CHAPTER 5

Intraventricular Conduction Abnormalities

NORMAL CONDUCTIONMany card iac condi t ions cause elect r ica l impulses to be conducted abnormal ly through the

vent r icu lar myocard ium, producing changes in QRS complexes and T waves. Therefore, i t is

important to understand the condi t ions requi red to fac i l i ta te normal in travent r icu lar impulse

conduct ion. These are as fol lows

The le f t and r ight ventr ic les are not in an enlarged s tate that would pro long the t ime

requi red for thei r act ivat ion and recovery ( Chapter 4 , “Chamber Enlargement”) .

Myocard ia l ischemia or in farct ion is not present or is of insuff ic ient magnitude to d isrupt

the spread of the act ivat ion and recovery waves ( Chapter 7 , “Myocard ial Ischemia and

In farc t ion”) .

There is rapid impulse conduct ion through the r ight- and le f t -ventr icular Purk in je

networks so that the endocardia l surfaces are act ivated almost s imultaneously (as

discussed later in th is chapter) .

There are no accessory pathways for conduct ion f rom the at r ia to the vent r ic les

(Chapter 6 , “Ventr icu lar Preexc i tat ion”) .

BUNDLE-BRANCH AND FASCICULAR BLOCKSince the act ivat ion of the vent r icu lar Purkinje system is not represented on the surface

electrocard iogram (ECG) (Fig. 1 .9 ), abnormal i t ies of i ts conduct ion must be detected indi rect ly

by the i r ef fects on myocardial act ivat ion and recovery. The most speci f ic changes ind icat ive of

such abnormal i t ies occur wi thin the QRS complex. A conduct ion disturbance within the r ight

bundle branch (RBB), lef t bundle branch (LBB), le f t bundle fasc ic les , or between the Purkinje

f ibers and the adjacent myocard ium may al ter the QRS complex and T wave ( Fig. 5 .1). A

conduction disturbance in the common bundle (Bundle of His) has similar effects on the entire

distal Purkinje system, and therefore does not alter the appearance of the QRS complex or T

wave.

Figure 5.1. LAF and LPF indicate the le f t anter ior and

le f t poster ior fasc ic les, respect ive ly . ( 1) , (2 ) , and (3 )

indicate the locat ions at which in travent r icu lar

conduct ion abnormal i t ies can produce al terat ions of the

QRS complex and T wave. (Modif ied f rom Wagner GS,

Waugh RA, Ramo BW. Card iac ar rhythmias. New York:

Churchi l l L iv ingstone, 1983;18. )

Block of an ent i re bundle branch requires that i ts ventr ic le be act ivated by myocard ia l spread

of e lectr ical act iv i ty f rom the other vent r ic le, wi th pro longat ion of the overa l l QRS complex.

Block of the ent i re RBB is termed complete r ight bundle-branch b lock (RBBB), whi le block of

the ent i re LBB is termed complete le f t bundle-branch b lock (LBBB). In both of these condi t ions,

the ventr ic les are act ivated success ively instead of s imultaneously. The other condi t ions in

which the ventr ic les are act ivated successive ly occur when one ventr ic le is preexci ted via an

accessory atr iovent r icu lar (AV) pathway ( Chapter 6 , “Ventr icu lar Preexc i tat ion”) and when

there are independent ventr icular rhythms ( Chapter 13 and Chapter 17). Under these

condi t ions, there is a fundamenta l s imi lar i ty in the d is tort ions of the ECG waveforms: the

durat ion of the QRS complex is prolonged and the ST segment s lopes into the T wave in the

di rect ion away from the vent r ic le in which the abnormal i ty is located ( Fig. 5 .2).

Figure 5.2. Compar ison of pat terns of QRS

morphology in lead V1 when the two vent r ic les are

act ivated success ively ra ther than s imul taneously :

A. A ventr icu lar beat . B. Bundle branch b lock. C. Vent r icu lar tachycard ia. D. Art i f ic ia l ly paced

vent r icu lar rhythm.

A vent r icu lar conduct ion delay with only s l ight pro longat ion of the QRS complex could be

termed incomplete RBBB or incomplete LBBB. However, i t is impor tant to remember f rom

Chapter 4 ( “Chamber Enlargement”) that en largement o f the r ight ventr ic le may produce a

distor t ion of the QRS complex that mimics incomplete RBBB ( Fig. 4 .9B), whereas enlargement

of the le f t ventr ic le may produce a pro longat ion of the QRS complex that mimics incomplete

LBBB (Fig. 4 .8A). S ince the LBB has mult ip le fasc ic les, another form of incomplete LBBB could

be produced by a d is turbance in one of i ts major fascic les.

The ventr icular Purkinje system is considered t r i fasc icular . I t cons is ts o f the RBB and the

anter ior and poster ior por t ions of the LBB. The prox imal RBB is smal l and compact , and may

therefore be considered ei ther a bundle branch or a fascic le. The prox imal LBB is also

compact , but is too large to be considered a fascic le. I t remains compact for 1 to 2 cm and

then fans into i ts two fascic les. 1 As Demoul in and Kulbertus have shown in humans, 2 there are

mult ip le anatomic var ia t ions in these fascic les among indiv iduals. Based on the i r anatomic

locat ions, the two fasc ic les are termed the lef t -anter ior fasc ic le (LAF) and le f t -poster ior

fasc ic le (LPF), as seen in Figure 5.3 . The LAF of the LBB courses toward the anter ior-super ior

papi l lary muscle, and the LPF of the LBB courses toward the poster ior - infer ior papi l lary

muscle. There are a lso Purkinje f ibers that emerge from the very proximal LBB that proceed

along the sur face of the interventr icular septum and in i t iate lef t - to-r ight spread of act ivat ion

through the interventr icular septum.

Figure 5.3. The le f t ventr ic le has been opened to reveal the LBB and i ts fascic les as or ig ina l ly

presented in Figure 1.7C. Note that the anter ior and poster ior fasc ic les of the LBB are also

designated super ior and in fer ior , respect ive ly , because these terms ind icate the ir t rue anatomic

posi t ions. (From Netter FH. The Ciba col lect ion of medica l i l lust ra t ions. vo l 5. Hear t . Summit ,

NJ: Ciba–Geigy, 1978:13. )

Rosenbaum and coworkers descr ibed the concept of b locks in the fasc ic les of the LBB, which

they termed le f t anter ior and le f t poster ior hemiblock .3 However, these two k inds of b lock are

more appropr iate ly termed le f t anter ior fascicu lar b lock (LAFB) and le f t poster ior fasc icu lar

block (LPFB). Iso la ted LAFB, LPFB, or RBBB is considered unifasc icular b lock. Complete LBBB

or combinat ions of RBBB with LAFB or wi th LPFB are bi fasc icu lar blocks , and the combinat ion

of RBBB wi th both LAFB and LPFB is considered t r i fascicu lar b lock .

UNIFASCICULAR BLOCKSThe term “uni fasc icular b lock” is used when there is ECG ev idence of b lockage of on ly the

RBB, LAF, or LPF. Iso la ted RBBB or LAFB occur commonly, whi le iso la ted LPFB is rare.

Rosenbaum and coworkers ident i f ied only 30 pat ients wi th LPFB, as compared with 900

pat ients wi th LAFB.3

Right Bundle-Branch BlockSince the r ight ventr ic le cont r ibutes min imal ly to the normal QRS complex, RBBB produces

l i t t le d is tor t ion of the QRS complex dur ing the t ime requi red for le f t -vent r icu lar act ivat ion.

Figure 5.4 i l lus trates the minimal distort ion of the ear ly port ion and marked distor t ion of the

la te port ion of the QRS complex that typ ica l ly occurs with RBBB. The min imal contr ibut ion of

the normal r ight-ventr icular myocard ium is complete ly subt racted f rom the ear ly por t ion of the

QRS complex and then added la ter, when the r ight ventr ic le is act ivated v ia the spread of

impulses f rom the lef t ventr ic le . Th is produces a late prominent pos i t ive wave in lead V1

termed R¢, because i t fo l lows the ear l ier pos i t ive R wave produced by the normal lef t - to-r ight

spread of act ivat ion through the in terventr icu lar septum ( Fig. 5 .4 and Table 5.1).

Figure 5.4. The cont r ibut ions f rom act ivat ion of the

in tervent r icu lar septum and the r ight and lef t ventr icular

f ree wal ls to the appearance of the QRS complex in

lead V1, wi th normal in t ravent r icu lar conduct ion (top)

and with RBBB (bottom) . The numbers refer to the f i rst ,

second, and thi rd sequent ia l 0.04-s per iods of t ime.

Only two 0.04-s per iods are required for normal

conduct ion, but a th ird is requi red when RBBB is

present .

Table 5.1. Cr i ter ia for Right Bundle Branch Block

RBBB has many var ia t ions in i ts ECG appearance, as i l lust ra ted by the examples in Figure

5.5A, Figure 5.5B and Figure 5.5C. In Figure 5.5A, the RBBB is considered “ incomplete”

because the durat ion of the QRS complex is on ly 0 .10 s ; but in Figure 5.5B and Figure 5.5C ,

the RBBB is considered “complete” because the duration of the QRS complex is ≥ 0.12 s.

Figure 5.5. Twelve- lead ECGs f rom a

17-year-o ld g ir l wi th an ost ium

secundum at r ia l septal defect (A) , an

81-year-o ld woman wi th f ibrosis of the

RBB (B) , and an 82-year-o ld man with

f ibros is o f both the RBB and the

anter ior fasc ic le of the LBB (C) . Arrows

in A , B , and C ind icate the prominent

terminal R¢ wave in V1, and aster isks

in A and C ind icate the r ightward and

le f tward ax is shi f ts, respect ive ly .

Left-Fascicular BlocksNormal act ivat ion of the lef t -vent r icu lar f ree wal l spreads simultaneously f rom two s i tes (near

the inser t ions of the papi l lary muscles of the mi tral valve) . Wavefronts of act ivat ion spread

from these endocardial s i tes to the over ly ing epicardium. Since the wavefronts t ravel in

opposi te di rect ions, they neut ra l ize each other 's in f luence on the ECG in a phenomenon ca l led

cancel lat ion . When b lock in e i ther the LAF or LPF is present, act ivat ion of the f ree wal l

proceeds from one si te instead of two. Since the cancel lat ion is removed, the waveforms of the

QRS complex change, as descr ibed below ( Fig. 5 .6 and Table 5.2 and Table 5.3).

Figure 5.6. Schematic le f t ventr ic le v iewed f rom i ts apex upward toward i ts base. The

in tervent r icu lar septum (S ) , lef t -ventr icular f ree wal l ( FW ) , and anter ior (A ) and infer ior ( I )

regions of the le f t vent r ic le are ind icated. The typ ical appearances of the QRS complexes in

leads I (top) and aVF (bottom) are presented for normal (A) , LAFB (B) , and LPFB le f t -

vent r icu lar act ivat ion (C) . Dashed l ines w i th in the inner c i rc les represent the fascic les; the two

wavy l ines c ross ing a fascic le indicate the si tes of b lock. Smal l crosshatched ci rc les represent

the papi l lary muscles; outer r ings represent the endocardial and epicard ia l surface of the le f t

vent r icu lar myocard ium. Arrows w i th in the outer r ings indicate the di rect ions of the wavefronts

of act ivat ion as they spread f rom the unblocked fasc ic les through the myocard ium.

Table 5.2. Cr i ter ia for Lef t Anter ior Fasc icu lar Block

Table 5.3. Cr i ter ia for Lef t Poster ior Fascicu lar B lock

Left Anterior Fascicular Block.I f the LAF of the LBB is blocked (Fig. 5 .6B), the in i t ia l act ivat ion of the lef t -vent r icu lar f ree wal l

occurs v ia the LPF. Act ivat ion spreading from endocard ium to the epicardium in th is region is

di rected in fer ior ly and r ightward. S ince the block in the LAF has removed the in i t ia l super ior

and lef tward act ivat ion, a Q wave appears in leads that have thei r posi t ive e lectrodes in a

super ior/ le f tward posi t ion ( i .e. , lead I) and an R wave appears in leads that have the ir posi t ive

electrodes in an in fer ior / r ightward pos i t ion ( i .e . , lead aVF). Fol lowing th is in i t ia l per iod, the

act ivat ion wave spreads over the remainder of the le f t -ventr icular f ree wal l in a

super ior/ le f tward d irect ion, producing a prominent R wave in lead I and a prominent S wave in

lead aVF. This change in the lef t -ventr icular act ivat ion sequence produces a lef tward sh i f t o f

the axis of the QRS complex to at least –45 degrees. The overa l l durat ion of the QRS complex

may be normal (Fig. 5 .7A) or pro longed by 0.01 to 0.04 s (Fig. 5 .7B).4

Figure 5.7. Twelve- lead ECGs f rom a 53-year-o ld woman wi th no medical problems (A) and a

75-year-o ld man with a long history of poor ly t reated hypertens ion (B) . Arrows ind icate the

deep S waves in leads I I , I I I , and aVF that ref lect ext reme le f t axis deviat ion.

Left Posterior Fascicular Block.I f the LPF of the LBB is blocked (Fig. 5 .6C), the s i tuat ion is reversed f rom that in LAF b lock,

and the in i t ia l act ivat ion of the le f t -vent r icu lar f ree wal l occurs v ia the LAF. Act ivat ion

spreading f rom the endocard ium to the epicardium in th is region is di rected super ior ly and

le f tward. S ince the b lock in the LPF has removed the in i t ia l infer ior and r ightward act ivat ion, a

Q wave appears in leads wi th the ir posi t ive e lect rodes in an infer ior / r ightward posi t ion ( i .e. ,

lead aVF) and an R wave appears in leads wi th thei r posi t ive e lectrodes in a super ior/ le f tward

di rect ion ( i .e. , lead I ) . Fo l lowing th is in i t ia l per iod, the act ivat ion spreads over the remainder o f

the le f t -vent r icu lar f ree wal l in an infer ior / r ightward d i rect ion, producing a prominent R wave in

lead aVF and a prominent S wave in lead I . Th is change in the lef t -vent r icu lar act ivat ion

sequence produces a r ightward shi f t o f the axis of the QRS complex to at least +90 degrees. 5

The durat ion of the QRS complex may be normal or s l ight ly prolonged ( Fig. 5 .8 ).

Figure 5.8. Twelve- lead ECG from a heal thy 77-year-o ld woman. Arrows ind icate the deep S

waves in leads I and aVL typ ical o f both LPFB and RVH.

The considerat ion that LPFB may be present requi res that there be no evidence of r ight-

vent r icu lar hyper trophy (RVH) f rom e i ther the precord ia l leads ( Fig. 5 .8 ) or f rom other c l in ica l

data. However , even the absence of RVH does not al low diagnosis o f LPFB, because RVH can

produce the same pat tern as LPFB in the l imb leads, and RVH is much more common than is

LPFB.

BIFASCICULAR BLOCKSThe term “b i fascicu lar block” is used when there is ECG evidence of invo lvement o f any two of

the RBBB, LAF, or LPF. Such evidence may appear at di f ferent t imes or may coexist on the

same ECG. Bi fascicu lar block is somet imes appl ied to complete LBBB, and is commonly

appl ied to the combinat ion of RBBB wi th e i ther LAFB or LPFB. The term “b i la tera l bundle-

branch block” is also appropr iate when RBBB and ei ther LAFB or LPFB are present. 6 When

there is bi fasc icular b lock, the durat ion of the QRS complex is prolonged to at least 0.12 s .

Left Bundle-Branch BlockFigure 5.9 i l lus trates the marked d istor t ion of the ent i re QRS complex produced by LBBB.

Complete LBBB may be caused by d isease in e i ther the main le f t bundle branch (LBB)

(prediv is ional ) or in both of i ts fascic les ( postd iv is ional ) . When the impulse cannot progress

along the LBB, elect r ica l act ivat ion must f i rst occur in the r ight ventr ic le and then t ravel

through the interventr icular septum to the le f t vent r ic le.

Figure 5.9. The format o f Figure 5.4 is repeated to i l lus tra te the contr ibut ions f rom act ivat ion

of the var ious aspects of the vent r icu lar myocard ium to the appearances of the QRS complex in

lead V1 with LBBB.

Normal ly , the intervent r icu lar septum is act ivated from lef t to r ight , producing an in i t ia l R wave

in the r ight precord ia l leads and a Q wave in leads I , aVL, and the lef t precord ia l leads. When

complete LBBB is present , however, the septum is act ivated f rom r ight to le f t . Th is produces Q

waves in the r ight precord ia l leads and e l iminates the normal Q waves in the le f tward-or iented

leads. The act ivat ion of the lef t ventr ic le then proceeds sequent ia l ly f rom the in terventr icular

septum, to the adjacent anter ior and in fer ior wal ls, and then to the poster ior - lateral f ree wal l .

Th is sequence of ventr icular act ivat ion in complete LBBB tends to produce monophasic QRS

complexes, wi th QS complexes in lead V1 and R waves in leads I , aVL, and V6 ( Table 5.4).

Table 5.4. Cr i ter ia for Lef t Bundle Branch Block

LBBB has many var ia t ions in i ts ECG appearance, as i l lust ra ted by the examples in Figure

5.10A, Figure 5.10B and Figure 5.10C. Figure 5.10A shows the typica l appearance of complete

LBBB. In Figure 5.10B the extreme LAD indicates that conduct ion is even s lower in the LAF

than in the LPF, and only minimal R waves are seen in leads V1 through V4. In Figure 5.10C

the aberrat ion of a markedly pro longed QRS complex is present , suggest ing the coexis tence of

le f t -vent r icu lar hypert rophy (LVH).

Figure 5.10. Twelve- lead ECGs f rom an 82-

year-o ld woman wi th no medical problems

(A) , a 71-year-o ld man wi th chronic hear t

fa i lure (B) , and a 74-year-o ld man wi th a

long history of hypertension (C) . Arrows in

A and C ind icate the typ ical character ist ics

of LBBB in leads I and V1, and arrows in B

indicate the deep S waves in leads I I , I I I ,

and aVF and decreased R waves in leads

V2–V4.

Right Bundle-Branch Block with Left Anterior Fascicular BlockJust as LAFB appears as a un i fascicu lar b lock much more commonly than does LPFB, i t more

commonly accompanies RBBB as a b i fascicu lar b lock. The diagnosis o f LAFB p lus RBBB is

made by observing the la te prominent R or R¢ wave in precord ial lead V1 of RBBB, and the

in i t ia l R waves and prominent S waves in l imb leads I I , I I I , and aVF of LAFB. The durat ion of

the QRS complex should be at least 0 .12 s and the f ronta l-plane axis of the complex should be

between –45 degrees and –120 degrees ( Fig. 5 .11). In Figure 5.11A only LAFB is present ,

whi le in Figure 5.11B the presence of RBBB indicates that a second fascic le has been b locked.

Figure 5.11. Twelve- lead ECGs f rom a 1-year prev ious (A) and a current (B) eva luat ion of a

73-year-o ld woman wi th no medica l problems and no other evidence of hear t d isease. Arrows

indicate the deep S waves in I I , I I I , and aVF that are character is t ic of LAFB in A , and a

prominent R¢ wave character ist ic o f RBBB in V1 in B .

Right Bundle-Branch Block with Left Posterior Fascicular BlockThe example of bi fascicu lar block consist ing of RBBB wi th LPFB rarely occurs. Even when

changes in the ECG are ent i rely typica l o f th is combinat ion, the d iagnosis should be

considered only i f there is no c l in ica l evidence of RVH. The d iagnosis of RBBB wi th LPFB

should be considered when precord ia l lead V1 shows changes typica l o f RBBB and l imb leads I

and aVL show the in i t ia l R waves and prominent S waves typ ical o f LPFB. The durat ion of the

QRS complex should be at least 0 .12 s and the f ronta l-plane axis of the complex should be at

least +90 degrees (Fig. 5 .12).7

Figure 5.12. Twelve- lead ECG from an 82-year-o ld woman wi th no complaints and no other

ev idence of hear t d isease. Arrows ind icate the prominant S waves in I and aVL and RR'

complex in V1.

SYSTEMATIC APPROACH TO THE ANALYSIS OF BUNDLE-BRANCH AND FASCICULAR BLOCKSThe fo l lowing s teps should be taken in analyzing bundle-branch and fascicu lar b locks:

Examine the contour of the QRS complex.

RBBB and LBBB have opposi te ef fects on the contour o f the QRS complex. RBBB adds

a new waveform di rected toward the r ight vent r ic le fo l lowing the complet ion of s l ight ly

al tered waveforms d irected toward the lef t vent r ic le ( Fig. 5 .4). Therefore, the QRS

complex in RBBB tends to have a t r iphas ic appearance. In lead V1, which is opt imal for

v isual iz ing r ight - versus le f t -s ided conduct ion delay, the QRS in RBBB has the

appearance of “rabbi t ears” (Fig. 5 .5). Typical ly , the “ f i rs t ear” (R wave) is shor ter than

the “second ear” (R¢ wave) . (A l though the term “rabbi t ears” in th is context re fers to a

t r iphas ic QRS, i t can a lso refer to two peaks found in monophasic QRS complexes.)

When RBBB is accompanied by block in one of the LBB fasc ic les, the pos i t ive

def lect ion in lead V1 is o f ten monophasic , as in Figure 5.12 .

In LBBB, a sequent ia l spread of act ivat ion through the interventr icular septum and lef t -

vent r icu lar f ree wal l rep laces the normal , compet ing and s imul taneous spread of

act ivat ion through these areas. As a resul t , the QRS complex tends to have a

monophasic appearance that is notched rather than smooth.

Al though LBBB and LVH have many ECG s imi lar i t ies, they also show marked

di f ferences. Whereas the normal Q waves over the le f t vent r ic le may be present or even

exaggerated in LVH, they are absent in LBBB. When the LBB is complete ly b locked, the

septum is ent i re ly act ivated f rom i ts r ight s ide. Figure 5.13 i l lus trates the appearance of

incomplete (Fig 5.13B) and complete (Fig. 5 .13C) LBBB in a pat ient wi th LVH (Fig.

5.13A).

Figure 5.13. The f ive representat ive ECG leads i l lus tra te the evolv ing ECG changes in

a pat ient wi th severe hypertension as the LVH is compl icated by LBBB. A. Age 60

years. B. Age 63 years. C. Age 67 years.

Measure the Durat ion of the QRS Complex.

Complete RBBB increases the durat ion of the QRS complex by 0.03 to 0.04 s , and

complete LBBB increases the durat ion of the complex by 0.04 to 0.05 s . Block wi th in

the LAF or LPF of the LBB usual ly prolongs the durat ion of the QRS complex by only

0.01 to 0.02 s (Fig. 5 .7B and Fig. 5 .8).4

Measure the Maximal Ampl i tude of the QRS Complex.

Bundle-branch b lock (BBB) produces QRS waveforms wi th lower vo l tage and more

def in i te notching than those that occur wi th ventr icular hypert rophy. However, the

ampl i tude of the QRS complex does increase in LBBB because of the relat ively

unopposed spread of act ivat ion over the le f t ventr ic le .

One general rule for d i f ferent ia t ing between LBBB and LVH is that the greater the

ampl i tude of the QRS complex, the more l ike ly is LVH to be the cause of th is. S imi lar ly ,

the more prolonged is the durat ion of the QRS complex, the more l ikely is LBBB to be

the cause of th is e f fect . Klein and co l leagues 8 have suggested that in the presence of

LBBB, ei ther of the fol lowing cr i ter ia are associated with LVH:

o S wave in V2 + R wave in V6 > 45 mm.

o Evidence of lef t -at r ia l en largement wi th a QRS-complex durat ion > 0.16 s .

Est imate the Di rect ion of the QRS Complex in the Two Planes of the ECG.

Since complete RBBB and complete LBBB a l ter conduct ion to ent i re vent r ic les, they might not

be expected to produce much net a l terat ion of the f rontal -p lane QRS ax is . However,

Rosenbaum studied pat ients wi th intermi t tent LBBB in which b locked and unblocked complexes

could be examined side by side.4 LBBB was of ten observed to produce a s igni f icant le f t -axis

sh i f t and somet imes even a r ight axis sh i f t . The ax is was unchanged in on ly a minor i ty o f

pat ients.

However , b lock in e i ther the LAF or LPF of the LBB alone produces marked ax is deviat ion. The

in i t ia l 0 .20 s of the QRS complex is d irected away from the b locked fasc ic les, and the middle

and late port ions are di rected toward the b locked fasc ic les, caus ing the overa l l d i rect ion of the

QRS complex to be shi f ted toward the s i te o f the block ( Fig. 5 .7 and Fig. 5 .8 ).5 When b lock in

ei ther of these LBB fasc ic les is accompanied by RBBB, an even la ter waveform is added to the

QRS complex, thereby further pro longing i ts durat ion. The di rect ion of th is f inal waveform in

the f rontal p lane is in the vic in i ty o f 180 degrees, as a resul t o f the RBBB ( Fig. 5 .5C).5

In BBB, the T wave is usual ly d i rected opposi te to the la ter por t ion of the QRS complex (e.g. ,

in Figure 5.14A, the T wave in lead I is inverted and the la ter par t of the QRS complex is

upr ight ; in Figure 5.14B the T wave is upr ight and the later part o f the QRS complex is

negat ive) . Th is opposi te polar i ty is the natura l resul t of the depolar izat ion–repolar izat ion

disturbance produced by the BBB, and is therefore termed secondary . Indeed, i f the di rect ion

of the T wave is s imi lar to that o f the terminal part o f the QRS complex ( Fig. 5 .14C), i t should

be considered abnormal . Such T-wave changes are pr imary and imply myocard ial d isease. The

diagnosis o f myocard ia l in farc t ion in the presence of BBB is considered in Chapter 10

(“Myocardial Infarction”).

Figure 5.14. Twelve- lead ECGs f rom

an 89-year-old woman dur ing a rout ine

heal th evaluat ion (A) , a 45-year-o ld

pi lo t dur ing an annual heal th

evaluat ion (B) , and a 64-year-o ld

woman on the f i rs t day af ter coronary

bypass surgery (C) . Arrows ind icate

the concordant d irect ions of the

terminal QRS complex and of the T

wave in leads V2–V4 in C .

One method of determin ing the c l in ica l s ign i f icance of T-wave changes in BBB is to measure

the angle between the ax is o f the T wave and that of the terminal par t o f the QRS complex.

Obvious ly , i f the two are opposi te ly d irected (as they are with secondary T-wave changes), the

angle between them wi l l be wide and may approach 180 degrees. I t has been proposed that i f

th is angle is less than 110 degrees, myocardia l d isease is present. In Figure 5.14B, the angle

is about 150 degrees, whereas in Figure 5.14C i t is only a few degrees.

CLINICAL PERSPECTIVE ON INTRAVENTRICULAR CONDUCTION DISTURBANCESBoth RBBB and LBBB are of ten seen in apparent ly normal ind iv iduals. 9 The cause of th is is

f ibros is o f the Purkinje f ibers, which has been descr ibed as Lenegre's d isease 10 or Lev 's

disease.11 The process of Purkinje f ibrosis progresses slowly : a 10-year fo l low-up s tudy of

heal thy aviators wi th BBB revealed no inc idence of complete AV b lock, syncope, or sudden

death.12 The patholog ic process may be accelerated by systemic hyper tension: i t preceded the

appearance of BBB in 60% of the ind iv iduals in the Framingham study. The mean age of onset

of the BBB was 61 years.13

Insight into the long- term prognosis for indiv iduals wi th chronic BBB but no other evidence of

card iac d isease comes from studies of the ECG changes preceding the development of

t ransient or permanent complete AV block. Fr iedberg and associates have documented the

common presence of some combinat ion of bundle-branch or fascicu lar block immediate ly

before onset of the AV b lock. The most common combinat ion was RBBB with LAFB. 14

The combined resul ts o f these stud ies suggest that Lenegre's or Lev's disease is a s lowly

develop ing process of f ibros is o f the Purkinje f ibers that has the ul t imate potent ia l of causing

complete AV b lock because of b i latera l bundle-branch involvement. S ince the Purk in je cel ls

lack the phys io logic capaci ty of the AV-nodal cel ls to conduct a t vary ing speeds, a sudden

progression f rom no AV b lock to complete AV b lock may occur. 15 When th is does occur ,

vent r icu lar act ivat ion can resul t on ly f rom impulse format ion wi th in a Purkin je cel l beyond the

si te o f the b lock. Severa l c l in ica l condi t ions may resul t , including syncope and sudden death.

Bundle-branch or fascicu lar block may a lso be the resul t o f other ser ious cardiac diseases. In

Centra l and South Amer ica, Chagas d isease, produced by in fect ion wi th Trypanosoma cruzi , is

almost endemic and is a common cause of RBBB wi th LAFB. 16 As indicated in Chapter 4 ,

RBBB is commonly produced by the d is tent ion of the r ight ventr ic le that occurs wi th volume

over loading. Trans ient RBBB may be produced dur ing r ight -hear t catheter izat ion, as i l lus tra ted

in Figure 5.15 .

Figure 5.15. RBBB is induced by t rauma to the RBB. A catheter has been advanced f rom the

leg via the infer ior vena cava, and i ts t ip l ies against the r ight vent r icu lar endocardium in the

vic in i ty o f the RBB. The resul tant RBBB is i l lus trated in the thi rd and four th beats of the

schemat ic lead V1 ECG record ing. (Modi f ied f rom Net ter FH. The CIBA col lect ion of medical

i l lust ra t ions. vo l 5. Heart . Summit , NJ: CIBA–Geigy, 1978:13. )

Any combinat ion of the bundle branches or prox imal fascic les may be b locked dur ing an

episode of myocard ia l ce l l death in a pat ient wi th coronary atheroscleros is . These s tructures

receive the ir b lood supply v ia the prox imal septa l per forat ing branch of the lef t anter ior

descending coronary ar tery (Fig. 5 .16). Therefore, the bundle branches and thei r prox imal

fasc ic les become involved when there is an occlus ion in ei ther the le f t main coronary ar tery or

the or ig in of i ts anter ior descending branch. Indiv iduals who survive to reach the hospi tal a f ter

occlus ion of such a major coronary ar tery may have any combinat ion of bundle-branch or

fasc icular blocks compl icat ing extensive myocardial infarct ion. Since the acute and long-term

mortal i ty ra tes in these pat ients are very high, they do not represent a s ign i f icant por t ion of the

overa l l populat ion of indiv iduals wi th chronic bundle-branch and fascicu lar block. 17

Figure 5.16. The prox imal por t ion of special ized conduct ion system is shown in re la t ion to i ts

blood supply f rom a r ight anter ior ob l ique view: A , AV node; B , Common bundle; C , LPF; D ,

LAF; E ; RBB. Note the lengths of the septa l per forat ing branches of the lef t anter ior

descending (LAD ) coronary ar tery in contrast to those of the poster ior descending ar tery ( PDA ) .

(From Rotman M, Wagner GS, Wal lace AG. Bradyarrhythmias in acute myocard ia l in farc t ion.

Ci rculat ion 1972;45:703–722, wi th permiss ion. Copyr ight 1972 Amer ican Hear t Associat ion.)

In termit tent BBB (pro longed QRS complexes present a t some t imes but not a t others) usual ly

represents a t ransi t ion stage before permanent b lock is estab l ished. Figure 5.17A and Figure

5.17B show examples of the sudden onsets of LBBB and RBBB, respect ive ly .

Figure 5.17. Precord ial leads V1 and V5 are shown f rom a 62-year-o ld woman dur ing rout ine

ECG moni tor ing af ter uncompl icated abdominal surgery (A) and a 54-year-o ld man dur ing 24-

hour ECG moni tor ing for a compla in t o f d izz iness (B) . Arrows ind icate the onsets in the V1

leads of typica l ly appearing LBBB in A and RBBB in B .

At t imes, in termi t tent BBB is determined by the heart rate. As the rate accelerates, the RR

in terva l shor tens and the descending impulse f inds one of the bundle branches s t i l l in i ts

re fractory per iod (Fig. 5 .18). With th is tachycard ia-dependent BBB , s lowing of the heart rate

al lows descending impulses to ar r ive af ter the refractory per iod of the ent i re conduct ion

system, and normal conduct ion is resumed.

Figure 5.18. Precord ial leads V1 and V5 dur ing rout ine ECG monitor ing of a 47-year-old woman

af ter breast cancer surgery. Arrows ind icate the appearance of incomplete RBBB fo l lowing the

shor ter cyc le interva ls .

A rarer form of intermi t tent BBB, which develops only when the cardiac cyc le lengthens rather

than shor tens (Fig. 5 .19), is termed bradycard ia-dependent BBB . In termit tent BBB is a form of

in termit tent aberrant conduct ion of e lect r ica l impulses through the ventr icular myocardium.

Figure 5.19. A l l beats are conducted s inus beats grouped in pa irs. Those ending the shor ter

cycles are conducted normal ly, whi le those ending the longer cycles are conducted wi th LBBB.

GLOSSARYAtherosclerosis:a th ickening of the inner ar ter ia l wal l caused by the deposi t ion of fat ty substances.

AV block:a block in the card iac conduct ion system that causes a d isrupt ion of at r ia l - to-ventr icular

electr ica l conduct ion.

Bifascicular block:an in traventr icular conduct ion abnormal i ty invo lv ing any two of : the RBB, the anter ior d iv is ion

of the LBB, and the poster ior div is ion of the LBB.

Bilateral bundle-branch block:an in traventr icular conduct ion abnormal i ty invo lv ing both the r ight and le f t bundle branches, as

indicated e i ther by the presence of some conducted beats wi th RBBB and others wi th LBBB, or

by AV block located d is ta l to the common bundle.

Bradycardia-dependent BBB:RBBB or LBBB that is in termit tent , appearing only wi th a s lowing of the at r ia l ra te .

Cancellat ion:El iminat ion of an abnormal i ty produced by a part icular cardiac problem by a s imi lar abnormal i ty

in another part o f the heart or by a di f ferent abnormal i ty in the same par t of the hear t , s ince

the ECG waveforms represent the summat ion of the wavefronts of act ivat ion and recovery

wi th in the hear t .

Chagas disease:

a trop ica l d isease caused by the f lagel la te organism Trypanosoma cruz i , which is marked by

prolonged h igh fever , edema, and enlargement o f the spleen, l iver, and lymph nodes, and is

compl icated by card iac involvement .

Fascicle:a group of Purk in je f ibers too smal l to be ca l led a “branch. ”

Fibrosis:a condi t ion in which Purkinje f ibers are t ransformed into nonconduct ing in ters t i t ia l f ibrous

t issue.

Left anterior fascicular block:a conduct ion abnormal i ty in the anter ior fasc ic le of the LBB.

Left poster ior fascicular block:to a conduct ion abnormal i ty in the poster ior fascic le of the LBB.

Lenegre's (Lev's) disease:both Lenegre and Lev descr ibed var ia t ions of f ibrosis o f the in travent r icu lar Purk in je f ibers in

the absence of o ther s igni f icant card iac disease.

Predivisional and postdivisional:terms refer r ing to block wi th in the LBB ei ther “pre-” or proximal to i ts d iv is ion in to fasc ic les, or

“post- ” and involv ing both the anter ior and poster ior fasc ic les.

Primary and secondary T-wave changes:in the presence of RBBB or LBBB, the term “pr imary T-wave changes” refers to abnormal T

waves that are di rected simi lar ly to the la t ter por t ion of the QRS complex, and “secondary T-

wave changes” refers to normal T waves that are d irected opposi te to the la t ter port ion of the

QRS complex.

Refractory period:the per iod fo l lowing e lect r ica l act ivat ion dur ing which a card iac cel l cannot be react ivated.

RR interval :the per iod between success ive QRS complexes.

Septal Q wave:a normal, in i t ia l ly negat ive QRS waveform that appears in le f tward-or iented ECG leads

because of ear l iest act ivat ion of the intervent r icu lar septum via the septa l fasc ic les of the LBB.

Syncope:a br ie f loss of consc iousness associated wi th t rans ient lack of cerebra l b lood f low.

Tachycardia-dependent BBB:RBBB or LBBB that is in termit tent , appearing only wi th an accelerat ion of the atr ia l ra te .

Trifascicular block:an in traventr icular conduct ion abnormal i ty invo lv ing the RBB and both the anter ior and

poster ior fascic les of the LBB.

Unifascicular block:an in traventr icular conduct ion abnormal i ty invo lv ing only one of the three pr inc ipal fasc ic les of

the in t ravent r icu lar Purkinje system.

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