Most cardiovascular clinicians know this, but here’s the rule in case you don’t. Whenever there is “sinus tachycardia” at 150 beats per minute (BPM), be suspicious that the real rhythm is atrial flutter with 2 to 1 (2:1) conduction. The usual atrial flutter rate is 300 BPM. With 2:1 conduction, the rate is close to 150 BPM; the flutter waves are buried in the QRS and not visible. Consequently, the EKG looks like a supraventricular or sinus tachycardia. The rhythm doesn’t have to be exactly 150, but around 150 and absolutely metronome-like in its regularity. There may be occasional 3:1 or 4:1 atrial-to-ventricular conduction, which gives the diagnosis away. Carotid sinus massage or IV adenosine will increase vagal tone and produce sufficient AV block to reveal the flutter waves. Remember to push first on the right carotid sinus and if that’s not successful, repeat the maneuver on the left, as discussed in my posting on Substack on September 19, 2023 https://pauldthompsonmd.substack.com/p/another-knowing-whats-rightfrom-left?utm_source=profile&utm_medium=reader2 The right carotid sinus primarily affects on the sinus node, whereas the left carotid sinus predominantly affects the A-V node. Many patients will only develop sufficient A-V block to reveal the flutter with left carotid pressure.