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PR Interval Measurement

Measures the time delay from atrial depolarization (P wave start) to ventricular depolarization (QRS start). Prolongation suggests AV nodal conduction delay.
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The statement of the theorem

The PR interval measures the conduction delay ΔtPR\Delta t_{PR} across the Atrioventricular (AV) node. This delay is mathematically defined as the time integral of the potential gradient ∇ϕ\nabla \phi as the electrical impulse traverses the nodal tissue ΩAV\Omega_{AV}: \nΔtPR=∫rSArHisdt=∫rSArHisdsvcond(r)\Delta t_{PR} = \int_{\mathbf{r}_{SA}}^{\mathbf{r}_{His}} dt = \int_{\mathbf{r}_{SA}}^{\mathbf{r}_{His}} \frac{ds}{\mathbf{v}_{cond}(\mathbf{r})} \nWhere dsds is the differential path length and vcond(r)\mathbf{v}_{cond}(\mathbf{r}) is the local conduction velocity, which is significantly reduced in the AV node compared to atrial or ventricular tissue. Prolongation implies a decrease in vcond\mathbf{v}_{cond} along the path.
Source: Wikipedia