Filed under: Premature Atrial Contraction (PAC)
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Patient: n/a
ECG description:
- Sinus tachycardia
- Supraventricular bigemeny
- One premature ventricular contraction
Discussion:
There is a baseline sinus tachycardia with a PR interval of 130ms, regularly interrupted by premature atrial contractions (PAC). Each PAC depolarizes the atria and resets the SA node, causing a change in automaticity and a noncompensatory extrasystolic pause. Judging by the PR interval as well as P axis and morphology of the premature beats, the ectopic pacemaker is atrial. The ectopic PR interval is 130ms, and it is plausible to think that the ectopic pacemaker is located near the SA node. The P wave axis is ca. 30 degrees, and the ectopic P wave axis is ca. 60 degrees, which means that the atria are depolarized anterogradely and in almost the same direction as from the SA node. QRS axis and morphology is slightly different in the QRS complex following the first premature beat and the second and third. Looking closely, we can see that P wave axis and morphology slightly differs from the first PAC to the next two. The PR interval however is the same. This could be due to multifocality, but since the PR interval is quite similar, the two foci must be very close to each other. After the third bigeminal beat, a broad QRS occurs. In spite of the aberrantly looking RBBB-like morphology, this is most likely a premature ventricular contraction (PVC). If this was aberrancy, it would be due a refractory right bundle branch that couldn’t cope with the rapid changes in automaticity caused by the PAC’s. However, the coupling interval before the broad complex is similar to the other coupling intervals, and this demonstrates that the RBB in fact handles the rapid changes in automaticity quite well. In the precordial leads, we can see a P wave following the PVC, suggesting that the atrias have been depolarized retrogradely from the PVC.
July 1, 2009
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Patient: Woman, 76 y/o. Admitted to the ER for palpitations. Medical history not available.
ECG description:
- Sinus rhythm
- Every fourth sinus beat is followed by a premature atrial contraction (PAC)
- PACs have fixed coupling interval, are preceded by an ectopic P wave and a short PR interval. P’ wave is upright in inferior leads.
- Normal, horizontal cardiac axis at ca. 10°
ECG interpretation:
Every fourth sinus beat is followed by a supraventricular extrasystole with a fixed coupling interval. This pattern is called quadrigeminy. The coupling interval is the interval between the sinus P wave and the ectopic P wave. While varying coupling intervals are often seen in parasystolia, a fixed coupling interval suggests the presence of an ectopic focus that due to increased automaticity decides to fire during normal sinus rhythm. The P wave preceding each premature beat is partially hidden in the preceding P wave, but the PR interval is clearly shorter than in the sinus cycles. This suggest that the PAC origins from a site nearby the sinus node, but closer to the AV junction. The PR interval exceeds 100ms, which suggests an atrial focus rather than a junctional, in which the PR interval would be expected to be shorter. In AV junctional extrasystolia, the atria is often depolarized in a retrograde fashion as the ectopic impulse starts around the AV junction and spreads upwards through the atrium. This normally produces inverted P waves in the inferior leads. Here however, the P wave is upright in the inferior leads, which also indicates an atrial focus.
May 25, 2009
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Patient: Male, 91 y/o. Admitted to the hospital after he fell in his home and was found after 24 hours on the floor. No complaints over chest pain. He is put under observation for commotio cerebri. Cerebral CT scan is negative. Known angina pectoris and has had one AMI in 1983 (vessel unknown). He has a prerenal kidney failure. Troponine level on admittance is 0.04. 8 hours later it is 0.03.
ECG description:
- Regulary irregular supraventricular rhythm
- Sinus rhythm
- Premature atrial contractions (PAC) in trigeminal pattern
- Normal cardiac axis
- Poor R wave progression
- Q wave in lead III
- 2mm horizontal ST-depression in leads V2-V4
ECG interpretation:
Sinus rhythm is interrupted by premature atrial contractions (PAC). Every second sinus beat is followed by a PAC, resulting in a trigeminal pattern. Each PAC resets the sinus node, and is therefore followed by a non-compensatory postextrasystolic pause, contributing to slowing the rhythm down. The ectopic P waves are best seen lead V1, where they are upright/positive. In the inferior leads (II, III, aVF), the same P waves are inverted. This indicates that the ectopic focus lies in or around the AV junction, depolarizing the atria in a retrograde fashion – away from the inferior leads, making the P wave negative. You will probably note that the first P wave in the ECG is negative in lead III. This could lead to confusion, as it would suggest that also this beat is ectopic. However, a diphasic or inverted sinus P wave in lead III is normal in small amounts of the population (Chou 2008:8). Also, note that the last P wave in the ECG has the same P wave axis and morphology as the rest of the ectopic P waves. Although it doesn’t seem to be much premature, it comes from the same ectopic pacemaker. It also breaks the trigeminal pattern.
Premature atrial contractions (PAC) and postextrasystolic pauses
Since we’re on the subject, let’s do a quick recap of the essentials regarding PACs and their pauses.
- A PAC originates from an ectopic site in the atria, and therefore produces a P wave morphology that differs from that of the sinus P wave. It will also produce a normal QRS, as the premature atrial impulse will conduct through His-Purkinje normally. Unless of course, some kind of intraventricular conduction delay is present or if aberrant conduction of the impulse occurs. A PAC produces a pause in the heart rhythm, which can be seen on the ECG. A PAC will usually depolarize the whole atrium (logically, since there is nothing there to stop the impulse from spreading all over the place). When the impulse therefore reaches the sinus node, the node will be “reset” and start a “new” sinus rhythm. This will be manifested on the ECG as a pause, which will be longer than the sinus cycle.
- Such a pause is either labeled a compensatory or a non-compensatory postextrasystolic pause. To differentiate these pauses, we need to look at the three intervals: the normal interval, the coupling interval and the postextrasystolic pause. The normal interval is the basic cycle in the ECG, in this ECG it is the sinus cycle, the cycle starting with the third P wave and ending with the fourth P wave in this ECG. The coupling interval is the interval between the sinus P wave and the premature P wave. The postextrasystolic pause is the pause after the premature beat, consisting of the interval between the premature P wave and the sinus P wave following the pause.
- Now, if the coupling interval + postextrasystolic pause is less than twice of the normal interval, the pause is non-compensatory. If the two added intervals equals twice or more than twice the length of the normal interval, the pause is compensatory. With PACs, non-compensatory pauses are the common finding. Only very rarely, a premature beat originating near or in the AV junction can spread only in the anterograde direction and avoid resetting the sinus node and thereby produce a compensatory pause.
- Now, let’s take a look at lead III from this ECG. By using a ruler or a caliper we will find that the coupling interval + the postextrasystolic pause is less than twice the length of the normal interval. Which is what we expected to find. This is a premature atrial contraction producing a non-compensatory pause.
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May 20, 2009
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Patient: n/a
ECG description:
- Sinus arrhythmia / sinus rhythm with varied rate. V-rate varies from 45-110 bpm.
- Left Axis Deviation (LAD). Cardiac axis approx. – 40°
- Left Anterior Fascicle Block
- Premature Atrial Contractions (PAC) in bigeminal pattern
- Widespread T wave abnormalities. T flat/negative in leads II, III, aVF, V5, V6
- Deep Q-wave in lead V1
Detection of Atrial Bigeminy
The 12 lead ECG displays multiple supraventricular extrasystoles. Each sinus beat is followed by a supraventricular extrasystole with a fixed coupling interval throughout the recording. Since the coupling interval is fixed and the P’ waves are morphologially similar, the term unimorph is used. Unimorph complexes signal that all the premature beats arise from the same automaticity focus, insinuating ectopic unifocality. Though, there is always a possibility that unimorph complexes can originate in different foci. Therefore, the term unimorph is used instead of unifocal.
Determining from the P wave axis and morphology, the ectopic focus is not sinoatrial. The narrow QRS suggests that the ectopic impulse is being conducted normally through the His-Purkinje fibers, ruling out a ventricular ectopy. The PR interval measures 120 ms, which lets us conclude that the irritable focus is atrial, not junctional. The inverted P waves suggests that the ectopic impulse is spreading retrogradely and inferiorly from its focus.
Each premature complex is followed by a postextrasystolic pause, slowing down the rhythm. By measuring the pause, the coupling interval and the normal sinus interval, the pause can be labeled compensatory or noncompensatory, with the latter being the normal finding with premature atrial contractions. As premature atrial impulses normally depolarize the whole atrium and thereby resets the SA Node, sinus pacing is usually interrupted, causing a pause after the extrasystole. This pause is a result of the sinoatrial slowing and is longer than, but not a multiple of the normal interval. With noncompensatory pauses, the coupling interval + the postextrasystolic pause is less than twice the normal interval. With compensatory pauses, the sum of the coupling interval and the postextrasystolic pause is exactly twice the normal interval length. Because of the bigeminal pattern here, there is no “normal” sinus cycle, making it difficult to establish whether the pauses are compensatory or non-compensatory.
January 31, 2009
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Patient: n/a
ECG description:
- Sinus rhythm with varied rate: 75-130 bpm
- Premature Atrial Contractions (PAC) presenting in couplets
- Left Axis Deviation (LAD). Cardiac Axis is deviated leftwards and superiorly at approx. 90°
- Left Anterior Fascicular Block (LAFB) due to LAD, deep S in III, no sign of LVH or MI
- Low Voltage in Limb Leads
- Poor R Wave Progression (PRWP)
Atrial Couplets, PACs and P waves
After one sinus cycle, the rhythm is interrupted by a PAC (complex no. 3 from the left). The change in P wave axis and morhpology of this complex suggests ectopy. The P’ wave is inverted in leads II, III and aVF, suggesting that the ectopic impulse originates in the left atria, spreading in a retrograde fashion. Determining by the PR interval, which is 100 ms, the ectopic pacemaker is atrial and not junctional, and sits closer to the AV Node than the SA Node.
The PAC is immediately followed by a new PAC, creating an atrial couplet. This second PAC seems to originate from another focus, as there is a change in P’ wave axis and configuration. The PR interval of this PAC is 110ms, and the P’ waves are upright in the inferior leads, suggesting that it spreads inferiorly and towards the left. The second PAC is followed by two sinus cycles, which is then followed by another PAC couplet. The PACs in this couplet seem to originate from the same ectopic foci as in the first couplet, although there is a variation in coupling interval length.
Atrial couplets can be benign, but are less common in healthy hearts, and should increase suspicion towards onset of atrial fibrillation. Ultimately, one would prefer to print a longer rhythm strip at this point, to see the phenomenon over a longer time interval. Unfortunately this is not available for this particular case.
The Postextrasystolic Pause
With supraventricular premature impulses, the dominant automaticity focus (normally the SA Node, as in this case) is usually reset by the premature impulse. The supraventricular impulse usually activates the whole atria and thereby also the SA Node. The early activation of the SA Node interrupts the pacing function of the node, and causes a delay in impulse generation. The next impulse will then be slightly delayed, causing the following RR interval to be prolonged. This is called a noncompensatory pause. If the SA Node is not reset, then its pacing function will not be disturbed, and the following RR interval will be an exact multiple of the normal interval, resulting in a compensatory pause.
PACs usually present with non-compensatory pauses, as ectopic atrial impulses will usually activate the whole atria, including the SA Node, and thereby interrupting the sinus pacing activity. In this EKG, the pause after the first PAC is interrupted early by another ectopic impulse, so this pause cannot be determined. The second PAC however (complex no. 4 from the left) is followed by a postextrasystolic pause that is prolonged, but still not an exact multiple of the normal sinus cycle length. This is a non-compensatory pause, which tells us that the SA Node has been reset. This helps to establish and diagnose an atrial origin for the ectopic beats.
January 30, 2009
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Patient: Woman, ca. 60 y/o, sudden onset of palpitations and dyspnea. Prehospital EKG showed rapid atrial fibrillation, but she converted spontaneously during transport. When presented in the ER, she still feels palpitations, although not as rapid and intense as they were.
ECG description:
- Sinus rhythm with varying rate, ca. 55-70 npm
- Multiple premature atrial complexes
- Left Axis Deviation (LAD). Axis at approx. – 40°.
- Left Anterior Fasicle Block (LAD, QRS<120ms, no sign of LVH)
- Deep Q waves in V1-V2
- Poor R wave Progression (PRWP)
ECG interpretation: Sinus rhythm with Left Anterior Fasicle Block and Multiple Premature Contractions
ECG comments: The postextrasystolic pause can be compensatory or non-compensatory, depending on whether the premature depolarization of the atria has reset the SA Node or not. When the premature atrial depolarization resets the SA Node, the node will use a variable time period to restart, causing a pause. This pause is longer than the normal PP interval, but not twice as long. If the SA Node is not reset, the pause will be an exact multiple or twice as long as the normal PP interval.
This is measured by adding the PP interval of the coupling interval with the PP interval of the pause. If the sum of these is equal to the normal PP interval, the pause is compensatory. If it is shorter than the normal PP interval, the pause is non-compensatory. See this previous post, where distinguishing between compensatory and non-compensatory pauses is explained.
Why are these PACs and not PJCs?
It is not very usual for PACs to present with compensatory pauses as in this ECG. PACs usually present with non-compensatory pauses, as an atrial depolarization usually resets the SA node. Still, I’d like to think that the ectopic beats here are atrial. If they were junctional, the P waves would fall close to or right after the QRS. With a PJC the PR interval is usually 10 ms or less when the P wave precedes the QRS complex. With PJC, P waves are also inverted in the inferior leads. Another suggestion that this ECG shows PACs, is that the configuration of the premature P wave differs from that of the sinus P waves, which is because the premature impulse originates in a different part of the atria and depolarizes them in a different way.
From measuring the normal intervals, coupling intervals and postextrasystolic pauses, you can see that all the pauses in this ECG are compensatory, except one: The second PAC in the first strip is followed by a pause that is less the sum of the normal PP interval and the coupling PP interval.
Lastly, this ECG shows a Left Anterior Fasicle Block, based on Left Axis Deviation, narrow QRS (>120ms) and no sign of Left Ventricle Hypertrophy measuring by the Sokolow-Lyon index).
December 30, 2008
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ECG description:
- Sinus rhythm, regularly irregular, at approximately 90 bpm
- PR interval normal and constant at 140ms
- Multiple premature atrial complexes
- Atrial trigeminy pattern with compensatory postextrasystolic pauses
- Axis normal at approx. 60°
ECG conclusion: Atrial Trigeminy with Compensatory Postextrasystolic Pauses.
Recognition of Premature Atrial Complexes (PAC)
Every second beat is followed by a premature atrial complex (PAC). A PAC is recognized by a premature QRS complex preceded by a premature P wave. As the pacing comes from the atria, the QRS complex is narrow and has the same morphology as the normal beats. PACs often gets mistaken for junctional premature beats, as the premature P wave easily gets hidden in the preceding T wave. Whenever presented with premature beats, always scan the T waves for hidden P waves. With completely hidden P waves, the giveaway is often a too tall T, which is taller than the other T waves in the same lead. Premature atrial impulses arise from irritable automaticity foci in the heart, and can be induced by several factors, as adrenaline (epinephrine) released by adrenal glands, increased sympathetic stimulation, precense of caffeine, amphetamines, cocaine or other β1 receptor stimulants. Also, digitalis toxicity can lead to PACs. When an automaticity focus in the atria becomes irritable due to one or several of the factors mentioned, it may spontaneously fire an impulse. Such a premature impulse will depolarize the surrounding myocardial tissue, and will show on the ECG as a premature beat. As irritable foci in the atria will lead to atrial contraction, a p wave will preceed the premature QRS complex. Since the impulse starts in the atria, it usually gets conducted normally through to the ventricles via the AV node, which shows on the ECG as a normal PR interval (unless underlying AV block of any kind) and a QRS complex that is similar to the other sinus beats on the ECG.
Different intervals during sinus arrhythmia
A premature beat is premature because it occurs earlier than expected. To be able to tell if a beat is premature, one needs to look at the preceding beats, comparing the normal beat intervals to the interval preceding the premature complex. If the RR or PP interval preceding an early beat is shorter than the normal RR or PP intervals, the beat is considered premature. There are three types of intervals during arrhythmias, that are important to understand the underlying mechanism when dealing with premature beats. These may be named differently in the books and literature, but the concept is the same:
Normal interval (N_int): The interval (PP or RR), ususally in milliseconds or millimetres between the beats in the underlying rhythm.
Coupling interval(C_int): The interval (PP or RR) between a normal sinus beat and the following premature beat.
Postextrasystolic pause (P_ex): The following pause after a premature beat. A postextrasystolic pause can be compensatory (where the SA node is not reset) and non-compensatory (where the SA node is reset).
The Postextrasystolic Pause: Difference between compensatory and non-compensatory
The cycle pause after a PVC is called a post-extrasystolic pauses. Such pauses are divided into two kinds: Compensatory Post-Extrasystolic Pauses and Non-Compensatory Post-Extrasystolic Pauses. These names are often too long to use, so the terms compensatory pause and non-compensatory pause are used instead.
Compensatory Post-Extrasystolic Pause
A PVC starts in the ventricles from an irritable, often hypoxic focus. Therefore, it only depolarizes the ventricles, not the SA Node. Therefore, the SA Node is not reset. So the SA Node fires as planned and on schedule. Often, if you use your caliper and measure PP intervals, you can spot that timely P within a PVC. The problem is though, that when the sinus node fires, the ventricles are still refractory, and the sinus impulse doesn´t get conducted. When this impulse reaches the ventricles, they´re not ready, and can´t depolarize. So there is a pause after the PVC as the ventricles finish repolarizing, making them receptive to the next sinus generated cycle. Remember that since the depolarization begins in the ventricular tissue, the SA Node will never know anything about this premature impulse. And if it doesn´t get depolarized by the impulse, it will not reset and will keep on pacing. And if the SA Node is not reset, the compensatory pause will be an exact multiple of the regular PP interval. So by measuring PP intervals, you can check if the pause is compensatory or not.
Non-Compensatory Post-Extrasystolic Pause
With non-compensatory pauses, the SA Node is reset and starts a new sinus cycle. The non-compensatory pause is not an exact multiple of the regular PP interval. The SA Node is usually reset by Premature Atrial Contractions (PACs) or Premature Junctional Contractions (PJCs). PVCs are sometimes followed by a non-compensatory pause, but only very rarely. Remember, for the SA Node to be depolarized by a PVC, there will have to be retrograde conduction through the AV Node. This is not very usual, but can happen. The take-home advice here, is that with non-compensatory pauses, you are usually dealing with a PAC or a PJC.
The Postextrasystolic Pause: Distinguishing compensatory pauses from non-compensatory pauses
By measuring and comparing intervals, and by setting up two simple formulas, we can check if a postextrasystolic pause is compensatory or not.
Compensatory: The pause is so long , that the distance between the two normal beats that surround the premature beat, is twice as long the normal interval in the underlying rhythm.
Non-compensatory: The pause is longer than the normal interval in the underlying rhythm, but not long enough for the coupling interval and the compensatory pause to double the length of the normal interval.
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As we can see here, the normal interval is 31mm. The sum of the coupling interval and the postextrasystolic pause is 62mm. The pause is compensatory, which means the SA node has not been reset. It continues to pace, undisturbed and unknowingly of the premature beat that just occured. By positioning a caliper on the normal interval and moving this distance two times to the right, you will land exactly on the P wave that follows the postextrasystolic pause. Remember that if the SA node had been reset, it would start a new cycle, and therefore the postextrasystolic pause would be shorter.
December 10, 2008
