Final answer:
The next intervention for a patient with a narrow complex SVT is to perform vagal maneuvers. If these are unsuccessful, adenosine may be administered.
Step-by-step explanation:
In this case, the next intervention for a patient with a narrow complex SVT (Supraventricular Tachycardia) is to perform vagal maneuvers to try and slow down the heart rate. Vagal maneuvers can include actions such as bearing down (as if having a bowel movement), applying cold water to the face, or coughing forcefully. These maneuvers stimulate the vagus nerve, which can help restore normal heart rhythm. If the vagal maneuvers are unsuccessful, the next step would be to administer a medication called adenosine to help restore a normal heart rate.