Approach to haemolysis
Haemolysis is a serious event in ECMO. There is most certainly an ECMO related complication identifiable. Less commonly a severe patient pathology causing haemolysis is present, this, however, is a diagnosis of exclusion and requires a careful review of the differential diagnosis of haemolysis.
ECMO related haemolysis is reversible and must be acted upon immediately to avoid a series of negative effects of haemolysis.
Early haemolysis is most commonly identified by
- A red tinge in the effluent of the CRRT or urine
- Increased plasma free haemoglobin (the rise with haemolysis is commonly exponential, so even a small rise should prompt a search for potential causes)
- Pump head thrombosis may become apparent (e.g. noisy pump head) before the haemolysis is manifest in other ways
Haemolysis systematic assessment
- Access insufficiency
- Excessive negative access pressures
- Recent cardiopulmonary bypass
- Thrombosed access cannula
- Inspect the circuit for clot and or fibrin deposits – tubing focus on junction, high-flow tubing
- Oxygenator thrombus: it is important to inspect the however small amount of thrombus with little temporal progression is not specific nor sensitive in ruling in or out oxygenator thrombosis as causative
- The transmembrane pressure, if reading errors are excluded, is very specific but insensitive to exclude oxygenator thrombosis
- Pump head thrombosis
If no clear ECMO or patient-related cause can be identified it is a reasonable intervention to change the circuit and scrutinise for any presence of oxygenator or pump head thrombus. Examination of the circuit post exchange is key to make the diagnosis.
Observation of clot
Inspection of the ECMO tubing and the oxygenator for clot and/or fibrin deposits is routinely and regularly done. Small clots commonly form on the inflow-side of the oxygenator even early in the time course with often uncertain significance. More sophisticated ways to determine actual oxygenator clot or volume (such as haemodilution techniques) are not available at our institution. In particular changes within the oxygenator itself can not be visualised, and the visible surface represents only a fraction of the surface area. However, large and progressive clot visible on the outside may correlate with oxygenator thrombosis and may trigger a circuit change in conjunction with other signs.
Visible clot and or fibrin strands in the tubing are less common but of greater concern and depending on their size and location raise the suspicion of eventually causing pump head thrombosis.
The transmembrane pressure indicates the resistance across the oxygenator. It is fairly proportional to blood flow even with blood flow in the higher range (>5L/min). Elevated transmembrane pressures generally indicate thrombosis in the oxygenator and should trigger a circuit change. Clear thresholds are pressures > 10mmHg/L/min (L/min of blood flow), but pressures of 8 -10 mmHg/L/min are also highly unusual and need to be scrutinised or any level of transmembrane pressure that gradual trends upwards.
The first step though in the PLS circuit, is to ensure the readings are correct. The pressure transducer easily clots once blood samples have been taken through the pre- and post-oxygenator connection. This is important given elevated transmembrane pressures are a very specific sign yet NOT sensitive to conclude about oxygenator function or presence of oxygenator thrombosis.