Patient Cases
Select any patient to begin. Every case is open — work in any order.
Nursing Handoff
Initial Impression
Before you see any data — what is your initial read on this patient?
Normal ABG Values
| Parameter | Normal Range | Units |
|---|---|---|
| pH | 7.35 – 7.45 | — |
| PaCO₂ | 35 – 45 | mmHg |
| HCO₃⁻ (serum) | 22 – 26 | mEq/L |
| PaO₂ | 80 – 100 | mmHg |
| SaO₂ | 95 – 100 | % |
| Base Excess | -2 to +2 | mEq/L |
| Anion Gap | 8 – 12 | mEq/L |
| Lactate | < 2.0 | mmol/L |
Baseline reference for all calculations: pH 7.40 · PaCO₂ 40 · HCO₃⁻ 24 · AG 12
Compensation Formulas
Metabolic Acidosis (Winter's Formula)
Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2
Tolerance ±2 mmHg. PaCO₂ above range = concurrent respiratory acidosis. Below range = concurrent respiratory alkalosis.
Metabolic Alkalosis
Expected PaCO₂ = 40 + 0.7 × (HCO₃⁻ − 24) ± 5
Tolerance ±5 mmHg. Maximum PaCO₂ from compensation ≈ 55 mmHg. Above = concurrent respiratory acidosis.
Acute Respiratory Acidosis
Expected HCO₃⁻ = 24 + 1 × (ΔPaCO₂ / 10)
HCO₃⁻ rises 1 mEq/L per 10 mmHg rise in PaCO₂. Immediate buffering. Minutes to hours.
Chronic Respiratory Acidosis
Expected HCO₃⁻ = 24 + 3.5 × (ΔPaCO₂ / 10)
Full renal compensation requires ≥ 24–48 hours.
Acute Respiratory Alkalosis
Expected HCO₃⁻ = 24 − 2 × (ΔPaCO₂ / 10)
HCO₃⁻ falls 2 mEq/L per 10 mmHg fall in PaCO₂.
Chronic Respiratory Alkalosis
Expected HCO₃⁻ = 24 − 5 × (ΔPaCO₂ / 10)
Only disorder where pH may fully normalize with compensation.
Anion Gap
Anion Gap Formula
AG = Na⁺ − (Cl⁻ + HCO₃⁻)
Normal: 8–12 mEq/L. Use serum HCO₃⁻ (electrolytes), not ABG-calculated value. Calculate AG in every acid-base disorder to avoid missing a hidden high-AG metabolic acidosis.
Albumin Correction
Corrected AG = AG + 2.5 × (4 − albumin g/dL)
Hypoalbuminemia lowers the measured AG. Always correct AG when albumin is below normal.
AG interpretation: AG 8–12 = normal. AG 13–19 = mildly elevated (borderline, requires clinical context). AG ≥ 20 = high AG metabolic acidosis until proven otherwise. AG ≥ 30 = almost certainly high AG acidosis.
Delta-Delta Ratio
Delta-Delta Formula
Δ/Δ = (AG − 12) / (24 − HCO₃⁻)
Apply ONLY when high-AG metabolic acidosis is confirmed. Cause matters: lactic acidosis expected ratio ≈ 1.5; ketoacidosis expected ratio ≈ 1.0.
| Ratio | Interpretation |
|---|---|
| < 1.0 | Concurrent normal-AG metabolic acidosis hidden underneath |
| 1.0 – 2.0 | Pure high-AG metabolic acidosis |
| > 2.0 | Concurrent metabolic alkalosis raising HCO₃⁻ above expected |
Critical rule: Delta-delta applies ONLY to lactic acidosis and ketoacidosis — not uremic or other high-AG causes. Lactic acidosis: expected ΔHCO₃⁻ = ΔAG / 1.5. Ketoacidosis: expected ΔHCO₃⁻ = ΔAG / 1.0.
Causes of Metabolic Acidosis
High AG — MUDPILES
MMethanol / Metformin
UUremia (renal failure)
DDiabetic ketoacidosis
PPropylene glycol / Paracetamol
IIsoniazid / Iron / CO
LLactic acidosis
EEthylene glycol / Ethanol
SSalicylates / Starvation
Normal AG — HARDUP
HHyperalimentation / Hyperchloremia
AAddison's disease
RRenal tubular acidosis
DDiarrhea / GI bicarbonate loss
UUreteral diversion
PPancreatic fistula / Post-RTA
Key rule: Calculate AG in every patient, regardless of the apparent primary disorder. A hidden high-AG metabolic acidosis can be completely masked by a coexisting metabolic alkalosis.
Metabolic Alkalosis — Classification
| Type | Urine Cl⁻ | Causes | Treatment |
|---|---|---|---|
| Saline-responsive | < 10 mEq/L | Vomiting, NG suction, diuretics (early), post-hypercapnia | Normal saline + KCl |
| Saline-resistant | > 20 mEq/L | Primary hyperaldosteronism, Cushing's, Bartter/Gitelman, diuretics (active use) | Treat underlying cause |
Key concept: Metabolic alkalosis requires two conditions — generation (something raises HCO₃⁻) AND maintenance (the kidney abnormally retains HCO₃⁻). The kidney cannot correct alkalosis without chloride. Treat the chloride deficit, not the pH directly.
Respiratory Acidosis & Alkalosis
| Disorder | Common Causes |
|---|---|
| Resp Acidosis | COPD, asthma, opioids/sedatives, neuromuscular disease, airway obstruction, obesity hypoventilation, auto-PEEP |
| Resp Alkalosis | Anxiety, pain, sepsis (early), pregnancy, hepatic encephalopathy, salicylate toxicity (early), mechanical over-ventilation, altitude |
In acute asthma: Initial ABG shows respiratory alkalosis (hyperventilation). A "normalizing" PaCO₂ during an asthma attack is an ominous sign — the patient is fatiguing, not improving.
Minimum PaCO₂ achievable: Young adults ≈ 10–15 mmHg. Older patients ≈ 20 mmHg. At maximum compensation, any further HCO₃⁻ fall or CO₂ rise can cause catastrophic pH collapse.
Oxygenation Indices
P/F Ratio (Horowitz Index)
P/F = PaO₂ / FiO₂
Normal > 400. Mild ARDS: 200–300. Moderate ARDS: 100–200. Severe ARDS: < 100.
A-a Gradient
A-a = (FiO₂ × 713 − PaCO₂ / 0.8) − PaO₂
Normal (room air): Age/4 + 4. Elevated A-a with hypoxemia = V/Q mismatch, shunt, or diffusion. Normal A-a with hypoxemia = hypoventilation or low FiO₂.
Correction Formulas
Sodium Correction for Hyperglycemia
Corrected Na⁺ = Measured Na⁺ + 2.4 × [(glucose − 100) / 100]
Add 2.4 mEq/L to measured Na⁺ for every 100 mg/dL glucose above 100.
AG Correction for Hypoalbuminemia
Corrected AG = Measured AG + 2.5 × (4 − albumin)
Each 1 g/dL fall in albumin below 4 lowers the AG by ~2.5 mEq/L.
Henderson Equation (Validity Check)
[H⁺] = 24 × PaCO₂ / HCO₃⁻
[H⁺] of 40 = pH 7.40. Always verify internal ABG consistency before interpreting. A pH that doesn't match the calculated [H⁺] invalidates the ABG.
3-Step ABG Interpretation Method
Step 1 — Verify validity, identify primary disorder
Check [H⁺] = 24 × PaCO₂ / HCO₃⁻. Then: pH low + PaCO₂ high = respiratory acidosis. pH low + HCO₃⁻ low = metabolic acidosis. pH high + PaCO₂ low = respiratory alkalosis. pH high + HCO₃⁻ high = metabolic alkalosis.
Step 2 — Check compensation
Apply the correct compensation formula. Compare expected vs. actual. If the actual compensatory value is outside the expected range: a second primary disorder exists. Compensation never fully normalizes pH (except chronic respiratory alkalosis).
Step 3 — Calculate AG, apply delta-delta if indicated
AG = Na⁺ − (Cl⁻ + HCO₃⁻). Correct for albumin. Elevated AG → identify cause (MUDPILES). Apply delta-delta only in lactic acidosis or ketoacidosis to detect hidden metabolic alkalosis or non-AG acidosis.
Henderson Equation & ABG Validity
Henderson Equation
[H⁺] = 24 × PaCO₂ / HCO₃⁻
| pH | [H⁺] nmol/L | pH | [H⁺] nmol/L |
|---|---|---|---|
| 7.10 | 79 | 7.40 | 40 |
| 7.20 | 63 | 7.45 | 35 |
| 7.25 | 56 | 7.50 | 32 |
| 7.30 | 50 | 7.55 | 28 |
| 7.35 | 45 | 7.60 | 25 |
Validity rule: Always verify pH matches [H⁺] before interpreting. A pH that does not match the calculated [H⁺] means the ABG is internally inconsistent — repeat both ABG and serum electrolytes before making any clinical decisions.
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CLINICAL DECISION SUPPORT
Ventilator
FiO₂ (fraction of inspired oxygen)
Every oxygenation calculation — P/F ratio, A–a gradient — requires knowing the FiO₂. Different O₂ devices deliver different FiO₂ values, and getting this right changes your interpretation entirely.
DeviceFlowFiO₂ %
Room air—21%
Nasal cannula1–2 L/min~28%
Nasal cannula3–4 L/min~36%
Nasal cannula5–6 L/min~44%
Simple face mask6–10 L/min~50%
Non-rebreather10–15 L/min~60–80%
High-flow nasal O₂Up to 60 L/min21–100% (titrated)
P/F ratio — the oxygenation index
P/F = PaO₂ ÷ FiO₂
Example: PaO₂ 80 on room air (FiO₂ 0.21) → P/F = 80/0.21 = 381 (normal)
Same PaO₂ 80 on 60% O₂ (FiO₂ 0.60) → P/F = 80/0.60 = 133 (severe ARDS)
≥ 400Normal
200–300Impaired — consider ARDS
< 200Moderate ARDS
< 100Severe ARDS
A–a gradient — detecting shunt and V/Q mismatch
PAO₂ = (FiO₂ × 713) − (PaCO₂ ÷ 0.8)
A–a = PAO₂ − PaO₂
A normal A–a gradient means any hypoxaemia is from hypoventilation alone (e.g. opioid overdose, CNS depression). An elevated A–a means something is wrong with the lung itself — shunt, V/Q mismatch, diffusion defect.
< 15 mmHgNormal (on room air)
15–35 mmHgMildly elevated — investigate
> 35 mmHgSignificant V/Q mismatch or shunt
Note: A–a gradient rises on higher FiO₂ even in normal lungs. Normal on 100% O₂ can be up to ~100 mmHg. This tool calculates and flags automatically.
Common modes
AC-VCAssist-Control Volume Control — each breath delivers a set tidal volume. Rate is guaranteed; patient can trigger extra breaths at the same TV.
AC-PCAssist-Control Pressure Control — each breath applies a set pressure. TV varies with lung compliance. Better for lungs at risk of barotrauma.
SIMVSynchronized Intermittent Mandatory Ventilation — a set number of mandatory breaths are synchronised with patient effort; patient breathes spontaneously between them.
PSVPressure Support Ventilation — patient triggers every breath; each breath is augmented by a set pressure. Used for weaning. No guaranteed backup rate.
CPAPContinuous Positive Airway Pressure — constant pressure applied throughout the respiratory cycle. Patient breathes spontaneously; no mandatory breaths delivered.
BiPAPBilevel Positive Airway Pressure — alternates between higher inspiratory (IPAP) and lower expiratory (EPAP) pressures. Used for NIV; also a ventilator mode.
Key parameters
FiO₂Fraction of Inspired Oxygen — expressed as % (21–100%). Controls oxygenation. Wean to lowest FiO₂ that maintains SpO₂ ≥ 92–95%.
PEEPPositive End-Expiratory Pressure — pressure maintained at end of expiration (cmH₂O). Recruits collapsed alveoli, improves oxygenation, prevents de-recruitment. Normal: 5 cmH₂O. High PEEP (>10) can impair cardiac output.
TVTidal Volume — volume of each breath (mL). Target 6–8 mL/kg ideal body weight (IBW) for lung-protective ventilation. Above 8 mL/kg risks ventilator-induced lung injury (VILI).
IBWIdeal Body Weight — calculated from height, not actual weight. Used to set TV because lung size correlates with height, not mass.
RRRespiratory Rate — breaths per minute. Controls PaCO₂ in volume-controlled modes. ↑ RR → ↑ ventilation → ↓ PaCO₂. Normal set rate: 12–20/min.
MVMinute Ventilation = TV × RR. The total volume moved per minute. PaCO₂ is inversely proportional to MV. Double the MV → halve the PaCO₂ (approximately).
Arterial Blood Gas
7.35–7.45
35–45
22–26
80–100
135–145
98–106
Optional
<2.0
3.5–5.0
3.5–5.0
≥ 95% · from ABG
A–a adj. by age
Normal −2 to +2 mEq/L
Serial ABG trend
No previous ABGs saved this session.
⚠ Decision support only. Not a substitute for clinical judgement. Always follow institutional protocols.
Enter ABG values to analyse
pH, PaCO₂, and HCO₃⁻ are required. Na⁺ and Cl⁻ unlock the anion gap. PaO₂ and FiO₂ unlock oxygenation indices.