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.

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Abstract P39 Figure 1

Change in pH and PaCO₂ in patients on NIV for AHRF

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