FRCA Notes

Renal Replacement Therapy

RRT was an SAQ topic in 2017, where marks were roughly evenly split between indications, modalities available and the mechanisms of filtration and dialysis.

Those looking for fine detail on the topic need look no further than Deranged Physiology's 21(!) separate pages on the topic.

Rather than extreme detail, this lone page is intended to merely get one through the Final FRCA exam.

Resources

  • RRT can be used in both acute and chronic renal failure
  • It replicates some of the functions of the kidney:
    • Water homeostasis
    • Electrolyte homeostasis
    • Acid/base balance
    • Removal of waste products and toxins
  • However, it replicates neither the role of the kidney in blood pressure homeostasis (RAAS system) nor its hormonal functions (EPO, D-hormone)
  • The aide memoire for the indications for acute RRT are the vowels:
    • A - acidaemia refractory to medical management
    • E - electrolyte imbalance e.g. refractory hyperkalaemia
    • I - ingestion of toxic compounds e.g. lithium
    • O - overload refractory to medical management e.g. from cardiac failure
    • U - uraemia, as well as other toxins such as hyperammonaemia

  • One might also have one's arm bent to provide RRT for patients who are normally dialysed but can't reach their normal dialysis slot e.g. due to acute illness

Timings

  • Timing of instigation is variable and context-dependent
  • Using sodium bicarbonate to maintain a pH >7.30 in patients with critical illness and stage 2/3 AKI improved 28-day mortality and reduced need for RRT (BICAR-ICU, 2018), so could be used as a temporising strategy

  • Discontinuation should be considered when there's evidence of recovery of native renal function e.g.:
    • Increased urine output (400ml/24hrs)
    • Progressive decline in serum creatinine
    • Progressive increase in creatinine clearance (>20ml/min)

Ultrafiltration

  • Removal of water (ultrafiltrate) and small molecules
  • Useful in managing fluid balance/volume overload e.g. refractory cardio-renal syndrome
  • Fluid is not returned to the patient

Haemofiltration

  • Larger membrane pores lead to:
    • A larger volume of ultrafiltrate
    • Removal of molecules up to 50kDa in size

  • Electrolytes, urea and creatinine are carried across the semi-permeable membrane by solvent drag (convection) i.e. alongside the mass movement of water
  • As a larger volume of fluid is removed, a sterile replacement fluid (reinfusate) with the desired electrolyte composition is administered

Haemodialysis

  • A slower process whereby a dialysate is used in a counter-current to the blood flow on the other side of the semi-permeable membrane
  • Solutes move out of the blood into the dialysate via diffusion

Haemodiafiltration

  • Utilises a blend of convection (solvent drag), ultrafiltration and diffusion to remove solutes and plasma water

Components

  • Two-way vascular access device
  • Extracorporeal circuitry
  • 'Artificial nephron'
  • Blood return mechanism
  • Mechanisms for anticoagulation, control of blood temperature and control of fluid balance
  • Appropriate pressure and flow sensors with safety alarms

Artifical nephron

  • Man-made semi-permeable membranes act as haemofilters
    • Biocompatible synthetic materials; polysulphone or polyamide
    • Cellulose-based membranes are associated with activation of inflammatory pathways and use of more biocompatible membranes may improve patient outcomes

  • Arranged as hollow tubules in a plastic cannister, through which blood is pumped
  • Provide a high surface area (0.3 - 1.9m2) in a small device
  • Pores in the outer casing of the cannister allow passage of dialysate over the effluent side of the membrane, and facilitate collection of ultrafiltrate

  • Factors affecting haemofilter performance include:
    • Pore size (small in dialysis, large in haemofiltration)
    • Pressure in the blood and effluent compartments (and consequently transmembrane pressure)
    • Solute concentration gradient
    • Surface area of contact between membrane and blood
    • Coagulation status of blood

Sieving coefficient

  • The sieving coefficient is the ratio of a substance in ultrafiltrate vs. blood (/perfusate)
    • It is a measure of how a molecule passes through the semipermeable membrane
    • Higher sieving coefficient = more efficient convection process

  • Molecules are categorised as low, middle or large with respect to their molecular weight
  • Most filters allow low and middle molecules to pass, preserving large molecules (albumin, plasma proteins, cells)

  • Continuous, low-flow therapies run for 24hrs/day
    • E.g. continuous arterio-venous or veno-venous haemofiltration (CAVH or CVVH), slow continuous ultrafiltration (SCUF)
    • Better cardiovascular stability (and consequentially better preserved cerebral perfusion)
    • More efficient solute removal
    • Enhanced clearance of inflammatory mediators, which may be beneficial in sepsis
    • Better suited to frequent changing fluid balance of critically ill patient

  • Intermittent, high-flow therapy runs for 4hrs at a time
    • E.g. intermittent haemodialysis (IHD)
    • Requires new filter and circuit for each session
    • Greater propensity for haemodynamic instability

  • There is no demonstrable difference in survival benefit between different modes, although:
    • KDIGO recommends CRRT in haemodynamically unstable patients
    • The use of CRRT at initiation reduces long-term dialysis dependence vs. IHD

CVVH

  • A convective processes which uses the hydrostatic pressure gradient to filter plasma, water and thus solute across the membrane
  • 'Solvent drag' occurs; it is independent of solute concentration but determined by the direction and magnitude of the trans-membrane pressure
  • Higher flow rate
    • Increases ultrafiltrate production
    • Increases solute clearance
  • Effluent is discarded and circulating volume replaced by a balanced crystalloid buffer solution
  • Requires a double-lumen catheter in a major vein and a pumped extra-corporeal circuit

CVVHD

  • A diffusive process across a semi-permeable membrane
  • Involves counter-current dialysate flow
  • The counter-current system maintains a waste solute concentration gradient i.e. always lower on the dialysate side of the membrane
  • Requires a double-lumen catheter in a major vein and a pumped extra-corporeal circuit

SCUF

  • Extracorporeal circuit but the filter only allows water and small molecules through
  • Fluid is not replaced

CAVH

  • Requires separate arterial and venous lines
  • Patient's blood pressure provides flow in circuit i.e. no pumps involved

  • Dosing quantified by the volume of blood processed ('cleared') by filtration and fluid replacement, expressed in ml/kg/hr
  • Typical rate for a critically ill patient would be 25 - 35ml/kg/hr
    • E.g. for an 80kg patient = 2,800ml/hr
    • This would require a blood flow of 50ml/min - this is not an issue as most circuits have flow rates 80 - 240ml/min

  • Some suggestion that higher (>25ml/kg/hr) doses do not improve mortality or patient outcomes than lower doses (20 - 25ml/kg/hr)
  • High dose (>50ml/kg/hr) exchange has been postulated as a potential treatment in refractory septic shock, though strong evidence is lacking