This ECG is taken from a patient with an implanted pacemaker who was experiencing near-syncope.  She was taken to the hospital by EMS, where the pacemaker was adjusted to obtain ventricular capture.  This ECG did not have a Lead II rhythm strip, so the 12-lead ECG is being presented.  The P waves have been marked with a "P", pacemaker spikes marked with an arrow, and the QRS complexes marked with a "J" because they are junctional.  Because we can see 12 leads, or viewpoints, the morphology of the P waves and QRS complexes changes each time the machine switches to a new lead.

The underlying rhythm is sinus, with nearly regular P waves occuring at a rate of about 72 beats per minute.  The QRS complexes are also regular, but they are dissociated from the P waves.  Because the rate is near or just under 40 bpm, and the QRS complexes are narrow, this represents a slow junctional rhythm.  Because both atrial and ventricular rhythms are regular, but not associated with each other, an interpretation of complete heart block (third-degree AV block) can be made.  This explains why the patient had a pacemaker implanted.

The pacer spikes, for the most part, track the P waves, which is how this pacemaker is programmed.  They are not followed by a paced QRS complex, however.  This is failure to capture.  The second and fourth P waves did not stimulate a pacer spike because of their proximity to the T wave of the junctional beat.  The mA (energy setting) was adjusted in the Emergency Dept., and the pacemaker did not require repositioning.  The patient regained a reliable paced rhythm.

This section of the ECG Guru is meant to be for your basic students.  Pacemakers now have become very complex, with many options and variable settings.  So complex, that I would not feel comfortable getting into any more detail than I have here (although visitors to the site are welcome to).  It is important that, if you deal with patients in an emergency setting, you do not tell the patient that "something is wrong with their pacemaker" until it has been evaluated by a qualified person who can electronically interrogate the device.  It can be very difficult to determine from an ECG how a pacemaker is programmed, and how it should be reacting.  Since this patient had symptoms related to the bradycardia, and since pacemaker spikes occurred free of any refractory period and did not produce QRS complexes, it is safe to say there needs to be an adjustment.

In an emergency, with serious symptoms present, a transcutaneous or transvenous temporary pacemaker can be used.  Medications such as Atropine, epinephrine, and norepinephrine are also used, depending upon the type of AV block and the resources available.

This is a nice teaching strip of a slowing sinus bradycardia that began around 40 bpm, and is slowing.  It is a good example of how the sinus node slows down - there is no abrupt change of rates, rather a change with each R-to-R interval.  There is also a first-degree AV block, reflecting slowing of conduction in the AV node.  The PR interval is slightly variable at .28 sec. to about .32 sec.  This is a good strip to begin talking about treatment of bradycardias with beginner students, as there is no second- or third-degee AVB, but the patient is very likely to be symptomatic now, or very soon.  Atropine would probably improve this rate in a symptomatic patient, but if there is time, a 12-Lead would be a good idea to rule out acute M.I.  Inadvertently raising the rate too much in the injured heart can lead to pump failure, while leaving the patient poorly-perfused in a bradycardia will starve the heart.  A transthoracic or temporary IV pacemaker might be a better choice for some patients because of our ability to choose the rate.

This example of sinus arrest, also called sinus pause, shows a spontaneous return to sinus rhythm.  There are many mechanisms by which pauses can occur on the ECG. One concept for beginner students to grasp is that, if the pause contains the equivalent of regular R-to-R intervals, and the first complex after the pause is "on time",  we can expect that the sinus node kept firing, but did not penetrate the atria (exit block). If the pause is irregular in length, with the first beat after the pause seeming to come in randomly, we can call this sinus arrest or pause, understanding that there are many different mechanisms that can be at work here. Because what little we can see of the underlying sinus rhythm is irregular, or speeding up, we cannot discern absolutely that this is sinus arrest. 

The bottom line for the patient, and for any level practitioner, is, "how is the patient tolerating this pause, and what does it mean to the patient's overall prognosis?  In the short term, the patient may require emergency pacing while the cause of the dysfunction is investigated. If pauses are long enough to lower cardiac output, they can cause fainting. More than a few automobile accidents have happened as a result of this type of dysrhythmia. 

For more advanced students, this short rhythm strip (Lead II) reveals sloping ST depression, and indicates the urgent need for a 12-lead ECG and other tests. 

Atrial fibrillation with a controlled ventricular response is often considered to be atrial fib that has been controlled with medication.  While this is often true, it is possible to see atrial fib with a ventricular rate between 50 and 100 bpm in a patient who has not been treated.  The hallmark signs of atrial fib are:  no P waves and irregularly-irregular rhythm.

Multiple, simultaneous leads are advised to be sure there are no P waves, and a long rhythm strip is often needed to prove the irregularly-irregular rhythm.  Remember, even though this rate is adequate (about 90 bpm), the loss of P waves means no "atrial kick".  The atria are not filling the ventricles with a forceful pumping action. Patients can lose a significant percentage - up to 25% - of their cardiac output because of this.  Coupled with the risk of stroke from the formation and embolization of blood clots from the sluggish circulation in the atria, it is much preferable to have sinus rhythm at the same rate.