When a patient's heart rhythm speeds up and slows down in a rhythmic pattern, how is this described?

The phenomenon of a patient's heart rhythm speeding up and slowing down in a rhythmic pattern is best described as "patterned." This term implies that there is a discernible sequence or rhythm to the fluctuations in heart rate, suggesting that the changes are not random but follow a specific, repeatable cycle. In a clinical context, this might be observed in certain conditions such as respiratory sinus arrhythmia, where the heart rate varies with the breathing cycle.

In contrast to this, terms like "arrhythmic" would imply a lack of regularity altogether, and "irregular" would denote variations that do not follow a consistent pattern. "Sinusoidal," while it may seem fitting due to its smooth, wave-like pattern, is more specifically associated with sine wave patterns often used in mathematical contexts rather than the cyclical changes in heart rate observed in patients. Therefore, choosing "patterned" accurately reflects the rhythmic and systematic nature of the fluctuations in heart rate described in the question.