These are not blog posts.
They are unfinished thoughts and decision frames that help me think during ER shifts.
They are unfinished thoughts and decision frames that help me think during ER shifts.
The first time I looked at an ABG, it felt like a new language.
An understanding senior helped me see the logic behind the numbers.Once that clicked, ABGs stopped feeling intimidating.
Let’s interpret what the numbers on an ABG actually mean.
Step 1: Context always comes first
Before you even look at the ABG, look at the patient.
Never read ABG numbers in isolation.
👉ABGs are ordered to answer clinical questions, not out of curiosity.
So always ask yourself:
why was this ABG taken in the first place
Step 2: Check internal consistency — does this ABG even make sense?
First, make sure the numbers agree with each other.
Remember the Henderson–Hasselbalch relationship:
Hydrogen ion depends on the balance between PaCO₂ and bicarbonate.
Rearranged bedside check:
In simple terms:
if you multiply H⁺ and bicarbonate and divide by PaCO₂, you should get roughly 24.
If this relationship does not roughly hold
→ suspect lab error
Quick bedside hydrogen ion estimate
Example:
pH = 7.23 → H⁺ ≈ 80 − 23 = 57 nmol/L
If pH, PaCO₂, and HCO₃⁻ don’t align physiologically → pause.
Before diagnosing anything exotic, rule out lab error.
Step 3: Look at the pH
Normal pH: 7.35–7.45 (Midpoint: 7.40)
Now look at the pH and only decide one thing: which side of normal is the patient on.
- pH < 7.40 → Acidemia
- pH > 7.40 → Alkalemia
pH tells you the state, not the cause.
Step 4: Identify the primary acid–base disorder
Always remember this idea:
Hydrogen ion goes up if PaCO₂ goes up or bicarbonate goes down.
- ↑ PaCO₂ or ↓ HCO₃⁻ → ↑ H ⁺ →↓ pH → acidosis
- ↓ PaCO₂ or ↑ HCO₃⁻ → ↓ H ⁺ →↑ pH ⁺ → alkalosis
Rule
HCO₃⁻ moves in the same direction as pH → metabolic
PaCO₂ moves in the opposite direction as pH → respiratory
When pH is abnormal and both PaCO₂ and HCO₃⁻ contribute to the pH change, it’s a mixed disorder. How do you decide which process is dominant— metabolic or respiratory?
Compare their relative deviation from normal. Whichever shows the greater % change is the dominant disorder.
Example: pH 7.25, PaCO₂ 60 (50%), HCO₃⁻ 16 (33%) →
respiratory acidosis is the dominant disorder.
⚠️ Use this only for checking the dominant disorder, not to assess compensation.
⚠️ Remember this is only a simplified bedside technique to assist interpretation
This step is about identifying who started the problem — lungs or kidneys.
Step 5: Is there appropriate compensation?
Next, ask whether the body is responding appropriately.
Compensation ≠ normal pH
Normal pH = think mixed disorder
If the problem is metabolic, the lungs should compensate by changing PaCO₂.
If the problem is respiratory, the kidneys should compensate by changing bicarbonate.
The body tries to move pH toward normal, but it never overshoots on purpose.
Use the formulas below to assess compensation
Metabolic disorders
Metabolic acidosis — Winter’s equation
Metabolic alkalosis
Respiratory disorders — “1-2-3-4-5 rule”
(Expected change in HCO₃⁻ for every 10 mmHg change in PaCO₂ from 40 mmHg)
Disorder | Acute | Chronic |
Respiratory acidosis | +1 | +4 |
Respiratory alkalosis | −2 | −5 |
Assess whether it is acute or chronic using the ΔH⁺ / ΔPaCO₂ ratio:
- < 0.3 → chronic respiratory disorder
- 0.3–0.8 → acute on chronic respiratory disorder
- > 0.8 → acute respiratory disorder
Step 6: If compensation doesn’t fit → mixed disorder
If what you calculate and what you see on the ABG do not match, assume there is
more than one acid–base problem
Hydrogen ion goes up if PaCO₂ goes up or bicarbonate goes down.
- ↑ PaCO₂ or ↓ HCO₃⁻ → ↑ H⁺ → acidosis
- ↓ PaCO₂ or ↑ HCO₃⁻ → ↓ H⁺ → alkalosis
PaCO₂ higher than expected → secondary respiratory acidosis
PaCO₂ lower than expected → secondary respiratory alkalosis
HCO₃⁻ higher than expected → secondary metabolic alkalosis
HCO₃⁻ lower than expected → secondary metabolic acidosis
Step 7: If primary metabolic acidosis → calculate Anion Gap
Now ask one simple question:
Where did the bicarbonate go?
- Normal (with K ⁺ ): 12 ± 4
- Normal (without K ⁺ ): 8 ± 4
Shortcut
Gap ↑ → think added acids
Gap normal → think bicarbonate loss or chloride gain
Step 8: Calculate delta ratio (HAGMA only)
Interpretation
0.8–1.2 → Pure HAGMA
> 1.2 → HAGMA + metabolic alkalosis
< 0.8 → HAGMA + NAGMA
Shortcut
ΔAG ≈ ΔHCO₃⁻ → pure HAGMA
ΔAG > ΔHCO₃⁻ → HAGMA + metabolic alkalosis
ΔAG < ΔHCO₃⁻ → HAGMA + NAGMA
- Too much gap for the bicarbonate drop? → alkalosis hiding
- Too little gap for the bicarbonate drop? → extra acidosis hiding
Example
A 45-year-old man comes in with vomiting and altered sensorium. He’s tachypnoeic and dehydrated. ABG is sent.
ABG values:
- pH: 7.38
- PaCO₂: 28 mmHg
- HCO₃⁻: 16 mmol/L
Electrolytes:
- Na⁺: 140
- Cl⁻: 100
- K⁺: 4
Step 1: Clinical Context
A 45-year-old man comes with repeated vomiting and poor intake for two days.
He is tachypnoeic but haemodynamically stable.
An ABG is sent because he appears unwell and acid–base disturbance is suspected.
Step 2: Internal consistency?
Before interpreting anything, check if the numbers agree.
First, estimate hydrogen ion from pH:
pH 7.38 → H ⁺ ≈ 80 − 38 = 42
Now apply the Henderson bedside check:
That fits.
So this ABG is internally consistent.
The numbers make physiological sense and are safe to interpret further.
Step 3: Look at the pH
The pH is 7.38 —
nearly normal, slightly toward acidemia.
A near-normal pH should always raise suspicion for a mixed disorder.
Step 4: Identify the primary process
Bicarbonate is low (16) →
Step 5: Calculate compensation
For metabolic acidosis, use Winter’s equation:
So expected PaCO₂ range is 30–34 mmHg.
Actual PaCO₂ is 28 mmHg.
That is lower than expected.
So there is a concurrent respiratory alkalosis.
Step 6: Check the anion gap
That’s a high anion gap metabolic acidosis.
Step 7: Calculate delta values
A delta ratio > 1.2 indicates a concurrent metabolic alkalosis.
Interpretation
- High anion gap metabolic acidosis
- Respiratory alkalosis
- Metabolic alkalosis
Made with Bullet