Final answer:
The rule of thumb for positioning a respirator for cardiac patients with an elevated head of the bed is to place the patient in a semi-Fowler's position. This involves elevating the head of the bed to approximately 30-45 degrees.
Step-by-step explanation:
In the context of positioning a respirator for cardiac patients with an elevated head of the bed (HOB), the rule of thumb is to position the patient in a semi-Fowler's position. This position involves elevating the head of the bed to approximately 30-45 degrees. By doing so, gravity helps improve ventilation and blood flow to the lungs, making it easier for the patient to breathe.