Calculating arterial blood gas values from venous blood gas values
The intention of this website is to provide important scientific information about and around v-TAC.
The sections below cover the following areas:
The v-TAC software calculates arterial blood gas values based on peripheral venous blood gas measurements and an arterial oxygen saturation (SpO2) measurement using pulse oximetry.
The v-TAC software arterialises peripheral venous blood gas values by mathematically adding O2 and removing CO2 using advanced physiological acid-base and oxygenation models, until the calculated oxygen saturation equals the arterial oxygenation (SpO2) measured by pulse oximetry.
The v-TAC software and method is based on research from Aalborg University, Denmark.The following three publications explain the background of the v-TAC software.
Rees SE et al. (2010). Mathematical modelling of the acid-base chemistry and oxygenation of blood: a mass balance, mass action approach including plasma and red blood cells. Eur J Appl Physiol 108: 483-494.
Rees SE et al. (2006). A method for calculation of arterial acid-base and blood gas status from measurements in the peripheral venous blood. Comput Meth Prog Bio 81(1): 18-25.
Rees SE, Andreassen S. (2005). Mathematical models of oxygen and carbon dioxide storage and transport: The acid-base chemistry of blood. Crit Rev Biomed Eng 33(3): 209-264.
The v-TAC software and method may be beneficial in many clinical situations and for many different departments, particularly when arterial lines are not available or their use is limited or not desired.
Hospital departments & clinical situations where v-TAC may improve patient care
Patients screening in the emergency department
The blood sample for venous blood gas can be drawn in combination with other regular blood tests, thus obviating arterial punctures and reducing time to care while not compromising on quality of information gained.1
Blood gas testing in cardio-respiratory and other wards
Using venous blood and v-TAC may ease the access to blood gas testing when arterial punctures are problematic or qualified resources are unavailable or limited. v-TAC can enable a task-transfer from medical doctors to other staff groups such as nurses, which may improve workflow in the wards. v-TAC can enhance nurse-autonomy and up-scale nurse-led care, where nurses take over more responsibility. This may improve compliance with guidelines and outcomes and reduce length-of-stay.2
Monitoring of patients response to treatment
v-TAC can reduce the need for repeated arterial punctures, frequent in certain treatments such as Non-Invasive Ventilation (NIV) and may improve timely monitoring of this patient group.1,2,3
Reducing use of arterial lines in the intensive care unit
v-TAC may reduce the use of arterial lines and thereby may reduce the risk of well-known side effects such as injuries to the arteries.4
By reducing the use of arterial lines, v-TAC may contribute to improved patient mobility.5
1. Ekström M et al. (2019). PLoS ONE 14(4): e0215413. doi:10.1371/journal.pone.0215413
2. Kamperidis P et al. (2018). BMJ Thorax doi:10.1136/thorax-2018-212555.429
3. Rees SE et al. (2012). Clin Chem Lab Med 50(12): 2149-2154. doi:10.1515/cclm-2012-0233
4. Low LL et al. (1995). Chest 108: 216-219. doi:10.1378/chest.108.1.216
5. Perme C et al. (2013). Cardiopulm Phys Ther J 24(2): 12-17. doi:10.1097/01823246-201324020-00003”
The v-TAC software and method can help to reduce the need for arterial blood gas sampling and improve the quality of information that can be gained from venous blood gases.
Repeated arterial punctures are often required to monitor response to a treatment, such as non-invasive ventilation.
This video addresses the use of venous blood and v-TAC to monitor pCO2 trends and the limitations of venous blood alone.
Weber M, Cave G. (2021). BMJ Open Resp Res 8: e000896. doi:10.1136/ bmjresp-2021-000896
Schütz N et al.(2019) Open Access Emerg 11: 305–312. doi:10.2147/OAEM.S228420
A study from 2022 by Dr Michael G. Davies, lead consultant at the Royal Papworth Hospital, Cambridge, UK, was investigating “Use of mathematically arterialised venous blood gas sampling: Comparison with arterial, capillary, and venous sampling”.
The first results from this study were presented at the European Respiratory Society 2022 congress and included a comparison of the sample failure rate (due to missed sample or patient refusal) of 2% for venous (v-TAC) blood sampling, compared to 13% for arterial sampling and 21% for capillary samplin.
Venous (v-TAC) Blood Sampling
2% Sample failure or patient refusal
88% Success rate in 1st attempt
8% Success rate in 2nd attempt
2% Success rate in 3rd or more attempt
Arterial Blood Sampling
13% Sample failure or patient refusal
67% Success rate in 1st attempt
15% Success rate in 2nd attempt
5% Success rate in 3rd or more attempt
Capillary Blood Sampling
21% Sample failure or patient refusal
55% Success rate in 1st attempt
19% Success rate in 2nd attempt
5% Success rate in 3rd or more attempt
Davies MG et al. (2022). Poster ERS 2022.
https://medically.roche.com/global/en/restricted/respiratory/ERS-2022/ers-2022-poster-mike-use-of-mathematically-arterialised.html [Last Accessed February 2023]
v-TAC in clinical practice
It is very simple to operate v-TAC in daily clinical practice. The software works seamlessly with existing blood gas analysers. After implementation, the operational workflow is as follows:
First, measure the arterial oxygen saturation (SpO2) with a pulse oximeter, subsequently, draw an anaerobic peripheral venous blood sample in a blood gas syringe.
To start the analysis, insert the venous blood sample into the blood gas analyser and enter the measured SpO2 value. The exact operation of the blood gas analyzer depends on the analyzer brand and type.
Before using v-TAC please consult the v-TAC Standalone User Guide for complete information.
The v-TAC Standalone software application is a server-based application that works seamlessly with existing blood gas analyzers from leading manufacturers.
The v-TAC Standalone software is not installed on the blood gas analyzer itself. Instead, the v-TAC software is a standalone software application designed to run on a small footprint Windows virtual server or a dedicated computer in the hospital’s IT environment.
A single instance of the v-TAC Standalone software can service all blood gas analysers in the hospital.
During normal operation, v-TAC Standalone software operates seamlessly as a black-box and does not require any attention or application management
Intended Use of the v-TAC Standalone software
v-TAC Standalone is an in vitro diagnostic medical device software intended to automatically convert peripheral venous blood gas values (pHv, pvO2, pvCO2) in combination with venous oximetry values (SvO2, tHbv, MetHbv, COHbv) and an arterial saturation value (SpO2a) by pulse oximetry, to quantitatively estimate arterial blood gas values (paO2, paCO2, pHa).
v-TAC Standalone is an aid for the calculation of the arterial blood gas values in hemodynamically stable adult patients (age 18 and above).
v-TAC Standalone is intended to be used with Blood gas analyzers that meet the acceptance criteria for analytical performance and functional requirements defined by Roche and Pulse oximeters certified according to ISO 80601-2-61.
v-TAC is intended to be used by healthcare professionals in a near patient testing environment and laboratory. Not for self-testing.
For further Safety information and List of limitations and contraindications, please consult the v-TAC Standalone User Guide.
The software is CE-marked as an IVDD product and available in countries accepting the CE-mark and with support for the local language(s).
Download PDF documents of the v-TAC Standalone software in the following languages:
Multiple studies have evaluated v-TAC performance by comparing v-TAC calculated arterial values with the gold standard arterial blood gas. Some studies also collected a second arterial sample (ABG2) immediately after the VBG sample. The purpose was to evaluate the repeatability of arterial blood gases in clinical practice.
The study population included a broad range of patients from emergency departments, pulmonary departments and intensive care units with various diagnoses, including COPD, sepsis, asthma, pneumonia and lung cancer.
v-TAC was validated on hemodynamically stable patients treated with, e.g. non-invasive ventilation (NIV) and oxygen therapy.
The typical approach was to collect a reference sample (ABG1) and simultaneously measure the arterial oxygen saturation (SpO2) using a pulse oximeter and collect a peripheral venous sample. In clinical practice, it is not possible to do the steps simultaneously, so typically SpO2 is measured first, then the arterial sample is collected, and as soon as possible after that the venous (VBG) sample is collected. In practice, this means the VGB sample is often collected 1-5 minutes after the SpO2 measurement and in some cases after more time has elapsed.
The repeatability of both arterial blood gas measurements and venous blood gas measurements is affected by pre-analytical errors in the time span from taking to analyzing the blood sample, and by analytical errors. Additionally, both arterial blood gas and venous blood gas are affected by biological fluctuations. When comparing two subsequent measurements, the biological change (e.g. hypo- or hyperventilation) has an impact on the result.
See separate section about the repeatability of arterial blood gas.
This publication validated v-TAC arterialized VBGs (v-TAC) against arterial blood gas analysis (ABG) for measuring pH, pCO2 and pO2 in a Swedish single center study. 87 paired sets of ABGs and VBGs with SpO2 from 46 stable inpatients in general respiratory and internal medicine wards with mainly COPD exacerbation, heart disease or bacterial infection were included.
The conclusions were: Calculated arterial blood gases (v-TAC) from a venous sample and pulse oximetry were comparable to ABG values and may be useful for evaluation of blood gases in clinical settings. This could reduce the logistic burden of arterial sampling, facilitate improved screening and follow-up and reduce patient pain.
Ekström M et al. (2019). PLoS ONE 14(4): e0215413. doi:10.1371/journal.pone.0215413
This study evaluated v-TAC arterialized VBGs (v-TAC) in patients in the ICU and patients on the pulmonary medical ward. v-TAC, peripheral venous (VBG) and capillary samples (CBG) from an earlobe warmed with vasodilating cream were compared to arterial blood gas analyses (ABG). Paired arterial and peripheral venous blood samples were obtained from 37 patients with various diagnoses in the ICU and from 63 patients in the pulmonal ward. From 39 patients in the ward, simultaneous capillary samples had been taken in addition.
The conclusions were:
Mathematical arterialization functions well in a range of patients in an ICU and ward outside the country of development of the method. Furthermore, accuracy and precision are similar to capillary blood samples. When considering a replacement for arterial sampling in ward patients, using capillary sampling or mathematical arterialization should depend on logistic ease of implementation and use rather than improved measurements of using either technique.
Thomsen LP et al. (2021). Respir 100(2): 164-172. doi:10.1159/000512214
Davies MG et al. (2022). Use of mathematically arterialised venous blood gas sampling: Comparison with arterial, capillary, and venous sampling. Poster ERS 2022.
Shastri L et al. (2021). Mathematically arterialised venous blood is a stable representation of patient acid–base status at steady state following acute transient changes in ventilation. J Clin Monit Comput
Weber M, Cave G. (2021). Trending peripheral venous pCO2 in patients with respiratory failure using mathematically arterialised venous blood gas samples. BMJ Open Resp Res 8: e000896.
Thomsen LP et al. (2021). Evaluation of Mathematical Arterialization of Venous Blood in Intensive Care and Pulmonary Ward Patients. Respir 100(2): 164-172.
Ekström M et al. (2019). Calculated arterial blood gas values from a venous sample and pulse oximetry: Clinical validation. PLoS ONE 14(4): e0215413.
Lumholdt M et al. (2019). Mathematical arterialisation of peripheral venous blood gas for obtainment of arterial blood gas values: a methodological validation study in the clinical setting. J Clin Monit Comput 33: 733–740.
Lumholdt M et al. (2018). Can routine blood gas screening identify patients with unsuspected acid-base conditions and lead to optimised triage group allocation? BMJ Open 8(Suppl 1): A1-A34.
Kamperidis P et al. (2018). Optimising Acute Non-Invasive Ventilation Care in the NHS; the v-TAC approach. BMJ Thorax doi:10.1136/thorax-2018-212555.429
Manuel A et al. (2017). A method for calculation of arterial blood gas values from measurements in the peripheral blood (v-TAC): The first UK study. Eur Resp J 50: PA4734.
Kelly AM et al. (2013). Agreement between mathematically arterialised venous versus arterial blood gas values in patients undergoing non-invasive ventilation: a cohort study. Emerg Med J 31:e46-e49.
Rees SE et al. (2012). Calculating acid-base and oxygenation status during COPD exacerbation using mathematically arterialised venous blood. Clin Chem Lab Med 50(12): 2149-2154.
Tygesen G et al. (2012). Mathematical arterialization of venous blood in emergency medicine patients. Eur J Emerg Med 19:363–372.
Oddershede L et al. (2011). The cost-effectiveness of venous-converted acid-base and blood gas status in pulmonary medical departments. ClinicoEconomics Outcomes Res (3): 1-7.
Rees SE et al. (2009). Converting venous acid-base and oxygen status to arterial in patients with lung disease. Eur Respir J (26): 1141-1147.
Toftegaard M et al. (2009). Evaluation of a method for converting venous values of acid-base and oxygenation status to arterial values. Emerg Med J (26): 268-272.
In pilots and clinical studies using measured arterial blood gas as the reference, take note of the following recommendations and precautions:
Collect arterial and venous blood samples simultaneously.
Ensure high quality in sample collection.
Exclude samples with signs of pre-analytical errors.
Ensure patient ventilatory stability before and during sample collection.
Use proper statistical methods to evaluate the results.
If the data is entered manually, verify the data carefully for correctness before analysis.
A frequently asked and relevant question is how sensitive v-TAC is to inaccurately measured SpO2.
General notes about pulse oximetry:
The use of pulse oximetry to estimate the arterial saturation level has a certain patient-to-patient variability. To receive ISO 80601-2-61 certification, pulse oximeters must have a performance of ±4%, but in clinical practice 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.
Summary about v-TAC sensitivity to in-accurately measured SpO2
For pH and pCO2, simulations indicate that SpO2 changes of ±10% will cause pH to change in a range of 0.02 pH units and pCO2 to change in a range of 3.5 mmHg. v-TAC will, to a high degree, correct for the arterial-to-venous difference caused by aerobic metabolism.
For pO2, the accuracy of the calculated pO2 depends on the accuracy of the SpO2 measurement and the specific SpO2 value where pO2 is less sensitive to inaccurate SpO2 values from approximately 95% and below and more sensitive to inaccurate SpO2 values from approximately 96% and above.
See Explainer videos below for more details.
v-TAC Standalone software version 1.5 User Guide
To evaluate if the v-TAC performance is acceptable to be a potential substitute for arterial blood gas testing in certain patient groups, it is important to understand the repeatability of arterial blood gas tests. We found three studies that investigated this question:
Common for all three studies by Mallat, Toftegaard and Daher is that the data was collected from patients with an indwelling arterial catheter, removing risk of collecting arterial blood with a mix of venous blood.
95% Limits of Agreement of repeated arterial blood gas tests compared to v-TAC performance:
*Two arterial samples drawn immediately after each other from an indwelling catheter
**Up to 75 mmHg
Conclusion: The v-TAC performance studies show that the performance of v-TAC versus a reference arterial blood gas test is comparable to the repeatability of consecutive arterial blood gas tests.
Mallat J et al. (2015). Medicine 94(3): e415 doi:10.1097/MD.0000000000000415
Toftegaard M et al. (2009). Emerg Med J (26): 268-272. doi:10.1136/emj.2007.052571
Daher A et al. (2022). Respiration. doi:10.1159/000524491
v-TAC Standalone software version 1.5 User Guide
A study by Shastri L et al. from 2021, shows how arterial acid-base values change more rapidly than venous acid-base values with fluctuating breathing patterns.
The conclusion was: “This study reaffirms the reliability of the physiology based mathematical model for transformation of venous blood acid–base status to arterialised equivalents. In addition, it also shows that arterialised venous blood is a more stable representative of steady state arterial blood gas values following acute, transient changes in ventilation”.
Shastri L et al. (2021). J Clin Monit Comput doi:10.1007/s10877-021-00764-3
Other articles supporting the v-TAC software and method
Shastri L et al. (2021). Is venous blood a more reliable description of acid-base state following simulated hypo- and hyperventilation? Scand J Trauma Resusc 29: 35.
Thomsen LP et al. (2020). Arterial and transcutaneous variability and agreement between multiple successive measurements of carbon dioxide in patients with chronic obstructive lung disease. Respir Physiol Neurobiol 280: 103486.
Schütz N et al.(2019) Can venous blood gas be used as an alternative to arterial blood gas in intubated patients at admission to the emergency department? A retrospective study. Open Access Emerg 11: 305–312.
McKeever TM et al. (2016). Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax 71: 210–215.
Mallat J et al. (2015). Repeatability of Blood Gas Parameters, pCO2 Gap, and pCO2 Gap to Arterial-to-Venous Oxygen Content Difference in Critically Ill Adult Patients. Medicine 94(3): e415.
Byrne AL et al. (2014). Peripheral venous and arterial blood gas analysis in adults: are they comparable? A systematic review and meta-analysis. Respirology 19: 168–175.
Kelly AM. (2013). Agreement between arterial and venous blood gases in emergency medical care: a systematic review. Hong Kong J Emerg Med 21: 166-171.
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