This is a Lead II rhythm strip from a patient with a biventricular pacemaker. The second and sixth beats are PVCs.  In this lead, the pacemaker spikes are very difficult to see, but they are present.  The pacemaker is operating in a "demand" mode for pacing the atria.  Some of the P waves appear to be the patient's own, and some appear to be caused by the pacing stimulus.  For example, the first beat appears to have no pacer spike before the P wave, and the second beat does have one (albeit tiny).  The morphology of the P waves appears to change, also.  

This pacemaker has been programmed to cause a QRS complex after every P, whether the P wave was made by the patient or by the pacemaker.  The patient originally had a left bundle branch block, making his QRS complexes very wide, and lowering his cardiac output.   The biventricular pacemaker paces both ventricles, synchronizing their depolarization and narrowing the QRS.  This improves cardiac output.  The physician has programmed this pacemaker to pace the ventricles after every P wave, whether native or paced.  The paced QRS happens slightly before the native (wide) QRS would have, giving the patient the benefit of narrow QRS complexes.  Biventricular pacemakers have been shown to improve cardiac output in patients with wide complexes.

This rhythm strip shows a good example of complete (third-degree) AV block with ventricular escape rhythm.  It will be easy for your basic students to "march out" the P waves.  They are regular at a rate of about 88/min., and they are either visible, or are "hiding" in the QRS complex.  The ventricular rhythm is wide and very slow, and completely dissociated from the sinus rhythm.

For your more advanced students, you may want to discuss the likely origin or "level" of the block.  Blocks above the Bundle of His can have JUNCTIONAL escape rhythms, while blocks that occur below the Bundle of His generally have ventricular escape rhythms.  Ask your students which type of CHB they would prefer to have:  suprahisian or subhisian - and why?

This is a normal sinus rhythm with atrial bigeminy, a term meaning that every other beat is a PAC.  If you look carefully, you can see slight differences in the sinus P waves and the atrial (premature) P waves.  The PACs penetrate and reset the sinus node, causing what looks like a delay after the PAC.  It is often just a return to the normal P to P interval, or nearly so.  If you teach basic students in a clinical setting, they will learn from palpating the peripheral pulse and feeling the pattern of bigeminal beats. Sometimes, the premature beat feels much weaker due to less filling time available to the ventricles.  Atrial bigeminy can have very benign causes, such as increased caffeine intake, or it can have more complex causes such as advanced heart disease or conduction blocks.  In some patients, atrial bigeminy, or any PACs, can be a precursor to more serious atrial dysrhythmias, such as atrial fibrillation.

This is a good basic rhythm strip example of atrial fibrillation with a rapid ventricular response showing the identifying characteristics of atrial fibrillation:  no P waves, an irregularly-irregular rhythm, and a "fibrillatory" baseline.  The wavy baseline will not be seen in all leads in all patients, so it is best to use the first two findings as diagnostic criteria.  Atrial fib often appears initially as a rapid rhythm, as the AV node is being bombarded by many impulses from multiple foci (pacemakers) in the atria.  Depending upon the AV node's ability to transmit these impulses,however, we could see a slow, normal, or rapid ventricular response. 

Atrial fib has very chaotic depolarization of the atrial muscle, resulting in quivering and ineffective pumping of the atria.  This loss of "atrial kick" can severely reduce ventricular filling, and can reduce cardiac output by as much as 25%.  In patients with a very rapid rate, cardiac output can be further reduced, causing CHF.  In addition, the fibrillating atria can form blood clots due to sluggish movement of blood.  These clots can embolize and cause stroke.  For these reasons, patients with atrial fib are anticoagulated and sometimes the atrial fib is stopped by medical, surgical, or electrical therapy.  Recurrence of atrial fib is common after treatment, and for some patients, control of the ventricular rate and anticoagulation become the preferred treatment.