Arterial Blood Gas (ABG) Analysis

Normal Values

Parameter	Normal Range
pH	7.35 - 7.45
PaCO2	35 - 45 mmHg
PaO2	70 - 95 mmHg (age-dependent)
HCO3-	22 - 26 mEq/L
Base Excess (BE)	-2 to +2 mEq/L
SaO2	95 - 100%

Step-by-Step Approach

Step 1 - Check the pH

pH	Interpretation
< 7.35	Acidemia
7.35 - 7.45	Normal
> 7.45	Alkalemia

A normal pH does not rule out a mixed disorder.

Step 2 - Identify the Primary Disorder

Look at PaCO2 and HCO3- together with the pH:

Disorder	pH	PaCO2	HCO3-
Metabolic Acidosis	↓	↓ (compensation)	↓ (primary)
Metabolic Alkalosis	↑	↑ (compensation)	↑ (primary)
Respiratory Acidosis	↓	↑ (primary)	↑ (compensation)
Respiratory Alkalosis	↑	↓ (primary)	↓ (compensation)

Key rule: If pH and PaCO2 move in opposite directions → primary respiratory disorder. If they move in the same direction → primary metabolic disorder.

Step 3 - Check for Appropriate Compensation

Compensation always moves in the same direction as the primary disturbance, but never fully corrects the pH.

Metabolic Acidosis - Respiratory Compensation (Winter's Formula)

Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2

PaCO2 above expected = concurrent respiratory acidosis
PaCO2 below expected = concurrent respiratory alkalosis

Metabolic Alkalosis - Respiratory Compensation

Expected PaCO2 = (0.7 × HCO3-) + 21 ± 2

Respiratory Acidosis - Renal Compensation

Acute: HCO3- rises by 1 mEq/L per 10 mmHg rise in PaCO2
Chronic: HCO3- rises by 3.5 mEq/L per 10 mmHg rise in PaCO2

Respiratory Alkalosis - Renal Compensation

Acute: HCO3- falls by 2 mEq/L per 10 mmHg fall in PaCO2
Chronic: HCO3- falls by 5 mEq/L per 10 mmHg fall in PaCO2

If compensation falls outside the expected range, a mixed disorder is present.

Step 4 - Calculate the Anion Gap (if metabolic acidosis present)

Anion Gap = Na+ - (HCO3- + Cl-) Normal = 8-12 mEq/L (some sources cite 8-16)

The anion gap = unmeasured anions (plasma proteins, phosphate, citrate, sulfate).

High Anion Gap Metabolic Acidosis (HAGMA)

Mnemonic: MUDPILES (or GOLDMARK)

M - Methanol
U - Uremia (chronic renal failure)
D - Diabetic ketoacidosis (DKA)
P - Propylene glycol / Paracetamol
I - Iron, Isoniazid
L - Lactic acidosis
E - Ethylene glycol
S - Salicylates

Normal Anion Gap Metabolic Acidosis (NAGMA / Hyperchloremic)

Mnemonic: HARDUPS

H - Hyperalimentation
A - Addison's disease
R - Renal tubular acidosis (RTA)
D - Diarrhea
U - Ureteroenteric fistula
P - Pancreatic fistula
S - Saline excess (dilutional)

Step 5 - Delta-Delta Ratio (for HAGMA)

If there is a high anion gap, use the delta-delta to check for a hidden concurrent metabolic disorder:

Delta-Delta = (AG - 12) / (24 - HCO3-)

Ratio	Interpretation
< 0.4	Concurrent normal AG metabolic acidosis
0.4 - 1.0	Mixed HAGMA + NAGMA
1.0 - 2.0	Pure HAGMA (expected)
> 2.0	Concurrent metabolic alkalosis

Step 6 - Assess Oxygenation

PaO2 normal: 70-95 mmHg (decreases with age)

PaO2 sitting = 104.2 - (0.27 × age in years)

Alveolar-Arterial (A-a) Gradient:

P(A-a)O2 = [FiO2 × (PB - 47)] - (PaCO2 / 0.8) - PaO2 Normal A-a gradient = (Age/4) + 4

A-a Gradient	Mechanism of Hypoxemia
Normal	Hypoventilation, high altitude
Increased	V/Q mismatch, diffusion impairment, shunt

Summary Table of Four Simple Disorders

Disorder	pH	PaCO2	HCO3-	Respiratory Compensation	Renal Compensation
Metabolic Acidosis	↓	↓	↓	Hyperventilation	↑ HCO3- reabsorption
Metabolic Alkalosis	↑	↑	↑	Hypoventilation	↑ HCO3- excretion
Respiratory Acidosis	↓	↑	↑	None	↑ HCO3- reabsorption
Respiratory Alkalosis	↑	↓	↓	None	↓ HCO3- reabsorption

(Bold = primary disturbance)

A Quick Note on Venous Blood Gases (VBG)

VBGs are increasingly used as a safer alternative. Venous pH averages ~0.03 lower than arterial. PvCO2 runs 3-8 mmHg higher than PaCO2 and correlates less reliably. VBGs are unreliable in hypotensive patients or severe hypercapnia, but a normal PvCO2 effectively excludes hypercapnia.

Sources: Costanzo Physiology 7th Ed., Goldman-Cecil Medicine

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