A 42-year-old woman is enrolled in a randomized controlled trial to study cardiac function in the setting of several different drugs. She is started on verapamil and instructed to exercise at 50% of her VO2 max while several cardiac parameters are being measured. During this experiment, which of the following represents the relative conduction speed through the heart from fastest to slowest?
AV node > ventricles > atria > Purkinje fibers
Purkinje fibers > ventricles > atria > AV node
Purkinje fibers > atria > ventricles > AV node
Purkinje fibers > AV node > ventricles > atria
Purkinje fibers > atria > ventricles > AV node
The conduction velocity of the structures of the heart are in the following order: Purkinje fibers > atria > ventricles > AV node. A calcium channel blocker such as verapamil would only slow conduction in the AV node.

The conduction velocity of cardiac tissue is determined by a wide variety of factors, including the number and identity of various ion channels, the number and type of gap junctions, and the size and composition of muscle fibers. Conduction through the Purkinje system is the fastest within the heart, allowing for coordinated contraction of the ventricles, and atrial muscle conducts faster than ventricular muscle. Conduction through the AV node is the slowest, to allow the ventricles enough time to fill with blood.

Bonke et al. review the evidence regarding the conduction velocity of parts of the heart. They discuss how the AV node is slower than the SA node. They recommend understanding how different parts of the heart conducts electricity.

Incorrect Answers:
Answers A-C & E: These choices do not correspond to the correct conduction velocities in each part of the cardiac conduction system. A calcium channel blocker would not change the relative velocities of conduction in Purkinje fibers, atria, and ventricles.

Bullet Summary:
The conduction velocity through the heart in order of speed is Purkinje fibers > atria > ventricles > AV node.