S122 Mathematically Arterialised Venous Blood Gas Sampling in the Management of Patients with Hypercapnic Respiratory Failure

IntroductionArterial blood gas (ABG) sampling is a key aspect of non-invasive ventilation (NIV) therapy. Sampling is painful, though local anaesthesia is rarely used in UK ward-based practice. Capillary (CBG) and venous (VBG) are alternative methods, though limited by reliability and accuracy concerns. National audits show that missed or delayed blood gas sampling can have an adverse impact on patient outcome. A newer method (v-TAC, Roche) is available, based on VBG measurements combined with SpO2. It has shown close agreement for pH and PCO2 for all ranges, and for PO2 values less than 10 kPa. Within an NIV treatment pathway, we have compared ABG sampling with v-TAC, CBG, and VBG.MethodsTime-matched serial ABG, v-TAC, CBG, and VBG samples in adult patients with known or suspected hypercapnic respiratory failure. Bland-Altman agreements, comparing ABG to CBG, VBG, and v-TAC, were performed for individual sampling episodes. The primary outcome was the reliability of the differing methods to detect changes in PaCO2 in response to NIV.Results119 time-matched samples were available for analysis. First-time sampling success was 88% for VBG/v-TAC, 67% for ABG, and 55% for CBG. Bland-Altman analysis for PCO2 and pH showed close agreement for v-TAC with ABG, but not for CBG or VBG. Mean PCO2 bias (SD) was; -0.01 (0.50) kPa for v-„TAC, -0.75 (0.69) kPa for CBG (i.e. CBG reads lower) and +1.00 (0.90) kPa for VBG. Pre and post NIV sampling comparisons were available for 32 subjects for v-TAC, and 23 for CBG. PaCO2 responses to NIV were similar for ABG (0.53kPa), v-TAC (0.55kPa) and VBG (0.49kPa), but lower for CBG (0.16kPa). v-TAC classified the post-NIV improvement or deterioration in PCO2 most accurately (91%, R2= 0.61, figure 1). ABGs were painful; post-hoc analysis suggested that 15% of samples showed a transient change in ventilation at the time of sampling.ConclusionsPaCO2 and pH results were interchangeable between v-TAC and ABG, whereas CBG and VBG showed poor agreement with ABG. These findings challenge some common UK ward-based practices, including use of repeated ABG sampling and CBG sampling as an acceptable surrogate. Please refer to page A212 for declarations of interest related to this abstract.