Venous Blood pH Calculator
Enter the bicarbonate (HCO3) and venous carbon dioxide partial pressure (PvCO2) to calculate venous blood pH using the Henderson-Hasselbalch equation. Optionally enter the measured venous pH to classify the acid-base disorder and estimate the corresponding arterial values. Results update as you type.
Formula
Worked example
A patient has HCO3 of 24 mEq/L and PvCO2 of 46 mmHg. Dissolved CO2 = 0.0308 x 46 = 1.4168 mmol/L. Ratio = 24 / 1.4168 = 16.94. pH = 6.1 + log10(16.94) = 6.1 + 1.229 = 7.329, in the mildly acidotic range. Estimated arterial pH = 7.329 + 0.03 = 7.359, within the arterial normal range (7.35-7.45).
How venous blood pH is calculated
Venous blood pH is estimated from the Henderson-Hasselbalch equation: pH = 6.1 + log10([HCO3] / (0.0308 x PvCO2)). The value 6.1 is the pKa of carbonic acid in plasma, and 0.0308 is the solubility coefficient of CO2 in plasma at body temperature (in mmol/L per mmHg). Bicarbonate (HCO3) is reported in mEq/L, which is numerically identical to mmol/L for a monovalent ion. PvCO2 in mmHg is equal in value to torr. Together, these two measured quantities define the acid-base state of the blood through the bicarbonate buffer system, which is the dominant buffer in human plasma.
VBG versus ABG: what each measures and when VBG is used
An arterial blood gas (ABG) requires an arterial puncture, which is more uncomfortable and carries a small risk of arterial injury. A venous blood gas (VBG) is drawn from any peripheral vein, making it faster and easier to obtain. Because pH and bicarbonate are nearly identical between venous and arterial blood under stable conditions, VBG is a validated screening tool for acid-base disorders, diabetic ketoacidosis, and hypercarbia. The key limitation is that VBG does not assess oxygenation: venous PO2 reflects oxygen consumption by tissues, not lung function. The population-average offsets (add 0.03 to venous pH, subtract 5 from PvCO2, subtract 1.5 from HCO3) give reasonable arterial estimates, but individual variation can be substantial in haemodynamically unstable patients, so ABG remains the reference standard when accurate oxygenation data are needed.
Interpreting acid-base disorders from VBG
Systematic VBG interpretation follows three steps. First, look at the pH: below 7.31 is acidosis, above 7.41 is alkalosis. Second, identify the primary driver: if pH is low and PvCO2 is high, the primary disorder is respiratory acidosis; if pH is low and HCO3 is low, the primary disorder is metabolic acidosis (the reverse applies for alkalosis). Third, assess compensation: the body responds to a primary acid-base problem with a secondary change in the other component, and expected compensation can be estimated using standard formulas. When both PvCO2 and HCO3 are abnormal in opposite directions, a mixed disorder is present. A normal pH with both parameters out of range suggests a fully compensated or mixed process.
Clinical uses and limitations of the Henderson-Hasselbalch equation
The Henderson-Hasselbalch equation is the cornerstone of acid-base chemistry in clinical medicine. It applies to the carbonate buffer system: CO2 + H2O forms H2CO3 (carbonic acid), which dissociates into H+ and HCO3. Because the pKa of this system (6.1) is not close to physiological pH, the buffer pair is most effective at the bicarbonate concentrations found in plasma, where the ratio [HCO3] / [H2CO3] is typically near 20, giving a pH near 7.4. The equation assumes steady-state conditions and standard body temperature; it is less accurate in severe hypothermia or extreme dysproteinaemia. For everyday clinical acid-base assessment it is highly reliable.
Normal venous blood gas reference ranges
| Parameter | Venous (VBG) | Arterial (ABG) | Unit |
|---|---|---|---|
| pH | 7.31-7.41 | 7.35-7.45 | |
| PCO2 | 41-51 | 35-45 | mmHg |
| HCO3 | 22-29 | 22-28 | mEq/L |
| Base excess | -2 to +2 | -2 to +2 | mEq/L |
Normal values for peripheral venous blood gas in adults. Venous values differ from arterial values by predictable population-average offsets.
Frequently asked questions
What is the normal pH range for venous blood?
The normal range for peripheral venous blood pH is 7.31 to 7.41, compared with 7.35 to 7.45 for arterial blood. Venous blood is slightly more acidic because it carries the CO2 and waste acids produced by metabolising tissues back to the lungs.
Why is venous pH lower than arterial pH?
As blood passes through the tissues it picks up CO2 produced by cellular metabolism. More dissolved CO2 shifts the bicarbonate buffer equilibrium toward more carbonic acid and more hydrogen ions, lowering pH. The typical venous-to-arterial pH difference is about 0.02 to 0.05 pH units; the average is around 0.03, which is why adding 0.03 to venous pH gives a reasonable estimate of arterial pH.
Can a VBG replace an ABG?
For acid-base assessment only, a VBG is usually sufficient. Studies show that VBG pH correlates closely with ABG pH across most clinical scenarios. However, VBG cannot reliably assess oxygenation because venous PO2 reflects tissue oxygen consumption rather than pulmonary gas exchange. ABG remains necessary when precise oxygenation data are required, for example in respiratory failure, mechanical ventilation management, or carbon monoxide poisoning.
What does a pH below 7.31 mean on a VBG?
A venous pH below 7.31 indicates acidosis. The next step is to identify whether the cause is respiratory (elevated PvCO2), metabolic (low HCO3), or a combination of both. Mild acidosis (7.20 to 7.31) warrants urgent evaluation; severe acidosis (below 7.20) is a medical emergency requiring immediate investigation and treatment.
Are mEq/L and mmol/L the same for bicarbonate?
Yes. Bicarbonate (HCO3) is a monovalent anion, meaning one mEq equals one mmol. Bicarbonate values are interchangeable between the two units at any concentration, so you can use values reported in either unit without conversion.
What are the VBG-to-ABG conversion offsets and how reliable are they?
The population-average offsets are: arterial pH = venous pH + 0.03; arterial PaCO2 = PvCO2 - 5 mmHg; arterial HCO3 = venous HCO3 - 1.5 mEq/L. These work well in haemodynamically stable patients. In low-flow states such as septic shock or cardiac arrest, the venous-arterial CO2 gap widens considerably and these offsets can significantly underestimate arterial PaCO2.