This strip offers something interesting for both your basic-level students and for your more advanced students. First, it is a good example of sinus rhythm with a premature beat. The PR interval was measured by the machine at .21 sec (218 ms). The premature beat is supraventricular - that is, it is not a PVC. Because of the slightly long PRI in this strip, it's P wave COULD be buried in the preceding T wave. That would make this a premature atrial contraction (PAC).