The meaning of a high plasma free haemoglobin: Retrospective review of the prevalence of haemolysis and circuit thrombosis in an adult ECMO centre over 5 years

Aims: In adults requiring extracorporeal membrane oxygenation (ECMO), we wanted to determine; i) the frequency of elevated plasma free haemoglobin (PFHb), ii) the reasons for circuit changes and iii) whether elevated PFHb was associated with higher in-hospital mortality. Materials and Methods: Patients requiring ECMO between January 2010 and August 2014 were identified from a prospectively collected ECMO database. Their scanned medical records and pathology results were reviewed. Relevant patient, biochemical and circuit data were collected on an Excel spreadsheet and analysed using Stata 13 (StataCorp, College Station, TX). The patients were analysed in three groups, depending on their peak PFHb during ECMO: 'Normal PFHb' (<0.1 g/L), 'Low level PFHb' (0.1 - 0.5 g/L), 'High level PFHb' (>0.5 g/L). Main Results: There were 184 ECMO runs (56 VV, 128 VA) - 61 'Normal PFHb', 99 'Low level PFHb', 24 'High level PFHb'. Circuit thrombosis (pump, oxygenator) or haemolysis requiring exchanges were significantly more common in VV ECMO compared to VA ECMO - 23.21% (13/56) vs. 0.78% (1/128), p<0.001. Elevated PFHb was associated with a longer duration of haemofiltration (p<0.001) and ECMO support (p<0.001). In-hospital mortality rates for the 'Normal PFHb', 'Low level PFHb' and 'High level PFHb' groups were 16.39% (10/61), 30.30% (30/99) and 37.50% (9/24), respectively, p=0.067. Conclusion: Elevated PFHb values during adult ECMO were common. Severe haemolysis or thrombosis requiring circuit changes were uncommon and occurred almost exclusively on VV ECMO. There was a non-statistically significant increase in in-hospital mortality with elevated PFHb and studies of larger registry data may clarify the prognostic value of PFHb in adult patients.