Acid-Base Balance & ABG Interpretation

pH & Buffer Systems

The body fights to keep pH in a razor-thin range — even small deviations are dangerous.

What Is pH?

pH measures how acidic or alkaline (basic) the blood is. The normal range is 7.35–7.45. The body functions best within this narrow window. Deviations cause enzyme dysfunction, altered mental status, arrhythmias, and ultimately death.

• pH <7.35 = Acidosis (too much acid or too little base)
• pH >7.45 = Alkalosis (too little acid or too much base)
• pH <6.8 or >7.8 = incompatible with life

Three Buffer Systems

The body uses three systems to maintain pH balance, each working at a different speed:

1. Chemical buffers (bicarbonate, phosphate, protein) — work in SECONDS. The bicarbonate buffer system (H₂CO₃ ↔ HCO₃⁻ + H⁺) is the most important clinically.
2. Respiratory system (lungs) — works in MINUTES. Adjusts CO₂ by breathing faster or slower. CO₂ + H₂O ↔ H₂CO₃ (carbonic acid). More CO₂ = more acid.
3. Renal system (kidneys) — works in HOURS TO DAYS. Adjusts HCO₃⁻ (bicarbonate) by retaining or excreting it. Also excretes or retains hydrogen ions (H⁺).

Normal ABG Values

Ranges vary by lab, facility, and textbook — for exams, use the range given in the question. See the full NCLEX-PN lab-value reference & sources →

Memorize these — you'll need them for every ABG question.

pHnormal range

7.35–7.45 — overall acid-base status. Controlled by both systems.

PaCO₂respiratory

35–45 mmHg — respiratory component (acid when elevated). Controlled by lungs.

HCO₃⁻metabolic

22–26 mEq/L — metabolic component (base). Controlled by kidneys.

PaO₂oxygenation

80–100 mmHg — oxygenation status (NOT acid-base). Controlled by lungs.

SaO₂ (SpO₂)saturation

95–100% — oxygen saturation of hemoglobin. Controlled by lungs.

Key Concepts

• CO₂ is an acid. When CO₂ rises, pH drops (more acidic). When CO₂ drops, pH rises (more alkaline).
• HCO₃⁻ is a base. When HCO₃⁻ rises, pH rises (more alkaline). When HCO₃⁻ drops, pH drops (more acidic).
• PaO₂ tells you about oxygenation — it does NOT determine acid-base status. A patient can be well-oxygenated and still be in acidosis.

The ROME Method — 3-Step ABG Interpretation

This is the only method you need. Master these 3 steps and you'll get every ABG question right.

Step 1: Look at the pH

• pH <7.35 → Acidosis
• pH >7.45 → Alkalosis
• pH 7.35–7.45 → Normal (may be compensated — check Step 2 and 3)

Step 2: Check CO₂ (Respiratory Component)

Does CO₂ explain the pH? Do they go in OPPOSITE directions?

• pH ↓ (acidosis) + CO₂ ↑ = Respiratory Acidosis (opposite directions ✓)
• pH ↑ (alkalosis) + CO₂ ↓ = Respiratory Alkalosis (opposite directions ✓)

Step 3: Check HCO₃⁻ (Metabolic Component)

Does HCO₃⁻ explain the pH? Do they go in the SAME direction?

• pH ↓ (acidosis) + HCO₃⁻ ↓ = Metabolic Acidosis (same direction ✓)
• pH ↑ (alkalosis) + HCO₃⁻ ↑ = Metabolic Alkalosis (same direction ✓)

Quick Example

pH 7.28, CO₂ 55, HCO₃ 24

Respiratory Acidosis

pH ↓, CO₂ ↑ — the lungs are NOT getting rid of enough CO₂.

What's Happening

The lungs are hypoventilating — breathing too slowly or too shallowly. CO₂ (an acid) builds up in the blood → pH drops. Think: anything that makes the patient breathe LESS.

Common Causes

• COPD — chronic CO₂ retention (most common chronic cause)
• Opioid/sedative overdose — respiratory depression (most common acute cause on exams)
• Pneumonia, atelectasis, pulmonary edema
• Chest wall injuries (flail chest)
• Neuromuscular diseases (Guillain-Barré, myasthenia gravis, ALS)
• General anesthesia recovery
• Airway obstruction (mucus plug, foreign body)

Signs & Symptoms

• Confusion, lethargy, drowsiness → coma (CO₂ narcosis)
• Headache (CO₂ causes cerebral vasodilation)
• Tachycardia, arrhythmias
• Warm, flushed skin (vasodilation from CO₂)
• Decreased respiratory rate and effort

Nursing Interventions

• Improve ventilation — deep breathing, incentive spirometry, repositioning
• Administer oxygen (carefully in COPD — low flow 1–2 L/min)
• Suction airway if needed
• If opioid-related → naloxone (Narcan)
• May need mechanical ventilation in severe cases
• Semi-Fowler's or high-Fowler's position to maximize lung expansion

Respiratory Alkalosis

pH ↑, CO₂ ↓ — the lungs are blowing off TOO MUCH CO₂.

What's Happening

The patient is hyperventilating — breathing too fast and too deep. They're exhaling too much CO₂ (an acid) → blood becomes too alkaline → pH rises.

Common Causes

• Anxiety/panic attack — the most common cause and most tested
• Pain
• Fever (increased metabolic rate → increased breathing)
• Pulmonary embolism (PE) — hypoxia triggers hyperventilation
• High altitude
• Early salicylate (aspirin) poisoning — stimulates the respiratory center
• Mechanical ventilation set too high

Signs & Symptoms

• Lightheadedness, dizziness
• Numbness and tingling (circumoral and extremities)
• Confusion, difficulty concentrating
• Tachycardia, palpitations
• Carpopedal spasm (hands cramping — similar to hypocalcemia because alkalosis decreases ionized calcium)
• Rapid, deep breathing

Nursing Interventions

• Calm the patient — if anxiety-related, coaching slow breaths is first-line
• Have patient breathe slowly — pursed-lip breathing or coaching "breathe with me"
• Treat the underlying cause (pain management, antipyretics for fever)
• Rebreathing into a paper bag is NO LONGER recommended (can worsen hypoxia)
• If on a ventilator → decrease rate or tidal volume

Metabolic Acidosis

pH ↓, HCO₃ ↓ — too much acid produced OR too much base lost.

What's Happening

Either the body is producing excess acid (ketones, lactic acid), the kidneys aren't excreting enough acid, or the body is losing too much bicarbonate (base). The metabolic system is the problem; the lungs try to compensate by hyperventilating.

Common Causes — "MUDPILES"

• Methanol ingestion
• Uremia (renal failure — can't excrete H⁺)
• Diabetic ketoacidosis (DKA) — #1 most tested cause
• Propylene glycol
• Isoniazid / Iron toxicity
• Lactic acidosis (shock, sepsis, tissue hypoxia)
• Ethylene glycol (antifreeze)
• Salicylates (aspirin OD — late stage)

Also: severe diarrhea (losing HCO₃⁻ through stool)

Signs & Symptoms

• Kussmaul respirations — deep, rapid breathing (lungs compensating by blowing off CO₂). This is the hallmark sign.
• Fruity breath odor (DKA — acetone from ketone production)
• Confusion, lethargy → coma
• Nausea, vomiting, abdominal pain
• Hyperkalemia (H⁺ moves into cells, K⁺ moves out — buffering mechanism)
• Warm, flushed skin

Nursing Interventions

• Treat the underlying cause — insulin for DKA, dialysis for renal failure, antibiotics for sepsis
• Sodium bicarbonate IV for severe acidosis (pH <7.1) — use cautiously
• IV fluids (NS for DKA)
• Monitor potassium closely — as acidosis is corrected, K⁺ shifts back into cells → risk of hypokalemia
• Protect airway if LOC is decreased

Metabolic Alkalosis

pH ↑, HCO₃ ↑ — too much acid lost OR too much base gained.

What's Happening

The body has lost too much acid (H⁺) or retained too much bicarbonate (base). The metabolic system is the problem; the lungs try to compensate by hypoventilating (breathing slower to retain CO₂/acid).

Common Causes

• Vomiting — #1 most tested cause (losing HCl/stomach acid)
• NG suction — same mechanism (suctioning out stomach acid)
• Excessive antacid use (too much base)
• Loop and thiazide diuretics (losing H⁺ and Cl⁻ with urine)
• Excessive sodium bicarbonate administration
• Hypokalemia (kidneys excrete H⁺ to retain K⁺ → pH rises)

Signs & Symptoms

• Slow, shallow respirations (lungs compensating by retaining CO₂)
• Confusion, dizziness, irritability
• Numbness, tingling (alkalosis decreases ionized calcium → neuromuscular irritability)
• Muscle cramps, tetany, tremors
• Hypokalemia symptoms (weakness, arrhythmias)
• Seizures in severe cases

Nursing Interventions

• Treat the underlying cause — antiemetics for vomiting, replace electrolytes
• Replace potassium and chloride (IV KCl)
• Isotonic IV fluids (0.9% NS — contains chloride)
• Discontinue antacids or bicarbonate if that's the cause
• Monitor electrolytes closely (K⁺, Cl⁻, Ca²⁺)
• Monitor respiratory status — slow breathing is compensatory; do NOT stimulate breathing (that would worsen alkalosis)

Compensation

The body always tries to fix itself — the opposite system kicks in to bring pH back toward normal.

How Compensation Works

• Respiratory problem → kidneys compensate (adjust HCO₃⁻) — SLOW (hours to days)
• Metabolic problem → lungs compensate (adjust CO₂) — FAST (minutes)
• The compensating system moves in the SAME direction as the primary change to bring pH back toward normal

Three States of Compensation

Uncompensated

pH abnormal, only one value (CO₂ or HCO₃) is off. Body hasn't started compensating yet.

Partially compensated

pH still abnormal, but BOTH CO₂ and HCO₃ are abnormal. Body is trying but hasn't fixed pH yet.

Fully compensated

pH is back in normal range (7.35–7.45) but BOTH CO₂ and HCO₃ are still abnormal. Look at which side of 7.40 the pH falls on to find the primary problem.

Fully Compensated — Finding the Primary Problem

When pH is normal but both CO₂ and HCO₃ are abnormal, use the "7.40 rule":

• pH 7.35–7.39 (low side of normal) → the primary problem is acidosis
• pH 7.41–7.45 (high side of normal) → the primary problem is alkalosis

Compensation Examples

pH 7.30, CO₂ 55, HCO₃ 24

Respiratory acidosis, uncompensated (pH ↓, CO₂ ↑, HCO₃ normal)

pH 7.32, CO₂ 55, HCO₃ 30

Respiratory acidosis, partially compensated (pH still abnormal, kidneys raising HCO₃ to compensate)

pH 7.37, CO₂ 55, HCO₃ 32

Respiratory acidosis, fully compensated (pH normal but on acidic side, CO₂ still ↑, HCO₃ ↑ to compensate)

pH 7.25, CO₂ 30, HCO₃ 14

Metabolic acidosis, partially compensated (pH ↓, HCO₃ ↓ = metabolic acidosis, CO₂ ↓ = lungs compensating by blowing off CO₂)

Practice Interpretation Scenarios

Work through these step by step using ROME.

Scenario 1: pH 7.48, CO₂ 30, HCO₃ 24

Identify the imbalance and compensation.

Scenario 2: pH 7.22, CO₂ 40, HCO₃ 15

Identify the imbalance and compensation.

Scenario 3: pH 7.50, CO₂ 45, HCO₃ 34

Identify the imbalance and compensation.

Scenario 4: pH 7.33, CO₂ 60, HCO₃ 31

Identify the imbalance and compensation.

Scenario 5: pH 7.37, CO₂ 50, HCO₃ 28

Identify the imbalance and compensation.

Clinical Connections

Link each imbalance to the clinical scenarios you'll see on the exam.

COPD patient with chronic CO₂ retention

Respiratory acidosis (often compensated) — damaged lungs can't exhale CO₂ effectively.

Post-op patient over-sedated on morphine

Respiratory acidosis (uncompensated) — opioids depress respiratory center → ↓RR → ↑CO₂.

Patient having a panic attack

Respiratory alkalosis — hyperventilation → blowing off too much CO₂.

DKA patient with Kussmaul respirations

Metabolic acidosis (partially compensated) — ketones are acids → ↓pH. Lungs compensate with fast deep breathing (↓CO₂).

Patient with renal failure

Metabolic acidosis — kidneys can't excrete H⁺ or regenerate HCO₃⁻.

Patient with prolonged vomiting

Metabolic alkalosis — losing HCl (stomach acid) → ↓H⁺ → ↑pH.

Patient on NG suction for 3 days

Metabolic alkalosis — same as vomiting: suctioning out stomach acid.

Patient with severe diarrhea

Metabolic acidosis — losing HCO₃⁻ (base) from intestines → ↓pH.

Patient on a ventilator with high rate

Respiratory alkalosis — mechanical hyperventilation → too much CO₂ blown off.

Patient with PE and acute dyspnea

Respiratory alkalosis (early) → acidosis (late) — early: hypoxia → hyperventilation → ↓CO₂. Late: respiratory failure → ↑CO₂.

Frequently Asked Questions

Common student questions on this topic.

Quick-Reference Cheatsheet

Every key point from this guide on a single printable page.