P39 Noninvasive pH with Transcutaneous PCO2monitoring as an alternative to arterial line sampling: a new patient friendly approach to monitoring Acute NIV

Arterial blood gas measurement is a standard way to initiate and monitor Noninvasive ventilation (NIV) in acute hypercapnic respiratory Failure. It is painful for patients and time and resource intensive for staff. In a pilot study we have demonstrated that transcutaneous CO2 monitoring provides reliable CO2 measurements in patients with Acute Hypercapnic Respiratory Failure (AHRF). Moreover this is less painful and preferred by patients. van Oppen et al., Respir Care. 2014 Nov 18. pii: respcare.03335. PCO2 time trends were concordant. Mean PCO2 bias was -2.33 mm Hg (95%LOA -9.60 to 5.03) mmHg, r = 0.89 (p < 0.001). Initiation of transcutaneous monitoring was less painful than the arterial equivalent (p = 0.008). Particularly in patients with AHRF due to COPD exacerbation pH plays an important role in initiating and guiding therapy. We explored whether TcCO2 can be used to predict pH thereby minimising the need for repeated arterial blood gases in this patient group. Based on Henderson Hasselbalch equation pH = 6.1+ log (HCO3/CO2). In the pilot study mentioned above Non-invasive pHtc was determined using ptcCO2 and predicted bicarbonate. Reference bicarbonate’ was recorded from ABG taken at NIV initiation. TcCO2 was monitored continuously over 12 h using Radiometer TOSCA TCM4. PaCO2 was obtained from arterial blood samples at 0, 4, 8 and 12 h. Mean pH bias was 0.012 (95%LOA -0.070 to 0.094), r = 0.84 (p < 0.001). We have subsequently reviewed records for 38 patients who received Acute NIV for AHRF. We retrospectively looked at change in pH, bicarbonate and CO2 over 24, 48 and 72 h. Using these data and Henderson Hasselbach equation we can show that in the first 48 h change in pH is almost exclusively explained by change in CO2 (Pearsons Correlation coefficient for change in CO2 and pH = 0.84; p < 0.05). Therefore in patients with pure Respiratory Acidosis transcutaneous CO2 would provide trend for pH as well as CO2, thereby minimising the need for arterial blood gas measurement and improve patient comfort. Reference 1 van Oppen JD, Daniel PS, Sovani MP. What is the potential role of transcutaneous carbon dioxide in guiding acute noninvasive ventilation? Respir Care. 2015;60:484–91