ABG
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ABG Clinical Analyzer v4

Analyze, learn, practice, and track your progress in one offline teaching app.

🧪 Analyzer📚 Study🩺 Cases🎯 Quiz🗂️ Cards📈 Progress
Created by Dr. Rajvi Chaudhary & Aditya Chaudhary • Educational prototype • Not a substitute for clinical judgment
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ABG Clinical Analyzer

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ABG Clinical Analyzer v4

Interpret, learn, practice, and track your progress — all in one offline teaching app.

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ABG values

Enter the three core values first
Core

Electrolytes & metabolic data

Useful for anion gap and differential pathways
Optional

Clinical context

Helps frame respiratory-support urgency
Bedside
Enter the ABG values above and tap Interpret ABG.
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ABG Study Library

Your supplied Study Library content, integrated with active learning.

📚 ABG Study Library

Select a topic to learn ABG interpretation step-by-step. Each topic keeps the physiology, causes, differential diagnosis, investigations, compensation and practical bedside approach together.

Metabolic Acidosis — Complete Approach

Metabolic acidosis is a primary reduction in bicarbonate associated with a tendency toward acidemia.

1. Recognize it

  • HCO₃⁻ is decreased.
  • pH is usually decreased, unless another alkalinizing process is present.
  • PaCO₂ should fall as respiratory compensation occurs.

2. Calculate the Anion Gap

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

The first major branch point is whether the metabolic acidosis has an elevated or relatively normal anion gap.

High Anion Gap Metabolic Acidosis

Accumulation of unmeasured acids.
Normal Anion Gap Metabolic Acidosis

Usually bicarbonate loss or impaired acid handling with relative chloride retention.

3. High Anion Gap Differential Diagnosis

  • Lactic acidosis — shock, hypoperfusion, severe systemic illness, seizures, medications and other causes.
  • Ketoacidosis — diabetic, alcoholic, starvation and other ketotic states.
  • Kidney failure — impaired renal acid excretion.
  • Toxins and medications — consider the exposure history and targeted toxicology testing when clinically appropriate.

4. Normal Anion Gap Differential Diagnosis

  • Diarrhea and other gastrointestinal bicarbonate losses.
  • Renal tubular acidosis.
  • Hyperchloremic states or large chloride loads.
  • Selected medications and renal disorders.

5. Investigations

  • Na⁺, K⁺, Cl⁻, HCO₃⁻.
  • BUN and creatinine.
  • Glucose.
  • Lactate when lactic acidosis is suspected.
  • β-hydroxybutyrate or ketones when ketoacidosis is suspected.
  • Medication and toxin review.
  • Urine electrolytes and urine pH when renal tubular disease is suspected.

6. Check Respiratory Compensation

Winter's Formula: Expected PaCO₂ ≈ 1.5 × HCO₃⁻ + 8 ± 2
  • PaCO₂ higher than expected → additional respiratory acidosis.
  • PaCO₂ lower than expected → additional respiratory alkalosis.

7. Look for a Mixed Metabolic Disorder

If the anion gap is elevated, compare the increase in the anion gap with the decrease in bicarbonate. A disproportionate relationship can suggest another metabolic process.

Bedside presentation:
“This patient has a metabolic acidosis with a ___ anion gap and ___ respiratory compensation. The leading cause is ___.”
Clinical warning: Severe acidemia, shock, altered mental status, rapidly worsening respiratory status or suspected poisoning requires urgent clinical assessment.

Metabolic Alkalosis — Complete Approach

Metabolic alkalosis is a primary elevation in bicarbonate associated with a tendency toward alkalemia.

Common Causes

  • Vomiting.
  • Nasogastric suction.
  • Diuretic therapy.
  • Volume depletion.
  • Chloride depletion.
  • Hypokalemia.
  • Mineralocorticoid excess.
  • Post-hypercapnic alkalosis.

Useful Clinical Classification

Chloride-responsive

Commonly associated with vomiting, gastric losses and volume depletion.
Chloride-resistant

Consider mineralocorticoid states, ongoing renal potassium loss and other causes.

Investigations

  • Potassium.
  • Chloride.
  • Magnesium.
  • Volume status.
  • Blood pressure.
  • Medication review.
  • Urine chloride when appropriate.

Compensation

PaCO₂ generally rises as bicarbonate rises. If the PaCO₂ is substantially different from the expected response, consider an additional respiratory disorder.

High-yield clue: Metabolic alkalosis + low potassium + low chloride should prompt questions about vomiting, gastric losses and diuretics.

Respiratory Acidosis — Complete Approach

Respiratory acidosis occurs when PaCO₂ rises because effective alveolar ventilation is inadequate.

Common Causes

  • COPD and severe obstructive lung disease.
  • Opioids and sedatives.
  • CNS depression.
  • Neuromuscular weakness.
  • Airway obstruction.
  • Respiratory muscle fatigue.
  • Severe obesity-related hypoventilation.

Acute vs Chronic

Acute

Limited renal compensation. Bicarbonate rises relatively little.
Chronic

Greater renal bicarbonate retention occurs over time.

Investigations

  • Repeat/trend blood gas when clinically indicated.
  • Medication and substance review.
  • Chest imaging when indicated.
  • Electrolytes and renal function.
  • Neurologic or neuromuscular assessment when appropriate.

Assess Severity

  • Mental status.
  • Ability to protect airway.
  • Work of breathing.
  • Oxygenation.
  • Hemodynamic stability.
  • Trend in pH and PaCO₂.
Red flags: worsening hypercapnia with acidemia, deteriorating mental status, inability to protect the airway, severe respiratory distress or hemodynamic instability.

Respiratory Alkalosis — Complete Approach

Respiratory alkalosis occurs when PaCO₂ falls because alveolar ventilation is increased.

Common Differential

  • Hypoxemia.
  • Pulmonary disease.
  • Pulmonary embolism when clinically appropriate.
  • Pneumonia or sepsis.
  • Pain.
  • Fever.
  • Pregnancy.
  • Hepatic disease.
  • CNS disorders.
  • Selected medications or toxins.

Investigations

  • Pulse oximetry and oxygenation assessment.
  • Chest imaging when indicated.
  • Infection evaluation when appropriate.
  • CBC and metabolic panel when clinically useful.
  • Lactate when systemic illness is suspected.
  • Pregnancy assessment when relevant.
  • Targeted PE evaluation based on clinical probability.
Important: Do not automatically label hyperventilation as anxiety. Consider pulmonary, infectious, metabolic and systemic causes first.

Mixed Disorders

Patients can have two or more primary acid–base disorders at the same time.

When Should You Suspect a Mixed Disorder?

  • Compensation is outside the expected range.
  • Anion gap is unexpectedly elevated.
  • Delta analysis does not fit a simple isolated disorder.
  • The clinical presentation suggests multiple processes.
  • pH appears deceptively normal despite markedly abnormal values.

Useful Tools

Winter's Formula

Used in metabolic acidosis to assess respiratory compensation.
Delta Analysis

Compares the change in anion gap with the change in bicarbonate.
Clinical example: A patient with sepsis may have lactic acidosis plus respiratory alkalosis. A patient with COPD may have chronic respiratory acidosis plus a metabolic disorder.

Anion Gap — What It Means

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

The anion gap estimates unmeasured anions in plasma and is particularly useful when metabolic acidosis is present.

Albumin Matters

Albumin is a major unmeasured anion. Low albumin can make the measured anion gap appear deceptively normal.

Delta Analysis

ΔAG = AG − normal AG
ΔHCO₃⁻ = normal HCO₃⁻ − measured HCO₃⁻

Comparing these changes can help identify an additional metabolic process in a patient with high-anion-gap acidosis.

Different references use slightly different normal values and correction conventions. Follow the convention used by your institution or your analyzer.

Oxygenation & Ventilation

Oxygenation

  • Review PaO₂ and SaO₂.
  • Always interpret PaO₂ together with FiO₂.
  • Use the P/F ratio when appropriate.
  • Consider the A–a gradient in appropriate clinical settings.

Ventilation

PaCO₂ reflects alveolar ventilation. Hypercapnia generally indicates inadequate effective ventilation, but severity must be interpreted together with pH, clinical trajectory and patient condition.

Five Bedside Questions

  1. Can the patient protect the airway?
  2. Is mental status deteriorating?
  3. Is work of breathing increasing?
  4. Is oxygenation worsening?
  5. Is the patient hemodynamically unstable?

Investigation Guide

ABG Pattern Investigations to Consider
High AG metabolic acidosis Electrolytes, renal function, glucose, lactate, β-hydroxybutyrate/ketones, medication and toxin review.
Normal AG metabolic acidosis Electrolytes, renal function, gastrointestinal history, urine electrolytes and urine pH when RTA is suspected.
Metabolic alkalosis Potassium, chloride, magnesium, volume status, blood pressure, medication review and urine chloride when appropriate.
Respiratory acidosis Blood gas trend, medication/substance review, chest evaluation, electrolytes and targeted neurologic or neuromuscular assessment.
Respiratory alkalosis Oxygenation assessment, pulmonary evaluation, infection/systemic work-up and targeted testing according to clinical probability.
Golden rule: Don't order every test for every ABG. Let the ABG pattern + history + examination guide the next investigation.

A Simple Learning Pathway

Level 1

pH → PaCO₂ → HCO₃⁻ → primary disorder
Level 2

Learn compensation.
Level 3

Anion gap + albumin correction + delta analysis.
Level 4

Oxygenation + ventilation + respiratory failure.

Where to Learn

  • Residency ICU, pulmonary and nephrology teaching sessions.
  • Standard internal medicine and critical-care textbooks.
  • Nephrology resources for difficult metabolic disorders.
  • Reputable open-access medical education resources.
  • Case-based ABG practice.

The Best Habit

Say your interpretation out loud:

“This ABG shows ___ with ___ compensation, with/without a mixed disorder, and the leading clinical cause is ___.”
Educational content for learning and bedside organization. Always verify formulas, reference ranges and management decisions against current institutional protocols and authoritative clinical resources. This section is not a substitute for supervision or clinical judgment.
CASE-BASED PRACTICE

ABG Case Lab

Work through clinical vignettes before revealing the interpretation and teaching pearl.

INTERACTIVE QUIZ

ABG Quiz Arena

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SPACED RECALL

ABG Flashcards

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