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Henderson-Hasselbalch (VBG pH)

Stable patient, peripheral draw

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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 (VBG pH)Use the calculator below to check your health metrics

Normal VBG

Stable patient, peripheral draw

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Metabolic Acidosis

Low HCO3, respiratory compensation

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Lactic Acidosis

Elevated lactate with HAGMA

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Respiratory Acidosis

Elevated pCO2, metabolic compensation

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Metabolic Alkalosis

Elevated HCO3

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Pre-Dialysis Check

Chronic kidney disease patient

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Clinical Scenarios

Click a scenario to load VBG values:

Normal VBG

Stable patient, peripheral draw

Routine screening - normal values

Metabolic Acidosis

Low HCO3, respiratory compensation

DKA patient presenting to ED

Lactic Acidosis

Elevated lactate with HAGMA

Septic shock patient

Respiratory Acidosis

Elevated pCO2, metabolic compensation

COPD patient with exacerbation

Metabolic Alkalosis

Elevated HCO3

Vomiting with volume depletion

Pre-Dialysis Check

Chronic kidney disease patient

ESRD patient before dialysis session

VBG Values

Venous pCO2
mmHg
Serum bicarbonate
mEq/L
Serum lactate
mmol/L

Electrolytes

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

Clinical Context

VBG Analysis Results

7.34

VBG pH

Normal pH Range

~7.38

Est. Arterial pH

~40

Est. Arterial pCO2

VBG Reliability

95%

Lactate Assessment

1 mmol/L - Normal

Low mortality risk

Normal or Fully Compensated

Compensation: Complete

16

Anion Gap

16.0

Corrected AG

VBG Limitations

  • VBG cannot assess oxygenation - PaO2 requires ABG
  • VBG may miss mild hypercapnia in respiratory disorders
  • Tourniquet time affects peripheral VBG lactate accuracy

VBG Analysis Visualization

Blood Gas Values

VBG Reliability

Lactate Levels

Step-by-Step Calculations

Step 1: Henderson-Hasselbalch (VBG pH)

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

Calculation: pH = 6.1 + log10(24 / (0.03 × 46))

Result: 7.34

Step 2: Estimate Arterial pH

Formula: ext{Arterial} ext{pH} approx ext{VBG} ext{pH} + 0.03 ext{to} 0.05

Calculation: Estimated ABG pH = 7.34 + 0.035

Result: 7.38

Step 3: Estimate Arterial pCO2

Formula: ext{Arterial} pCO2 approx ext{VBG} pCO2 - 6 ext{mmHg}

Calculation: Estimated ABG pCO2 = 46 - 6

Result: 40 mmHg

Step 4: Calculate Anion Gap

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

Calculation: AG = 140 - (100 + 24)

Result: 16 mEq/L

Step 5: 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

VBG vs ABG Comparison

ParameterVBG Typical ValuesABG Typical ValuesDifference
pH7.32-7.427.35-7.45VBG ~0.03-0.05 lower
pCO2 (mmHg)41-5135-45VBG ~3-8 higher
HCO3 (mEq/L)22-2822-28Equivalent
PO2 (mmHg)30-5080-100NOT comparable
Lactate (mmol/L)0.5-2.20.5-2.0Similar

What is Venous Blood pH?

Venous blood pH (VBG) is a measurement of the hydrogen ion concentration in venous blood. While arterial blood gases (ABG) are considered the gold standard for acid-base assessment, VBG offers a less invasive alternative that correlates reasonably well with ABG in many clinical situations.

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Less Invasive

VBG requires only venipuncture, avoiding the pain and complications of arterial puncture. Ideal for screening and serial monitoring.

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Good Correlation

In stable patients, VBG pH correlates well with ABG pH (within 0.03-0.05 units). HCO3 is essentially equivalent between VBG and ABG.

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Key Limitations

Cannot assess oxygenation (PaO2). Less reliable in shock states. May miss subtle pCO2 elevations in respiratory disorders.

How to Interpret VBG Results

VBG Interpretation Approach

  1. 1

    Assess Hemodynamic Status

    VBG-ABG correlation depends on perfusion. In shock, consider ABG.

  2. 2

    Calculate VBG pH

    Use Henderson-Hasselbalch. VBG pH <7.32 suggests acidemia.

  3. 3

    Estimate Arterial Values

    Add ~0.03-0.05 to pH, subtract ~6 from pCO2 for estimates.

  4. 4

    Calculate Anion Gap

    Same calculation as ABG. Elevated AG indicates HAGMA.

  5. 5

    Review Lactate

    VBG lactate is clinically useful. >4 mmol/L indicates significant hypoperfusion.

When to Use VBG vs ABG

VBG is Appropriate When:

  • ✓Screening for metabolic disorders (DKA, renal failure)
  • ✓Monitoring treatment response serially
  • ✓Hemodynamically stable patients
  • ✓Pre-dialysis assessment in ESRD
  • ✓Lactate monitoring in sepsis
  • ✓Pediatric patients (avoid arterial sticks)

ABG is Preferred When:

  • ✗Assessing oxygenation (PaO2, A-a gradient)
  • ✗Respiratory failure assessment
  • ✗Shock or hemodynamic instability
  • ✗Ventilator management decisions
  • ✗ARDS diagnosis/monitoring
  • ✗Precise pCO2 needed (COPD, NIV titration)

Key VBG Formulas

1. Henderson-Hasselbalch (VBG)

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

2. VBG to ABG Conversion

ABG pH ≈ VBG pH + 0.03 to 0.05
ABG pCO2 ≈ VBG pCO2 - 6 mmHg

Less reliable in shock states - differences may be greater

3. Anion Gap

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

Same calculation for VBG as ABG. Normal: 8-12 mEq/L

Frequently Asked Questions

Can VBG replace ABG in all situations?

No. VBG cannot assess oxygenation (PaO2, P/F ratio, A-a gradient). In respiratory failure, shock, or when precise pCO2 is needed, ABG remains essential.

How accurate is VBG for detecting hypercapnia?

VBG pCO2 is typically 3-8 mmHg higher than ABG. A normal VBG pCO2 effectively rules out significant hypercapnia, but mildly elevated VBG pCO2 may not accurately reflect arterial levels.

Is VBG lactate as reliable as ABG lactate?

Yes, VBG lactate correlates well with ABG lactate. It's excellent for initial assessment and serial monitoring in sepsis and other hypoperfusion states.

Does tourniquet time affect VBG results?

Prolonged tourniquet time can falsely elevate lactate and may affect pCO2. Minimize tourniquet time for accurate results. If concerned, use central venous sampling.

What is the role of central venous blood gas?

Central VBG (from CVP line) offers better correlation with ABG than peripheral VBG, especially in unstable patients. It's useful in ICU for monitoring but still can't assess oxygenation.

Clinical Pearls

VBG for Screening

A normal VBG pH (7.32-7.42) and HCO3 effectively rules out significant metabolic acidosis. It's an excellent screening tool.

Lactate Trends Matter

Serial VBG lactate monitoring is valuable in sepsis. Lactate clearance >10% at 6 hours is associated with improved outcomes.

Bicarbonate is Equivalent

VBG HCO3 is essentially identical to ABG HCO3. You can confidently use VBG bicarbonate for metabolic assessment.

Shock Changes Everything

In shock states, VBG-ABG correlation breaks down. Central-peripheral pCO2 gap widens. Get an ABG when hemodynamically unstable.

Related Calculators

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Acid Base Calculator

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Anion Gap Calculator

Serum anion gap analysis

Winters' Formula

Expected pCO2 calculation

Bicarbonate Deficit

HCO3 replacement

Serum Osmolality

Osmolar gap calculation

GFR Calculator

Kidney function assessment

Sodium Correction

Corrected sodium levels

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

🏥 Health Facts

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— CDC

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