Download - Interpretation of the ABG
Interpretation of the ABGSapna Shah
August 19, 2021
Objectives
To apply a systematic approach to identifying a patient’s acid‐base disorders
To understand acid‐base physiology
To be able to answer ABIM board‐style questions that address acid‐base disorders
Stepwise Approach: pLACO
p: L: A: C: O:
pH Labs (pH, pCO2) Anion GapCompensationIs there another/third process? Osmolality?
Acidosis: process that tends to lower pH Alkalosis: process that tends to raise pH
Acidosis Respiratory pH ↓ PaCO2 ↑
Acidosis Metabolic pH ↓ PaCO2 ↓
Alkalosis Respiratory pH ↑ PaCO2 ↓
Alkalosis Metabolic pH ↑ PaCO2 ↑
AG = Na+ – (Cl‐ + HCO3‐)
Balance between bases
Blood pH
H2CO3HCO3‐7.35‐7.45Electrically
neutral
Total cations (+)
Total anions (‐)
=
(↑pH) and acids (↓pH)
Blood pH
Total cations (+)
Total anions (‐)=
Na+
Cl‐
HCO3‐
13724
104
mEq
/L
Cation Anion
(3‐11mEq/L)
137 – (104+24) = 9 mEq/L
Anion GapUnmeasured Anions
① Organic Acids② Neg charged plasma proteins
Albumin
So what causes metabolic acidosis?
Metabolic acidosis
H2CO3HCO3‐
Build‐up of acid‐ Produced or ingested in ↑↑↑ amounts ‐ Excessive HCO3
‐ loss → kidneys or GI tract
HCO3‐
↓↓↓ HCO3‐ in the blood
How is metabolic acidosis classified?
Anion Gap
High or Normal
Na+
Cl‐
HCO3‐
mEq
/L
Cation Anion
HCO3‐ + H+ H2CO3Carbonic acid
H2O + CO2
①↑↑ produc on in blood②Acid build‐up③Organic acids accumulated in the blood
Anion Gap
High or Normal
Na+
Cl‐
HCO3‐
mEq
/L
Cation Anion
↑↑ produc on in blood ① Lactic acidosis
‐ Decreased O2 delivery to tissues
‐ ↑↑ anaerobic metabolism and build‐up of lactic acid
‐ Uncontrolled diabetes mellitus ② Diabetic ketoacidosis
‐ Lack of insulin forces cells to use fat for energy instead of glucose
‐ Fats ‐ → ketoacids ‐ (acetoacetic acid) (β‐hydroxybutyric acid)
Anion Gap
High or Normal
Na+
Cl‐
HCO3‐
mEq
/L
Cation Anion
Acid build‐up① Inability of the kidneys to excrete, even when produced in normal amounts
‐ Chronic renal failure ‐ Organic acids, like uric
acid or sulfur containing amino acids, accumulate
Anion Gap
High or Normal
Na+
Cl‐
HCO3‐
mEq
/L
Cation Anion
Organic acid ingestions ①Oxalic acid (Ethylene glycol)② Formic acid (Methanol)③ Hippuric acid (Paint, Glue)
(H+)
H+ + HCO3‐** Plasma maintains electron neutrality **‐ For each new negatively charged anion there is one less bicarbonate ion
Organic acids are not part of AG equation, thus the AG will be high
Na+
Cl‐
HCO3‐
Cation Anion
Anion Gap
High or Normal
↓↓HCO3‐ offset by Cl‐ ions AG normal ‐ Severe diarrhea
* HCO3‐ rich intestinal/pancreatic sections aren’t reabsorbed
‐ Type 2 RTA * PCT unable to reabsorb HCO3‐* In response kidneys reabsorb more Cl‐ions ‐> hyperchloremic metabolic acidosis
How does the body compensate?
Mechanism to keep pH in balance
Moving H+ from blood into cells ‐ Cells exchange H+ for K+‐ Helps with acidosis, but also causes
hyperkalemia‐ With organic acids (lactic acid,
ketoacids) H+ enters with the organic anion and is not exchanged with K+
Mechanism to keep pH in balance
Respiratory system
carotid artery, aortic archChemoreceptors
↓pHBrainstem
↑Respiratory rate and depth of breathing
CO2 moves out of the body
So what causes metabolic alkalosis?
T
Metabolic alkalosis
Loss of H+ Gain of HC03‐
Gastrointestinal tract‐ Vomiting ‐ Gastric secretions are acidic (H+) Kidneys
‐ Too much aldosterone
Urine Blood
KidneysIncreased reabsorption of HCO3
‐
Stimulated by volume contraction of excessive loss of extracellular fluid Loop diuretics, thiazide diuretics, severe dehydration
Hypokalemia
H2O retention
Renin‐angiotensin‐aldosterone
How does the body compensate?
Mechanism to keep pH in balance
Moving H+ from cells into blood‐ Cells exchange H+ for K+‐ Helps with alkalosis, but also causes
hypokalemia
Mechanism to keep pH in balance
Respiratory system
carotid artery, aortic archChemoreceptors
↑pHBrainstem
↓Respiratory rate and depth of breathing
CO2 moves into the body
A vomiting, ill‐appearing patient with alcohol use disorder has laboratory results showing:
Na: 137 pH: 7.40 K: 3.8 pCO2: 41Cl: 90 pO2: 85 HCO3: 22
P: pHL: Labs (pH, pCO2)A: Anion GapC: Compensation O: Is there another process? Osmolality?
Winter’s Formula: Expected pCO2 = 1.5 (HCO3) + 8 +/‐ 2
Delta Gap: ‐ Difference between pts AG and normal AG ‐ This amount is considered an HCO3
‐ equivalent‐ For every unit rise in AG, the HCO3
‐ should lower by 1‐ If HCO3
‐ is higher than expected, there is metabolic alkalosis
So what causes respiratory acidosis?
Normal respiratory physiology
HCO3‐ + H+ H2CO3Carbonic acid
H2O + CO2
pCO2 kept in a narrow range to maintain pHLungs maintain ventilation rate needed to remove CO2
Respiratory acidosis
Normal mechanism of ventilation is disturbed Minute ventilation is inadequate to balance pH
Brainstem Respiratory centers
‐ Stroke, medication overdose‐ Respiratory centers slow their rate
of firing, breathing slows
Neuromuscular disorder
Myasthenia gravis → nerves don’t s mulate muscleTrauma, obesity → diaphragm/chest muscles don’t
work correctly
Airway obstruction
Object in bronchus prevents lung from ventilating
Impaired gas exchange
Impaired gas exchange between alveoli and capillaries
Alveoli are damaged (COPD) Fluid in alveoli (pneumonia) Fluid between alveoli and capillary walls (pulm edema)
Lungs can’t efficiently remove CO2
Respiratory acidosis
CO2 accumulates in the blood
pCO2 rises (usually >45)pH falls (usually <7.35)
How does the body compensate?
Mechanism to keep pH in balance
Excess CO2 diffuses across cell membranes, especially into RBCs, binds water and forms carbonic acid, which then diffuse to HCO3
‐ and H+
Acute phase
HCO3‐ + H+ H2CO3Carbonic acid
H2O + CO2
Escapes into circulation
Remains intracellular
‐ Limited intracellular buffers for H+
‐ Thus minimal CO2 moves into cells ‐ Generate 1mEq HCO3
‐ for every 10mmHg increase in pCO2
Mechanism to keep pH in balance
Chronic phase (3‐5d)
Sense pH too low
Cells in PCT start generating and reabsorbing more HCO3
‐ in the blood stream
Generate 4mEq HCO3‐ for every 10mmHg
increase in pCO2
Much more substantial increase in pH
So what causes respiratory alkalosis?
Respiratory alkalosis
Normal mechanism of ventilation is disturbed Minute ventilation is higher than needed to balance pH
Brainstem Respiratory centers
↑ chemoreceptor firing
‐ Hypoxia‐ Pneumonia‐ Pulmonary embolism‐ High altitude
Abnormal response:Normal response:‐ Anxiety/panic attack‐ Sepsis‐ Salicylate overdose
Lungs removes too much CO2
Respiratory alkalosis
pCO2 falls (usually <35)pH rises (usually >7.45)
Mechanism to keep pH in balance
Acidic molecules in cells, especially RBCs, release H+ where they bind to HCO3
‐ in order to carbonic acid, which then diffuse to H2O and CO2
Acute phase
HCO3‐ + H+ H2CO3Carbonic acid
H2O + CO2
Moves extracellular
‐ Limited intracellular acidic molecules compared to serum HCO3
‐
‐ Thus minimal H+ move out of cells ‐ Decrease 2mEq HCO3
‐ for every 10mmHg decrease in pCO2
↓ levels as it binds H
Mechanism to keep pH in balance
Chronic phase (3‐5d)
Sense pH too high
Cells in PCT excrete more HCO3‐
Decrease 5mEq HCO3‐ for every 10mmHg
decrease in pCO2
Much more substantial decrease in pH
What’s the difference between an ABG/VBG?
ABG pH = VBG pH + 0.04ABG pCO2 = VBG pCO2 – 4
*ABG better for septic shock
*Need ABG to assess OXYGENATION
So, lets apply these concepts in practice..
Stepwise Approach: pLACO
p: L: A: C: O:
pH Labs (pH, pCO2) Anion GapCompensationIs there another/third process? Osmolality?
Acidosis: process that tends to lower pH Alkalosis: process that tends to raise pH
Acidosis Respiratory pH ↓ PaCO2 ↑
Acidosis Metabolic pH ↓ PaCO2 ↓
Alkalosis Respiratory pH ↑ PaCO2 ↓
Alkalosis Metabolic pH ↑ PaCO2 ↑
AG = Na+ – (Cl‐ + HCO3‐)
A woman with CKD stage 4 is admitted to the hospital with protracted vomiting. The following laboratory data are obtained::
Na: 145 pH: 7.40 K: 4 pCO2: 42Cl: 100 pO2: 92 HCO3: 25
P: pHL: Labs (pH, pCO2)A: Anion GapC: Compensation O: Is there another process? Osmolality?
A young man is admitted to the hospital with protracted vomiting. The following laboratory data are obtained:
Na: 146 pH: 7.51 K: 2.8 pCO2: 50Cl: 92 pO2: 96HCO3: 38
P: pHL: Labs (pH, pCO2)A: Anion GapC: Compensation O: Is there another process? Osmolality?
A patient presents complaining of a tight feeling in their chest, shortness of breath and some tingling in their fingers and around their mouth. Na: 144 pH: 7.49 K: 4 pCO2: 27Cl: 108 pO2: 99HCO3: 24
P: pHL: Labs (pH, pCO2)A: Anion GapC: Compensation O: Is there another process? Osmolality?
Resources
• https://www.merckmanuals.com/professional/endocrine‐and‐metabolic‐disorders/acid‐base‐regulation‐and‐disorders
• https://www.thoracic.org/professionals/clinical‐resources/critical‐care/clinical‐education/abgs.php