P wave changes on exercise in patients with isolated mitral stenosis

Patients with mitral stenosis usually showed a marked increase in the P negativity following exercise. The P terminal force in Lead V₁ in 20 cases with isolated mitral stenosis was ?0.090 mm. sec. before exercise, which changed to ?0.177 mm. sec. following the single Master two-step test.Normal adults never showed such changes on exercise. The phenomenon was considered to be due to the posterior rotation of the P wave vector in the horizontal plane, which was induced by the enlargement of the left atrial wall on exercise.