Here is a good example of 60-cycle interference artifact on a sinus rhythm strip.  The artifact is caused by electrical interference from a nearby electrical appliance. Modern monitors are able to filter interference out, but it is still occasionally seen.  Even though we can still discern P waves in this strip, and we can see that the rhythm is irregular, possibly sinus arrhythmia, the artifact prevents us from accurately evaluating the strip.  Every effort should be made to identify the offending device.  In "ECG Basics", we attempt to stay "basic", but if any of our Gurus would like to comment on this in a more technical fashion, it is welcome.

Paramedic Erik Testerman has generously donated several excellent teaching ECGs to the Guru, and we will be featuring all of them soon.  This week, we show you the ECGs from a 59-year-old man with a blood glucose of 30 mg/dl.  He had no complaints .  After a bolus of Dextrose 10%, his blood glucose was 105 mg/dl.  He gave a past medical history of diabetes mellitus, hypertension, and left bundle branch block. Vitals were reported as normal and stable, except for the slow heart rate.

The first ECG, taken in the field, show a second-degree AV Block.  The conduction ratio is 2:1.  That is, there are two P waves for every QRS complex.  With this ratio, it is sometimes difficult to determine whether the patient has Type I (usually AV nodal) or Type II (Infranodal) AVB.  In order to diagnose Type I AVB (Wenckebach), we need to see TWO P waves in a row conducted, to see the prolongation of the PR interval.  It is not correct, however, to call ALL 2:1 AV blocks "Type II".  Often, simply taking a longer rhythm strip will expose a period of 3:2 conduction, showing progressive prolongation of the PRI.

This ECG, however, gives us some clues that it is probably TYPE II.  The patient has a left bundle branch block.  Type II AVBs are infranodal - that is, they affect the structures below the AV node:  the His Bundle and the Bundle Branches.  Type II AVBs represent INTERMITTENT TRI-FASCICULAR BLOCK, and that is common in the presence of RBBB  and LBBB (a bi-fascicular block).  This ECG probably represents an existing LBBB with an intermittent RBBB - When the right bundle is blocked, the patient has a tri-fascicular block, and no conduction to the ventricles.

Another clue that this is Type II is that the NON-CONDUCTED P waves fall CLEAR of the preceding T waves, meaning that they had ample opportunity to conduct, not being in the absolute refractory period.

Fortunately, the rhythm strip, taken one minute later, uncovers the diagnosis!  The sixth and seventh QRS complexes are conducted with a 3:2 ratio, showing PR intervals that stay the same, proving the rhythm is Type II. 

For this patient, the heart block and resulting bradycardia don't seem to be causing symptoms.  But infranodal blocks can easily progress to complete heart block and should be treated with implanted pacemakers.  The EMS crew in this case had transcutaneous pacer pads on the patient as a precaution, but he remained well-perfused and with a good BP the whole time. 

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.