CVS Practice Test

The slope of phase 4 in cardiac myocyte action potentials refers to the resting potential and the pre-depolarization phase before the action potential is generated. This phase is primarily influenced by the influx of sodium ions and the activity of ion channels responsible for pacemaker activity. If the slope of phase 4 is altered, it directly affects the rate of spontaneous depolarization, which has a significant impact on the heart rate.

An increase in the slope of phase 4 would lead to a faster depolarization, resulting in an increased heart rate, known as positive chronotropic effect. Conversely, a decrease in the slope would slow down depolarization and lead to a reduced heart rate, known as negative chronotropic effect. Thus, changes in the slope of phase 4 are closely associated with alterations in heart rate due to their influence on the timing of action potential generation in the pacemaker cells of the heart.

In summary, adjusting the slope of phase 4 has a direct and significant effect on heart rate, making it the most relevant clinical change.