Arterial Blood Glasses

Arterial blood gases (ABGs) reveal oxygenation and acid–base status in one snapshot by pairing PaO₂/SaO₂ with pH, PaCO₂, and HCO₃⁻, then checking whether compensation or a mixed disorder is present. These notes are clinical job aids for quick use at the bedside, in the ED, ICU, and step-down.

Safety anchors: Verify device & FiO₂ at draw • Remove air bubbles • Label site/time • Trend values, don’t chase single numbers • Escalate for severe acidemia/alkalemia or refractory hypoxemia

Clinical judgment: Collect cues → interpret with a 4-step scaffold → estimate compensation → name the pattern/mixed state → choose and sequence actions

Use this pack: Scan tables, practice Quick Checks, then test yourself with the NGN mini-item + answer keys.

Policy note: follow facility policy for ABG sampling, anticoagulation precautions, oxygen targets, and escalation triggers.

1) ABG Basics 📋

Master the building blocks before jumping to patterns.

1.1 Normal Ranges

Item	Normal
pH	7.35–7.45
PaCO₂	35–45 mmHg (↑ = acid)
HCO₃⁻	22–26 mEq/L (↑ = base)
PaO₂	80–100 mmHg (context: age/FiO₂)
SaO₂	≥95% (context matters)

1.2 Oxygenation Indices (context matters)

• P/F ratio = PaO₂ / FiO₂ (decimal): ≥400 good • 200–300 mild impairment • <200 significant impairment

• A–a gradient (rule of thumb on room air) ≈ (Age/4) + 4 mmHg. Higher-than-expected suggests V/Q mismatch, shunt, or diffusion defect.

1.3 Quality at the Draw

Right patient/time/device & FiO₂ • Heparinized syringe • Expel bubbles • Gentle mix • Prompt run • Hemostasis 5–10 min (longer if anticoagulated)

2) 4-Step Interpretation 🔍

1. Oxygenation: PaO₂/SaO₂ vs FiO₂ (consider P/F).

2. pH: acidic (<7.35) or alkaline (>7.45).

3. Primary cause: PaCO₂ (respiratory) vs HCO₃⁻ (metabolic).

4. Compensation/mixed: expected vs actual changes.

Memory aid — R.O.A.M.: Rule out hypoxemia • pH direction • Assign primary • Match compensation

3) Acid–Base Patterns 🧪

Pattern	pH	PaCO₂	HCO₃⁻	Common causes	Bedside implications
Respiratory acidosis (acute)	↓	↑	↔/slight ↑	Hypoventilation, CNS depression, opioid OD, COPD/asthma flare, pneumonia	Support ventilation, open airway, treat precipitant
Respiratory acidosis (chronic)	↘/near nml	↑	↑↑	Chronic COPD, neuromuscular disease	Kidneys retain HCO₃⁻; titrate O₂ thoughtfully
Respiratory alkalosis (acute)	↑	↓	↔	Pain/anxiety, early hypoxemia/PE, iatrogenic over-vent	Treat driver; reassess oxygenation
Respiratory alkalosis (chronic)	↗/near nml	↓	↓	Long-standing hyperventilation	Correct cause; avoid over-ventilation
Metabolic acidosis	↓	↓ (comp)	↓	DKA, lactic acidosis/sepsis, renal failure, diarrhea, toxins	Kussmaul; fluids/insulin/pressors per orders; monitor K⁺
Metabolic alkalosis	↑	↑ (comp)	↑	Vomiting/NG suction, loop diuretics, hyperaldosteronism, hypokalemia	Replace Cl⁻/K⁺; address losses; med review

Memory aid — ROME: Respiratory Opposite (pH vs PaCO₂) • Metabolic Equal (pH & HCO₃⁻ move together)

4) Compensation Rules 📐

Primary disorder	Expected compensation (bedside rule)
Acute resp. acidosis	HCO₃⁻ ↑ ~1 mEq/L per +10 mmHg PaCO₂
Chronic resp. acidosis	HCO₃⁻ ↑ ~3–4 mEq/L per +10 mmHg PaCO₂
Acute resp. alkalosis	HCO₃⁻ ↓ ~2 mEq/L per −10 mmHg PaCO₂
Chronic resp. alkalosis	HCO₃⁻ ↓ ~4–5 mEq/L per −10 mmHg PaCO₂
Metabolic acidosis	Winter’s: expected PaCO₂ ≈ 1.5×HCO₃⁻ + 8 (±2)
Metabolic alkalosis	PaCO₂ ↑ ≈ 0.7 mmHg per +1 mEq/L HCO₃⁻ (±2)

If compensation overshoots/undershoots expectations → suspect a mixed disorder.

5) Tic-Tac-Toe Method 🎯

Place each value on a 3×3 grid (columns Acid | Normal | Alkaline; rows pH, PaCO₂, HCO₃⁻). Two marks in the same Acid/Alkaline column = primary disorder. The odd one = compensation (or mixed if it doesn’t fit).

Blank grid

	Acid	Normal	Alkaline
pH	<7.35	7.35–7.45	>7.45
PaCO₂	>45	35–45	<35
HCO₃⁻	<22	22–26	>26

6) Worked Cases 🧩

Case A — Primary Respiratory Acidosis (metabolic compensation)

ABG: pH 7.29, PaCO₂ 60, HCO₃⁻ 28, PaO₂ 62 (2 L NC)

	Acid	Normal	Alkaline
pH	7.29 ✔️
PaCO₂	60 ✔️
HCO₃⁻			28 ✔️

Likely diagnosis: COPD exacerbation, hypoventilationImplications: Upright; bronchodilator/RT support; controlled O₂ to target; consider NIV; treat precipitant.

Case B — Respiratory Alkalosis (acute)

ABG: pH 7.50, PaCO₂ 30, HCO₃⁻ 23, PaO₂ 58 (RA)

	Acid	Normal	Alkaline
pH			7.50 ✔️
PaCO₂			30 ✔️
HCO₃⁻		23 ✔️

Likely diagnosis: Pain/anxiety, early hypoxemia/PEImplications: Position up; treat driver; titrate O₂; evaluate for PE/injury if persistent.

Case C — Metabolic Acidosis (respiratory compensation)

ABG: pH 7.22, PaCO₂ 28, HCO₃⁻ 11, PaO₂ 74 (4 L NC)

	Acid	Normal	Alkaline
pH	7.22 ✔️
PaCO₂			28 ✔️
HCO₃⁻	11 ✔️

Winter’s check: 1.5×11+8 = 24.5±2; actual 28 → slightly inadequate (fatigue/ventilation issue).Likely diagnosis: DKA, sepsis/lactate, renal failureImplications: Airway/vent support PRN; fluids; insulin/antibiotics per orders; monitor K⁺ & glucose.

Case D — Metabolic Alkalosis (respiratory compensation)

ABG: pH 7.55, PaCO₂ 46, HCO₃⁻ 36, PaO₂ 92 (2 L NC)

	Acid	Normal	Alkaline
pH			7.55 ✔️
PaCO₂	46 ✔️
HCO₃⁻			36 ✔️

Likely diagnosis: Vomiting/NG suction, diuretics, hypokalemiaImplications: Replace Cl⁻/K⁺; antiemetics; review diuretics; reduce NG losses.

Case E — Mixed (Respiratory Alkalosis + Metabolic Acidosis)

ABG: pH 7.60, PaCO₂ 20, HCO₃⁻ 18, PaO₂ 52 (RA)

	Acid	Normal	Alkaline
pH			7.60 ✔️
PaCO₂			20 ✔️
HCO₃⁻	18 ✔️

Likely diagnosis: Sepsis with lactic acidosis; salicylate toxicityImplications: Treat the cause; guided O₂/ventilation; lactate/tox screen; close monitoring.

7) Edge Cases — “Normal” pH with a Lean ⤵️⤴️

Edge Cases

N1 — pH 7.36 ↘ (acidic edge)

1 Pattern: Chronic respiratory acidosis (compensated) • PaO₂/FiO₂: 68 / 0.28

	Acid	Normal	Alkaline
pH		7.36 ↘ ✔️
PaCO₂	60 ✔️
HCO₃⁻			33 ✔️

Implications: Controlled O₂ to target • Upright • Bronchodilators/clear secretions • Consider NIV if tiring • Treat trigger.

N2 — pH 7.44 ↗ (alkaline edge)

2 Pattern: Chronic respiratory alkalosis (compensated) • PaO₂/FiO₂: 80 / RA

	Acid	Normal	Alkaline
pH		7.44 ↗ ✔️
PaCO₂			28 ✔️
HCO₃⁻	19 ✔️

Implications: Find/treat hyperventilation drivers (pain, anxiety, pregnancy, liver dz, altitude) • Avoid over-ventilation.

N3 — pH 7.40 ≈ (neutral)

3 Pattern: Fully compensated chronic respiratory acidosis • PaO₂/FiO₂: 70 / 0.40

	Acid	Normal	Alkaline
pH		7.40 ≈ ✔️
PaCO₂	55 ✔️
HCO₃⁻			34 ✔️

Implications: Likely COPD baseline • Titrate O₂ thoughtfully • Watch for acute-on-chronic change (rising CO₂, ↑WOB).

N4 — pH 7.35 ↘ (acidic edge of normal)

4 Pattern: Mixed — respiratory alkalosis + metabolic acidosis • PaO₂/FiO₂: 92 / RA

	Acid	Normal	Alkaline
pH		7.35 ↘ ✔️
PaCO₂			25 ✔️
HCO₃⁻	13 ✔️

Implications: “Normal” pH hides danger • Think sepsis/salicylates • Lactate/tox screen • Treat cause • Reassess ABG/ETCO₂.

N5 — pH 7.45 ↗ (alkaline edge of normal)

5 Pattern: Metabolic alkalosis with respiratory compensation • PaO₂/FiO₂: 94 / 0.28

	Acid	Normal	Alkaline
pH		7.45 ↗ ✔️
PaCO₂	50 ✔️
HCO₃⁻			35 ✔️

Implications: Replace Cl⁻/K⁺ • Assess volume status • Adjust NG/diuretics per orders • Antiemetics • Rhythm monitoring.

Common Diagnoses & Targeted Actions 📋

Diagnosis	Typical ABG	First nursing priorities
COPD exacerbation	Resp. acidosis ± chronic comp; hypoxemia	Upright, bronchodilators, controlled O₂, consider NIV, secretion management
DKA	Metabolic acidosis + low PaCO₂ (comp)	Fluids, insulin, K⁺ protocol, glucose/anion gap trends
Sepsis/lactate	Metabolic acidosis (often mixed)	O₂, fluids/pressors per orders, cultures/abx, lactate trends
Pulmonary embolism	Resp. alkalosis; PaO₂ low	O₂, rapid workup, anticoag per orders
Pneumonia	Hypoxemia; variable acid–base	O₂ to target, antibiotics, lung expansion, secretion management
Vomiting/NG suction	Metabolic alkalosis	Replace Cl⁻/K⁺, minimize losses, antiemetics

9) Documentation & Handoff 🗎

ABG values with device & FiO₂ • patient appearance/respiratory effort • interventions taken (ventilation/O₂/meds) • response & repeat measures • plan and escalation thresholds (SBAR)

Quick Checks (Retrieval) 🧠✅

• List the four steps of ABG interpretation in order.

• Use ROME to classify: pH 7.22, PaCO₂ 28, HCO₃⁻ 11.

• Calculate Winter’s expected PaCO₂ for HCO₃⁻ = 12. Does PaCO₂ 34 fit?

• Name two causes of respiratory alkalosis with low PaO₂.

• What P/F ratio suggests significant impairment?

Answer Key & Rationales 🧠✅

1. Oxygenation → pH → primary cause → compensation/mixed.

2. Metabolic acidosis (pH↓, HCO₃⁻↓) with respiratory compensation (PaCO₂↓).

3. 1.5×12+8 = 26±2 → expected 24–28; PaCO₂ 34 is higher (inadequate comp/mixed).

4. Early PE, severe pneumonia/hypoxemia (plus pain/anxiety).

5. <200.A

NGN-Style Mini Item 🧩

6. Scenario: Post-op, anxious, RR 30, SpO₂ 92% on room air. ABG: pH 7.51, PaCO₂ 29, HCO₃⁻ 23, PaO₂ 58.

Select all priority actions:

• ☐ Place non-rebreather at 15 L immediately

• ☐ Upright, coach breathing, treat pain/anxiety

• ☐ Titrate O₂ to target; reassess with SpO₂ and symptoms

• ☐ Prepare for intubation without trial of therapy

• ☐ Evaluate for PE or pneumonia if hypoxemia persists

Closing Practice Pearls ✨

• A great pH with poor PaO₂ is still an emergency—oxygenation first.

• Always pair ABG with FiO₂ at draw; device mismatch misleads interpretation.

• Compensation ≠ cure—a “normal” pH can hide a dangerous mixed disorder.

• Use P/F ratio to track oxygenation severity, not just PaO₂ alone.

• If compensation overshoots/undershoots, think mixed.

• Treat the patient + pattern, then recheck after every intervention.

Answer Key & Rationales 🧠✅

1. Oxygenation → pH → primary cause → compensation/mixed.

2. Metabolic acidosis (pH↓, HCO₃⁻↓) with respiratory compensation (PaCO₂↓).

3. 1.5×12+8 = 26±2 → expected 24–28; PaCO₂ 34 is higher (inadequate comp/mixed).

4. Early PE, severe pneumonia/hypoxemia (plus pain/anxiety).

5. <200.A

6. Correct: ✅ Upright/coach • ✅ Titrate O₂ & reassess • ✅ Evaluate for PE/pneumonia if persistentWhy: Acute respiratory alkalosis with hypoxemia; treat drivers and oxygenate to target while investigating causes.