Learning Objectives 15 AV Heart Blocks LO 15.1 Describe the origin and key features of block of the AV node. LO 15.2 Identify the causes, effects, and appearance of 1st-degree AV heart block. LO 15.3 Identify the causes, effects, and appearance of 2nd-degree AV heart block, type I. Chapter Outline LO 15.4 Identify the causes, effects, and appearance of 2nd-degree AV heart block, type II. LO 15.5 Identify the causes, effects, and appearance of 3rd-degree AV heart block. LO 15.6 Identify the causes, effects, and appearance of atrioventricular dissociation. מנח Block of the Atrioventricular Node 1st-Degree Atrioventricular Heart Block 2nd-Degree Atrioventricular Heart Block, Type 2nd-Degree Atrioventricular Heart Block, Type II 3rd-Degree Atrioventricular Heart Block Atrioventricular Dissociation 2A 11:1000 Case History ni A 65-year-old man presents to the emergency department complaining of palpi- tations. He denies chest pain, shortness of breath, or light headedness. He had a heart attack five years ago and has hypertension controlled by medication. After obtaining his vital signs, normal except for a slow pulse, the charge nurse attaches the ECG leads and turns on the monitor. The emergency room (ER) physician enters the room and examines the tracing while the nurse starts an IV. The physician calls the paramedic student into the exam room and hands her the ECG strip and asks her to interpret it. The student indicates that the rate is slow, but the rhythm is regular. She also notices that the PR interval is normal but that there are more P waves than QRS complexes. REKVISISJONER 418 Section 3 A Origin and Clinical Aspects of Dysrhythmias LO 15.1 Block of the Atrioventricular Node AV heart blocks are partial delays or complete interruptions in the cardiac conduction pathway between the atria and ventricles (Figure 15-1). The block can occur at the AV node, the bundle of His, or the bundle branches. With AV heart block, cardiac output can be negatively affected if the ven- tricular rate slows. This can occur either due to electrical impulses from the Chapter 15 A AV Heart Blocks 419 atria being blocked from entering the ventricles, or, as what can occur with complete AV heart block, the ventricular escape beats arise from a site that is REMINDER lower in the ventricles (producing a slower rate). The initial electrical impulse begins high The most common causes of heart block are ischemia, MI, degenerative in the right atrium, in disease of the conduction system, congenital anomalies, and medications the SA node. It travels (especially digitalis preparations). Dysrhythmias resulting from AV heart through the atria via intraatrial and intranodal pathways to the AV node. blocks include the following: 1st-degree AV heart block Figure 15-1 AV heart blocks 2nd-degree AV heart block, type I (Wenckebach) 2nd-degree AV heart block, type II 3rd-degree AV heart block KEY CONCEPT Heart blocks are partial delays or complete inter- ruptions in the cardiac conduction pathway between the atria and ventricles. . hihihi QRS QRS QRS QRS 1st-degree AV heart block is a consistent delay in conduction through the AV node 1st-Degree Atrioventricular Heart Block Description نے انیلہ اختیارا۔ WANT MORE INFORMATION? Visit the Fast & Easy ECGs Website at www.mhhe.com/ shadeECG2e TP ilin Ist-degree AV heart block is not a true block because all impulses are con- ducted from the atria to the ventricles, but rather it is a consistent delay of conduction at the level of the AV node (Figure 15-2). It results in prolonga- tion of the PR interval that remains the same duration from beat to beat. QRS QRS QRS QRS QRS QRS QRS 2nd-degree AV heart block, type I, is an intermittent block at the level of the AV node that results in a progressive lengthening of the PR interval LO 15.2 Causes While 1st-degree AV heart block may occur in healthy persons for no apparent reason (particularly athletes), it is sometimes associated with other conditions such as myocardial ischemia, acute MI, increased vagal (parasympathetic) tone, and digitalis toxicity. Other causes of 1st-degree AV heart block are shown in the following box. P TP الماليدا النترا QRS QRS QRS QRS QRS 2nd-degree AV heart block, type II, is an intermittent block at the level of the bundle of His or bundle branches resulting in some atrial impulses not being conducted to the ventricles Causes of 1st-Degree Heart Block Cause Examples thick whichi Cardiac diseases QRS 3rd-degree AV heart block is a complete block of the conduction at or below the AV node, and impulses from the atria cannot reach the ventricles Use of certain drugs Myocardial ischemia or infarction (often inferior wall MI), injury or ischemia to the AV node or junction, myocarditis, degenerative changes (age related) in the heart Beta-adrenergic blockers (i.e., propranolol), calcium channel blockers (i.e., verapamil) , digoxin, antidysrhythmics (i.e., amiodarone, quinidine, procainamide) به اساس اہل السناسل Autonomic nervous system stimulation Increased vagal tone Noncardiac disorders QRS QRS Hypokalemia and hyperkalemia, hypothermia, hypothyroidism QRS QRS QRS QRS QRS AV dissociation occurs when the atria and the ventricles are under the control of separate pacemakers and beat independently of each other Chapter 15 A AV Heart Blocks 421 420 Section 3 A Origin and Clinical Aspects of Dysthythmias In 1st-degree AV heart block impulses arise from the SA node but their passage through the AV node is delayed. Treatment As cardiac output is not affected, no specific treatment is indicated for 1st- degree AV heart block. However, efforts are directed toward identifying and treating the cause LO 15.3 Delay Delay Delay 2nd-Degree Atrioventricular Delay Delay Delay Heart Block, Type I Description نی نی یا 2nd-degree AV heart block, type I, also called Wenckebach or Mobitz Type I, is an intermittent block at the level of the AV node (Figure 15-3). The PR interval (representing AV conduction time) increases until a QRS com- plex is not generated (dropped). By then, AV conduction recovers and the 시 sequence repeats. Rate underlying rhythm may be slow, normal, or fast Rhythm underlying rhythm is usually regular P waves normal and all look alike; one precedes each QRS complex QRS complexes of normal duration and all look alike Some describe the pathophysiology of this dysrhythmia as a weakened AV junction that grows more tired with each heartbeat (thus producing In 2nd-degree AV heart block, type 1 (Wenckebach), impulses arise from the SA node but their passage through the AV node is progressively delayed until the impulse is blocked. PR intervals longer than 0.20 seconds in duration and constant ST segments normal and isoelectric I waves normal shape and upright, but may be abnormal if QRS complex is prolonged QT intervals within normal limits of 0.36 to 0.44 second U waves may or may not be present Figure 15-2 1st-degree AV heart block. Delay More delay ------ Even more delay ---- Delay Impulse is blocke More delay ----- ? QUESTION 1. What is the normal duration for the PR interval? Note: Answers to the Sidebar Questions can be found at the end of the chapter Effects 1st-degree AV heart block is of little or no clinical significance because all impulses are conducted to the ventricles. The patient experiencing this be condition is usually asymptomatic. 1st-degree AV heart block may temporary, particularly if it stems from ischemia early in the course of MI or certain medications . Conversely , it may also progress to higher degree block, especially in the presence of inferior wall MI. Appearance The most obvious characteristic of 1st-degree AV heart block is PR intervals greater than 0.20 seconds in duration and constant. The underlying rhythm is usually regular whereas the rate is that of the underlying rhythm. It can occur in bradycardic, normal rate, and tachycardic rhythms. The P waves are normal; one precedes each QRS complex, and the QRS complexes a normal limits. Rate underlying rhythm may be slow, normal, or fast ORS complexes usually of normal duration and all look alike, periodically one is absent P waves normal and all look alike, but not all followed by QRS complex Rhythm patterned irregularity QT intervals within normal limits of 0.36 to 0.44 second U waves may or may not be present are within I waves normal size and configuration ST segments normal and isoelectric PR intervals progressively longer in duration until a QRS complex is dropped, then cycle begins again KEY CONCEPT The most prominent char- acteristic of Ist-degree AV heart block is a PR interval that is greater than 0.20 seconds in duration. Figure 15-3 1st-degree AV block is more of a condition than a dysrhythmia. For this reason we typically describe it as part of the underlying rhythm (e.g., sinus rhythm, sinus tachycardia, or sinus bradycardia with 1st-degree AV block). 2nd-degree AV heart block, type I (Wenckebach). 422 Section 3 A Origin and Clinical Aspects of Dysrhythmias upright and uniform, but there are more P waves than QRS complexes progressively longer PR interval following each P wave). Finally, the AV junction is too tired to carry the impulse, and a QRS complex is dropped only a P wave appears). The lack of conduction through the AV junction allows it to rest; thus the next PR interval is shorter. Then as each subsequent impulse is generated and transmitted through the AV junction, there is a progressively longer PR interval until, again, a QRS complex is dropped. This cycle repeats. a QRS complex is finally dropped can vary among individuals. For example, one The number of beats in which the P wave is followed by a QRS complex before patient may have a dropped beat after every third beat, and another patient may drop the QRS complex after every fifth complex. Chapter 15 A AV Heart Blocks 423 again. The P waves are as some of the QRS complexes are blocked. KEY CONCEPT With 2nd-degree AV heart The atrial rate is that of the underlying rhythm whereas the ventricular rate block, type 1, the key fea- is slightly less than the atrial rate (slower than normal). The QRS complexes are within normal limits unless the block is infranodal, in which case the QRS ture that helps identify it are PR intervals that complexes will be wide. Infranodal refers to a block caused by an abnormality become progressively lon- below the AV node, either in the bundle of His or in both bundle branches. An ger until a QRS complex infranodal block has more serious clinical implications than a block at the level is dropped. There are also more P waves than QRS of the AV node. It most often occurs in older persons. Symptoms include fre- complexes, and the QRS quent episodes of fainting and a pulse rate of 20 to 40 bpm. The P-P interval is complexes are normal. constant while the R-R interval increases until a QRS complex is dropped. Causes 2nd-degree AV heart block, type I, often occurs in acute MI or acute myocar- ditis. Common causes are shown in the following box. Treatment If patient is asymptomatic, no specific treatment is needed. Patients who are symptomatic (e.g. , chest pain, hypotension) should receive oxygen, an IV lifeline, and administration of atropine and transcutaneous pacing should be considered. Causes of 2nd-Degree AV Heart Block, Type 1 Cause Examples LO 15.4 Cardiac diseases AV nodal ischemia secondary to right coronary artery occlusion, myocardial ischemia or MI (inferior wall MI), cardiac surgery, myocarditis, rheumatic fever Use of certain drugs Beta-adrenergic blockers, calcium channel blockers, digitalis, quinidine, procainamide 2nd-Degree Atrioventricular Heart Block, Type II Description 2nd-degree AV heart block, type II, is an intermittent block at the level of the AV node, bundle of His, or bundle branches resulting in atrial impulses that are not conducted to the ventricles (Figure 15-4). It is less common than type I, but it is more serious as it may progress to complete AV heart block. It differs from type I in that the PR interval is constant prior to a beat being "dropped” Increased vagal tone Autonomic nervous system stimulation Noncardiac disorders Electrolyte imbalance (hyperkalemia) Causes Effects 2nd-degree AV heart block, type I, may occur in otherwise healthy persons. By itself , it is usually transient and reversible, resolving when the underlying condition is corrected. However, particularly if it occurs early in MI, it may progress to more serious blocks. If dropped beats occur frequently , the patient may show signs and symptoms of decreased cardiac output. Those types Type II block is usually associated with anterior-wall MI, degenerative changes in the conduction system, or severe coronary artery disease. Com- mon causes of type II block are shown in the following box. of Common Causes of 2nd-Degree AV Heart Block, Type II Cause Examples heart block that result in dropped QRS complexes or a slower ventricular rate can lead to a decrease in cardiac output (decreased heart rate x stroke volume = decreased cardiac output). Cardiac diseases Anterior wall MI, severe coronary artery disease, organic heart disease, degenerative changes in the conduction system, cardiac surgery, myocarditis, rheumatic fever Beta-adrenergic blockers, calcium channel blockers, digitalis, quinidine, procainamide ? QUESTION 2. What type of irregularity does 2nd-degree AV heart block, type I, produce? Appearance The characteristics that stand out in 2nd-degree AV heart block, type I, are a patterned, irregular rhythm; cycles of progressively longer PR intervals; and more P waves than QRS complexes. The irregularity appears as a pattern (the cycle seems to occur over and over); it is often described as grouped beating. The PR intervals become progressively longer until a P wave fails to conduct, resulting in a "dropped” QRS complex. After the blocked beat, the cycle starts Use of certain drugs Autonomic nervous system stimulation Increased vagal tone Noncardiac disorders Electrolyte imbalance (hyperkalemia) 424 Section 3 A Origin and Clinical Aspects of Dysrhythmias In 2nd-degree AV heart block, type II, impulses arise from the SA node but some are blocked in the AV node. Blocked Chapter 15 A AV Heart Blocks 425 The key characteristic that helps you differentiate between the two types of 2nd-degree AV block is whether the PR interval changes or remains constant. KEY CONCEPT The key feature that helps identify 2nd-degree AV heart block, type II, is Patients who are symptomatic (e.g., chest pain, hypotension) should receive more P waves than QRS supplemental oxygen, an IV lifeline, and you should consider transcutaneous complexes and constant PR pacing. Atropine may be used in type II AV block with new wide QRS com- intervals for the conducted beats. Blocked Treatment plexes while setting up the transcutaneous pacemaker. LO 15.5 that Ly 3rd-Degree Atrioventricular Heart Block Description 3rd-degree AV heart block is a complete block of the conduction at or below the AV node so that impulses from the atria cannot reach the ventricles (Fig- ure 15-5). It is also called complete AV heart block. The SA node serves as Rate ventricular rate may be slow, normal, or fast; atrial rate is within normal range Rhythm may be regular or irregular (depends or whether conduction ratio remains the same) P waves normal and all look alike; not all followed by a QRS complex ORS complexes usually of normal duration and all look alike; periodically one is absent In 3rd-degree AV heart block there is a complete block at the AV node resulting in the atria being depolarized by an impulse that arises from the SA node and the ventricles being depolarized by an escape pacemaker that arises somewhere below the AV node. PR intervals are of constant duration for conducted beats ST segments normal and isoelectric I waves normal size and configuration QT intervals within normal limits of 0.36 to 0.44 second U waves may or may not be present Block Blocked Figure 15-4 2nd-degree AV heart block, type II. Escape --- Escape --- Escape Escape Effects Type II is a serious dysrhythmia, usually considered malignant in the emergency setting when it is symptomatic. Slow ventricular rates result in decreased cardiac output and may produce signs and symptoms of hypoper fusion (low blood pressure, shortness of breath, congestive heart failure, pul- monary congestion, and decreased level of consciousness). It may progress to a more severe heart block and even to ventricular asystole. Appearance The feature that stands out in 2nd-degree AV heart block, type II, is the presence of more P waves than QRS complexes. The number of P waves for each QRS complex (e.g., 2:1, 3:1, or 4:1), may be fixed or it may vary (e.g., 2:1, then 3:1, or 4:1, or vice versa). The PR interval may be normal or but it is constant for each conducted complex. The QRS complexes a within normal limits, but they may also be wide (if the block is infranodal). The rhythm is typically regular. It will be irregular if the conduction ratio underlying rhythm whereas the ventricular rate is less than the atrial rate. (number of P waves to each QRS complex) varies. The atrial rate is that of Rate ventricular rhythm may be slow, normal, or fast; atrial rate is within normal range Rhythm atrial and ventricular rhythms are regular but not related to one another P waves are normal and look alike, but are not related to and appear to march through the QRS complexes QRS complexes are normal duration if escape focus is junctional and widened if escape focus is venticular U waves may or may not be present ? QUESTION 3. How can you distinguish between a nonconducted PAC and 2nd-degree AV heart block? r prolonged are usually QT intervals within normal limits of 0.36 to 0.44 second PR intervals are not measurable Twaves may be normal size and configuration or abnormal if QRS is widened ST segments may be normal configuration or abnormal if QRS is widened Figure 15-5 3rd-degree AV heart block. Chapter 19 A Bundle Branch Block 557 556 Section 4 A 12-Lead ECGs Figure 19-5 QRS complexes seen with bundle branch block are wider than normal and may appear as two complexes joined together The two R waves are named in sequential order-R and R R R Lead 1 The QRS complex is at least 0.12 seconds (three small squares) or greater QRS configuration Broads wave in 1, V and V in V1, © KEY CONCEPT With bundle branch block, because the two QRS com- plexes are not in sync, they appear superimposed on one another, and the ECG records this combined elec- trical activity as a widened QRS complex, sometimes with two peaks. on one another. The ECG records this combined electrical activity as notched or slurred QRS complexes or QRS complexes having two peaks (sometimes referred to as an M like or a rabbit ears appearance). When we see two R waves, we name them in sequential order, the R and R'. The R' is referred to as R prime and represents delayed depolarization of the blocked ventricle (Figure 19-5). RR If only a portion of the left bundle branch is blocked, this is called a fascicular block. SR RSR WANT MORE INFORMATION? Visit the Fast & Easy ECGs Website at www.mhhe.com/ shade ECG2e. Also, for a quick reference source to help you recall the indicators of right and left bundle branch block and hemiblock while in the field or clinical set- ting, refer to the ECG Pocket Guide. Figure 19-6 In RBBB, conduction through the right bundle is blocked, causing depolarization of the right ventricle to be delayed; conduction does not start until the left ventricle is almost fully depolarized. Possible sites of block within the ventricular conduction system include • The right bundle branch-right bundle branch block The left bundle branch-left bundle branch block • The left anterior fascicle-left anterior fascicular block (LAFB), also called left anterior hemiblock (LAHB) The left posterior fascicle-left posterior fascicular block (LPFB), also called left posterior hemiblock (LPHB) Any combination of these or along with prolongation of the PR interval (first degree AV block) through the anterior and posterior fascicles and into the Purkinje fibers of the left ventricle. The impulse then travels from the left ventricle across the septum and depolarizes the right ventricle. Its movement across the ventricles 19 slower than normal because it is moving cell by cell instead of via the faster conduction pathways. LO 19.4 ? QUESTION 4. Which leads are best for assessing the electrical activity in the right ventricle? Right Bundle Branch Block With right bundle branch block (RBBB), conduction through the right bundle branch is blocked, causing depolarization of the right ventricle to be delayed; conduction does not start until the left ventricle is almost fully depo- larized (Figure 19-6). The impulse starts off normally. It is initiated in the SA node and travels through the atria, depolarizing them. It and then transverses the AV node and passes through the bundle of His. It then moves down the left bundle, initi- ates septal depolarization, and at the same time attempts to travel down the right bundle branch. Because the right bundle is blocked, the impulse travels RBBB occurs with anterior wall MI, coronary artery disease, hypertension, scar tissue that develops after heart surgery, viral or bacterial infection of the heart muscle (myocarditis), and pulmonary embolism. It may also be caused, as can any type of heart block, by drug toxicity. Further, it can also be due to a heart abnormality that's present at birth (congenital), such as atrial septal defect (a hole in the wall separating the upper chambers of the heart). RBBB causes the QRS complex to have a unique shape. This occurs because of late depolarization of the right ventricle. In RBBB the QRS begins normally with activation of the septum from left to right, which produces a small R wave in leads V, and V, (as the impulse Chapter 19 A Bundle Branch Block 569 558 Section 4 A 12-Lead ECGS travels toward the positive electrode and a small Q wave in leads V and Vo moves through the left ventricle, it produces an S wave in V, and V2 (as it is moving away from the positive electrode) and an R wave in V, and V. (as it is (as the impulse travels away from the positive electrode). Then, as the impulse toward the positive electrode) and a broad S wave in lead V and V. (as it is the right ventricle produces a tall R' wave in leads V, and V2 (as it is moving traveling toward the positive electrode). Then, the delayed depolarization of KEY CONCEPT moving away from the positive electrode), as well as the other leads overlying To diagnose RBBB, check the left ventricle (I and a V.). This produces a wide QRS complex that appears for an R, R' wave in leads in one of several different shapes (see Figure 19-6); an rSR' complex with a V, or V2. The appearance of wide S and R' wave, an rs complex with a wide R' wave, or a wide R wave right bundle block has been with an M-shaped appearance. Some liken its appearance to rabbit ears. Also, likened to the letter M. in most leads, the T waves deflect opposite that of the QRS deflection. V In the left lateral leads, late right ventricular depolarization produces recipro- cal late broad S waves. QRS configuration in V, V, QRS configuration in Vs Ve Characteristics of RBBB. • Tall, wide QRS complexes in leads V, and Vthat appear in one of several different shapes: -an SR' complex with a wide S and R' wave -an rs' complex with a wide R' wave a wide Rwave with an M-shaped appearance • Reciprocal late broad S waves in I, aVL, V5, and V6 • AT wave that deflects opposite of the QRS complex in most leads الاسم LO 19.5 Figure 19-7 In LBBB, conduction through the left bundle is blocked, causing depolarization of the left ventricle to be delayed; it does not start until the right ventricle is almost fully depolarized. Left Bundle Branch Block In left bundle branch block (LBBB), depolarization of the left ventricle is delayed (Figure 19-7). Again, the impulse starts off normally. It is initiated in the SA node and travels through the atria, depolarizing them. It then trans- verses the AV node, passes through the bundle of His, and attempts to move down the right and left bundle branches. It passes through the right bundle normally but is blocked from passing through the left bundle. Following right ventricular depolarization, the impulse travels across the septum and depolar- izes the left ventricle. Its movement across the ventricle is slower than normal because depolarization is moving cell by cell instead of via the faster conduc- tion pathways of the anterior and posterior fascicles and Purkinje fibers. LBBB can be caused by anterior wall Ml; hypertensive heart disease; aortic stenosis ; degenerative changes of the conduction system; or thickened, stiff- ened, or weakened heart muscle (cardiomyopathy). Activation of the ventricles in the left bundle branch is greatly different than what occurs in the normal heart. This leads to a much different-looking QRS complex. the current is mov- In leads V, and V2 (which overlie the right ventricle), the initial movement of the impulse from right to left instead of left to right through the septum can produce a small Q wave (which is often not seen) as the impulse is traveling away from the positive electrode. Then as the right ventricle depolarizes, it can appear as a very small R wave (or it may even be absent) as the impulse is now moving toward the positive electrode. The abnormal depolarization of the left ventricle produces a wide, largely negative S wave a ing away from the positive electrode. This is referred to as a wide rS complex. If the R wave is absent, it is an entirely negative and wide QS complex. Normally, in leads V, VI, and a V. (which overlie the left ventricle), the impulse moves from right to left through the septum by way of the right ven- are seen as a Q wave and tall R wave. In LBBB, because the tricle (instead of from left to right), there is an absence of a Q wave in these QRS complexes Chapter 19 A Bundle Branch Block 581 560 Section 4 A 12-Lead ECGS leads. Then, as the impulse is traveling toward the positive electrode, the slow left ventricular depolarization produces a wide, tall, slurred R wave (see Figure 19-7). The R waves can also be flattened on top or notched with two tiny points). However, the M-shaped appearance is less likely to be seen than in RBBB. Figure 19-8 Incomplete bundle branch block. KEY CONCEPT To diagnose LBBB, check for a wide QRS complex and an R, R' wave in leads Vsor Vo Also, in most leads, the T wave deflection is opposite that of the QRS deflec- tion, without a Q or S wave. H 0.11 seconds or less in duration Characteristics of LBBB. • Tall, wide, slurred R waves in I, avi, V5, and V, (the Rwaves can also be flattened on top • Wide, largely negative rS complexes or entirely negative OS complexes in V, and V, • A T wave that deflects opposite of the QRS complex in most leads or notched) . LO 19.6 Bundle Branch Block and Repolarization Similar to repolarization changes seen in hypertrophy, repolarization abnormali- ties may occur in both RBBB and LBBB. These changes mimic those seen with myocardial ischemia/injury, such as ST segment depression and T-wave inversion. but the anterior and posterior fascicles of the left bundle branch are the most clinically significant. Whereas the ECG appearance of anterior and poste- rior hemiblocks differs from that of bundle branch blocks, the mechanism is essentially the same—the normal conduction pathway is blocked, and the affected myocardium receives its electrical stimulation via cell-to-cell depolarization and retrograde conduction from the remaining intact parts of the conduction system. Whereas there is minimal prolongation with hemi- block, it is not enough to widen the QRS complex to any appreciable degree . However, the morphology of the QRS complex does change. Axis deviation is the key ECG characteristic of hemiblocks. It occurs because, when one fascicle is blocked, the electrical current travels down the other to stimulate the heart. This causes the axis to shift accordingly. When diagnos- ing hemiblock, be sure to rule out other causes of axis deviation, such as ven- tricular hypertrophy. KEY CONCEPT Hemiblocks cause axis deviation. LO 19.7 ? QUESTION 5. What ECG changes tell us left axis deviation is present? In RBBB, ST segment depression and T-wave inversion (see Figure 19-6) is seen in the right precordial leads (V, and V2). Likewise, in LBBB, ST segment depression and T-wave inversion (Figure 19-7) may be seen in most leads, par- ticularly the left precordial leads (Vs and V.). Incomplete Bundle Branch Block At times either the right or the left bundle branch conducts the electrical impulse more slowly than normal, but is not totally blocked. When this hap- pens on the side of the slow conduction, the electrical impulse reaches the ventricle slightly later than normal. As a result, the QRS complex may have a similar appearance to bundle branch block and is slightly wider than nor- mal (Figure 19-8), but it is not as wide as with complete bundle branch block (having a duration of no greater 0.11 seconds). This slight widening of the QRS is often referred to as incomplete bundle branch block. It may also be called an intraventricular conduction defect (or an IVCD). Whereas incomplete bundle branch block may indicate underlying heart dis- ease, incomplete bundle branch block is often of no significance, especially when it occurs on the right side (i.e., incomplete RBBB). Hemiblock As previously described, the left bundle branch consists of three separate branches or fascicles . A block in conduction through one of the fascicles is called hemiblock. There are many fascicles in the heart's conduction system, LO 19.9 The anterior fascicle is longer and thinner and has a more fragile blood sup- ply than the posterior fascicle, so LAHB is far more common than LPHB. While LAHB can be seen in both normal and diseased hearts, LPHB almost always is associated with heart disease. Left Anterior Hemiblock With LAHB, conduction down the left anterior fascicle is blocked (Figure 19-9). For this reason, the electrical impulse rushes down the left posterior fascicle to the inferior surface of the heart. Depolarization of the left ventricle occurs, progressing in an inferior-to-superior and right-to-left direction. The axis of ventricular depolarization is therefore redirected upward and slightly to the left, producing tall positive R waves in the left lateral leads and deep S waves inferiorly. This results in left axis deviation. Left Posterior Hemiblock In LPHB, the electrical impulse rushes down the left anterior fascicle, resulting in ventricular myocardial depolarization occurring in a superior-to-inferior LO 19.8 KEY CONCEPT LAHB results in left axis deviation Chapter 19 A Bundle Branch Block 563 562 Section 4 A 12-Lead ECGs QRS configuration in lead QRS configuration in lead 1 Small R Tall R Lead 1 Figure 19-9 With LAHB, conduction down the left anterior fascicle is blocked, resulting in all the current rushing down the left posterior fascicle to the inferior surface of the heart. There are tall positive R waves in the left lateral leads (lead 1), deep S waves inferiorly (lead ave), and left axis deviation. Figure 19-10 With LPHB, conduction down the left posterior fascicle is blocked, resulting in ventricular myocardial depolarization occurring in a superior-to-interior and left-to-right direction. There are deep Swaves in the left lateral leads (lead 1), tall positive Rwaves inferiorly lead ael, and right axis Deep S 9 Small Q deviation. -90 Left axis deviation QRS configuration in lead a Ve +180° QRS configuration in lead av 09 Lead av Lead a VF Tall R Small R Right axis deviation V Small Deep S +90° KEY CONCEPT LPHB results in right axis deviation and left-to-right direction (Figure 19-10). Therefore, the main electrical axis is directed downward and to the right, producing tall R waves inferiorly and deep S waves in the left lateral leads. This results in right axis deviation. In contrast to complete LBBB and RBBB, the QRS complex in hemiblock S is not prolonged Patients with any type of ventricular conduction block and especially those with a combination of blocks are at high risk of developing complete heart block. because these patterns lack unique anatomic and pathologic substrates, we have included coverage of these terms because they are still widely used. Bifascicular block is a conduction disturbance in which two of the three main fascicles of the His/Purkinje system are blocked . Most often, it refers to a combination of RBBB and either LAHB (more commonly) or LPHB. Some authors include LBBB in the definition of bifascicular block because the block occurs above the bifurcation of the left anterior and left posterior fascicles of the left bundle branch. Trifascicular block is a conduction disturbance in which there are three fea- tures seen on the ECG (Figure 19-11): Prolongation of the PR interval (first degree AV block) LO 19.10 Bifascicular and Trifascicular Block Although a 2009 American Heart Association Electrocardiography and Arrhyth- mias Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society (AHA/ACCF/HRS) sci- entific statement on the standardization and interpretation of the electrocardior gram recommended against using the terms bifascicular and trifascicular block RBBB Either LAFB or LPFB Trifascicular block is uncommon.
Purchase answer to see full attachment