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Henderson-Hasselbalch Equation

pH 7.40, pCO2 40, HCO3 24

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Evidence-based calculations Used in clinical settings worldwide Regular monitoring recommended

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Why: This calculation helps assess important health parameters for clinical and personal wellness tracking.

How: Enter your values above and the calculator will apply validated formulas to compute your results.

Evidence-based calculationsUsed in clinical settings worldwide
Sources:Medical LiteratureWHO Guidelines

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Understanding Henderson-Hasselbalch EquationUse the calculator below to check your health metrics

Normal ABG

pH 7.40, pCO2 40, HCO3 24

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Metabolic Acidosis

pH ~7.25, pCO2 25, HCO3 12

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Respiratory Acidosis

pH ~7.28, pCO2 60, HCO3 27

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Metabolic Alkalosis

pH ~7.50, pCO2 48, HCO3 36

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Respiratory Alkalosis

pH ~7.52, pCO2 25, HCO3 20

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Mixed Disorder

pH ~7.15, pCO2 55, HCO3 12

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Clinical Scenarios

Click a scenario to load ABG values:

Normal ABG

pH 7.40, pCO2 40, HCO3 24

Healthy adult on room air

Metabolic Acidosis

pH ~7.25, pCO2 25, HCO3 12

DKA patient with respiratory compensation

Respiratory Acidosis

pH ~7.28, pCO2 60, HCO3 27

COPD exacerbation with acute retention

Metabolic Alkalosis

pH ~7.50, pCO2 48, HCO3 36

Vomiting with volume depletion

Respiratory Alkalosis

pH ~7.52, pCO2 25, HCO3 20

Hyperventilation, early sepsis

Mixed Disorder

pH ~7.15, pCO2 55, HCO3 12

Combined respiratory and metabolic acidosis

Blood Gas Values

Partial pressure of CO2
mmHg
Serum bicarbonate
mEq/L
Partial pressure of O2
mmHg
Fraction of inspired O2
%

Electrolytes

Serum sodium
mEq/L
Serum chloride
mEq/L
For AG correction
g/dL

Clinical Context

ABG Analysis Results

7.40

Calculated pH

Normal pH

16.0

Corrected AG

452

P/F Ratio

Normal or Fully Compensated

Complete

Compensation Assessment

Expected: 22 - 26

✓ Appropriate compensation

Recommendations

  • Repeat ABG in 2-4 hours to assess response to treatment
  • Correlate with clinical picture and other laboratory values

ABG Analysis Visualization

ABG Values

Anion Gap Comparison

Step-by-Step Calculations

Step 1: Henderson-Hasselbalch Equation

Formula: ext{pH} = 6.1 + log10([HCO3-] / (0.03 imes pCO2))

Calculation: pH = 6.1 + log10(24 / (0.03 × 40))

Result: 7.40

Step 2: Calculate Anion Gap

Formula: ext{AG} = ext{Na}+ - ( ext{Cl}- + HCO3-)

Calculation: AG = 140 - (100 + 24)

Result: 16 mEq/L

Step 3: Correct Anion Gap for Albumin

Formula: ext{Corrected} ext{AG} = ext{AG} + 2.5 imes (4 - ext{Albumin})

Calculation: Corrected AG = 16 + 2.5 × (4 - 4)

Result: 16.0 mEq/L

Step 4: A-a Gradient

Formula: A-a = (FiO2 imes 713 - pCO2/0.8) - PaO2

Calculation: A-a = (0.21 × 713 - 40/0.8) - 95

Result: 4.7 mmHg

Step 5: P/F Ratio

Formula: P/F = PaO2 / FiO2

Calculation: P/F = 95 / 0.21

Result: 452

ABG Reference Ranges

ParameterLowNormalHigh
pH<7.35 (Acidemia)7.35-7.45>7.45 (Alkalemia)
pCO2 (mmHg)<3535-45>45
HCO3 (mEq/L)<2222-26>26
PaO2 (mmHg)<6080-100-
Anion Gap (mEq/L)<88-12>12

What is Arterial Blood pH?

Arterial blood pH is a measure of the hydrogen ion concentration in arterial blood and is the cornerstone of acid-base assessment. The pH is calculated from the relationship between carbon dioxide (pCO2) and bicarbonate (HCO3-) using the Henderson-Hasselbalch equation.

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Henderson-Hasselbalch

pH = 6.1 + log([HCO3-] / (0.03 × pCO2)). This equation relates pH to the ratio of metabolic (HCO3) and respiratory (pCO2) components.

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Buffer Systems

The body maintains pH through multiple buffer systems, with the bicarbonate buffer (HCO3-/CO2) being the most important for clinical assessment.

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Dual Regulation

Lungs regulate pCO2 (minutes), kidneys regulate HCO3 (days). This allows compensation for primary disorders in the opposite system.

How to Interpret ABG Results

The 5-Step ABG Interpretation Method

  1. 1

    Assess the pH

    Is it acidemia (<7.35) or alkalemia (>7.45)?

  2. 2

    Identify the Primary Disorder

    Which value (pCO2 or HCO3) explains the pH change?

  3. 3

    Check Compensation

    Is the opposite system compensating appropriately?

  4. 4

    Calculate the Anion Gap

    AG = Na - (Cl + HCO3). Elevated AG indicates HAGMA.

  5. 5

    Calculate Delta Ratio

    Identifies hidden mixed disorders when AG is elevated.

When to Order an ABG

🫁

Respiratory Distress

Acute respiratory failure, COPD exacerbation, asthma, pneumonia assessment.

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Metabolic Emergencies

DKA, HHS, lactic acidosis, suspected toxic ingestion, renal failure.

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Shock States

Septic, cardiogenic, or hemorrhagic shock for acidosis monitoring.

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Drug Overdose

Salicylate, methanol, ethylene glycol, opioids - determines severity.

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Ventilator Management

Assess ventilator settings, weaning readiness, ARDS management.

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Cardiac Arrest

Post-ROSC assessment, guide bicarbonate therapy decisions.

Key ABG Formulas

1. Henderson-Hasselbalch Equation

pH = 6.1 + log([HCO3-] / (0.03 × pCO2))

2. Anion Gap

AG = Na+ - (Cl- + HCO3-)

Normal: 8-12 mEq/L. Corrected AG = AG + 2.5 × (4 - albumin)

3. Winters' Formula (Expected pCO2)

Expected pCO2 = 1.5 × HCO3 + 8 ± 2

For metabolic acidosis - if pCO2 differs, suspect mixed disorder

4. Delta Ratio

Delta Ratio = (AG - 12) / (24 - HCO3)

<1: Mixed HAGMA + NAGMA | 1-2: Pure HAGMA | >2: HAGMA + Met Alkalosis

Frequently Asked Questions

Can I use VBG instead of ABG?

VBG can screen for acid-base disorders. VBG pH is ~0.03-0.05 lower, pCO2 is ~3-8 mmHg higher, and HCO3 is similar. However, ABG is required for accurate oxygenation assessment.

What is a mixed acid-base disorder?

When two or more primary disorders occur simultaneously. Suspected when compensation is inappropriate, when delta ratio is abnormal, or when the clinical picture doesn't match a single disorder.

Why correct the anion gap for albumin?

Albumin is an unmeasured anion. Low albumin falsely lowers the AG, potentially masking HAGMA. Each 1 g/dL decrease in albumin below 4 decreases AG by ~2.5 mEq/L.

How quickly does compensation occur?

Respiratory compensation for metabolic disorders: hours. Metabolic compensation for respiratory disorders: days (acute = minimal, chronic = full). Full compensation never normalizes pH completely.

What does the P/F ratio tell us?

PaO2/FiO2 ratio assesses oxygenation efficiency. Normal >400. ARDS criteria: mild 200-300, moderate 100-200, severe <100. It standardizes oxygenation across different FiO2 levels.

Clinical Pearls

pH Rules the Diagnosis

Always start with pH. It tells you acidemia vs alkalemia. The body never overcompensates - if pH is normal with abnormal values, think mixed disorder.

AG Always Indicates Acidosis

An elevated anion gap always means metabolic acidosis is present, even if the pH is normal or alkalemic (mixed disorder).

MUDPILES for HAGMA

Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates. This mnemonic covers major HAGMA causes.

Critical pH Values

pH <7.2 or >7.55 requires urgent intervention. Severe acidemia impairs cardiac function; severe alkalemia causes arrhythmias and tetany.

Related Calculators

Acid Base Calculator

Complete ABG interpretation

Anion Gap Calculator

Serum anion gap analysis

Winters' Formula

Expected pCO2 calculation

Venous Blood pH

VBG pH calculation

Bicarbonate Deficit

HCO3 replacement

Serum Osmolality

Osmolar gap calculation

Urine Anion Gap

NAGMA differentiation

GFR Calculator

Kidney function assessment

For informational purposes only — not medical advice. Consult a healthcare professional before acting on results.

🏥 Health Facts

— WHO

— CDC

HI THERE
👋Henderson-Hasselbalch Equation is a health metric used in clinical and wellness assessments to support informed decision-making.
WHY IT MATTERS
💡This calculation helps assess important health parameters for clinical and personal wellness tracking.
HOW TO USE
🎯Enter your values above and the calculator will apply validated formulas to compute your results.
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