- All forms of RRT can precipitate thrombus formation by exposing blood to a non-biological surface
- Contact between blood and either the filter itself (which has a high surface area and is the most thrombogenic component) or plastic tubing can activate the clotting cascade
- Clot may occur in either:
- The filter, causing trans-membrane high pressure alarms
- The vascular access catheter, causing access high pressure alarms
- Clotting not only leads to reduced filter efficiency, but may cause iatrogenic anaemia through loss of filter circuits
Anticoagulation During Renal Replacement Therapy
Anticoagulation During Renal Replacement Therapy
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- As with all extra-corporeal circuits, RRT requires anticoagulation to stop thrombus formation
- Ensure adequate driving pressure i.e. venous pressure
- Ensure adequate flow rates through the vascath
- RIJ > femoral > LIJ
- Appropriate catheter position and care
- Increasing pre-filter fluid (e.g. 70/30 mix instead of 50/50 mix)
- Lowers haematocrit and reduces change of filter clot
- Reduces efficacy of filtration process
- Heparin can be given into the circuit before the filter, which results in less systemic anticoagulation
- Filters are also primed with heparin, then infused with a target APTTr of <1.4
- It can also be given systemically
- Benefits:
- Reversibility with protamine
- Titratable with APTTr
- APTTr testing is commonly performed as part of routine clotting studies
- Cheap
- Issues:
- Heparin resistance due to critical-illness associated antithrombin III deficiency
- Heparin-induced thrombocytopaenia
- Haemorrhagic complications
- Ionised calcium is required for the function of factors 2 (prothrombin), 7a, 9a and 10/10a i.e. the vitamin K dependent clotting factors
- Citrate chelates calcium (historically clotting factor IV)
- The net effect is inhibition of thrombin formation
- In order to achieve this effect, the filter must have an ionised calcium concentration of <0.4mmol/L
- The citrate-calcium complex is removed in the filter by both convection and diffusion
- Citrate is also rapidly metabolised in the Krebs cycle in the liver and renal cortex
- Benefits include:
- Generally felt to be as safe as heparin
- Increased filter lifespan
- Reduced need for blood products
- Suitable in those for whom heparin is contraindicated
- Provides a whopping 0.59kcal/mmol to help your patients get swole
- Preferred in various countries including Australasia and Italy
- Issues
- Potential metabolic alkalosis by increasing the strong ion difference
- Potential high anion gap metabolic acidosis
- Reduced ionised calcium levels
- Hypomagnesaemia, as magnesium binds to the citrate-calcium complex
- PGE2 ('Flolan') can be given to inhibit platelet function
- It benefits from a short half-life and therefore fairly rapid reversal once infusion terminated
- Issues include:
- Potent vasodilation → cardiovascular instability
- Impairs HPV → hypoxia if relying on HPV to prevent excessive shunt
- Intracranial hypertension