Final answer:
The initial dose of diltiazem for treating an irregular, narrow complex tachycardia in a 68-year-old male is usually 0.25 mg/kg as a bolus, with a second dose of 0.35 mg/kg possible if needed.
Step-by-step explanation:
The treatment of a 68-year-old male experiencing an irregular, narrow complex tachycardia with diltiazem starts with the administration of an initial dose. The recommended starting dose of intravenous diltiazem for this condition is typically 0.25 mg/kg as a bolus over 2 minutes. If the initial dose is not effective and does not result in any adverse effects, a second dose of 0.35 mg/kg may be administered 15 minutes later. Caution is advisable, and this should be done under careful monitoring of blood pressure and heart rate, due to the risk of hypotension and bradycardia.