0071643192.pdf

■ Ketone wasting

■ Toluene

■ High anion gap (anion gap >20) caused by increased concentrations of

anions other than K+and Cl−

Three rules assist in identifying a mixed disorder:

1. Neither respiratory nor renal compensation completely normalizes the pH.


2. The PCO 2 in patients with a metabolic acidosis is predicted by Winter’s for-
   mula: PCO 2 ∼∼1.5 HCO 3 −+8.
   ■ PCO 2 below predicted by Winter’s formula =respiratory alkalosis.
   ■ PCO 2 above predicted by Winter’s formula =respiratory acidosis.


3. Δgap=measured anion gap – normal anion gap. The Δgap should ap-
   proximate the decrease in HCO 3 −; if the drop in HCO 3 −cannot be ex-
   plained completely by the Δgap a nongap acidosis is also present.

Metabolic Acidosis (åpH+åHCO 3 - )

An extremely common presentation both in the emergency department and

on the exam, patients are often ill- or septic-appearing on presentation.

SYMPTOMS ANDEXAM

■ Tachypnea is a compensatory response in any patient with a metabolic aci-

dosis, but may be greatest (ie, Kussmaul) in DKA or salicylate poisoning.

■ Severe hypoxia or hypotension may precipitate lactic acidosis.

■ Intoxicated appearance or visual complaints and a high anion gap acido-

sis should prompt an evaluation of serum osmolar gap to screen for toxic

alcohols.

■ Volume overload may be due to acute renal failure with associated acidosis.

■ Adrenal insufficiency

■ Diarrhea

DIAGNOSIS ANDCAUSES

■ A metabolic acidosis is present in any patient with a pH <7.35 and

HCO 3 −<20 mEq/L. In patients with mixed disorders, the pH may be

normal or >7.40. Use the anion gap and the serum K+to help narrow the

differential.

■ Normal anion gap [Na+−(K++Cl-)]∼∼ 12

■ Look for chloride losses.

■ Hypokalemic normal gap acidosis:

■ Renal losses: Renal tubular acidosis or acetazolamide

ENDOCRINE, METABOLIC, FLUID, AND ■ GI losses: Diarrhea or malabsorption

ELECTROLYTE DISORDERS

Causes of a high

anion gap—

MUDPILES

Methanol,Metformin

Uremia

Diabetic (or alcoholic)

ketoacidosis

Paraldehyde

INH/iron/inhalant

(ie, CO) poisoning

Lactic acidosis (sepsis,

shock, hypoxia,

seizures, cyanide)

Ethylene glycol

Salicylates,Solvents

There are only three

endogenous causes of an

anion gap acidosis: Lactate,

ketones, and uremia. All other

causes are exogenous.

Compensation for an acid-base

disorder never completely

normalizes the pH. A pH of 7.45

in a patient with a low HCO 3 −

indicates a second disorder (in

this case a primary respiratory

alkalosis) (see Table 7.4).

TABLE 7.4. Commonly Tested Acid-Base Disorders

PHPCO 2 HCO 3 - ANIONGAP CAUSES

↓or↑↓↓ ↓ ↑ Salicylates

↓↓↓ ↑DKA, toxic alcohol, sepsis

↓↑↑– – Narcotic OD, COPD (pure

respiratory acidosis)