Comparison of mathematically arterialised venous blood gas sampling with arterial, capillary, and venous sampling in adult patients with hypercapnic respiratory failure: a single-centre longitudinal cohort study.

BACKGROUND: Accurate arterial blood gas (ABG) analysis is essential in the management of patients with hypercapnic respiratory failure, but repeated sampling requires technical expertise and is painful. Missed sampling is common and has a negative impact on patient care. A newer venous to arterial conversion method (v-TAC, Roche) uses mathematical models of acid-base chemistry, a venous blood gas sample and peripheral blood oxygen saturation to calculate arterial acid-base status. It has the potential to replace routine ABG sampling for selected patient cohorts. The aim of this study was to compare v-TAC with ABG, capillary and venous sampling in a patient cohort referred to start non-invasive ventilation (NIV). METHODS: Recruited patients underwent near simultaneous ABG, capillary blood gas (CBG) and venous blood gas (VBG) sampling at day 0, and up to two further occasions (day 1 NIV and discharge). The primary outcome was the reliability of v-TAC sampling compared with ABG, via Bland-Altman analysis, to identify respiratory failure (via PaCO2) and to detect changes in PaCO2 in response to NIV. Secondary outcomes included agreements with pH, sampling success rates and pain. RESULTS: The agreement between ABG and v-TAC/venous PaCO2 was assessed for 119 matched sampling episodes and 105 between ABG and CBG. Close agreement was shown for v-TAC (mean difference (SD) 0.01 (0.5) kPa), but not for CBG (-0.75 (0.69) kPa) or VBG (+1.00 (0.90) kPa). Longitudinal data for 32 patients started on NIV showed the closest agreement for ABG and v-TAC (R2=0.61). v-TAC sampling had the highest first-time success rate (88%) and was less painful than arterial (p<0.0001). CONCLUSION: Mathematical arterialisation of venous samples was easier to obtain and less painful than ABG sampling. Results showed close agreement for PaCO2 and pH and tracked well longitudinally such that the v-TAC method could replace routine ABG testing to recognise and monitor patients with hypercapnic respiratory failure. TRIAL REGISTRATION NUMBER: NCT04072848; www. CLINICALTRIALS: gov.

Postpartum pneumomediastinum manifested by surgical emphysema. Should we always worry about underlying oesophageal rupture?

Spontaneous pneumomediastinum during labour is a rare, usually benign and self-limiting condition. It often presents with chest or neck pain and surgical emphysema. The latter sign is easy to demonstrate but often missed during clinical assessment if the condition is not included in the differential diagnosis of chest pain and dyspnoea in peripartum. The authors describe a case of 20-year-old primigravida who developed surgical emphysema following prolonged vaginal delivery. The chest x-ray revealed pneumomediastinum, and small left apical pneumothorax. She was investigated with CT of the chest and contrast swallow, both of which excluded oesophageal perforation. The management was conservative and she made a complete recovery. Spontaneous oesophageal rupture is a potential cause of pneumomediastinum and leads to high morbidity and mortality if not diagnosed early. However, it is extremely uncommon in labour, especially without a preceding history of vomiting. Unless a strong clinical suspicious exists, routine investigations and or treatment of suspected oesophageal perforation are unnecessary.