OBI Medical

Robustness of v-TAC

v-TAC Input Checks

Before the mathematical process is initiated, several checks take place.

First the SpO2 value must be within the range of 80% to 100% and, secondly, the SpO2 value must be greater than the svO2 value minus 4%. As the third step, the venous blood gas values must be physiologically plausible. If any of the checks fail, an error message explaining the cause of the error will appear, and no conversion will take place.

The 4% tolerance on SpO2 is to accommodate the following situation: In patients where the arterial blood flushes through the tissues with very small metabolism, the venous values will be close to arterial values. However, due to tolerance on pulse oximetry and blood gas testing, the SpO2 value measured may be slightly below the svO2.  In such cases, the svO2 value is used for the v-TAC conversion.



Arterio-venous (A-V) difference

When arterial blood flows through the tissues of a peripheral limb, oxygen can be consumed, and acid can be produced in different forms through aerobic and anaerobic metabolism, whereas the reverse process is not physiologically plausible. Therefore, the direction of arterial-venous differences in values is predictable.

It is evident that small A-V differences in saturation level are accompanied by small A-V differences in pH, pCO2 and pO2 and, vice versa, large differences in the A-V saturation level are accompanied by large A-V differences in pH, pCO2 and pO2.

Although the exact A-V differences are very complex and vary significantly from patient to patient, the correlation between the difference in the A-V oxygen saturation and the differences in the A-V pH, pCO2 and pO2 are relatively linear in the individual patient. The physiological model in the v-TAC software accounts for this.

Due to the SpO2 input checks and the design of the v-TAC software, the v-TAC software can only calculate arterial blood gas values of pHa,c to be higher than or equal to the pHv and paCO2,c to be lower than or equal to the pvCO2 and, finally, paO2,c to be higher than or equal to pvO2.

v-TAC​ used on cappillary og arterial blood

If capillary or arterial blood is used as input on purpose or coincidently, the sO2 level measured in the capillary or arterial blood will be very close to or equal the SpO2 level measured by pulse oximetry. As a result, the v-TAC software will present output blood gas values that, in practice, will have only very minor or no corrections made compared to the original values.

Consequences og faulty or inaccurate SpO2 measures

The use of pulse oximetry to estimate the arterial saturation level is known to have a certain patient-to-patient variability. The accuracy of a standard pulse oximeter is typically ±4%, but in clinical praxis it may be as much as 10%. Underestimation of SpO2 is not uncommon, e.g. if the pulse oximeter picks up a poor signal due to poor peripheral perfusion, incorrect positioning of the probe or similar. Another source of error is incorrect entering of the measured SpO2 value on the blood gas analyser.

To illustrate the effect of inaccurate/faulty SpO2 measurements, three examples have been selected based on real patient data.

Example 1:

COPD patient with average A-V difference. SpO2 measured to 88% (slightly overestimated, SaO2=85.3%). White cells show results of SpO2 +/- 5% and 10% simulation. In this example, -10% not possible, due to 80% lower limit.

Example 2:

COPD patient with very small A-V difference. SpO2 measured to 92% (SaO2=92.4%). White cells show results of SpO2 +/- 5% and 10% simulation. In this example, -5% and -10% are not possible, due to venous sO2=90%, and +10% is not possible because it exceeds 100%.

Example 3:

Asthma patient with very large A-V difference (∆pHA-V = 0.063, ∆pCO2A-V = 2.48 kPa). SpO2 measured to 99% (saO2=97.3%). White cells show results of SpO2 +/- 5% and 10% simulation. In this example, +5% and +10% are not possible because they exceed 100%.

The table below illustrates the typical impact of v-TAC calculated results from variation of SpO2.


pHa,c and paCO2,c calculated values are robust to inaccurate/faulty SpO2 input values. The accuracy of paO2,c is dependent on the accuracy of the SpO2 measurement.