What must the amplitude of the impulse exceed to achieve myocardial capture?

To achieve myocardial capture, the amplitude of the impulse must be large enough to cause depolarization of the cardiac cells. This means that the electrical signal delivered by the pacing device must meet or exceed the threshold necessary to stimulate the heart muscle effectively. When the impulse reaches this threshold, it initiates a contraction of the heart muscle, resulting in myocardial capture.

The concept of depolarization is crucial in pacing, as it refers to the change in electrical charge that occurs in cardiac cells, leading to muscle contraction. If the amplitude is insufficient, depolarization will not occur, and the heart will not respond, which is why it’s critical for the impulse to be strong enough to achieve this response.

Other options do not accurately capture the necessary physiological principle. For example, achieving an immediate response is not solely dependent on the amplitude but also on the timing and synchronization of the pacing impulse. Consistency across all patients can vary due to individual differences in heart anatomy and size, thus it is not a requirement for capture. Lastly, the threshold must be exceeded, rather than be less than it, to ensure a successful depolarization and subsequent heart contraction.