A seventy-six (76) year old man is admitted to the ICU following a laparotomy for faecal peritonitis. He has developed Multiple System Organ Failure over two days, requiring ventilatory and inotropic support. He is oliguric, increasingly acidotic, uraemic and has a rising serum creatinine.
(c) List the available dialytic therapies and their associated advantages/disadvantages. Which mode would you choose in this man and how does it achieve solute clearance?
College Answer
Available techniques (not limited to the candidates unit) include intermittent haemodialysis, peritoneal dialysis, and the variants contained within continuous renal replacement therapy (CRRT). Intermittent Haemo-Dialysis (IHD; solute clearance = diffusion): quick control, less ICU staff involvement (? cost implications), minimise exposure to anticoagulation BUT haemodynamic instability, potentially dramatic fluid and electrolyte shifts, exposure to extracorporeal circuit and filter membrane, vascular access problems, poor control between dialyses, timing dependent on dialysis staff.
Peritoneal dialysis (PD; solute clearance = convection): continuous control of fluids, avoids vascular access problems, gentle technique, cheap, does not require electricity/machinery/renal unit, timing only dependent on ICU staff BUT requires intact peritoneum, fluid dwelling may impair respiratory function, peritoneal access problems (especially infection), potential inability to control severe uraemia and electrolyte disturbances, hyperglycaemia.
Continuous Renal Replacement Therapy (CRRT; solute clearance = mixture of convection and diffusion): continuous control of fluids and electrolytes (and ? other substances), easily titratable (changing fluid replacement and arterial flow rate or gradient for ultrafiltrate), greater haemodynamic stability (compared with IHD), timing only dependent on ICU staff BUT complex, vascular access problems, usually requires anticoagulation, exposure to extracorporeal circuit and filter membrane.
Specific subtypes of CRRT should be discussed, including:
Continuous Arterio-Venous Haemofiltration (CAVH; solute clearance = convection): simple, can do without pump, electrolyte balance determined by replacement fluid BUT requires arterial access, arterial flow rates and ultrafiltrate flow limit clearance.
Continuous Veno-Venous Haemofiltration (CVVH; solute clearance = convection): avoids arterial access problems, blood flow rate controlled by pump, good clearance of middle molecules, electrolyte balance determined by replacement fluid BUT requires pump and large bore venous access
Continuous Veno-Venous Haemo-Dia-Filtration (CVVHDF; solute clearance = convection and diffusion): increased solute clearance (dependent on dialysate flow), flow rates controlled by pump, electrolyte balance determined by replacement fluid and dialsysate BUT less efficient clearance of middle molecules, still requires pump and large bore venous access.
Discussion
This question would benefit from a tabulated answer.
But firstly, to answer the last question - in this patient CVVHDF would be an appropriate therapy, as heis likely to be hemodynamically unstable, and may not tolerate SLEDD. As hemodynamic stability is reestablished SLEDD can be commenced as a means of intermittently correcting uraemia and fluid overload. The other presented options are either outdated (CAVH, really?) or inappropriate (IHD).
As for the main part of the question; it asks the candidate to compare advantages disadvantages and mechanisms for all available dialytic therapies. In more conservative questions (eg. Question 19 from the second paper of 2008, or Question 10 from the first paper of 2011) only a small selection of modalities is selected for discussion, which would make for a sensibly brief answer. The extensive tabulated answers from these questions will not be repeated here.
Instead, links are offered:
The college answer to this question also incorporates peritoneal dialysis, CAVH, and separates into CVVHF from CVVHDF.
Thus, below is a table which compares the modalities which are missing from the tables linked above.
| Modality | CVVHF | CVVHDF | PD | CAVH |
| Access | Vas cath | Vas cath | PD abdominal catheter | Arterial catheter |
| Flow rate | Low flow rate | Low flow rate | nil; rather, fluid dwell time is the important feature | Arterial flow rate |
| Anticoagulation | Continuous | Continuous | None | Intermittent boluses may be required |
| Fluid removal | Slow | Slow | Slow | Slow |
| Electrolyte removal | Slow; by convection (mainly) and diffusion |
Slow; by convection and diffusion |
Very slow; by diffusion alone | Slow; by convection (mainly) |
| Efficiency of solute clearance | Low However, good solute clearance is ultimately achieved over a prolonged course |
Low However, good solute clearance is ultimately achieved over a prolonged course |
Poor efficiency of fluid and electrolyte clearance | Low However, good solute clearance is ultimately achieved over a prolonged course |
| Hemodynamic impact | Well tolerated | Well tolerated | Tolerated by most patients | Unsuitable for hemodynamically unstable patients - arterial flow rate may be too low |
| Cost | Expensive | Expensive | Cheap | Cheap |
| Advantages |
Good clearance of middle molecules Well tolerated hemodynamically Good control over fluid removal and solute exchange |
Good clearance of middle molecules Well tolerated hemodynamically Good control over fluid removal and solute exchange Good control over acid-base balance |
Does not require anticoagulation. Patient's blood is not exposed to the circuit Intermittent, thus less labour intensive; Allows periods of mobility for the patient Well tolerated unless very unstable |
Anticoagulation may not be required; Pump may not be required |
| Disadvantages |
Expensive Requires anticoagulation Prolonged immobilization Electrolyte and acid-base control is better with CVVHDF |
Expensive Requires anticoagulation Prolonged immobilization Slow and inefficient |
Poor solute clearance Requires abdominal access Potential for peritonitis |
Requires arterial access Dependent on arterial flow rates without a pump Poor solute clearance |
References
D'Intini, Vincent, et al. "Renal replacement therapy in acute renal failure." Best Practice & research clinical anaesthesiology 18.1 (2004): 145-157.
O'Reilly, Philip, and Ashita Tolwani. "Renal Replacement Therapy III: IHD, CRRT, SLED." Critical care clinics 21.2 (2005): 367-378.
Wei, S. S., W. T. Lee, and K. T. Woo. "Slow continuous ultrafiltration (SCUF)--the safe and efficient treatment for patients with cardiac failure and fluid overload." Singapore medical journal 36.3 (1995): 276-277.
Kanno, Yoshihiko, and Hiromichi Suzuki. "Selection of modality in continuous renal replacement therapy." (2010): 167-172. -This seems to be an entire issue of Contributions to Nephrology
(Vol. 166) by Claudio Ronco.