Pacemaker stimuli can also arise from other parts of the heart – the atria, the AV junction, or the ventricles. The terms “ectopy” or “ectopic beat” are used to describe these non-sinus beats.Ectopic beats are often premature; i.e. they come in early or before the next sinus beat is due.
The premature beat by itself does not cause any symptom; the next normal beat following the premature may cause palpitation. The consequences of a premature beat are:
1) PB occurs earlier than expected beat, hence cannot pump a significant amount of blood.
2) PB prevents the occurrence of subsequent normal beat; and
3) The PB is usually followed by a pause.
There are two types of premature beats:
1) Supraventricular
2) Ventricular
The supraventricular premature beats originate in the atria or the atrioventricular junction and hence produce a narrow QRS complex unless they are conducted aberrantly in which case, there is QRS widening. Ventricular prematures originate beyond the brancing of the conduction tissue and hence prodce an abnormally prolonged QRS complex.
Atrial Premature Beats APB (Premature Atrial Complexes)
Description
A premature atrial contraction results from an ectopic stimulus that arises from somewhere in either the left or the right atrium, but not in the sinus node. The atria are depolarized from the ectopic stimulus, but the remainder of the conduction is typically normal through the AV Node-Junction and downward into the bundle branches (i.e. normal PR and QRS morphology and intervals). APBs occur as single or repetitive events and have unifocal or multifocal origins. Possible Causes2APBs are very common and may occur in persons with a normal heart or in persons with virtually any type of organic heart disease. APBs do not imply that a person has cardiac disease and may be seen with caffeine intake and with emotional stress. Other causes include:
• Administration of sympathomimetic agents (epinephrine, theophylline)
• Electrolyte abnormalities
• Myocardial ischaemia or injury
• Digoxin toxicity
• Hyperthyroidism
ECG Criteria
1) Heart Rate: Typically normal.
2) Rhythm: Underlying rhythm is typically regular with early premature beats.
3) P-waves: Atrial depolarization is premature, occurring before the next normal
P-wave: Since the impulse originates outside the SA node, the P-wave may have a different shape-often notched, peaked or buried in the proceeding T-wave.
4) PR Interval: Maybe normal, shorter or longer than normal PR interval, depending on origin of the APB.
5) QRS Width: Typically normal but may be prolonged if the PAC is aberrantly conducted through the ventricles.
The premature beat by itself does not cause any symptom; the next normal beat following the premature may cause palpitation. The consequences of a premature beat are:
1) PB occurs earlier than expected beat, hence cannot pump a significant amount of blood.
2) PB prevents the occurrence of subsequent normal beat; and
3) The PB is usually followed by a pause.
There are two types of premature beats:
1) Supraventricular
2) Ventricular
The supraventricular premature beats originate in the atria or the atrioventricular junction and hence produce a narrow QRS complex unless they are conducted aberrantly in which case, there is QRS widening. Ventricular prematures originate beyond the brancing of the conduction tissue and hence prodce an abnormally prolonged QRS complex.
Atrial Premature Beats APB (Premature Atrial Complexes)
Description
A premature atrial contraction results from an ectopic stimulus that arises from somewhere in either the left or the right atrium, but not in the sinus node. The atria are depolarized from the ectopic stimulus, but the remainder of the conduction is typically normal through the AV Node-Junction and downward into the bundle branches (i.e. normal PR and QRS morphology and intervals). APBs occur as single or repetitive events and have unifocal or multifocal origins. Possible Causes2APBs are very common and may occur in persons with a normal heart or in persons with virtually any type of organic heart disease. APBs do not imply that a person has cardiac disease and may be seen with caffeine intake and with emotional stress. Other causes include:
• Administration of sympathomimetic agents (epinephrine, theophylline)
• Electrolyte abnormalities
• Myocardial ischaemia or injury
• Digoxin toxicity
• Hyperthyroidism
ECG Criteria
1) Heart Rate: Typically normal.
2) Rhythm: Underlying rhythm is typically regular with early premature beats.
3) P-waves: Atrial depolarization is premature, occurring before the next normal
P-wave: Since the impulse originates outside the SA node, the P-wave may have a different shape-often notched, peaked or buried in the proceeding T-wave.
4) PR Interval: Maybe normal, shorter or longer than normal PR interval, depending on origin of the APB.
5) QRS Width: Typically normal but may be prolonged if the PAC is aberrantly conducted through the ventricles.
 |
| Complete VS Incomplete Pause |
APBs can have three different outcomes depending on the degree of prematurity (i.e., coupling interval from previous P-wave), and the preceding cycle length.
1) Nonconducted (blocked), i.e., no QRS complex because the APB finds AV node still refractory. This manifests on surface ECG as a pause. The most common cause of an unexplained pause on ECG is non-conducted APB,
2) Conducted with aberration, i.e., APB makes it into the ventricles but finds one or more of the conducting fascicles or bundle branches refractory. The resulting QRS is usually wide, and is sometimes called an Ashman beat,
3) Normal conduction, i.e., similar to other QRS complexes in the ECG.
The fate of a PAC depends on:
1) coupling interval from the last P-wave and
2) preceding cycle length or heart rate.
The pause after a APB is usually incomplete; i.e., the APB 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).
Premature Junctional Complexes (PJC)
Similar to APBs 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., <0.12s).
Ventricular Premature Beats or Premature Ventricular Complexes (VPBs) VPBs may be unifocal, multifocal or multiformed. Multifocal VPBs have different sites of origin, which means their coupling intervals (measured from the previous QRS complexes) are usually different. Multiformed VPBs usually have the same coupling intervals (because they originate in the same ectopic site but their conduction through the ventricles differ. Multiformed VPBs are common in digitalis intoxication.
VPBs may occur as isolated single events or as couplets, triplets, and salvos (4-6 VPBs in a row), also called brief ventricular tachycardias. VPBs may occur early in the cycle (R-on-T phenomenon), after the T-wave (as seen above), or late in the cycle—often fusing with the next QRS (fusion beat). R-on-T VPBs may be especially dangerous in an acute ischaemic situation,because the ventricles may be more vulnerable to ventricular tachycardia or fibrillation.For fusion to occur the sinus P-wave must have made it to the ventricles to start the activation sequence, but before ventricular activation is completed the “late” PVC occurs. The resultant QRS looks a bit like the normal QRS, and a bit like the PVC; i.e., a fusion QRS. The events following a VPB are of interest. Usually a VPB is followed by a complete compensatory pause because the sinus node timing is not interrupted; one sinus P-wave isn’t able to reach the ventricles because they are still refractory from the VPB; the following sinus impulse occurs on time based on the sinus rate. In contrast, APBs are usually followed by an incomplete pause because the APB usually enters the sinus node and resets its timing; this enables the following sinus P-wave to appear earlier than expected. Not all VPBs are followed by a pause. If a VPB occurs early enough (especially if the heart rate is slow), it may appear sandwiched in between two normal beats. This is called an interpolated VPB. The sinus impulse following the VPB may be conducted with a longer PR interval because of retrograde concealed conduction by the VPB into the AV junction slowing subsequent conduction of the sinus impulse. Finally a VPB may retrogradely capture the atrium, reset the sinus node, and be followed by an incomplete pause. Often the retrograde P-wave can be seen on the ECG, hiding in the ST-T-wave of the PVC.
The most unusual post-PVC event is when retrograde activation of the AV junction re-enters the ventricles as a ventricular echo.VPBs usually stick out like “sore thumbs”, because they are bizarre in appearance compared to the normal complexes. However, not all premature sore thumbs are PVCs.
1) Nonconducted (blocked), i.e., no QRS complex because the APB finds AV node still refractory. This manifests on surface ECG as a pause. The most common cause of an unexplained pause on ECG is non-conducted APB,
2) Conducted with aberration, i.e., APB makes it into the ventricles but finds one or more of the conducting fascicles or bundle branches refractory. The resulting QRS is usually wide, and is sometimes called an Ashman beat,
3) Normal conduction, i.e., similar to other QRS complexes in the ECG.
The fate of a PAC depends on:
1) coupling interval from the last P-wave and
2) preceding cycle length or heart rate.
The pause after a APB is usually incomplete; i.e., the APB 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).
Premature Junctional Complexes (PJC)
Similar to APBs 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., <0.12s).
Ventricular Premature Beats or Premature Ventricular Complexes (VPBs) VPBs may be unifocal, multifocal or multiformed. Multifocal VPBs have different sites of origin, which means their coupling intervals (measured from the previous QRS complexes) are usually different. Multiformed VPBs usually have the same coupling intervals (because they originate in the same ectopic site but their conduction through the ventricles differ. Multiformed VPBs are common in digitalis intoxication.
VPBs may occur as isolated single events or as couplets, triplets, and salvos (4-6 VPBs in a row), also called brief ventricular tachycardias. VPBs may occur early in the cycle (R-on-T phenomenon), after the T-wave (as seen above), or late in the cycle—often fusing with the next QRS (fusion beat). R-on-T VPBs may be especially dangerous in an acute ischaemic situation,because the ventricles may be more vulnerable to ventricular tachycardia or fibrillation.For fusion to occur the sinus P-wave must have made it to the ventricles to start the activation sequence, but before ventricular activation is completed the “late” PVC occurs. The resultant QRS looks a bit like the normal QRS, and a bit like the PVC; i.e., a fusion QRS. The events following a VPB are of interest. Usually a VPB is followed by a complete compensatory pause because the sinus node timing is not interrupted; one sinus P-wave isn’t able to reach the ventricles because they are still refractory from the VPB; the following sinus impulse occurs on time based on the sinus rate. In contrast, APBs are usually followed by an incomplete pause because the APB usually enters the sinus node and resets its timing; this enables the following sinus P-wave to appear earlier than expected. Not all VPBs are followed by a pause. If a VPB occurs early enough (especially if the heart rate is slow), it may appear sandwiched in between two normal beats. This is called an interpolated VPB. The sinus impulse following the VPB may be conducted with a longer PR interval because of retrograde concealed conduction by the VPB into the AV junction slowing subsequent conduction of the sinus impulse. Finally a VPB may retrogradely capture the atrium, reset the sinus node, and be followed by an incomplete pause. Often the retrograde P-wave can be seen on the ECG, hiding in the ST-T-wave of the PVC.
The most unusual post-PVC event is when retrograde activation of the AV junction re-enters the ventricles as a ventricular echo.VPBs usually stick out like “sore thumbs”, because they are bizarre in appearance compared to the normal complexes. However, not all premature sore thumbs are PVCs.
No comments:
Post a Comment
Note: Only a member of this blog may post a comment.