Diagnosis of Conduction Defects

Conduction defects can be observed during surgery. They can result from the passage of the pulmonary artery catheter through the right ventricle, or they can be a manifestation of myocardial ischemia. Because high-grade (second- and third-degree atrioventricular blocks) conduction defects often have deleterious effects on hemodynamic performance, their intraoperative recognition is important.

Three types of conduction system blocks are possible: sinoatrial block, atrioventricular heart block, and intraventricular conduction block ( Table 34-8 ). The His bundle ECG has greatly improved the understanding of conduction through the heart.^[94] In sinoatrial block, the block occurs at the sinus node. Because atrial excitation is not

TABLE 34-8 -- Conduction defects

Sinus node block

AV conduction defects

First-degree AV block

Second-degree AV block

Mobitz type I

Mobitz type II

Third-degree (complete) heart block

Intraventricular conduction defects

RBBB

Incomplete

Complete

LBBB

Incomplete

Complete

Left fascicular block

LAHB

LPHB

Bifascicular block

RBBB + LAHB

RBBB + LPHB

Alternating LBBB/RBBB

AV conduction defect + LBBB or RBBB

Trifascicular block (bilateral bundle branch block + AV conduction defect)

Indeterminate (bizarre defect, not included in any of the foregoing categories)

AV, atrioventricular; LAHB, left anterior hemiblock; LBBB, left bundle branch block; LPHB, left posterior hemiblock; RBBB, right bundle branch block.

From Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.

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The second type of heart block is an atrioventricular heart block, or atrioventricular block, which may be incomplete or complete.^[95] First- and second-degree atrioventricular blocks are usually considered incomplete, whereas a third-degree atrioventricular block is considered to be complete heart block. First-degree atrioventricular block is often found in healthy hearts, but it is also associated with coronary artery disease or digitalis administration. It is characterized by a PR interval longer than 0.21 second. All atrial impulses progress through the atrioventricular node to the Purkinje system. This form of heart block ordinarily requires no treatment ( Fig. 34-17 ). Second-degree atrioventricular block is associated with the conduction of some of the atrial impulses to the atrioventricular node and into the Purkinje system. It is further subdivided into two specific types.^[96] Mobitz type 1 block, or Wenckebach block, is characterized by progressive lengthening of the PR interval until an impulse is not conducted and the beat is dropped ( Fig. 34-18 ). This form of block is relatively benign and often reversible, and it does not require a pacemaker. It may be caused by digitalis toxicity or myocardial infarction and is usually transient. Mobitz type 1 block reflects disease of the atrioventricular node.

The other form of second-degree heart block is Mobitz type 2 block, which may reflect disease of the bundle of His and Purkinje tissues, especially when the QRS complex is broad. In this, the less common and more serious form of second-degree heart block, dropped beats occur without any progressive lengthening of the PR interval ( Fig. 34-19 ). This type of block has a serious prognosis, because it frequently progresses to complete heart block and may require pacemaker insertion before major surgical procedures. Third-degree atrioventricular block, also called complete heart block, occurs when all electrical activity from the atria fails to progress into the Purkinje system. The atrial and ventricular contractions have no relationship with each other, although each chamber contracts regularly. The ventricular rate is approximately 40 beats/min. The QRS complex may be normal if the pacemaker site is in the atrioventricular node, but it is usually widened to longer than 0.12 second when the pacemaker site is located in the ventricle ( Fig. 34-20 ). The heart rate is usually too slow to maintain adequate cardiac

Figure 34-17 First-degree atrioventricular block is diagnosed by the presence of a PR interval longer than 0.21 second. (Adapted from Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Figure 34-18 Mobitz type 1, or Wenckebach, block is diagnosed by the progressive lengthening of the PR interval until an impulse is not conducted and a dropped beat occurs. (Adapted from Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

The third type of block is an intraventricular conduction disturbance, which is usually classified as LBBB, RBBB, or hemiblock. LBBB is the most serious of these conduction disturbances. Impulses reach the ventricles exclusively through the right bundle branch—hence the wide QRS complex of more than 0.12 second and a wide-notched R wave in leads I, aVL, and V₆ ( Fig. 34-21 ). The most important leads to study in bundle branch blocks are I, V₁ , and V₆ . The pattern of LBBB in V₆ is similar to that of left ventricular hypertrophy but exaggerated. An LBBB pattern is always associated with significant cardiac disease. In an RBBB, the QRS complex exceeds 0.11 second, and leads V₁ to V₃ have broad rSR' complexes, whereas leads I and V₆ have wide S waves ( Fig. 34-22 ). RBBB may be of no clinical significance, as opposed to LBBB. However, it is frequently associated with chronic lung disease or atrial septal defects.

Hemiblock is a term used when one of two divisions of the left bundle is blocked, because if both divisions are blocked, complete LBBB exists. Even though hemiblocks are a form of intraventricular block, the QRS complex is not prolonged. Marriott's^[97] criteria for left anterior hemiblock are as follows ( Fig. 34-23 ): (1) left axis deviation (usually -60 degrees); (2) small Q in leads I and aVL and small R in leads II, III, and aVF; (3) a normal QRS duration; (4) a late intrinsicoid deflection in lead aVL (0.045 s); and (5) an increased QRS voltage in limb leads. In contrast, the criteria for a left posterior hemiblock are as follows ( Fig. 34-24 ): (1) right axis deviation (usually +120 degrees);

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Figure 34-19 Mobitz type 2 second-degree heart block is demonstrated when dropped beats occur without progressive lengthening of the PR interval. (Adapted from Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Figure 34-20 Complete, or third-degree, heart block is demonstrated by the total dissociation between atrial and ventricular complexes with a ventricular rate about 40 beats per minute. (Adapted from Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Figure 34-21 Left bundle branch block. Notice the rS pattern in V₁ and the notched rR' pattern in V₆ . In V₅ and V₆ , a moderate depression of the ST segment is seen. (Adapted from Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Figure 34-22 Right bundle branch block. Notice the rSR' pattern in lead V₁ , with slurring of R' reflecting late right ventricular depolarization. (From Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

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Figure 34-23 Left anterior hemiblock. Notice the left axis deviation and the terminal S in the inferior leads. (From Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Figure 34-24 Left posterior hemiblock in association with biatrial enlargement and nonspecific ST- and T-wave abnormalities. (From Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone, 1987.)

Trifascicular blocks usually consist of one of the foregoing bilateral bundle branch blocks (i.e., RBBB plus a left fascicular block) in addition to a prolonged PR interval ( Fig. 34-27 ). Bundle of His ECGs are necessary to determine whether the atrioventricular conduction disturbance is localized in the atrioventricular node or whether it is distal, possibly representing an incomplete fascicular block in the last remaining fascicle.