Reading EKGs

Reading EKGs > Rhythm

Escape Rhythms & Premature Beats

Escape Rhythms

The SA-node is able to remain the dominant pacemaker through automaticity and overdrive suppression, as described in the Electrophysiology Section.  However, when the SA-node fails to depolarize, surrounding cells with automaticity are able to spontaneously depolarize, taking over as the pacemaker for the heart.  These secondary pacemaker cells each have an intrinsic rate, allowing us to predict which cells are pacing the heart.

When not overdrive-suppressed, the atria is able to spontaneously depolarize and pace the heart at a rate of 60-80 bpm, which is termed Atrial Rhythm or Atrial Escape Rhythm.  If the atria fails to spontaneously depolarize, then the AV-Junction is able to pace the heart at a rate of 40 - 60 bpm, termed junctional escape rhythm.  Should the SA-node, atria, and AV-Junction fail to pace the heart, the last hope is the ventricles themselves, which can spontaneously depolarize at a rate of 20-40 bpm, and is predictably referred to as a Ventricular Escape Rhythm.

Pacemaker

Rate (bpm)

SA-Node

60 – 100

Atria

60 - 80

AV-Junction

40 - 60

Ventricles

20 - 40

By analyzing the rate, you can start to predict where the pacemaker is located.  As an example, a rate of 65 with abnormal P-waves would suggest an atrial rhythm, but not a sinus rhythm.  With abnormal P-waves, the atrium is clearly depolarizing, but is initiated somewhere outside of the SA-node, with a different conduction pattern.

A rate of 45 with inverted P-waves following a normal QRS would suggest a junctional escape rhythm.  The inverted P-waves reflect depolarization that is initiated in the AV-node, traveling in a retrograde fashion up the conduction system.  Please note that P-waves are not always present, either due to poor retrograde conduction or they are buried in the QRS complex.  Additionally, they may be found before or after the QRS complex, depending on the transit time through the AV-node. 

With junctional escape rhythms, the QRS complex is typically narrow.  Activation begins at the AV-node, which is supraventricular (above the ventricles).  As such, conduction is able to proceed from the AV-node to the Bundle of His and the rest of the ventricular conduction system in a normal fashion.

Heart rates (or ventricular depolarization rates) less than 40 bpm with a wide complex  suggest a ventricular escape rhythm.  Here, the ventricles are not activated in the typical pattern, and as such, do not depolarize as quickly or efficiently.  In addition, ventricular depolarization is unlikely to undergo retrograde conduction through the AV-node and back into the atrium.

Premature Beats

Similar to escape beats, premature beats are ectopic beats that do not originate in the SA-node.  However, unlike escape beats that only occur after a sustained sinus pause, premature beats occur sporadically within a normal conduction cycle, appearing earlier than a normal beat. 

These premature beats are the result of an area of enhanced automaticity that leads to spontaneous depolarization.  Depending on the timing, these premature waves of depolarization can have various effects on the rhythm.  They can be identified at the atrial, junctional, or ventricular level, each with slightly different presentations.

Premature Atrial Beats

Atrial premature beats, sometimes referred to as Premature Atrial Contractions / Complexes (PACs) are the most complicated.  They are marked by an abnormally shaped P-wave, labeled P', that occurs prior to the next predicted P-wave.  The shape varies with the location of the irritability focus, and can be found anywhere within the depolarization cycle.  However, if there is an unexplained pause and no P', check the T-waves and QRS complexes.  Depending on the timing, the P' may be buried within another wave.  Check for abnormally large T-waves or unusual shapes within the QRS for clues.

If the PAC is able to depolarize the SA-node, then the normal cycle will reset.  The rate present prior to the PAC will resume following the PAC and subsequent repolarization of the SA-node.  However, there will be a brief episode of irregularity where the PAC occurred.  During this interval, you will not see a sinus beat because it is buried in the rest of the PAC.

As an example, a patient with a resting heart rate of 60 bpm would have a contraction every 1s.  Starting at time 0, you would predict that the SA node would fire at time 0,1s,2s,3s etc.  However, instead of firing at time 3s, an abnormal P-wave appears at 2.5s.  No contractions are present at 3s, but at 3.5s, 4.5s, 5.5s the normal SA-depolarization occurs.  The atrial rate resumed following the PAC, but the timing is now slightly different than initially predicted.  This change in timing is due to the PAC resetting the SA-node.

When located further from the SA-node, or the timing coincides with a refractory period, the PAC may fail to depolarize the SA-node.  When this happens, a compensatory pause occurs.  This pause is double the PP interval, and is related to the concept of refractory periods.

In this example, the SA-node again fires at time 0, 1s, 2s, etc and the PAC fires at time 2.5s.  However, it fails to depolarize the SA-node, only effecting part of the atrial tissue.  At time 3s, the SA-node depolarizes on schedule, but is unable to spread throughout the atrium due to the refractory state of the surrounding myocardium.  As a result, there is little to no change on the EKG, because it is not sensitive enough to detect depolarization of the conduction pathways, only the much larger myocardial depolarization.

While the SA-node did fire at time 3s, nothing was recorded.  It will continue to fire every second, and at time 4s, another NSR will be recorded.  The net result is a P' wave that appears between a P-P interval that is double the normal length of time (P at 3s, refractory atrium at 4s, P at 5s).  This phenomenon is referred to as a compensatory pause.
Up until this point, we have only addressed changes that occur within the atrium.  However, it is possible for the PAC to conduct down through the AV-node and into the ventricles.  The timing of the PAC will determine if this conduction is possible, as well as the resulting QRS morphology. 

If the AV-node is refractory when the PAC occurs, it is impossible to conduct a ventricular beat.  The resulting waveform is referred to as a non-conducted PAC.  If the AV-node and ventricular conduction system are fully repolarized, the PAC is able to conduct, resulting in a normal, narrow QRS complex that appears earlier than expected. 

However, if the AV-node is repolarized but the ventricular system is only partially repolarized, then an abnormal QRS complex ensues, termed PAC with aberrancy.   Depolarization is able to travel down through the AV-node and enter the repolarized ventricle, but unable to access the refractory portion.  The net result is a slow, widened QRS that is recorded as one portion of the conduction system depolarizes earlier than the refractory section.

PACs have the greatest number of possible permutations.  The good news is that once you understand the theory behind the changes, learning about junctional and ventricular premature beats is simple.

Atrial Bigeminy and Trigeminy

Before discussing the junctional and ventricular premature beats,  we need to address the concept of geminy, which refers to pairs or couples.  Premature beats may occur regularly, pairing with an existing rhythm.  These PACs frequently reset the SA-node, resulting in a short pause, but not a full compensatory pause.  In Atrial Bigeminy, the PAC follows every normal sinus beat.  The pattern is:

P – P' – reset – P – P' – reset – P – P'

With Atrial Trigeminy, there are two normal beats followed by a PAC.  The pattern is:

P – P – P' – (reset) – P – P – P' (reset) – P

 

Premature Junctional Beat

Premature junctional beats (PJB) are similar in concept to PACs, except that they originate at the AV-junction instead of the atrium.  With PJBs, you will see a sinus beat followed by a QRS depolarization that is not preceded by a P-wave.  Just like PACs, PJBs may conduct to the ventricles with or without aberrancy, depending on the state of ventricular repolarization.  Additionally, PJBs may also depolarize the atrium in a retrograde fashion, similar to junctional escape rhythms.

As you may have predicted, there are also geminy patterns with PJBs.  A PJB that always follows a sinus beat is termed Junctional Bigeminy, and a PJB that follows two sinus beats is described as Junctional Trigeminy.   This rhythms may or may not have retrograde P-waves, and reset the SA-node, creating a partial compensatory pause.

Premature Ventricular Beats

Premature ventricular beats, more commonly referred to as premature ventricular contractions (PVC), are the final level of premature automaticity.  PVCs also originate from a focus of irritability, and are able to depolarize the ventricle.  Unlike PACs and PJBs, they do not frequently cross the AV-node, leaving the atrial rhythm intact.  PVCs do not utilize the His-Purkinje conduction system, resulting in large, slurred QRS complexes.  PVCs frequently result in a compensatory pause due to the duration of ventricular repolarization, not resetting of the SA node (you may continue to see uninterrupted P-waves). 

A PVC that routinely follows a normal QRS is Ventricular Bigeminy, and a PVC that follows two normal QRS beats is Ventricular Trigeminy

Ventricular Tachycardia

Not all PVCs are isolated or coupled to the normal rhythm.  When multiple PVCs occur sequentially, they are described as a Run of Ventricular Tachycardia (VT).  To qualify as a run of VT, there must be at least 3 consecutive PVCs.  These PVCs can originate from a single focus, giving a monomorphic appearance (Monomorphic VT), or originate from multiple foci, resulting in Polymorphic VT. 

With VT, occasional fusion beats may occur, and are the result of simultaneous atrial and ventricular depolarization.  With fusion beats, the conduction travels through the AV-node and begins to depolarizes the ventricular conduction system while a PVC begins to depolarize the ventricular myocardium from the opposite direction.  The net result is a fusion of the two beats, with the first portion of the QRS appearing normal, while the latter portion appears broad and slurred.  Fusion beats confirm the diagnosis of VT, which can occasionally be mistaken for aberrantly conducted beats. 

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