Techniques are provided for use with a pulmonary artery pressure (PAP) monitor having an implantable PAP sensor. In one example, a PAP signal is sensed that is representative of beat-by-beat variations in PAP occurring during individual cardiac cycles of the patient. The PAP monitor detects intervals within the signal corresponding to the durations of cardiac cycles, then detects cardiac rhythm irregularities based on the intervals. For example, the PAP monitor can detect and distinguish atrial fibrillation, ventricular fibrillation and ventricular tachycardia based on the stability of the intervals of the PAP signal along with other information such as ventricular rate. The PAP monitor can also detect and distinguish premature contractions based on durations of the intervals. Examples where the PAP monitor is a component of an implantable cardiac rhythm management device (CRMD) are also provided.