What is the diagnosis for a patient experiencing ST-segment elevation in lead V4R after nitroglycerin administration?

In a patient presenting with ST-segment elevation in lead V4R after nitroglycerin administration, the most plausible diagnosis is right-ventricular myocardial infarction (MI). Lead V4R is specifically positioned over the right ventricle, and ST-segment elevation in this lead indicates that there may be an ischemic process affecting the right side of the heart.

The administration of nitroglycerin, which is a vasodilator, can sometimes exacerbate right ventricular MI. This is due to the potential decrease in preload and blood pressure associated with nitroglycerin, which can lead to a deterioration in right ventricular function if there is already impaired perfusion or hemodynamics. In cases where there is a significant right-sided ischemia due to inferior wall MI, right-ventricular involvement often accompanies it, particularly if there is a thrombus in the right coronary artery.

Therefore, the identification of ST-segment elevation in V4R following nitroglycerin use strongly supports the diagnosis of right-ventricular MI, highlighting the relationship between the electrical changes observed on the ECG and the physiological state of the heart in response to medication.