- Clearance describes the volume of blood that is cleared of a substance per unit of time
- The determinant of clearance in CRRT depends upon the mechanisms that are being utilised
- In continuous haemofiltration:
- A volume of ultrafiltrate is produced following the passage of the pressurised plasma over the membrane
- Clearance is of a solute determined by the:
- Rate at which ultrafiltrate is produced (Quf)
- Sieving coefficient of the filter membranes (determined by the ratio of solute concentrations in the ultrafiltrate)
- Most small molecules have a sieving coefficient of 1, meaning they pass freely through the membrane (though this is less true of middle molecules)
- Therefore, the clearance is analogous to the ultrafiltrate production rate (Quf)
- In continuous haemodialysis:
- The concentration gradient across the membrane determines clearance
- The gradient is affected by the dialysate flow rate (Qd) and the blood flow rate (Qb)
- Qd is much slower than Qb, and the dialysate becomes fully saturated.
- Therefore, the dialysate rate (Qd) becomes the rate-limiting factor for solute removal and is analogous to clearance
- Overall consequently, clearance is equivalent to the combination of the ultrafiltration rate (Quf) and dialysate (Qd), which is termed the effluent rate (Qef)
- This is often discussed in terms of the ‘dose’ of a continuous replacement therapy which is given
- In continuous haemofiltration:
- In ultrafiltration, the effluent rate is determined by the flow of plasma into the membrane and the fraction of this which is filtered out as effluent (known as the filtration fraction)
- To maintain a constant effluent rate:
- At low blood flows a large filtration fraction is required
- At high flows, a small filtration fraction can be used
- The renal replacement circuits have inbuilt algorithms to deliver a desired effluent flow rate based upon the blood flow from the patient