What is the most appropriate treatment for a patient with sudden-onset racing heart after failing the Valsalva maneuver?

The most appropriate treatment for a patient with sudden-onset racing heart, particularly if they exhibit signs of a paroxysmal supraventricular tachycardia (SVT), is adenosine 6 mg IV. Adenosine is a rapid-acting drug that can interrupt the reentry pathways in the heart, thereby terminating the reentrant tachycardia. It is specifically indicated for acute management of SVT and works by temporarily blocking conduction through the atrioventricular (AV) node, which can reset the electrical impulses and restore a normal rhythm.

While other options like amiodarone, metoprolol, and synchronized cardioversion are valid treatments for various arrhythmias, they are typically used in different clinical scenarios or for more complex arrhythmias. Amiodarone is often reserved for more persistent or wide-complex tachycardias, while metoprolol is not a first-line treatment in acute situations. Synchronized cardioversion may be indicated if the patient is hemodynamically unstable, but for someone who responds well to methods such as the Valsalva maneuver, the immediate administration of adenosine provides a quick and effective solution for SVT.