Final answer:
The most appropriate initial action for a nurse caring for a patient with chest pain, low blood pressure, and cardiac rhythm abnormalities is to provide transcutaneous pacing.
Step-by-step explanation:
The most appropriate initial action for a nurse caring for a client in the critical care unit with complaints of chest pain, a blood pressure of 78/40, shortness of breath, and third-degree AV block on the heart monitor would be to provide transcutaneous pacing. Transcutaneous pacing is a temporary method of pacing the heart by attaching external electrodes to the patient's chest. This initiates electrical impulses to stimulate the heart and maintain an adequate heart rate.