OBI Medical

Principles of the method

Peripheral venous blood differs from arterial blood by its content of oxygen and carbon dioxide. Arterial blood values can therefore be calculated by removing CO2 and adding O2 in a constant ratio (RQ) that reflects the metabolism of tissue.

The method simulates addition of oxygen to the peripheral venous blood until simulated oxygen saturation matches that measured non-invasively from a pulse oximeter. The amount of oxygen added in this simulation is then used to calculate the amount of CO2 removed, and as a consequence the complete arterial oxygen and acid-base status can be determined.


The following is a subset of a publication by Rees et al. A method for calculation of arterial acid-base and blood gas status from measurements in the peripheral blood (see Publications)

In order to perform the conversion five steps are carried out:

Step A:

First an anaerobic venous blood sample is drawn and measurements of pHv, pCO2,v, SO2,v, pO2,v, Hbv, Methaemoglobin (MetHbv), and carboxyhaemoglobin (COHbv) are taken to provide a picture of the acid-base status of the peripheral venous blood.

Step B:

The venous measurements pHv, pCO2,v, pO2,v, SO2,v, Hbv, MetHbv, and COHbv are used to calculate the total CO2 concentration (tCO2,v), total O2 concentration (tO2,v), base excess (BEv), and the concentration of 2,3-diphosphoglycerate (DPGv) in the venous blood. This can be performed using mathematical models of the acid base chemistry of blood.

Step C:

Using the variables describing venous blood (tCO2,v; tO2,v; Hbv; BEv; DPGv; tNBBp,v) calculation of the respective variables in arterial blood can now be performed.

Step D:

Calculated values of arterialised blood tCO2,a; tO2,a; Hba; BEa; tNBBp,a and DPGa are then used to calculate the remaining variables describing arterialised blood, i.e. pHa, pCO2,a, pO2,a, and SO2,a, in a reverse of the process described in step B.

Step E:

The calculated arterialised oxygen saturation SO2,a is then compared with that measured by the pulse oximeter (SpO2), the difference between the two giving an error = SO2,a −SpO2. By varying the value of Delta-O2 and repeating steps C to E a value of Delta-O2 can be found for which the error is zero. At this point, the Delta-O2 represents the concentration of O2 added, and RQ multiplied by Delta-O2 the concentration of CO2 removed, so as to transform venous to arterialised blood. For this value of Delta-O2, calculated values of all variables describing arterialised blood (pHa; pCO2,a; pO2,a and SO2,a) should be equal to measured arterial values.


Please read the original and full published article to learn all details of the science behind the method, including sensitivity analysis and much more. Learn more

Please refer to v-TACTM precision to learn more about the accuracy and precision of the method. Learn more