0071643192.pdf

■ Anatomic right-to-left shunt = blood flow that bypasses the lungs alto-

gether as in a VSD with pulmonary hypertension.

■ Right-to-left shunt →low PaO 2 and an increased A-a gradient on blood

gas. If anatomic, it will not improvewith supplemental oxygen.

O 2 DELIVERY TOTISSUES

■ Three principal components determine O 2 delivery to tissues:

■ O 2 content:Amount of O 2 dissolved in blood and carried on hemoglobin

■ Cardiac output

■ Oxyhemoglobin saturationat a given PaO 2

■ Demonstrated in the oxyhemoglobin dissociation curve

■ Right shift of curve = O 2 more readily given up to tissue.

■ Causes of right shift:Acidosis, hyperthermia, increased 2,3-DPG

■ Causes of left shift:Alkalosis, hypothermia, abnormal hemoglobin,

decreased 2,3-DPG

Cardiac Conduction and Rhythm Assessment

GENERALAPPROACH

Arrhythmias are a problem encountered frequently by the emergency physi-

cian. A general approach to assessing rhythms is essential in determining

management.

Is the patient in cardiac arrest?

■ Four rhythms produce cardiac arrest: Ventricular fibrillation, pulseless

ventricular tachycardia, pulseless electrical activity, and asystole

■ See “Management of Cardiac Arrest.”

Is the rhythm stable or unstable?

The definition of stability relates to the ability of the heart to produce suffi-

cient cardiac output.

Signs of instability:

■ Cardiac ischemia manifested by chest pain or acute CHF

■ Circulatory compromise (hypotension, diaphoresis, weak pulses)

■ Neurologic dysfunction caused by decreased cerebral blood flow

■ Generally global with altered mental status; rarely (if ever) focal

Is the rhythm too fast (> 100 bpm) or too slow (< 60 bpm)?

Is the rhythm wide or narrow complex?

Rhythm complex width is defined as the width of the QRS. QRS ≥0.12 seconds is

considered wide complex.

■ Wide complex rhythms result from conduction outside the normal path-

ways (bundle branch block, pacemaker, initiation below the AV node) or

the presence of a metabolic abnormality or toxin (hyperkalemia, TCA

overdose).

■ Narrow complex rhythms are conducted through normal pathways and

almost always begin above or within the AV node.

RESUSCITATION

Hypoxemia due to an

anatomic shunt does not

correct with 100% O 2.

Carbon monoxide replaces O 2

by binding to hemoglobin

AND shifts the oxyhemoglobin

dissociation curve to the left,

decreasing the availability of

O 2 to tissues.

Four rhythms produce cardiac

arrest: Ventricular fibrillation,

pulseless ventricular

tachycardia, pulseless electrical

activity, and asystole.