v-TAC Standalone software

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 modeling 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. doi:10.1007/s00421-009-1244-x
• 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. doi:10.1016/j.cmpb.2005.10.003
• 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. doi: 10.1615/CritRevBiomedEng.v33.i3.10

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. Patients receiving treatments, for example non-invasive ventilation (NIV) or oxygen therapy, were also studied.

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.

Highlighted publications:

• Shastri L et al. The use of venous blood gas in assessing arterial acid-base and oxygenation status - an analysis of aggregated data from multiple studies evaluating the venous to arterial conversion (v-TAC) method. Expert Rev Respir Med. 2024 July; 18(7):553-559.
• Davies M et al. (2023). BMJ Open Respir Res 10:e001537. doi:10.1136/bmjresp-2022-001537
• 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. doi:10.1007/s10877-021-00764-3.
• 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. doi:10.1136/ bmjresp-2021-000896.
• Thomsen LP et al. (2021). Evaluation of Mathematical Arterialization of Venous Blood in Intensive Care and Pulmonary Ward Patients. Respir 100(2): 164-172. doi:10.1159/000512214.
• Ekström M et al. (2019). Calculated arterial blood gas values from a venous sample and pulse oximetry: Clinical validation. PLoS ONE 14(4): e0215413. doi:10.1371/journal.pone.0215413.

Additional publications:

• 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. doi:10.1007/s10877-018-0197-1.
• 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. doi:10.1136/10.1136/bmjopen-2018-EMS.65. 
• 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. doi:10.1183/1393003.congress-2017.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. doi:10.1136/emermed-2013-202879.
• 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. doi:10.1515/cclm-2012-0233.
• Tygesen G et al. (2012). Mathematical arterialization of venous blood in emergency medicine patients. Eur J Emerg Med 19:363-372. doi:10.1097/MEJ.0b013e32834de4c6.
• 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. doi:10.2147/CEOR.S14489.
• 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. doi:10.1183/09031936.00140408.
• 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. doi:10.1136/emj.2007.052571.

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 analyzers in the hospital.

During normal operation, v-TAC Standalone software operates seamlessly as a background process and does not require any attention or application management

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

General notes about pulse oximetry:¹⁰

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. 

Summary about v-TAC sensitivity to inaccurately measured SpO₂:¹⁰

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.

To evaluate whether 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 comparability of two repeated arterial blood gas tests. We found three studies that investigated this question.^11-13

Common for all three studies by Mallat, Toftegaard, and Daher is that the data were collected from patients with an indwelling arterial catheter, removing the risk of collecting arterial blood with a mix of venous blood.

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.

A study by Shastri et al. from 2021 shows how arterial acid-base values change more rapidly than venous acid-base values with fluctuating breathing patterns.¹

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). Is venous blood a more reliable description of acid-base state following simulated hypo- and hyperventilation? Scand J Trauma Resusc 29: 35. doi: 10.1186/s13049-021-00848-8
• 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. doi: 10.1016/j.resp.2020.10348
• 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. doi:10.2147/OAEM.S228420
• McKeever TM et al. (2016). Using venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax 71: 210–215. doi:10.1136/thoraxjnl-2015-207573
• 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. doi:10.1097/MD.0000000000000415
• 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. doi:10.1111/resp.12225
• 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. doi: 10.1177/102490791302000307