PR interval is constant in:

- Mobitz type II of the second degree block has a constant PR interval, with every 'nth' beat is dropped; that is, inconsistently one impulse from atria does not reach ventricles. - Mobitz type I of the second degree block shows lengthening PR interval in successive beats, with finally in one beat P wave is not followed by QRS complex. - In a third variety of the second degree block, atria-to-ventricle excitation is a fixed small number ratio (e.g., 3:2). For instance, every third impulse from SA node will fail to reach ventricles.