ABG Interpretation

Normal ABG Values

Component	Normal Range	Indicates
pH	7.35 – 7.45	Overall acid-base status
PaCO2	35 – 45 mmHg	Respiratory component (acid)
HCO3−	22 – 26 mEq/L	Metabolic component (base)
PaO2	80 – 100 mmHg	Oxygenation status
SaO2	95% – 100%	Oxygen saturation

Step-by-Step ABG Interpretation (Tic-Tac-Toe Method)

1. Step 1: Look at pH
   • < 7.35 = Acidosis
   • > 7.45 = Alkalosis
   • 7.35 – 7.45 = Normal (check if compensating)
2. Step 2: Evaluate PaCO2 (Respiratory Component)
   • > 45 = Respiratory Acidosis (hypoventilation → CO2 retention)
   • < 35 = Respiratory Alkalosis (hyperventilation → CO2 blown off)
3. Step 3: Evaluate HCO3− (Metabolic Component)
   • < 22 = Metabolic Acidosis (bicarbonate loss or acid gain)
   • > 26 = Metabolic Alkalosis (bicarbonate gain or acid loss)
4. Step 4: Match — Which component matches the pH?
   • If pH is acidotic and CO2 is high → Respiratory Acidosis
   • If pH is acidotic and HCO3 is low → Metabolic Acidosis
   • If pH is alkalotic and CO2 is low → Respiratory Alkalosis
   • If pH is alkalotic and HCO3 is high → Metabolic Alkalosis
5. Step 5: Determine Compensation
   • Uncompensated: pH is abnormal, only one component is abnormal
   • Partially compensated: pH is abnormal, both components are abnormal
   • Fully compensated: pH is normal, both components are abnormal

ROME Mnemonic

Letter	Meaning	Explanation
R	Respiratory	Opposite: When CO2 goes up, pH goes down (and vice versa). CO2 is an acid.
O	Opposite
M	Metabolic	Equal: When HCO3 goes up, pH goes up (and vice versa). HCO3 is a base.
E	Equal

Common Causes

Disorder	Common Causes
Respiratory Acidosis (↑ CO2)	COPD, respiratory depression (opioids), pneumonia, airway obstruction, neuromuscular disease
Respiratory Alkalosis (↓ CO2)	Hyperventilation (anxiety, pain, fever), mechanical overventilation, PE, early salicylate poisoning
Metabolic Acidosis (↓ HCO3)	DKA, lactic acidosis, renal failure, diarrhea, salicylate overdose. Use MUDPILES: Methanol, Uremia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates
Metabolic Alkalosis (↑ HCO3)	Vomiting, NG suction, diuretics, excessive antacids, hypokalemia

Compensation Mechanisms

Primary Disorder	Compensating System	Compensating Response
Respiratory Acidosis	Kidneys (slow: 24–48 hrs)	Retain HCO3, excrete H+
Respiratory Alkalosis	Kidneys (slow: 24–48 hrs)	Excrete HCO3, retain H+
Metabolic Acidosis	Lungs (fast: minutes)	Hyperventilate (blow off CO2) — Kussmaul respirations
Metabolic Alkalosis	Lungs (fast: minutes)	Hypoventilate (retain CO2) — limited response

Practice Examples

#	pH	PaCO2	HCO3	Interpretation
1	7.30	55	24	Uncompensated Respiratory Acidosis (COPD exacerbation)
2	7.50	30	24	Uncompensated Respiratory Alkalosis (anxiety/hyperventilation)
3	7.25	40	16	Uncompensated Metabolic Acidosis (DKA)
4	7.50	40	32	Uncompensated Metabolic Alkalosis (prolonged vomiting)
5	7.33	55	30	Partially Compensated Respiratory Acidosis
6	7.37	50	30	Fully Compensated Respiratory Acidosis
7	7.28	28	14	Partially Compensated Metabolic Acidosis

Key Nursing Considerations

• Use a heparinized syringe for ABG collection; apply pressure for 5 minutes post-draw
• Perform Allen's test before radial artery puncture
• Transport specimen on ice and analyze within 15 minutes
• For patients on O2: document FiO2 at time of draw