ABG Interpretation Guide | GCC Nurse Reference

Parameter	Normal Range	Acidotic	Alkalotic
pH	7.35 – 7.45	<7.35	>7.45
PaCO₂	35 – 45 mmHg	>45 (resp.)	<35 (resp.)
HCO₃¯	22 – 26 mEq/L	<22 (met.)	>26 (met.)
PaO₂	80 – 100 mmHg	<80 = hypoxaemia
SaO₂	>95%	<90% = significant
A-a Gradient	<10–15 mmHg	Age-adjusted = age/4 + 4
Anion Gap	8 – 12 mEq/L	>12 = elevated AG

📋 6-Step Systematic ABG Interpretation

Is the pH normal, acidotic, or alkalotic?

pH <7.35 → Acidosis
pH 7.35–7.45 → Normal (but may have compensated disorder)
pH >7.45 → Alkalosis
If pH normal but PaCO₂ or HCO₃ abnormal → likely fully compensated disorder

Is the primary problem respiratory or metabolic?

Respiratory acidosis: pH ↓ + PaCO₂ ↑ (hypoventilation)
Respiratory alkalosis: pH ↑ + PaCO₂ ↓ (hyperventilation)
Metabolic acidosis: pH ↓ + HCO₃ ↓
Metabolic alkalosis: pH ↑ + HCO₃ ↑

Is compensation occurring?

Use expected compensation formulas. If measured value ≠ expected → mixed disorder.

Respiratory Acidosis (acute): ΔHCO₃ = +1 per 10 mmHg ↑ PaCO₂

Respiratory Acidosis (chronic): ΔHCO₃ = +3.5 per 10 mmHg ↑ PaCO₂

Respiratory Alkalosis (acute): ΔHCO₃ = −2 per 10 mmHg ↓ PaCO₂

Respiratory Alkalosis (chronic): ΔHCO₃ = −5 per 10 mmHg ↓ PaCO₂

Metabolic Acidosis: Expected PaCO₂ = (1.5 × HCO₃) + 8 ± 2 [Winter’s Formula]

Metabolic Alkalosis: Expected PaCO₂ = (0.7 × HCO₃) + 21 ± 2

Calculate the A-a Gradient

PAO₂ = (FiO₂ × 713) − (PaCO₂ / 0.8)

A-a Gradient = PAO₂ − PaO₂

Normal: <10–15 mmHg on room air (increases with age)
Age-adjusted normal: age ÷ 4 + 4 mmHg
Elevated A-a gradient → V/Q mismatch, shunt, diffusion defect
Normal A-a gradient with hypoxaemia → hypoventilation, high altitude

If metabolic acidosis → calculate Anion Gap

Anion Gap = Na⁺ − (Cl⁻ + HCO₃)

Corrected AG = AG + 2.5 × (4.0 − albumin g/dL)

Normal AG (8–12): non-anion gap metabolic acidosis (NAGMA)
Elevated AG (>12): anion gap metabolic acidosis (AGMA)
MUDPILES: Methanol, Uraemia, DKA, Propylene glycol, INH/Iron, Lactic acidosis, Ethylene glycol, Salicylates

If elevated AG → calculate Delta-Delta Ratio

Delta-Delta = (AG − 12) ÷ (24 − HCO₃)

<0.4 → NAGMA superimposed on AGMA
0.4–0.8 → Mixed high AG and normal AG metabolic acidosis
1.0–2.0 → Pure AGMA (expected)
>2.0 → AGMA + metabolic alkalosis (HCO₃ higher than expected)

📈 Compensation Reference Table

Disorder	Primary Change	Expected Compensation	Degree
Respiratory Acidosis (acute)	PaCO₂ ↑	HCO₃ +1 per 10 mmHg ↑ CO₂	Minutes
Respiratory Acidosis (chronic)	PaCO₂ ↑	HCO₃ +3.5 per 10 mmHg ↑ CO₂	Days
Respiratory Alkalosis (acute)	PaCO₂ ↓	HCO₃ −2 per 10 mmHg ↓ CO₂	Minutes
Respiratory Alkalosis (chronic)	PaCO₂ ↓	HCO₃ −5 per 10 mmHg ↓ CO₂	Days
Metabolic Acidosis	HCO₃ ↓	PaCO₂ = (1.5 × HCO₃) + 8 ±2	Hours
Metabolic Alkalosis	HCO₃ ↑	PaCO₂ = (0.7 × HCO₃) + 21 ±2	Hours

🌐 Respiratory Acidosis Patterns

● Respiratory Acidosis — COPD

COPD Exacerbation

pH 7.28 | PaCO₂ 68 | PaO₂ 52 | HCO₃ 30 | SaO₂ 86%

Chronic CO₂ retention with acute-on-chronic deterioration. Elevated HCO₃ reflects chronic compensation. Target SpO₂ 88–92% in known COPD (avoid over-oxygenation).

● Respiratory Acidosis — Drug

Opioid Overdose

pH 7.18 | PaCO₂ 80 | PaO₂ 45 | HCO₃ 24 | SaO₂ 78%

Acute respiratory depression. Normal HCO₃ (no compensation yet). Requires airway management and naloxone. A-a gradient normal (pure hypoventilation).

● Respiratory Acidosis — NMJ

Neuromuscular Disease (GBS, MG)

pH 7.30 | PaCO₂ 58 | PaO₂ 72 | HCO₃ 27 | SaO₂ 92%

Respiratory muscle weakness causes CO₂ retention. Monitor NIF (negative inspiratory force) — intubate if NIF <–25 cmH₂O. Mildly elevated HCO₃ if subacute.

🌐 Respiratory Alkalosis Patterns

● Respiratory Alkalosis — PE

Pulmonary Embolism

pH 7.52 | PaCO₂ 28 | PaO₂ 62 | HCO₃ 22 | SaO₂ 91%

Hypoxia-driven hyperventilation. Elevated A-a gradient. Normal or low PaO₂ despite supplemental O₂ suggests significant V/Q mismatch. Classic triad: tachycardia, pleuritic pain, dyspnoea.

● Respiratory Alkalosis — Anxiety

Anxiety / Hyperventilation

pH 7.58 | PaCO₂ 24 | PaO₂ 105 | HCO₃ 22 | SaO₂ 99%

Normal A-a gradient. PaO₂ may be elevated. Symptoms: perioral tingling, carpopedal spasm, light-headedness. Treat with reassurance and controlled breathing. Exclude organic causes first.

● Respiratory Alkalosis — Other

Pregnancy / High Altitude / Liver Failure

pH 7.48 | PaCO₂ 30 | PaO₂ 88 | HCO₃ 21 | SaO₂ 97%

Progesterone drives hyperventilation in pregnancy (normal). High altitude: hypoxic drive. Liver failure: hyperammonaemia stimulates respiratory centre. All show low PaCO₂ with compensatory HCO₃ fall.

🌐 Metabolic Acidosis — High Anion Gap (MUDPILES)

● AGMA — DKA

Diabetic Ketoacidosis

pH 7.10 | PaCO₂ 18 | PaO₂ 95 | HCO₃ 8 | AG 22

Kussmaul breathing (compensatory hyperventilation). Very high AG. Check glucose, ketones, urine ketones. Treat with IV fluids, insulin infusion, potassium replacement. K⁺ often low at presentation.

● AGMA — Lactic Acidosis

Sepsis / Shock (Type A Lactic Acidosis)

pH 7.22 | PaCO₂ 22 | PaO₂ 80 | HCO₃ 10 | AG 24 | Lactate 6.8

Tissue hypoperfusion → anaerobic metabolism. High AG. Lactate >4 mmol/L with sepsis = septic shock. Aggressive fluid resuscitation and antibiotics. Monitor lactate clearance.

● AGMA — Uraemia

Uraemic Acidosis (Renal Failure)

pH 7.26 | PaCO₂ 24 | PaO₂ 88 | HCO₃ 12 | AG 20

Accumulation of sulphates, phosphates, urates. Moderate AG elevation. Often accompanied by hyperkalemia, fluid overload. Bicarbonate supplementation and dialysis may be needed.

● AGMA — Salicylate

Salicylate / Aspirin Toxicity

pH 7.40 | PaCO₂ 18 | PaO₂ 96 | HCO₃ 12 | AG 22

Classic mixed picture: primary respiratory alkalosis (salicylate stimulates brainstem) + metabolic acidosis. pH may appear normal despite severe toxicity. Toxic salicylate level >300 mg/L. Treat: IV NaHCO₃, urinary alkalinisation.

🌐 Metabolic Acidosis — Normal Anion Gap (HARDUPS)

● NAGMA — GI Loss

Severe Diarrhoea / GI Fistula

pH 7.28 | PaCO₂ 30 | PaO₂ 90 | HCO₃ 14 | Cl 116 | AG 10

GI tract loses HCO₃. Compensatory hyperchloraemia (hyperchloraemic acidosis). Normal AG. Common in diarrhoeal illness, ileostomy, small bowel fistula. Replace HCO₃ losses with IV/oral rehydration.

● NAGMA — RTA

Renal Tubular Acidosis (RTA)

pH 7.30 | PaCO₂ 28 | PaO₂ 92 | HCO₃ 15 | AG 10

Impaired renal acid excretion or HCO₃ reabsorption. Normal AG. Urinary anion gap positive in distal RTA (type 1) and type 4; negative in type 2 (proximal). Treat underlying cause + HCO₃ supplements.

🌐 Metabolic Alkalosis Patterns

● Metabolic Alkalosis — Vomiting

Vomiting / NG Suction

pH 7.55 | PaCO₂ 48 | PaO₂ 85 | HCO₃ 38 | Cl 88

Loss of HCl from stomach. Hypochloraemia, hypokalaemia. Kidneys retain HCO₃ to preserve Cl⁻. Treat with IV NaCl (chloride-responsive alkalosis) + K⁺ replacement.

● Metabolic Alkalosis — Diuretics

Loop / Thiazide Diuretics

pH 7.50 | PaCO₂ 46 | PaO₂ 88 | HCO₃ 34 | K⁺ 2.8

Volume depletion activates RAAS → Na⁺/H⁺ exchange → HCO₃ retention. Hypokalaemia drives H⁺ into cells (extracellular alkalosis). Replace K⁺, consider spironolactone.

● Metabolic Alkalosis — Conn's

Primary Hyperaldosteronism / Post-hypercapnia

pH 7.52 | PaCO₂ 50 | PaO₂ 80 | HCO₃ 40 | K⁺ 2.5

Conn's: excess aldosterone drives H⁺ excretion. Post-hypercapnia: after correction of chronic CO₂ retention the kidneys still hold HCO₃. Chloride-resistant alkalosis. Treat underlying cause.

🌐 Mixed Disorders

● Mixed — COPD + Heart Failure

Respiratory Acidosis + Metabolic Alkalosis

pH 7.42 | PaCO₂ 62 | PaO₂ 60 | HCO₃ 40

pH deceptively normal. High PaCO₂ + very high HCO₃. Both drive each other. Diuretics for HF cause alkalosis on top of CO₂ retention. Delta-Delta >2.

● Mixed — Sepsis

Metabolic Acidosis + Respiratory Alkalosis

pH 7.38 | PaCO₂ 22 | PaO₂ 75 | HCO₃ 12 | Lactate 4.5

Classic sepsis pattern. pH near-normal masks severity. PaCO₂ lower than expected for metabolic acidosis → concurrent respiratory alkalosis. High lactate confirms tissue hypoperfusion.

● Mixed — Post Arrest

Post-Cardiac Arrest

pH 7.02 | PaCO₂ 65 | PaO₂ 55 | HCO₃ 10 | Lactate 12

Combined respiratory acidosis (hypoventilation) + metabolic acidosis (lactic/ischaemic). Maximally deranged. Ventilation corrects CO₂; perfusion/time corrects lactate. Targeted Temperature Management if ROSC achieved.

📝 Clinical Practice Cases

Read each scenario, review the ABG values, form your interpretation, then reveal the answer.

Case 1 — COPD Exacerbation

A 68-year-old male with known COPD presents with 3 days of increasing breathlessness and productive cough. He is on home oxygen 2 L/min. RR 28, confused, using accessory muscles.

pH 7.26PaCO₂ 74PaO₂ 50 HCO₃ 32SaO₂ 83%FiO₂ 0.28

Interpretation: Acute-on-chronic respiratory acidosis — COPD exacerbation

pH 7.26 → Acidosis
PaCO₂ 74 → Respiratory cause (elevated CO₂)
HCO₃ 32 → Partially compensated (chronic metabolic compensation)
Expected HCO₃ for chronic: 24 + 3.5 × (74-40)/10 = 24 + 11.9 = 35.9 — HCO₃ 32 is slightly less, meaning some acute component
A-a gradient elevated (on 28% FiO₂, PAO₂ ≈ 148, A-a ≈ 98 mmHg) — significant V/Q mismatch

Action: NIV (BiPAP) indicated. Target SpO₂ 88–92%. Nebulisers, steroids, antibiotics. Avoid high-flow O₂. Alert medical team urgently — GCS change is critical.

Case 2 — Diabetic Ketoacidosis

A 22-year-old female with type 1 diabetes presents with 2 days of vomiting, polyuria and abdominal pain. RR 32 deep. BSL 28 mmol/L. Urine ketones 3+.

pH 7.08PaCO₂ 16PaO₂ 98 HCO₃ 5Na 138Cl 100

Interpretation: Severe metabolic acidosis — DKA with appropriate respiratory compensation

pH 7.08 → Severe acidosis
HCO₃ 5 → Primary metabolic acidosis
AG = 138 - (100 + 5) = 33 — markedly elevated AGMA
Winter's formula: Expected PaCO₂ = (1.5 × 5) + 8 ±2 = 14–16 ✓ — appropriate compensation (Kussmaul breathing)
Delta-Delta = (33-12) / (24-5) = 21/19 = 1.1 → pure AGMA

Action: DKA protocol — IV 0.9% NaCl resuscitation, fixed-rate insulin infusion 0.1 U/kg/hr, K⁺ replacement (hold insulin if K⁺ <3.5), hourly glucose monitoring. Do NOT give NaHCO₃ unless pH <6.9.

Case 3 — Post-op Hypoventilation

A 54-year-old male is 2 hours post general anaesthesia for laparotomy. He is drowsy (opioid PCA), SpO₂ 90% on 4 L/min O₂. RR 8 shallow.

pH 7.22PaCO₂ 72PaO₂ 65 HCO₃ 25SaO₂ 91%FiO₂ 0.36

Interpretation: Acute respiratory acidosis — opioid-induced hypoventilation

pH 7.22 → Acidosis. PaCO₂ 72 → Respiratory cause
HCO₃ 25 — essentially normal → acute (no metabolic compensation yet)
Expected HCO₃ (acute): 24 + (72-40)/10 × 1 = 27.2 — close to 25, consistent with acute
A-a gradient: PAO₂ = (0.36 × 713) - (72/0.8) = 256.7 - 90 = 166.7; A-a = 166.7 - 65 = 102 — elevated, atelectasis post-op

Action: Stimulate patient, encourage deep breathing. Reduce/hold PCA. Consider naloxone 0.1–0.4 mg IV titrated. Elevate head of bed. Notify anaesthetist/intensivist if no improvement. Prepare for airway support.

Case 4 — Pulmonary Embolism

A 38-year-old woman 5 days post-Caesarean section develops sudden onset pleuritic chest pain and breathlessness. HR 122, RR 26, BP 105/70, SpO₂ 91% on room air.

pH 7.54PaCO₂ 26PaO₂ 65 HCO₃ 21SaO₂ 92%FiO₂ 0.21

Interpretation: Acute respiratory alkalosis — PE until proven otherwise

pH 7.54 → Alkalosis. PaCO₂ 26 → Respiratory cause (hyperventilation from hypoxic drive)
A-a gradient on room air: PAO₂ = (0.21 × 713) - (26/0.8) = 149.7 - 32.5 = 117.2; A-a = 117.2 - 65 = 52.2 — significantly elevated
P/F ratio = 65/0.21 = 309 — borderline mild impairment
Elevated A-a with hypoxia + post-partum = PE until proven otherwise

Action: High-flow O₂. Urgent CTPA or V/Q scan. Anticoagulation (LMWH) unless contraindicated. Echo to assess RV strain. Consider thrombolysis if haemodynamically unstable.

Case 5 — Septic Shock

A 72-year-old diabetic male presents with 2-day history of fever, dysuria, confusion. T 39.8°C, HR 118, BP 82/50, RR 30, SpO₂ 94% on 10 L/min O₂.

pH 7.35PaCO₂ 24PaO₂ 72 HCO₃ 13Na 136Cl 104Lactate 5.2

Interpretation: Mixed disorder — AGMA (lactic acidosis) + respiratory alkalosis, pH deceptively normal

pH 7.35 — normal but misleading. AG = 136 - (104 + 13) = 19 — elevated AGMA
Winter's for metabolic acidosis: Expected PaCO₂ = (1.5 × 13) + 8 ±2 = 27.5 – 29.5. Actual PaCO₂ 24 → lower than expected → concurrent respiratory alkalosis
Delta-Delta = (19-12)/(24-13) = 7/11 = 0.64 — mixed AGMA + NAGMA possible or mixed with RAlk
Lactate 5.2 mmol/L confirms tissue hypoperfusion = septic shock criteria

Action: Sepsis 6 bundle immediately. 30 mL/kg IV crystalloid bolus. Blood cultures x2 then broad-spectrum antibiotics within 1 hour. Vasopressors if MAP <65. Lactate remeasure at 2 hours. ICU referral.

Case 6 — Salicylate / Aspirin Overdose

A 19-year-old female is found unresponsive after reportedly taking "lots of aspirin." Tinnitus reported. RR 32, T 38.5°C, diaphoretic.

pH 7.40PaCO₂ 20PaO₂ 96 HCO₃ 12Na 140Cl 106

Interpretation: Mixed respiratory alkalosis + metabolic acidosis — classic salicylate toxicity

pH 7.40 — deceptively normal! Both disorders co-exist
AG = 140 - (106 + 12) = 22 — elevated, high AG metabolic acidosis
PaCO₂ 20 — far lower than expected for metabolic acidosis (Winter's: expect 26 ±2) → concurrent primary respiratory alkalosis
Salicylates: directly stimulate brainstem respiratory centre → resp. alkalosis; AND uncouple oxidative phosphorylation → metabolic acidosis + hyperthermia

Action: Check salicylate level (toxic >300 mg/L), paracetamol, glucose. IV NaHCO₃ to alkalinise urine (pH 7.5–8) → traps salicylate in urine. Haemodialysis if level >700 mg/L, renal failure, or deteriorating. Do NOT intubate unless necessary (loses resp. alkalosis compensation).

Case 7 — Anxiety Hyperventilation

A 24-year-old nurse presents to A&E with tingling in hands/feet, carpopedal spasm. She reports a panic attack during a busy shift. SpO₂ 100% on room air. Exam otherwise normal.

pH 7.62PaCO₂ 20PaO₂ 112 HCO₃ 20SaO₂ 100%FiO₂ 0.21

Interpretation: Acute respiratory alkalosis — psychogenic hyperventilation

pH 7.62 — markedly alkalotic. PaCO₂ 20 — respiratory cause
HCO₃ 20 — partial metabolic compensation
A-a gradient: PAO₂ = (0.21 × 713) - (20/0.8) = 149.7 - 25 = 124.7; A-a = 124.7 - 112 = 12.7 — normal
PaO₂ actually supra-normal due to over-ventilation. Normal A-a excludes PE, pneumonia, pulmonary pathology
Carpopedal spasm from ionised hypocalcaemia due to alkalosis

Action: Reassurance, controlled breathing (not paper bag in hospital setting — risk of hypoxia). Treat as anxiety attack. Exclude organic causes (PE, metabolic disorder). Tetany resolves with normalisation of pH.

Case 8 — Altitude Sickness

A 35-year-old hiker arrives at a GCC emergency room after trekking at high altitude in Nepal (4,500 m). Headache, nausea, ataxia, confusion (HACE suspected).

pH 7.50PaCO₂ 28PaO₂ 44 HCO₃ 21SaO₂ 76%

Interpretation: Respiratory alkalosis with significant hypoxaemia — altitude acclimatisation + HACE

pH 7.50 — mild alkalosis. PaCO₂ 28 — hypocapnia from hypoxic-driven hyperventilation
HCO₃ 21 — renal compensation begun (kidneys excrete HCO₃ at altitude)
PaO₂ 44 — severely hypoxic (low FiO₂ at altitude)
SaO₂ 76% — critical oxygenation failure
HACE (High Altitude Cerebral Oedema): neurological symptoms, ataxia, confusion at altitude

Action: Immediate descent (most important). High-flow O₂ 10–15 L/min. Dexamethasone 8 mg then 4 mg q6h (reduces cerebral oedema). Consider Gamow bag if descent delayed. Acetazolamide 250 mg BD for AMS prophylaxis.

Case 9 — Acute Liver Failure

A 45-year-old male with hepatic encephalopathy from acute liver failure. GCS 12, jaundiced. Ammonia 180 µmol/L. On lactulose.

pH 7.51PaCO₂ 29PaO₂ 82 HCO₃ 22Na 132Cl 98

Interpretation: Respiratory alkalosis — hyperammonaemic stimulation of respiratory centre in liver failure

pH 7.51 → Alkalosis. PaCO₂ 29 — respiratory cause. HCO₃ 22 — normal (no metabolic component yet)
AG = 132 - (98 + 22) = 12 — normal (upper limit). No AGMA at this stage
Liver failure: ammonia crosses BBB → stimulates central respiratory centre → respiratory alkalosis
PaO₂ 82 — borderline normal. Later can develop hepatopulmonary syndrome with widened A-a

Action: Monitor ammonia closely. Lactulose titration (target 2–3 soft stools/day). Rifaximin. Nutritional support. Monitor for progression to lactic acidosis (poor prognosis). Early liver transplant evaluation if appropriate.

Case 10 — Post-Cardiac Arrest (ROSC)

A 58-year-old male had a witnessed VF arrest. CPR for 22 minutes. ROSC achieved. Now on mechanical ventilation. Temperature 35°C (post-arrest protocol). Remains comatose.

pH 6.98PaCO₂ 62PaO₂ 88 HCO₃ 9Lactate 14FiO₂ 0.60

Interpretation: Severe combined respiratory + metabolic acidosis — post-cardiac arrest

pH 6.98 — critical acidosis. Both PaCO₂ 62 (↑ resp. acidosis) and HCO₃ 9 (↓ met. acidosis) → mixed/combined disorder
Lactate 14 mmol/L — severe lactic acidosis from global ischaemia
P/F ratio = 88/0.60 = 147 — moderate ARDS range (post-resuscitation lung injury)
Expected compensation impossible in arrest context — all deranged simultaneously

Action: Optimise ventilation to normalise PaCO₂ 35–45 (avoid hypocapnia post-arrest). Targeted Temperature Management 34–36°C for 24 hr. Serial lactates — clearance >10%/2 hr is favourable. Coronary angiography if STEMI. NaHCO₃ only if pH <7.0 persists. ICU care, neuroprognostication at 72 hr.

⚡ ROME Mnemonic

ROME — Direction of pH and component changes

Respiratory → Opposite: In respiratory disorders, pH and PaCO₂ move in opposite directions.
(Acidosis: pH ↓, PaCO₂ ↑ | Alkalosis: pH ↑, PaCO₂ ↓)

Metabolic → Equal: In metabolic disorders, pH and HCO₃ move in the same direction.
(Acidosis: pH ↓, HCO₃ ↓ | Alkalosis: pH ↑, HCO₃ ↑)

⚡ MUDPILES — High AG Metabolic Acidosis

MUDPILES Causes

Methanol / Metformin (rare)
Uraemia (renal failure)
Diabetic ketoacidosis
Propylene glycol / Paracetamol OD
Isoniazid (INH) / Iron overdose
Lactic acidosis (sepsis, shock, ischaemia)
Ethylene glycol (antifreeze)
Salicylates (aspirin overdose)

HARDUPS — Normal AG (Hyperchloraemic) Metabolic Acidosis

Hyperalimentation (TPN)
Acetazolamide / Addison's disease
Renal tubular acidosis (RTA type 1, 2, 4)
Diarrhoea (GI HCO₃ loss)
Ureteroenterostomy / Urinary diversion
Pancreatic fistula
Saline excess (dilutional acidosis)

⚡ Compensation Cheat Card

Disorder	Formula	Notes
Met. Acidosis	PaCO₂ = (1.5 × HCO₃) + 8 ±2	Winter's formula; Kussmaul breathing
Met. Alkalosis	PaCO₂ = (0.7 × HCO₃) + 21 ±2	Hypoventilation; limited (hypoxia limits)
Resp. Acid. (acute)	ΔHCO₃ = +1 per ↑10 PaCO₂	Buffering, minutes
Resp. Acid. (chronic)	ΔHCO₃ = +3.5 per ↑10 PaCO₂	Renal, 3–5 days
Resp. Alk. (acute)	ΔHCO₃ = −2 per ↓10 PaCO₂	Buffering, minutes
Resp. Alk. (chronic)	ΔHCO₃ = −5 per ↓10 PaCO₂	Renal, 3–5 days

⚡ P/F Ratio — Oxygenation Assessment

P/F >300 — Normal oxygenation. No ARDS criteria.

P/F 200–300 — Mild ARDS (Berlin criteria). Supplemental O₂, monitoring.

P/F 100–200 — Moderate ARDS. Consider NIV or intubation. PEEP ≥5 cmH₂O.

P/F <100 — Severe ARDS. Intubation likely required. Prone positioning, high PEEP strategy.

P/F Ratio = PaO₂ (mmHg) ÷ FiO₂ (fraction 0.21–1.0)

Note: PaO₂ must be from ABG, not SpO₂. FiO₂ must be known accurately. P/F ratio requires PEEP ≥5 cmH₂O for ARDS criteria (Berlin 2012).

⚡ GCC-Specific Practice Tips

When to Call the Doctor (DHA / SCHS / QCHP)

● pH <7.25 or >7.60 (any cause)
● PaCO₂ >60 with altered consciousness
● PaO₂ <60 despite supplemental O₂
● SaO₂ <88% not responding to O₂
● Lactate >4 mmol/L
● Suspected mixed disorder
● Any ABG requiring NIV / intubation decision
● New ARDS pattern (P/F <300)
● Post-arrest or haemodynamic instability

Documentation Tips for GCC

● Document FiO₂ and O₂ delivery device with every ABG
● Record time of sample, patient position, temperature
● State your interpretation: e.g. "Acute-on-chronic respiratory acidosis"
● Note action taken and doctor notified (name + time)
● Compare to previous ABG trends where available
● Log in DHA nursing notes: ABG parameters + clinical response
● For ICU patients: integrate with ventilator settings documentation
● QCHP examinations: practice 6-step approach for OSCEs

⚡ A-a Gradient Quick Rules

Finding	A-a Normal	A-a Elevated
Hypoxia cause	Hypoventilation, High altitude	V/Q mismatch, shunt, diffusion defect
Common conditions	Opioids, NMJ disease, COPD (pure hypovent.)	PE, pneumonia, pulmonary oedema, ARDS, ILD
Formula (room air)	PAO₂ = (0.21 × 713) − (PaCO₂ / 0.8) → A-a = PAO₂ − PaO₂
Age-adjusted normal	(Age ÷ 4) + 4 mmHg