Arterial Blood Gas (ABG) Analysis

Normal ABG Values

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

Age correction for PaO2 (sitting): PaO2 = 104.2 - (0.27 × age in years) Age correction for A-a gradient: P(A-a)O2 = (age/4) + 4

Step-by-Step Interpretation

Step 1: Assess pH

< 7.35 = Acidemia
> 7.45 = Alkalemia
7.35-7.45 = Normal (but may still have a mixed disorder)

Step 2: Identify the Primary Disorder

Look at PaCO2 and HCO3-:

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

Key rule: The compensation is always in the same direction as the primary disturbance. For example, in metabolic acidosis (↓HCO3-), the respiratory compensation is hyperventilation (↓PaCO2).

Step 3: Check for Adequate Compensation

Use these formulas to determine if compensation is appropriate:

Primary Disorder	Expected Compensation Formula
Metabolic Acidosis	Expected PaCO2 = (1.5 × HCO3-) + 8 ± 2 (Winter's formula)
Metabolic Alkalosis	Expected PaCO2 = (0.7 × HCO3-) + 21 ± 2
Acute Respiratory Acidosis	HCO3- rises 1 mEq/L per 10 mmHg rise in PaCO2
Chronic Respiratory Acidosis	HCO3- rises 3.5 mEq/L per 10 mmHg rise in PaCO2
Acute Respiratory Alkalosis	HCO3- falls 2 mEq/L per 10 mmHg fall in PaCO2
Chronic Respiratory Alkalosis	HCO3- falls 5 mEq/L per 10 mmHg fall in PaCO2

If measured PaCO2 is above the Winter's formula result in metabolic acidosis, there is a superimposed respiratory acidosis.

Step 4: Calculate the Anion Gap (in Metabolic Acidosis)

Anion Gap (AG) = [Na+] - ([HCO3-] + [Cl-])

Normal AG = 8-16 mEq/L (typically cited as ~12 mEq/L)

High Anion Gap Metabolic Acidosis (HAGMA)

Mnemonic: MUDPILES or GOLDMARK

Cause	Key Feature
Diabetic ketoacidosis	Ketones in urine/blood
Lactic acidosis	Lactate > 2 mmol/L
Salicylate poisoning	History, tinnitus
Methanol poisoning	Osmolar gap present
Ethylene glycol poisoning	Osmolar gap + oxalate crystals
Chronic renal failure	Phosphate/sulfate retention

Normal Anion Gap Metabolic Acidosis (NAGMA) = Hyperchloremic

Common causes: Diarrhea, Renal Tubular Acidosis (RTA)

In NAGMA, HCO3- loss is replaced by Cl- (measured anion), so the gap stays normal.

Step 5: Assess Oxygenation

Alveolar-arterial (A-a) gradient:

P(A-a)O2 = FiO2 × (PB - PH2O) - (PaO2 + PaCO2/0.8)

At room air (FiO2 = 0.21, sea level): simplified to roughly

P(A-a)O2 = 150 - PaO2 - (PaCO2/0.8)

Mechanisms of Hypoxemia

Mechanism	A-a Gradient	Example
V/Q mismatch	Increased	Pneumonia, PE
Diffusion impairment	Increased	Interstitial lung disease
Right-to-left shunt	Increased	AVM, intracardiac shunt
Hypoventilation	Normal	Neuromuscular weakness, opioid overdose
Low inspired O2 (altitude)	Normal	High altitude
Hepatopulmonary syndrome	Increased	Liver cirrhosis

Henderson-Hasselbalch Equation

pH = 6.1 + log10 ([HCO3-] / 0.003 × PaCO2)

This shows that pH is determined by the ratio of HCO3- to CO2, not by the absolute values alone.

Venous Blood Gas (VBG) vs ABG

VBGs are increasingly used as a safer alternative, but with limitations:

Venous pH averages ~0.03 lower than arterial pH (good correlation)
PvCO2 averages 3-8 mmHg higher than PaCO2 (less reliable correlation)
VBGs are unreliable in: hypotensive patients, severe hypercapnia
A normal PvCO2 can be used to rule out hypercapnia

Common Mixed Disorders

A mixed disorder exists when the measured compensation falls outside the expected range, OR when two primary disturbances coexist.

Examples:

Respiratory acidosis + Metabolic alkalosis: COPD patient on chronic diuretics
Metabolic acidosis + Respiratory alkalosis: Sepsis (lactic acidosis + hyperventilation)
Metabolic acidosis + Metabolic alkalosis: DKA patient with protracted vomiting

Quick Summary Table

Disorder	pH	Primary Change	Compensation
Metabolic Acidosis	↓	↓ HCO3-	Hyperventilation (↓ PaCO2)
Metabolic Alkalosis	↑	↑ HCO3-	Hypoventilation (↑ PaCO2)
Respiratory Acidosis	↓	↑ PaCO2	Renal ↑ HCO3- retention
Respiratory Alkalosis	↑	↓ PaCO2	Renal ↑ HCO3- excretion

Sources: Goldman-Cecil Medicine (Table 89-1, ABG & A-a gradient), Costanzo Physiology 7th Ed. (Acid-Base Disorders, p. 330-332)

This is a shared conversation. Sign in to Orris to start your own chat.