expect to find in the laboratory evaluation of this patient?
   By Jer5150 - Own work, CC BY-SA 3.0,  Reproduced from Wikimedia Commons 

External References

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Atrial fibrillation

calcium channel blockers for p. 361

Atrioventricular block

calcium channel blockers p. 323, 361

Calcium channel blockers p. 323

angina p. 323

antiarrhythmic drugs p. 361

contractility in p. 290

cutaneous flushing p. 247

gingival hyperplasia p. 249

hypertension p. 323

hypertrophic cardiomyopathy p. 315

migraine headaches p. 532

Raynaud phenomenon p. 480

Constipation p. 567

calcium channel blockers p. 323

Dihydropyridine calcium channel blockers p. 253

Dizziness p. 548

calcium channel blockers p. 323

Edema

calcium channel blockers as cause p. 323

Gingival hyperplasia

calcium channel blockers p. 323

Heart failure p. 316

calcium channel blockers p. 361

Hyperprolactinemia p. 248, 332, 540

calcium channel blockers and p. 323

Peripheral edema

calcium channel blockers p. 323

Prinzmetal angina

calcium channel blockers for p. 323

Raynaud phenomenon p. 481

calcium channel blockers for p. 323

Smooth muscle (vascular)

calcium channel blocker action p. 323

Supraventricular tachycardia

calcium channel blockers for p. 361

Transcript

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Antiarrhythmic medications help control arrhythmias, or abnormal heartbeats.

There are four main groups of antiarrhythmic medications: class I, sodium-channel blockers; class II, beta-blockers; class III, potassium-channel blockers; class IV, calcium-channel blockers; and miscellaneous antiarrhythmics, or unclassified antiarrhythmics. Now, we’ll focus on class IV and miscellaneous antiarrhythmics in this video.

First, let’s start with two main types of cells within the heart; pacemaker cells and non-pacemaker cells.

Pacemaker cells build the electrical conduction system of the heart, which consists of the sinoatrial node, or SA node; the atrioventricular node, or AV node; the bundle of His; and the Purkinje fibers.

Pacemaker cells have a special property called automaticity, which is the ability to spontaneously depolarize and fire action potentials.

On the other hand, non-pacemaker cells, also known as cardiomyocytes, make up the atria and ventricles; and they give the heart its ability to contract and pump blood throughout the body.

Now, in contrast to non-pacemaker cells, whose action potential has 5 phases, an action potential in pacemaker cells have only 3 phases.

Here’s a graph of the membrane potential vs. time. Phase 4, also known as the pacemaker potential, starts with the opening of the pacemaker channels.

The current through these channels is called pacemaker current or funny current (If), and it mainly consists of sodium ions.

These sodium ions cause the membrane potential to begin to spontaneously depolarize and as the membrane potential depolarizes, voltage-dependent T-type calcium channels open up, thereby further depolarizing the pacemaker cell.

As calcium enters the cell, voltage-dependent L-type calcium channels open up, causing more calcium to rush into the cell, ultimately depolarizing the membrane to its threshold potential. This marks the start of phase 0, which is also known as the depolarization phase.

Now phase 0 is caused by an influx of calcium ions through the voltage-dependent L-type calcium channels that started opening at the end of phase 4. But, this influx of calcium ions isn’t that rapid, so the slope of phase 0 is gradual.

Also during phase 0, the pacemaker channels and voltage-dependent T-type calcium channels close.

Finally, during phase 3, which is the repolarization phase, L-type calcium channels close and potassium channels open up, resulting in a net outward positive current.

At the end of repolarization, pacemaker channels open up and we start over with phase 4 again.

During phase 4 there’s also an outward movement of potassium ions as the potassium channels responsible for the repolarization phase continue to close.

Finally, it’s important to note that besides pacemaker cells, L-type calcium channels are also found in non-pacemaker cells and they’re responsible for phase 2 or the "plateau" phase of their action potential.

Furthermore, calcium that passes through these channels, along with calcium that’s released from the sarcoplasmic reticulum, are essential for the contraction of the cardiac myocytes that make up the rest of the heart.

Now, the automaticity of the heartbeat is led by the pacemaker cells that have the fastest phase 4, which are normally the pacemaker cells found in the SA node.

The SA node fires an electrical signal that propagates throughout both atria, making them contract.

The signal gets delayed a bit as it goes through the AV node, then goes through the Bundle of His to the Purkinje fibers of both ventricles, making them contract as well.

When the electrical signal of the heart doesn’t follow this path, it’s called an irregular heartbeat or arrhythmia.

For example, let’s say a part of ventricle begins to fire off action potentials at a rate that’s even faster than the SA node. This area of the heart essentially flips roles with the SA node, firing so fast that the pacemaker cells in the SA node don’t get a chance to fire. At that point, the heartbeat is being driven by the ventricles.

Alright, switching gears and moving on to pharmacology. Calcium channel blockers bind and inhibit voltage-dependent L-type calcium channels and they’re subdivided into two main groups: dihydropyridines and non-dihydropyridines.

Dihydropyridines, like amlodipine, nicardipine, and nifedipine, are highly selective for calcium channels on the vascular smooth muscle tissue; so they’re primarily used to treat hypertension.

On the other hand, non-dihydropyridines are the class IV antiarrhythmics and they include verapamil and diltiazem. These medications work by targeting the pacemaker cells and non-pacemaker cells in the heart.

In pacemaker cells, they decrease the amount of calcium entering the cell during phase 4 and 0, causing a slower pacemaker potential and slower depolarization.

Moreover, by prolonging phase 4 and 0, class IV antiarrhythmics also prolong effective refractory period or ERP, which is the period of time that the cell is unexcitable to new stimulus.

This way, they reduce firing of the SA node, eventually decreasing the heart rate.

But besides decreasing the activity of the SA node, they also decrease conduction velocity through the AV node.

On the ECG, this shows up as a longer PR interval, which is the time between the onset of atrial depolarization and the onset of ventricular depolarization.

On the other hand, in non-pacemaker cells, like cardiac myocytes, class IV antiarrhythmics decrease the amount of calcium entering the cell.

This way, they decrease the amount of calcium available inside the cell, weakening the force generated during heart contraction.

Now, verapamil is highly selective for cardiac calcium channels; thus, it’s primarily used to treat angina pectoris and nodal arrhythmias, such as supraventricular tachycardia.

Supraventricular tachycardia is a type of arrhythmia caused by an impulse that originates above the heart’s ventricles and there are a few different types: paroxysmal supraventricular tachycardia, atrial fibrillation, and atrial flutter.

Sources

  1. "Katzung & Trevor's Pharmacology Examination and Board Review,12th Edition" McGraw-Hill Education / Medical (2018)
  2. "Rang and Dale's Pharmacology" Elsevier (2019)
  3. "Hurst's the Heart, 14th Edition: Two Volume Set" McGraw-Hill Education / Medical (2017)
  4. "Effects of L-type calcium channel and human ether-a-go-go related gene blockers on the electrical activity of the human heart: a simulation study" Europace (2014)
  5. "Therapeutic drug monitoring: antiarrhythmic drugs" British Journal of Clinical Pharmacology (1998)
  6. "Goodman and Gilman's The Pharmacological Basis of Therapeutics, 13th Edition" McGraw-Hill Education / Medical (2017)