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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
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