ReAcT CO2: A Study to Assess TcCO2 to Guide Acute NIV

Arterial blood sampling is needed to monitor carbon dioxide and PH but is often painful. The aim of this study is to determine whether continuous carbon dioxide monitoring with a skin probe reduces the need for arterial blood sampling by at least 30%. The investigators will also study the safety and effectiveness of skin probe monitoring to manage non-invasive ventilation (NIV).

Patients with breathing difficulty need arterial blood sampling to check carbon dioxide levels. This is difficult, painful, associated with complications and often met with delays. The investigators have shown in a previous observational study, that using a skin probe to monitor carbon dioxide levels in patients with acute breathing difficulty was far more comfortable, significantly less painful and importantly closely reflected arterial carbon dioxide levels. Decisions regarding breathing support with mask (noninvasive ventilation or NIV) were however based on arterial blood samples collected simultaneously.

In this feasibility study over 72 hours, patients will either have arterial blood sampling or skin probe to monitor carbon dioxide levels.

The aim of the study is to determine whether continuous carbon dioxide monitoring with a skin probe reduces the need for arterial blood sampling by at least 30%. Investigators will also study the safety and effectiveness of skin probe monitoring to manage NIV.

Towards this the investigators will study patients admitted with sudden worsening of breathing that has led to a build up of carbon dioxide. Patients will be recruited within 8 hours, although preferably within 4 hours of starting NIV, preferably in the Emergency Department(ED). Their carbon dioxide levels will then be monitored by skin probe (TcCO2 group) or arterial blood gas (ABG) sampling.

Clinicians can perform ABGs in TcCO2 group at their discretion. Our main end point is the total number of ABGs performed in each group. Clinicians will complete a questionnaire for unscheduled ABGs in TcCO2 group.

The investigators will also measure any side effects, pain experienced by patients in each group and ease of using each technique as well as explore barriers to using transcutaneous monitoring and identify any drawbacks of this method.

Eligibility Criteria:
Inclusion Criteria:

* Patients aged18 years and over with hypercapnia, requiring acute NIV and who are admitted to the Emergency Department or Medical HDU and, who need arterial blood sampling for clinical reasons.
* Recruited ideally within 4 hours (but no later than 8 hours) of initiation of acute NIV in hospital.

Exclusion Criteria:

* Reduced level of consciousness or agitation.
* Metabolic or mixed acidosis, serum bicarbonate \< 24 mmol/L.
* Haemodynamic instability or reduced skin perfusion.
* Patients requiring more than 6litres/min (ormorethan60%) inspired oxygen through ventilator to maintain SpO2 of 88-92%.
* Need for multi-organ support e.g. inotropes or vasopressors or renal replacement therapy
* Arterial pH equal or less than 7.1 or carbon dioxide levels \>15kPa as they are at risk of NIV failure and likely to require mechanical ventilation.
* Participating in any other interventional study