Haemofiltration and dialysis Flashcards
What are the core components and safety features of an RRT system
Core components:
* Extracorporeal circuit in which blood circulated by peristaltic pump or patients arterial pressure
* System for blood return
* Artificial kidney
Important additional features:
* Anticoagulation system: heparin, citrate or prostacyclin infusions or special anticoagulant coated circuits
* Fluid balance and other treatment controls
* Blood temperature control
* Pressure and flow sensors with safety alarms: vital to prevent air embolism and accidental loss from disconnections
Haemofilter: function, structure,
- Glomerular filtration of blood and subsequent adjustment of the filtrate in the renal tubule is crudely but effectively copied by passing the patient’s blood over a semipermeable membrane in a ‘haemofilter’.
- This artificial membrane provides a selective barrier across which water, ions and larger molecules can move
- Factors including pore size and pressure difference across the membrane affect movement of molecules
Structure
* San be made from cellulose, or more biocompatible synthetic materials e.g. polysulphone or polyamide
* Usually a parallel collection of hollow fibres packed within a plastic cannister.
* Blood is passed or pumped from one end to another through these tubules
* Design provides a large blood contact surface area (0.3-1.9m^2) in a relatively compact device
* Ports in the outer casing are used to collect the filtrate and/or pass dialysate fluid across the effluent side of the membrane tubules
What factors affect the function of semipermeable membranes
- Pore size (smaller used in dialysis, larger used in haemofiltration)
- Surface area of membrane in contact with blood
- Pressure in blood compartment and pressure in effluent compartment, transmembrane pressure
- Solute concentration gradient
- Solute molecule size
- Coagulation status
Sieving coefficient: definition, how does it change with molecular weight
Sieving coefficient
* measures how different molecules pass across a semipermeable membrane
* = the ratio of concentration of the molecule in the ultrafiltrate to its original concentration in the perfusate (or blood in RRT)
Sieving coefficient decreases with increasing molecular weight - see diagram, myoglobin (weight 16.7kD) has a lower sieving coefficient than urea (60 Daltons)
e.g. if a membrane has a high sieving coefficient for particular solute, the molecule is likely to be smaller than the membrane pore size
How is molecular size measured with regards to RRT and the sieving coefficient. What molecule sizes pass across standard membranes used in RRT?
Molecular size is measured in in kiloDaltons (kD)
1 Dalton = 1 atomic mass unit (the weight in grams of one moles of a substance)
Most membranes used in RRT are designed to allow low to middle molecular size molecules to pass across, thus preserving the larger molecules such as plasma proteins, albumin (62kD) and cellular elements
Three basic techniques for using semipermeable membranes
- Ultrafiltration
- Haemofiltration
- Haemodialysis
Note all these techniques can be used simultaneously in modern kidney machines
Ultrafiltration
- The basic removal of plasma water (ultrafiltrate) with smaller molecules from the bloodstream
- Principally used to correct volume overload: therefore fluid is not infused back into the patient
Slow continuous ultrafiltration (SCUF) uses any extracorporeal circuit but a filter that permits only water and small ions into the ultrafiltrate.
Haemofiltration
- Also known as solvent drag or convection
- Filtration with larger membrane pores -> producing a larger ultrafiltrate volume
- Water, electrolytes, urea and creatinine are carried across the membrane and removed from the circulation. Larger pore size also permits removal of some larger molecules, up to 50kDa
- Greater loss of fluid from the patient requires replacement fluid or reinfusate: sterile replacement solution with the desired plasma concentration of electrolytes
Haemodialysis
- A slower process than haemofiltration
- Dialysate solution perfuses the non-blood side of the membrane in a countercurrent to the blood flow
- Solutes move out of the blood into the dialysate down a concentration gradient by diffusion
Delivering RRT in critical care: continuous vs intermittent
In critical care, RRT can be delivered by continuous low flow therapy or intermittent high flow systems
Continuous low flow therapy
* Typically run 24h a day for more than 1d, but may be stopped for procedures and filter or circuit changes
* Benefits: better cardiovascular stability and more efficient solute removal because of the steady biochemical correction and gradual fluid removal. Also more suited to frequently changing fluid balance situation in patients with multi-organ dysfunction
Intermittent high flow systems
* Often 4h sessions
* New filter and circuit for each session
RRT in critical care: mechanisms and differences between
* Continuous venovenous haemofiltration (CVVHF)
* Continuous venovenous haemodiafiltration (CVVHDF)
* Continuous ateriovenous haemofiltration (CAVH)
CVVHF: uses a double-lumen catheter in a major vein and a pumped extracorpoeal circuit. Uses convection for solute removal
CVVHDF: same circuit as CVVHF but uses a filter that allows a countercurrent of dialysis in addition to an effluent port. Combines diffusion with convection for solute removal
CAVH: Relies on patient’s blood pressure ot provide flow through the filter, rquires separate arterial and venous lines. Uses convection for solute removal
Type of CRRT circuit?
Slow continuous ultrafiltration (SCUF)
Uses any extracorporeal circuit but uses a filter that permits only water and small ions into the ultrafiltrate. Fluid is not replaced
Type of CRRT circuit?
Continuous, venovenous haemofiltration (CVVHF)
* Double lumen catheter in a major vein, pumped extracorporeal circuit
* Convection for solute removal
Type of CRRT circuit?
Continuous, venovenous haemodiafiltration (CVVHDF)
* Same circuit as CVVHF + a filter that allows a countercurrent of dialysate in addition to an effluent port.
* Combines diffusion with convection for solute removal
Type of CRRT circuit?
Continuous, arteriovenous haemofiltration (CAVH)
- relies on the patient’s blood pressure to provide flow through the filter
- requires separate arterial and venous lines.
- uses convection for solute removal
