Lecture 22 Acid Base Disorders Mah

ACID BASE CHEMISTRY

DEFINITIONS:

Acid: donates protons (H+)

Base: accepts protons (H+)

pH: expression of [H+]

BUFFERS: the ability of a system to maintain pH within a narrow range when an acid or base is added to it

Normal human pH: 7.4 (7.35 – 7.45) o Incompatible with life: < 6.8 and > 7.8

Physiologic buffers: o Carbonic acid/bicarbonate

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-

o Monobasic/dibasic phosphate o Ammonium/ammonia o Lactic acid/lactate o Proteins (hemoglobin, albumin)

ARTERIAL BLOOD GASES

Gold standard for monitoring acid-base status and oxygenation

Most common sampling site: radial artery

Invasive

Painful

Parameter Normal

pH 7.4

PaCO2 40

HCO3- 24

Anion gap ≤ 12

COMPENSATION PROCESSES: Primary disorder

Compensation process

Cause of disorder Timing of compensation

Metabolic acidosis

Respiratory alkalosis

Gain acid (or lose base) decreased HCO3

-

Within minutes

Metabolic alkalosis

Respiratory acidosis

Lose acid (or gain base) increased HCO3

-

Respiratory acidosis

Metabolic alkalosis

Hypoventilation increased PaCO2

May take 3-4 days to begin

Respiratory alkalosis

Metabolic acidosis

Hyperventilation decreased PaCO2

Metabolic = kidneys Respiratory = lungs

Lungs and kidneys expected to compensate for each other

5 STEP PROCESS 1. WHAT IS THE “-EMIA”? 2. WHAT IS THE MAJOR “-OSIS”?

Major “-osis” is the primary acid-base disorder

Need to determine if metabolic or respiratory 3. WHAT IS THE ANION GAP?

Anion gap = [Na+] – [Cl-] – [HCO3=]

Normal anion gap ≤ 12

In a suspected metabolic acidosis…

Elevated anion gap = acid added = anion gap metabolic acidosis

Normal anion gap = bicarbonate loss = non-anion gap metabolic acidosis

4. IS THERE A SUPERIMPOSED METABOLIC DISORDER?

Compare expected HCO3 and measured value

5. IS THERE A SUPERIMPOSED RESPIRATORY DISORDER?

ONLY IF PRIMARY DISORDER IS METABOLIC

Not possible to have resp acidosis & alkalosis at the same time

Compare expected PaCO2 and measured value NOTE: more info on anion gaps are explained in the slides

STEPS 1 AND 2:

pH

pH < 7.4Acidemia

PCO2 > 40 Respiratory

acidosis

HCO3 < 24Metabolic

acidosis

7.4

Normal, compensated, or mixed disorder

> 7.4

Alkalemia

PCO2 <40Respiratory

alkalosis

HCO3 > 24Metabolic alkalosis

STEP 1

STEP 2

COMPENSATION RULES (STEPS 4 & 5):

Lecture 22 Acid Base Disorders Mah

pH

pH < 7.4

Acidemia

PCO2 > 40

Respiratory acidosis

Acute

Increase 10:1

ChronicIncrease 10:3

HCO3 < 24Metabolic acidosis

Decrease 1:1

7.4

Normal, compensated, or mixed disorder

> 7.4

Alkalemia

PCO2 < 40Respiratory alkalosis

Acute

Decrease 10:2

Chronic

Decrease 10: 5

HCO3 > 24Metabolic alkalosis

Increase 1: 0.7

STEP 1 STEP 2 STEPS 4 & 5

HOW TO READ COMPENSATION RULES Primary ABG value affected : Expected compensation

Metabolic disorders: HCO3 : PaCO2

Respiratory disorders: PaCO2 : HCO3