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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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