Extrasystoles
Mechanisms of extrasystoles
Reentry
Most extrasystoles, particularly if they are monomorphic, bear a constant relationship to the preceding QRS complex (a fixed coupling interval).
The vast majority of these complexes probably represent a reentrant mechanism (each beat represents one trip around a reentrant pathway).
Parasystole
Parasystole occurs when an ectopic focus fires independently of the basic rhythm.
If the parasystolic focus is not reset by the basic rhythm, the focus is said to be protected
There is a constant interval between ectopic depolarizations, but the ectopic focus will manifest itself only whenever it finds the atrium or ventricle not refractory.
Parasystole is very uncommon.
Escape
Escape is a normal phenomenon that occurs when there is a sufficient pause to allow a lower pacemaker to depolarize.
Common examples are junctional or ventricular escape mechanisms.
Unclassified—some extrasystoles do not fall easily into any of these categories and may remain undiagnosed.
Atrial extrasystoles (Day 7-01) (Day 7-02) (Day 7-03)
Most atrial extrasystoles, or premature atrial complexes (PACs), are reentrant.
Atrial extrasystoles are preceded by a P wave that usually has abnormal morphology, indicative of the abnormal direction of atrial depolarization.
The QRS complex is narrow unless there is a preexisting intraventricular conduction defect.
DAY 7-01
DAY 7-02
DAY 7-03
Occasionally, the QRS complex may be wide (aberrant) when one of the bundle branches is not fully repolarized.
An atrial extrasystole usually resets the sinus mechanism and, therefore, is not followed by a compensatory pause (see below).
If an atrial extrasystole is very early, the ventricle may be refractory and not depolarize (“the commonest cause of a pause is a nonconducted atrial extrasystole”—Marriott).
Ventricular extrasystoles (Day 7-04) (Day 7-05) (Day 7-06)
Most ventricular extrasystoles are also reentrant
Ventricular extrasystoles are not preceded by a P wave
The QRS complex is obviously wide
Ventricular extrasystoles usually do not reset the atrial rate and are frequently followed by a compensatory pause.
A ventricular extrasystole may cause retrograde depolarization of the AV node, which results in a lengthening of the subsequent PR interval—a phenomenon known as concealed retrograde conduction.
DAY 7-04
DAY 7-05
DAY 7-06
Preexcitation
The origin of accessory pathways
In utero, the atria and ventricles are eventually separated by a fibrous plate called the AV ring.
The function of the AV ring is to provide support for the mitral and tricuspid valves and to electrically insulate the atria and ventricles.
The AV node is the only structure that should allow conduction through the AV ring.
Overexuberant separation of the atria and ventricles produces congenital 3° AV block (see Day 4).
If there is incomplete separation, residual muscle fibers may bridge the AV ring and form accessory electrical pathways.
Characteristics of accessory pathways
Accessory pathways usually do not have the conduction delay properties of the AV node.
Another way of saying this is that the refractory period of the accessory pathway is typically shorter than the AV node.
Accessory pathways can be located anywhere around the AV ring, and may be multiple.
ECG manifestations of accessory pathways
The delta wave
After atrial systole, the shorter refractory period of the accessory pathway may produce an early depolarization of part of the right or left ventricle.
The early depolarization of part of the ventricle causes the QRS complex to intrude into the PR interval producing the characteristic delta wave.
The location of the delta wave helps localize the site of the accessory pathway.
The presence of the delta wave and its attendant arrhythmias is known as the Wolff-Parkinson-White (WPW) syndrome.
Limitations to ECG interpretation (Day 7-07) (Day 7-08)
The delta wave may be inverted in various leads and may appear as a Q wave, producing a pseudoinfarct pattern.
QRS voltage is an unreliable indicator of LVH
ST and T wave changes are unreliable indicators of ischemia unless serial ECGs are available.
DAY 7-07
DAY 7-08
Accessory pathways and arrhythmias
Reentrant arrhythmias involving an accessory pathway
The accessory pathway may form one limb of a reentrant loop, with the AV node as the other limb.
A reentrant arrhythmia can occur with the wave of depolarization going down the AV node and retrograde up the accessory pathway (antegrade conduction).
As long as there is no concomitant intraventricular conduction defect (IVCD), the QRS complex with antegrade conduction down the AV node will be narrow (no delta wave). (Day 7-09)
DAY 7-09
If the depolarization proceeds down the accessory pathway and back up through the AV node (retrograde conduction), the QRS complex will be wide (and resemble VT). (Day 7-10)
Maneuvers or medications, which block the AV node, may terminate these arrhythmias (but see below).
Atrial fibrillation and atrial flutter with an accessory pathway (Day 7-11)
If atrial fibrillation or flutter occurs in the presence of an accessory pathway, many impulses may be conducted through the pathway so that the ventricular response is very rapid.
The QRS complex is usually wide and bizarre, and may be irregular if atrial fibrillation is present.
Interventions, which increase AV nodal block may speed conduction through the accessory pathway and are contraindicated.
These arrhythmias should be treated with urgent electrical cardioversion and subsequent ablation of the accessory pathway.
DAY 7-10
DAY 7-11
Sample Tracings
ECG 1
ECG 2
ECG 3
ECG 4
ECG 5
ECG 6
ECG 7
ECG 8
ECG 9
ECG 10
ECG 11
ECG 12
ECG 13
ECG 14
ECG 15
ECG 16
ECG 17
ECG 18
ECG 19
ECG 20
Extrasystoles and Preexcitation Syndromes
ECG 1
Atrial rate: 50
Ventricular rate: 50
Rhythm: Sinus bradycardia
P wave: Normal
PR interval: 110 msec
QRS complex:
Axis: 90°
Duration: 140 msec
Voltage:
Morphology:
ST segment:
T wave:
QT interval: 510 msec
U wave:
Diagnosis: Sinus bradycardia with WPW. It is inappropriate to consider the diagnosis of a lateral MI, even though there is a prominent Q wave in aVL, in the presence of WPW.
ECG 2
Atrial rate: 72
Ventricular rate: 72
Rhythm: Sinus rhythm with frequent PACs
P wave: Normal
PR interval: 140 msec
QRS complex:
Axis: -20°
Duration: 100 msec
Voltage: Increased in I
Morphology: Normal
ST segment: Normal
T wave: Normal
QT interval: 420 msec
U wave:
Diagnosis: Sinus rhythm with frequent PACs with aberrant conduction and possible LVH by voltage criteria. The premature beats might at first be mistaken for PVCs, but they are all preceded by a P wave.
ECG 3
Atrial rate: 55
Ventricular rate: 75
Rhythm: Sinus rhythm with frequent PVCs
P wave: Normal
PR interval: 160 msec
QRS complex:
Axis: 45°
Duration: 100 msec
Voltage: Normal
Morphology: Normal
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