>200 msec) and requires no urgent intervention. Search

for and address any predisposing conditions. Bradycardic

rhythms that feature more P waves than QRS complexes

typically represent second- or third-degree AV blocks.

Second-degree AV block is divided in Mobitz types I

(Wenckebach) and II. Type I presents with a PR interval

that progressively elongates until an impulse is not conducted to the ventricles, resulting in a dropped QRS on the

ECG. A progressively decreasing interval between c onsecutive R waves is classic for this dysrhythmia. In seconddegree AV block type II, although the PR interval remains

constant, occasional P waves will not be conducted to the

ventricles, resulting in a dropped QRS. Type II is more

CHAPTER 16

Figure 1 6-2. Dysrhythmia diag nostic algorithm.

serious than type I and typically represents a conduction

blockade distal to the AV node. Complete disruption of

signal conduction between the atria (P waves) and ventri ­

cles (QRS complexes) represents third-degree or complete

heart block. The P waves and QRS complexes march inde ­

pendently of one another with no consistency between the

two. The QRS complexes are referred to as escape beats and

can be either narrow (junctional) or wide (ventricular)

depending on their site of origin (Figure 16-3).

Two additional bradycardias warrant mention. Junctional

bradycardia is a slow regular rhythm with a narrow QRS

complex and absent or abnormal P waves owing to its origin

within the AV node. As this condition typically occurs because

of medication side effects (eg, beta-blockers), carefully elicit a

medical history to help identify the etiology. I dioventricular

rhythms (ventricular escape rhythms) originate in the ven ­

tricles and appear as regular wide QRS complex rhythms at a

rate of 20-40 bpm with no discernible P waves.

..... Tachydysrhythmias

In stable patients, assess for the regularity of the rhythm

and distinguish between a supraventricular (narrow QRS)

versus ventricular etiology (QRS > 100 msec) (Figure 16-4).

A rhythm that is fast, narrow, and regular is typically either

sinus tachycardia, atrial flutter, or SVT. Slowing the heart

DYSRHYTHMIAS

A

8

(

Figure 1 6-3. A. Second-degree AV block (Mobitz type I; Wenckebach). B. Second-degree AV block (Mobitz

type I I). C. Third-degree AV block. (Reproduced with permission from Tintina lli JE, Kelen GO, Sta pczynski JS.

Emergency Medicine: A Comprehensive Study Guide. 6th ed. New York: McGraw-Hill, 2004.)

rate with vagal maneuvers or adenosine aids in identifying

the underlying rhythm and may treat SVT. Although

appropriate concern should arise when using adenosine in

patients with either known or ECG findings concerning for

pre-excitation ( eg, WPW), it is generally safe provided that

the QRS complexes are narrow. Of note, SVT with aberrancy (conduction through an accessory pathway or bun ­

dle branch block) is typically difficult to distinguish from

VT. Always assume VT until proven otherwise, especially in

elderly patients with underlying cardiac disease.

Sinus tachycardia will appear with a P wave preceding each

QRS and regular R-R intervals. Sinus tachycardia is often

secondary to noncardiac issues, including pain, fever, anxiety,

PE, illicit drug use (cocaine), alcohol withdrawal, thyrotoxico ­

sis, volume depletion, and anemia. Focus on identifying and

treating the underlying cause of the tachycardia.

SVT can be subdivided into AV nodal re-entry tachycardias (AVNRT) and atrioventricular re-entry tachycar ­

dias (AVRT) depending on the anatomy of the re-entry

circuit. AVNRT has a re-entry loop contained within the

AV node itself, whereas AVRT requires the presence of an

accessory pathway ( eg, WPW) to complete the re-entry

loop. With AVRT, signals that travel downward through the

AV node and re-enter the atria through the accessory pathway (orthodromic) will exhibit narrow QRS complexes.

The reverse situation is known as antidromic conduction

and will exhibit wide QRS complexes. Wide QRS complexes are also present in patients with underlying bundle

branch blocks (SVT with aberrancy). Regardless of SVT

subtype, discernible P waves preceding the QRS complexes

will be absent and the R-R intervals will be r egular.

Atrial flutter has a classic "saw tooth" appearance from