Why should patients with heart blocks not have their temperatures taken rectally?

Patients with heart blocks should avoid rectal temperature measurements primarily due to the risk of rectum-vagal stimulation. This type of stimulation can lead to an increase in vagal tone, which potentially exacerbates the existing heart block. The vagus nerve is responsible for regulating heart rate, and when stimulated excessively, it can lead to bradycardia or even further heart block, posing a significant risk to the patient's cardiac rhythm and overall stability.

While discomfort is a consideration for any invasive procedure, and reliability and time are factors in clinical practice, the key concern with rectal temperatures in patients with heart blocks is the potential for adverse cardiovascular effects due to vagal stimulation.