General

Definitions

• Acidosis process that increases [H 'J; alkalosis process that decreases [H]

• Primary disorders: metabolic acidosis or alkalosis, respiratory acidosis or alkalosis

• Compensation respiratory: hyper- or hypoventilation alters P.COî to counteract 1 metabolic process renal: excretion retention of H ' HCOj by kidneys to counteract 1° respiratory process respiratory compensation occurs in min; renal compensation takes hrs to days compensation never fully corrects pH; if pH normal, consider mixed disorder

Workup

• Determine primary disorder / pH. P.COj, HCOj

• Determine if degree of compensation is appropriate

Primary Disorders

Primary disorder

Problem

PH

HCOj

P.CO,

Metabolic acidosis

gain of H' or loss of HCOj

1

«

t

Metabolic alkalosis

gain of HCOj or loss of H

t

«

t

Respiratory acidosis

hypoventilation

I

t

1

Respiratory alkalosis

hyperventilation

t

I

1

Compensation for Acid, Base Disorders

Primary disorder

Expected compensation

Metabolic acidosis

(also, PaCOi last two digits of pH)

Metabolic alkalosis

! P.COi - 0.75 ■ JHCOj

Acute respiratory acidosis

t HCOj 0.1 x ¿P.CO, (also. 1 pH .008 - ¿P.CO,)

Chronic respiratory acidosis

t HCOj 0.4 x iP.COi (also. 1 pH .003 >: iP.COj)

Acute respiratory alkalosis

i HCOj - 0.2 x AP.CO, (also, t pH .008 'iP,CO!)

Chronic respiratory alkalosis

¡HCOJ O.^xiP.COl

Mixed disorders (more than one primary disorder at the same time)

• If compensation less or greater than predicted, may be 2 disorders:

P,COj too low — concomitant 1 resp.alk. P,COj too high concomitant 1° resp. acid. HCOj too low — concomitant met acid. HCOj too high concomitant 1° met alk.

normal PjCOj & HCOj. but Î AG AG met. acid. » met. alk.

normal P»COj, HCOj, & AG no disturbance or non-AG met. acid. +• met. alk.

• Cannot have resp. acid, (hypoventilation) and resp. alk. (hyperventilation) simultaneously

N.B.. If ABG not available, can use VBG, but note that pH 0.04 4, PaCOj 8 mmHg t. and

HCO) 2 mEq T. (Adapted from Brenner BM. ed. Brenner & Rector's The Kidney. 5th ed.. 1996: Fern F. ed.

Procbcol Gutde to The Care of the Medico/ Patient. 5th ed.. 2001)

N.B.. If ABG not available, can use VBG, but note that pH 0.04 4, PaCOj 8 mmHg t. and

HCO) 2 mEq T. (Adapted from Brenner BM. ed. Brenner & Rector's The Kidney. 5th ed.. 1996: Fern F. ed.

Procbcol Gutde to The Care of the Medico/ Patient. 5th ed.. 2001)

Consequences of Severe Acid-Base Disturbances

Organ System

Acidosis (pH <7.20) Alkalosis (pH >7.60)

Cardiovascular

i contractility, arteriolar vasodilation Arteriolar vasoconstriction 1 MAP & CO; 1 response to catecholamines I coronary blood flow T risk of arrhythmias t risk of arrhythmias

Respiratory

Hyperventilation, i resp muscle strength Hypoventilation

Metabolic

TK J K. iCa. Mg. PO«

Neurologic

A MS A MS. seizures

(NEJM 1998:338:26.107)

(NEJM 1998:338:26.107)

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