Two Twins

"When too much ECG analysis is barely enough....." (attributed to 'Rampaging' Roy Einthoven)1.

This is the ECG of an 85 year old woman who was feeling dizzy. There was no chest discomfort, her blood pressure was 170/85 mm Hg, and initial blood tests (including serum troponin concentration) were normal.

Figure 1

+ Describe the main ECG changes

Figure 2

Figure 2 shows the ECG with the ectopic beats removed. The main findings in Figure 2 are:

  • Sinus rhythm
  • Normal frontal plane QRS axis
  • Left ventricular hypertrophy
  • Infero-lateral T wave flattening
  • 1 mm ST elevation in Leads V1 to V4

+ What is/are the underlying rhythm(s) ?

A rapid evaluation of the rhythm strip leads to a working diagnosis of (an underlying) sinus rhythm, and ventricular bigeminy. We could stop at this point: we have the diagnosis or "true name"2 of the condition. But there is more to understanding this rhythm than correctly identifying it. A more detailed and informative analysis can be made using a laddergram.

There are ten QRS complexes, with six of the complexes (1,3, 5, 7, 9,10) preceded by a P wave. The amplitude and width of these P waves is within normal limits (less than 2.5 mm and less than 0.12 seconds respectively), the PR interval is slightly variable (between 0.12-0.14 seconds for complexes 1,3, 5 and 7) and between 0.14-0.16 seconds for complexes 9 and 10. The shape of the P waves varies slightly (e.g. compare P1 with P5), but the P-P intervals between P1 and P7 are constant at 1820-1840 msec.

What is the underlying atrial rhythm - sinus or multiple atrial ectopic beats? The constancy of the P-P interval between P1 and P7 favour sinus rhythm.

Complexes 1, 3 , 5 and 7 are each followed by a complex with a different shape (complexes 2, 4, 6 and 8 respectively).

The features of complexes 2, 4, 6 and 8 are :

  • they are not preceded by a P wave,
  • their R wave amplitude is much greater than the amplitude of the R waves of the sinus beats
  • their QRS width is greater than that of the sinus beats
  • they have ST depression and T wave inversion.

Complexes 2, 4, 6 and 8 are ventricular ectopic complexes (VECs)3

These complexes have the same shape i.e. they are thus uniform ventricular ectopic complexes. Another term is unifocal ventricular ectopic complexes, the assumption being that the same shape means that the ectopics are arising from the same site or focus.

The first 8 complexes show ventricular bigeminy i.e. every second beat is a ventricular ectopic complex. The interval between the sinus complex and the following ventricular ectopic complex (the coupling interval) is constant at 500-560 msec. A constant coupling interval is a common feature of ventricular ectopic beats.

Complexes 9 and 10 are two consecutive sinus complexes. The P-P interval between complexes 9 and 10 is 1060 msec, which we will assume is the usual P-P interval during sinus rhythm.

The bigeminal rhythm between complexes 1 and 8 (a repeated pattern of two complexes and a pause) has a ventricular rate of about 80 beats per minute. The P-P interval between complexes 9 and 10 corresponds to a sinus rhythm with a heart rate of about 57 beats per minute.

The ST-T segment of each ventricular ectopic complex has a upward deflection near its lowest point. This is a P' wave produced by retrograde passage of the ventricular ectopic complex through the atrioventricular node into the atrium. About 45 percent of ventricular ectopic complexes pass retrogradely into the atria (Kistin AD, Landowne M. Retrograde conduction from premature ventricular contractions, a common occurrence in the human heart. Circulation. 1951;3:738-751)

+ What are the possible interactions between a ventricular ectopic complex and the sinus rhythm?

  1. The ventricular ectopic complex can be "sandwiched" between two sinus complexes without being followed by a pause. This is also called a interpolated ventricular ectopic complex
  2. The ventricular ectopic complex is followed by a pause. This can be a :

A. Complete compensatory pause (Figure 4)


Figure 4

The ventricular ectopic complex (4) has entered the atrioventricular node, making it refractory i.e. unable to conduct the next sinoatrial impulse (S4) from the sinoatrial node. The next conducted sinus impulse will be S5. The R-R interval between complexes 3 and 5 is (7 + 17) units i.e. 24 units. This is equal to the sum of the R-R intervals of two normal complexes:

R-R(1-2) + R-R(3 - 4) = R-R(4-5) + R-R(5-6).

This is called a complete compensatory pause.

B. Incomplete Compensatory Pause (Figure 5)

Figure 5

The ventricular ectopic complex (4) has passed through the atrioventricular node and reached the sinoatrial node. This may inscribe a inverted P wave on the ECG, or an upright P wave if the sinoatrial node is discharged by the ventricular ectopic complex. The result is suppression of the next due sinoatrial impulses (S4), with the following impulse (S4') occurring later than S4. However the S4' impulse occurs before the S5 impulse that would have occurred if there had been no change in the cardiac rhythm. If we measure the same R-R intervals as we did in the previous example we find the R-R interval between complexes 3 and 5 is (3 + 16) units i.e. 19 units, and that between complexes 1 to 3 is 24 units.

In other words:

R-R(1-2) + R-R(3 - 4) < R-R(4-5) + R-R(5-6)

This is called a incomplete compensatory pause.

The concept of complete and incomplete compensatory pauses are irrelevant to everyday analysis of ectopic beats, but they are useful in understanding the concept of retrograde activation of the atria or the sinoatrial node or both by ventricular ectopic complexes.

3.. Concealed conduction and echo beats (Bigeminal geek alert)

A ventricular ectopic complex that passes into the atrioventricular node but does not enter the atria may affect the conduction of sinoatrial impulses through the node, an effect that is called "concealed conduction". (Langebdorf R. Concealed conduction: The effect of blocked impulses on the formation and conduction of subsequent impulses. Am Heart J 1948; 35:542).

An example of concealed conduction is shown in Figure 6,

Figure 6

Sinus rhythm with ventricular ectopic complexes producing concealed conduction. The numbers are mm, and CI is coupling interval. The tracing comes from Rhythm Quizlets Self Assessment (HJL Marriott. 1987. Lea & Febiger). P waves are identified by blue circles; a white circle is a presumptive P wave whose position is based on a P-P interval of 25 mm. The P waves are conducted to the ventricles with a normal PR interval (green rectangle) or a more prolonged PR interval (P waves 3, 6 and 9). The prolongation of the PR interval is due to ventricular ectopic complexes (2, 5 and 8) that have entered the atrioventricular node and reduced the anterograde conduction velocity.

Echo beats occur when a ventricular ectopic complex passes both retrogradely through the atrioventricular node and in an anterograde direction via a re-entry pathway back to the ventricles. (Figure 7)

Figure 7

Consecutive rhythm strips (Lead II). A nodal rhythm (N) is present, with retrograde atrial conduction causing P' waves and re-entry in the atrioventricular node with anterograde conduction causing echo impulses (E). The laddergram analysis applies to the lower rhythm strip. (The rhythm strip is from: Barold SS, Linhart JW and Samet P. Reciprocal beating induced by ventricular pacing. Circulation. 1968;38:330-340) Echo beats may also be caused by a ventricular pacemaker (Fig 8) or by a ventricular ectopic complex.

Figure 8

Rhythm strip from lead II. A ventricular pacemaker (PM) has produced four ventricular complexes. Two of these pacemaker complexes (6 and 9) pass into the atrioventricular node and cause retrograde activation of the atria (causing a P' wave) and also pass anterograde to produce a echo (or reciprocal) beat. (The rhythm strip is from: Barold SS, Linhart JW and Samet P. Reciprocal beating induced by ventricular pacing. Circulation. 1968;38:330-340)

Important Points

We have used this ECG:

  1. To review the terminology of ventricular ectopic complexes
  2. To define the ECG characteristics of ventricular ectopic complexes
  3. To introduce the concept of ventriculoatrial passage of ventricular ectopic complexes

    • which helps us understand complete and incomplete compensatory pauses
    • and logically leads to the (advanced) concepts of concealed conduction and echo beats

Comments

(1) Greig Pickhaver and John Doyle are Australian comedians who wrote and hosted the satirical radio program called This Sporting Life on Triple J from 1986 to 2008. The radio show featured two characters: Roy and HG. Roy (John Doyle) was "'Rampaging' Roy Slaven", a retired sportsman turned expert commentator with a penchant for banality and cliche and for repeating the obvious. HG (Greig Pickhaver) was "HG Nelson" a excitable and maniacal announcer with the loud voice and sincerity of a house auctioneer. The show was a affectionate parody of Australia's obsession with sport - "making the serious trivial and the trivial serious" (Pickhaver).

The catch cry of each show was "When too much sport is barely enough!"

(2) A true name is a name of a thing or being that expresses, or is somehow identical with, its true nature. In mythology or literature knowing the true name of a person or spirit is often necessary before you can control or defeat them. The true name of things is a key theme in "Wizard of Earthsea" (written by Ursula Le Guin and published in 1968). This novel is "The most thrilling, wise and beautiful children's novel ever, it is written in prose as taut and clean as a ship's sail. Every word is perfect, like the spells Ged has to master. It poses the deep questions about life, death, power and responsibility that children need answering.......Both story and language lie at its heart, for it contains allusions to fragmented legends about the tragedies of heroes and heroines, and the world of Earthsea itself was summoned by speech. This gives Le Guin's world the mysterious depths of Tolkien's, but without his tiresome back-stories and versifying.......Nobody has ever described the wonder and terror of dragons, dancing on the wind 'like a vast black bat, thin-winged and spiny-backed', with such conviction" (from a review of the Wizard of Earthsea by Amanda Craig in 2003 in the Guardian)

The science fiction short story "The Nine Billion Names of God" (Arthur C Clarke, 1953) is about a Tibetan lamasery where the monks seek to list all the names of God. They decide to use modern technology to help them, and hire two computer operators to install and program a computer. When all of God's names are discovered the universe ends. (Human Genome Program, anyone?)

(3) Ventricular means a site that is distal to the bifurcation of the His bundle. (The His bundle is located in the interventricular septum, but is regarded as part of the atrioventricular junction). The term supraventricular thus includes the sinoatrial node, the atria, the atrioventricular node and the His bundle.

Pacemaker cells are present in the normal conduction system distal to the bifurcation of the His bundle. If the function of the normal pacemaker cells above the His bundle is impaired or there is a block to conduction involving the atrioventricular node or His bundle , a ventricular escape complex or a ventricular escape rhythm (called a idioventricular rhythm) will (should) develop.

Ectopic means "an abnormal place or position".

Complex means the QRS pattern on the ECG tracing produced by the electrical activity of the heart.

A ventricular ectopic complex is thus a electrical event that arises from a abnormal location in the ventricle i.e. from a focus outside the normal ventricular conduction pathway. The ventricular ectopic complex is premature i.e. occurs before the next expected impulse of the basic rhythm. This earlier onset distinguishes an ectopic complex from the delayed onset of a ventricular escape complex.

"Ventricular ectopic complex" is (probably) the most accurate true name, but the use of the term may not mean mastery of the topic. Furthermore there are many alternative names in use: ventricular extrasystole, premature ventricular contraction, premature ventricular beat, ventricular ectopic beat or ventricular premature depolarisation.

None of these terms is absolutely satisfactory e.g. "contraction" means shortening of cardiac muscle cells which follows an electrical impulse, and "beat" means heart beat which describes a haemodynamic event rather than a electrical one. Leo Schamroth, a well known South African cardiologist,  had this to say about one term: "The term 'premature ventricular contraction' .... is nonspecific, vague , inadequate and undesirable" (The Disorders of Cardiac Rhythm. Second Edition 1980 Blackwell Scientific Publications). The alternative terms are too entrenched for us to worry about their deficiencies.

Schamroth also had this to say about "bigeminy" and "trigeminy":

"When ventricular extrasystoles [his preferred term for ventricular ectopic complexes] occur alternately with conducted sinus beats , it results in a bigeminal rhythm - a repeated basic pattern of two beats and a pause. ..... Bigeminal rhythm may also result from other mechanisms such as 3:2 S-A or 3:2 A-V block. ....... [The terms] 'bigeminy' and 'trigeminy' ..... are electrocardiographic misnomers,  Bigeminy means two pairs - (bi: the Latin for twice, or double; and gemini: twins, Castor and Pollux - the third sign of the Zodiac). Bi geminy therefore means two twins, literally a grouping of two pairs. .......Nevertheless, these terms [bigeminy, trigeminy and quadrigeminy], by repeated usage over many years, have come to signify basic cadences of two, three, or four beats, etc...."(Leo Schramoth The Disorders of Cardiac Rhythm. Second Edition 1980 Blackwell Scientific Publications)