• Order a 1 2-lead electrocardiogram on stable patients

and address potentia l etiologies, including acute

coronary syndrome, electrolyte abnormal ities, toxic

ingestions, and medication side effects.

The normal electrocardiogram (ECG) waveform contains a P wave, QRS complex, and T wave. The P wave represents atrial depolarization. It is immediately followed by

the PR interval, which normally lasts between 1 20 and 200

msec in duration. The QRS complex represents ventricular

depolarization and is normally <100 msec in duration.

Delays in intraventricular conduction result in a widened

(> 100 msec) QRS complex. The ST segment represents the

plateau of ventricular depolarization and is normally iso ­

electric in appearance. Finally, the T wave represents ven ­

tricular repolarization. Of note, the segment extending from

the end of a T wave to the beginning of the next P wave,

known as the TP segment, should be used as the isoelectric

baseline when performing any type of ECG analysis.

Bradydysrhythrnias occur either because of depressed

sinus node activity or inhibited electrical signal conduction.

These are common in patients with structural heart damage,

excessive vagal tone, taking certain cardioactive medications,

or with specific electrolyte abnormalities (eg, hyperkalemia).

Tachydysrhythmias occur because of enhanced automaticity

from either the SA node or an ectopic focus and can originate

from both atrial and ventricular sources. Supraventricular

tachycardia (SVT) occurs when re-entry loops are present in

the AV node or accessory conduction pathways.

Rhythms with a wide QRS complex represent ventric ­

ular depolarization that occurs outside of the normal

CHAPTER 16

Table 1 6-1. PIRATES: Causes of atrial fibri l lation.

P PE, pneumonia, pericarditis

Ischemia (coronary artery disease and myocardial infarction)

R Rheumatic heart disease, respiratory failure

A Alcohol ("hol iday heart")

T Thyrotoxicosis

Endocrine (Ca), enlarged atria (mitral valve disease,

cardiomyopathy)

s Sepsis, stress (fever)

conduction system, whereas those with normal QRS durations originate from a focus either superior to or within

the AV node that then travel through standard conduction

pathways.

Cardiac dysrhythmias vary by etiology, severity, and

treatment. Atrial fibrillation (AF), for example, is common and has multiple causes (Table 1 6-1). Although

occasionally symptomatic and/or requiring emergent

intervention, many patients are typically unaware when

they are in AF. Asymptomatic bradycardia is also a very

common rhythm, especially in young, athletic patients. It

can be a normal fmding in some people or result from

medication use at therapeutic levels. Other bradydysrhythmias, such as third-degree heart block, always elicit

emergent concern. Tachydysrhythmias vary in a similar

manner, from an isolated asymptomatic atrial tachycardia

to an emergently life-threatening ventricular fibrillation,

the initial dysrhythmia for the majority of patients in

cardiac arrest.