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Take the Test |
 - ECG Rhythm Abnormalities_files/Bullet7.gif){kind=small}
Occur as single or repetitive events and have unifocal or
multifocal origins. The ectopic P wave (called P') is often hidden in the ST-T wave
of the preceding beat. (Dr. Marriott, master ECG teacher and author, likes to
say: "Cherchez le P on let T" which in French means: "Search for
the P on the T wave", but it's more sexy in French!) The P'R interval is normal or prolonged because the AV junction
is often partially refractory when the premature impulse enters it. PAC's can have three different outcomes
depending on the degree of prematurity (i.e., coupling interval from
previous P wave), and the preceding cycle length. This is illustrated
in the "ladder" diagram where normal sinus beats (P) are followed by three
possible PACs; in the diagram the refractory periods of the AV node and bundle
branches are indicated by the width of the boxes):
click here to view
A "ladder" diagram is an easy way of conceptualizing the conduction of impulses through the heart, and the resulting complexes (i.e., P waves and QRS waves).
Outcome #1. Nonconducted (blocked); i.e., no QRS complex because the PAC finds AV node still refractory. (see PAC labeled 'a' in the upper diagram 1)
The pause after a PAC is usually incomplete;
i.e., the PAC usually enters the sinus node and resets its timing, causing the
next sinus P to appear earlier than expected. (PVCs, on the other hand, are
usually followed by a complete pause because the PVC does not
usually perturb the sinus node; see ECG below.)
click here to view
Similar to PAC's in clinical implications, but occur less
frequently. The PJC focus, located in the AV junction, captures the atria
(retrograde) and the ventricles (antegrade). The retrograde P wave may appear
before, during, or after the QRS complex; if before, the
PR interval is usually short (i.e., <0.12 s). The ECG tracing and ladder
diagram shown below illustrates two classic PJC's with retrograde P waves
following the QRS.
click here to view
click here to view
Atrial activity is poorly defined; may see course or fine
undulations or no atrial activity at all. If atrial activity is seen,
it resembles an old saw (when compared to atrial flutter that
often resembles a new saw). Ventricular response is irregularly irregular and may
be fast (HR >100 bpm, indicates inadequate rate control),
moderate (HR = 60-100 bpm), or slow (HR <60 bpm, indicates
excessive rate control, AV node disease, or drug toxicity). A regular ventricular response with A-fib usually
indicates complete AV block with an escape or accelerated ectopic
pacemaker originating in the AV junction or ventricles (i.e., must consider
digoxin toxicity or AV node disease). The differential diagnosis includes atrial flutter with
an irregular ventricular response and multifocal atrial
tachycardia (MAT), which is usually irregularly irregular. The
differential diagnosis may be hard to make from a single lead rhythm strip;
the 12-lead ECG is best for differentiating these three arrhythmias.
click here to view
Regular atrial activity with a "clean" saw-tooth
appearance in leads II, III, aVF, and usually discrete 'P' waves in lead V1.
The atrial rate is usually about 300/min, but may be as slow as 150-200/min or
as fast as 400-450/min. Untreated A-flutter often presents with a 2:1 A-V conduction
ratio. This is the most commonly missed supraventricular tachycardia because
the flutter waves are often difficult to find when there is 2:1 ratio.
Therefore, always think "atrial flutter with 2:1 block" whenever there is a
regular supraventricular tachycardia @ ~150 bpm! (You won't miss it if you
look for it in a 12-lead ECG)
click here to view
In this ECG rhythm strip, arrows point to atrial flutter waves @ 280bpm with ventricular rate @ 140bpm (atrial flutter with 2:1 block)
The ventricular response may be 2:1, 3:1 (rare), 4:1, or irregular
depending upon the AV conduction properties and AV node slowing drugs on board
(e.g., digoxin, beta blockers). Ectopic, discrete looking, unifocal P' waves with atrial rate
<250/min (not to be confused with slow atrial flutter) Ectopic P' waves usually precede QRS complexes with P'R
interval < RP' interval (i.e., not to be confused with paroxysmal
supraventricular tachycardia with retrograde P waves appearing shortly after
the QRS complexes). Ventricular response may be 1:1 or with varying degrees of AV
block (especially in digitalis toxicity, as shown in this 3-lead ECG with 2:1
block).
click here to view
Ectopic atrial rhythm is similar to ectopic atrial tachycardia,
but with HR < 100 bpm. Discrete, multifocal P' waves occurring at rates of 100-250/min
and with varying P'R intervals (should see at least 3 different P wave
morphologies in a given lead). Ventricular response is irregularly irregular (i.e., often
confused with A-fib). May be intermittent, alternating with periods of normal sinus
rhythm. Seen most often in elderly patients with chronic or acute
medical problems such as exacerbation of chronic obstructive pulmonary
disease. If atrial rate is <100 bpm, call it multifocal atrial
rhythm Basic Considerations: These arrhythmias are circus
movement or reciprocating tachycardias because they
utilize the mechanism of reentry. The onset is sudden, usually
initiated by a premature beat, and the arrhythmia also stops abruptly - which
is why they are called paroxysmal. They are usually narrow-QRS
tachycardias unless there is preexisting bundle branch block or rate-related
aberrant ventricular conduction. There are several types of PSVT depending on
the location of the reentry circuit. AV Nodal Reentrant Tachycardia (AVNRT): This is the most
common form of PSVT accounting for approximately 50% of all symptomatic PSVTs.
The diagram illustrates the probable mechanism involving dual AV nodal
pathways, alpha and beta, with different
electrical properties. In the diagram alpha is a fast AV nodal
pathway with a long refractory period (RP), and beta is the slow
pathway with a short RP. During sinus rhythm alpha is always
used because it conducts faster. An early PAC, however, finds
alpha still refractory and must use the slower
beta pathway to reach the ventricles. By the time it traverses
beta, however, alpha has recovered allowing
retrograde conduction back to the atria. The retrograde P wave (called an
atrial echo for obvious reasons) is often simultaneous with the
QRS and, therefore, not seen on the ECG, but it can reenter the AV junction
because of beta's short RP.
click here to view
If conditions are right, a circus movement or reciprocating tachycardia results as seen in the above ECG and ladder diagram. Rarely, an "uncommon" form of AVNRT occurs with the retrograde P wave appearing in front of the next QRS (i.e., RP' interval > 1/2 RR interval), implying antegrade conduction down the faster alpha, and retrograde conduction up the slower beta.
AV Reciprocating Tachycardia (Extranodal bypass pathway): This
is the second most common form of PSVT and is seen in patients with WPW
syndrome. The WPW ECG, seen in the diagram, shows a short PR,
delta wave, and somewhat widened QRS.
click here to view
This type of PSVT can also occur in the absence of manifest WPW on a preceding ECG if the accessory pathway only allows conduction in the retrograde direction (i.e., concealed WPW). Like AVNRT, a PAC that finds the bypass track temporarily refractory usually initiates the onset of PSVT. The PAC conducts down the normal AV pathway to the ventricles, and reenters the atria retrogradely through the bypass track. In this type of PSVT retrograde P waves appear shortly after the QRS in the ST segment (i.e., RP' < 1/2 RR interval). Rarely the antegrade limb for PSVT uses the bypass track and the retrograde limb uses the AV junction; the PSVT then resembles a wide QRS tachycardia and must be differentiated from ventricular tachycardia.
Sino-Atrial Reentrant Tachycardia: This is a rare form
of PSVT where the reentrant circuit is between the sinus node and the right
atria. The ECG looks like sinus tachycardia, but the tachycardia is
paroxysmal; i.e., it starts and ends abruptly. Junctional Escape Beats:These are passive,
protective beats originating from subsidiary pacemaker cells in the AV
junction (usually in the Bundle of His). The pacemaker's basic firing rate is
40-60 bpm; junctional escapes are protective events that occur whenever the
primary pacemaker (i.e., sinus node) defaults or the AV node blocks the atrial
impulse. The ECG strip shows intermittent sinus slowing with two junctional
escapes.
click here to view
Junctional Escape Rhythm: This is a sequence of 3 or
more junctional escapes occurring by default at a rate of 40-60 bpm. There may
be AV dissociation or the atria may be captured retrogradely by the junctional
pacemaker. In the ECG example below the retrograde P waves are not seen and
must be hidden in the QRS's; the significant "Q" wave with ST elevation in the
bottom strip suggests an acute MI.
click here to view
Accelerated Junctional Rhythm: This is an
active junctional pacemaker rhythm caused by events that perturb
pacemaker cells (e.g., ischemia, drugs, and electrolyte abnormalities). The
rate is 60-100 bpm).
click here to view
Nonparoxysmal Junctional Tachycardia: This usually
begins as an accelerated junctional rhythm but the heart rate gradually
increases to >100 bpm. There may be AV dissociation, or retrograde atrial
capture may occur. Ischemia (usually from right coronary artery occlusion) and
digitalis intoxication are the two most common causes. In the example below
junctional tachycardia is seen with ('B') and without exit block ('A').
click here to view
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Return to the beginning of Lesson V Move on to Ventricular Arrythmias
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