Brockenbrough phenomenon, also called the Brockenbrough-Braunwald-Morrow sign, is a hemodynamic finding of dynamic left-ventricular outflow tract obstruction, classically in obstructive hypertrophic cardiomyopathy (HOCM).

After a premature ventricular contraction (PVC), the next beat is more forceful because of post-extrasystolic potentiation. In HOCM, this increased contractility worsens the dynamic obstruction, producing:

This paradoxical fall in aortic pulse pressure despite a stronger contraction helps distinguish dynamic LVOT obstruction from fixed aortic stenosis, where the stronger post-PVC beat generally increases aortic pulse pressure.

Fuster and Hurst's The Heart, 15th Edition, p. 1356. A concise hemodynamic description is also available.

A premature ventricular contraction (PVC) is an extra heartbeat that starts early in one of the ventricles, the heart’s lower pumping chambers, instead of following the usual electrical pathway from the atria.

Because it occurs early, it is often followed by a short pause. The next normal heartbeat can feel stronger. People may describe a PVC as a:

On an ECG, a PVC typically appears as an early, wide, abnormal-looking QRS complex, usually followed by a compensatory pause.

PVCs are common and often harmless, particularly in people without structural heart disease. Potential triggers include caffeine, alcohol, stress, poor sleep, nicotine, and stimulant medicines. However, frequent PVCs or PVCs accompanied by fainting, chest pain, shortness of breath, or known heart disease should be medically assessed. The Mayo Clinic overview gives a patient-oriented explanation.