Validity and clinical use of mathematical arterialized venous blood gas with the v-TAC approach for evaluation of arterial blood gas in patients with respiratory compromise

Introduction:

Arterial Blood Gas (ABG) is gold standard to assess acid-base and blood gas status for patients with severe respiratory compromise, such as COPD patients during exacerbation, to assess progression of illness and response to treatment, using pH, pCO₂ and pO₂ as the measure. Capillary blood gas (CBG) is widely accepted as a more patient friendly and less invasive substitute for ABG.

A new method, named v-TAC, has become available for clinical use. v-TAC calculates arterial acid-base and blood gas values from a peripheral venous blood gas (VBG) measurement, combined with an oxygen saturation measurement (SpO₂) from a pulse oximeter.

The purpose of this project is to study if the v-TAC method is a potential substitute for ABG and CBG for patients with respiratory compromise in an internal medicine department.

Methods:

We performed a prospective study with a total of 93 patients with severe respiratory failure. 32 were undergoing ventilation treatment in the ICU, of them 26 were intubated. 61 were selected from the ward. An ABG and a VBG with corresponding oxygen sat measurement were taken simultaneously as convenience samples. For comparison, and in combination with the ABG and VBG, a CBG was also measured on 37 patients from the ward. All samples were analysed as quickly as possible on the same blood gas analyser. The results were analysed using Bland-Altman plots and statistical methods.

Results:

R², Bias and ± 95% Limits of agreement for v-TAC vs. CBG respectively was: pH 92.4%, 0.004 ± 0.036 vs. 79.3%, -0.003 ± 0.042; pCO₂ 95%, -1.16 ± 4.64 mmHg vs. 89.3%, 0.02 ± 4.75 mmHg; pO₂ N/A, 0.69 ± 12.7 mmHg vs. N/A, 5.8 ± 17.3 mmHg.

Fig. 1: For pO₂: Samples where ABG and v-TAC agrees that pO₂ is greater than 75 mmHg (n = 40) are not shown and not included in calculation of v-TAC bias and CI. Samples where v-TAC reports pO₂ greater than 75 mmHg and ABG reports pO₂ less than or equal to 75 mmHg (n = 6) are shown separately (x).

For pH and pCO₂, the performance of v-TAC and CBG vs. ABG was comparable. For pO₂, the v-TAC 95% limits of agreement was marginally better compared to CBG performance, and with bias close to zero.

The results are in line with results from earlier studies comparing ABG with v-TAC, and ABG with CBG.

Conclusions:

v-TAC can be used as a substitute for arterial and capillary blood gas for patients with respiratory compromise. As venous blood is usually easier accessible, compared to arterial and arterialized capillary blood, v-TAC may potentially lead to workflow improvements and better patient comfort.