ABG Interpretation and Acid-Base Disorders
7.35-7.45
Normal pH
Acidosis below, alkalosis above
35-45
PaCO2
mmHg normal range
22-26
HCO3
mEq/L normal range
The 3-Step Method
follow these steps in order, every single timeLook at the pH
Below 7.35 = acidosis. Above 7.45 = alkalosis. This tells you the direction of the primary problem.
Which value matches the pH direction?
If pH is acidotic, is CO2 high (respiratory) or HCO3 low (metabolic)? The matching value is your primary disorder.
Is the other system compensating?
Is the opposite value also abnormal in the corrective direction? If yes, compensation is occurring. If pH is normal with two abnormal values = fully compensated.
A 'normal' pH with abnormal PaCO2 and HCO3 means FULL compensation, not a normal ABG. The body has successfully adjusted but the underlying disorder is still present.
Anion Gap Metabolic Acidosis
Anion Gap = Na+ minus (Cl- + HCO3-). Normal is 8-12 mEq/L.
MUDPILES Mnemonic
M Methanol
U Uremia
D DKA
P Propylene glycol
I Isoniazid/Iron
L Lactic acidosis
E Ethylene glycol
S Salicylates
Key Values to Memorize
pH 7.35-7.45
PaCO2 35-45 mmHg
HCO3 22-26 mEq/L
PaO2 80-100 mmHg
SaO2 95-100%
Anion Gap 8-12
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Practice CCRN QuestionsExam Traps
Compensated does NOT mean normal
A pH of 7.40 with a PaCO2 of 30 and HCO3 of 18 is NOT a normal ABG. It is a fully compensated metabolic acidosis. Both values are abnormal but the body has adjusted the pH back to normal range. The exam will present this and ask if it is normal.
PaO2 is about oxygenation, not acid-base
PaO2 and SaO2 tell you how well the patient is oxygenating. They have nothing to do with acid-base status. Do not include PaO2 in your acid-base interpretation. The exam will try to distract you with PaO2 values.
MUDPILES for anion gap metabolic acidosis
Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates. If the anion gap is elevated (>12), use MUDPILES to find the cause. Normal anion gap acidosis is usually from GI or renal losses.
Kussmaul breathing signals metabolic acidosis compensation
Deep, rapid breathing (Kussmaul) is the respiratory system trying to blow off CO2 to compensate for metabolic acidosis. Classic in DKA. The exam may describe the breathing pattern and expect you to identify the acid-base disorder.
Oxygenation vs Acid-Base
PaO2 and SaO2 tell you about OXYGENATION. They do NOT tell you about acid-base status. Do not mix them up.
Compensation Levels
Full compensation = pH normalized. Partial compensation = pH still abnormal but trending. No compensation = only one system responding.
pH sits in the middle of a seesaw. CO2 (acid) sits on one end. HCO3 (base) sits on the other. When one side gets too heavy, the pH tilts toward that side.
The lungs control CO2. Breathing faster blows off CO2 (less acid). Breathing slower retains CO2 (more acid). This happens in minutes. It is the fast compensation system.
The kidneys control HCO3. They can retain or excrete bicarbonate to shift the balance. This takes hours to days. It is the slow but more powerful compensation system.
Step 1: which way is the pH tilting? Step 2: which side (CO2 or HCO3) matches the tilt? That is your primary disorder. Step 3: is the other side trying to compensate?
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