P267 Association of the length of non-invasive ventilation (NIV) with arterial bicarbonate level in COPD patients with acute hypercapnic respiratory failure (AHRF)

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Following the British Thoracic Society (BTS) NIV audit 2011 we noted that our institution's length of stay was longer than the national average. Factors related to length of stay are complex and related to a lot of non-medical factors, however length (duration) of NIV treatment is not. Although the associations of mortality of COPD patients requiring NIV are well-documented (Non-invasive ventilation (NIV) in chronic obstructive pulmonary disease (COPD) exacerbations with AHRF with pH<7.26. Thomas A, et al. Thorax 2010;65:4. A33), the determinants of the length of NIV have not been clearly elucidated, which we decided to investigate.


A retrospective analysis of the initial ABG values on 67 episodes of NIV for COPD at a dedicated respiratory NIV unit from 1 November 2010 to 30 June 2011 was carried out. Analysis of blood gases and duration of use of NIV was documented and analysed.


In an 8-month period, 67 patients were admitted to the NIV unit with AHRF with COPD. There were 6 (8.95%) in-hospital deaths in this group. Mean (range) pH on admission was 7.26 (7.08–7.34), mean (range) pCO2 was 9.75 kPa (6.03–15.5), mean (range) arterial bicarbonate level (HCO3) 27.2 mmol/l (19.9–45.2). The mean peak Inspiratory Positive Airway Pressure (IPAP) used was 18.7 cm H2O and peak Expiratory Positive Airway Pressure (EPAP) was 5.4 cm H2O. Plotting a graph with HCO3 and length of NIV we see that it has a linear relationship (see Abstract P267 figure 1). Length of NIV increases by 0.294 days for every 1 mmol/l increase in HCO3 above the mean.


This scientific survey indicates that the length of NIV therapy in COPD patients in AHRF increases with a higher HCO3. Though outcome and mortality is closely linked to the pH, length of NIV is more closely linked to the HCO3. This is explained by the fact that people with higher HCO3 are likely to have had chronic respiratory failure for longer and likely to take longer to recover from the respiratory failure.

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