Compare Continuous Venovenous Haemofiltration (CVVHF), Sustained Low Efficiency Dialysis (SLED) and Intermittent Hemodialysis (IHD) with respect to:
a) Mechanism of solute clearance
b) Advantages
c) Disadvantages
CVVH |
SLED |
IHD |
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Mechanism of solute clearance |
Solvent removal occurs as a consequence of a pressure gradient across a semi permeable membrane. Solute removal occurs only by convection (solvent drag). |
Solute removal occurs predominantly by diffusion down a concentration gradient created by dialysate fluid on the other side of the semi permeable membrane. |
Solute clearance by diffusion |
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Advantages |
Achieves better clearance of middle molecules (< 15 Kd) than CVVHD/IHD, fluid management easier and flexible, lesser hemodynamic instability as compared to IHD. |
Can be done at night so patient can be mobilized during the day. Period of anticoagulation reduced. Possible cost savings by using online water and ability for one machine to deliver 2 treatment episodes per day. |
Shortest treatment time, anticoagulation often not required, cost savings by using online water |
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Disadvantages |
Patient immobilized, need for continuous anticoagulation, higher nursing requirement |
Inferior clearance of middle molecules, reduced fluid management flexibility. Higher risk of disequilibrium syndrome |
Least clearance of middle molecules, least flexible fluid management, highest risk of disequilibrium syndrome. Possible greater haemodynamic instability |
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Additional Examiners’ Comments:
Candidates’ knowledge and understanding of a core topic was overall poor. Some candidates were not able to describe the mechanisms involved. Many described CVVHDF rather than CVVHF
The college answer is of a surprisingly high quality. Little can be done to improve on it, except to furnish the points with references. And to rearrange the table, to make it look like a different table.
Modality and mechanism | Advantages | Disadvantages | |
CVVH
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IHD
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SLED
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Some footnotes of interest:
* The college answer makes much of the improved clearance of middle molecules by CVVH, but in actual fact all molecules are cleared better by this modality. An excellent comparison, almost purpose-made for this SAQ, comes from Liao et al (2003). The authors found that clearance of small solutes by CVVH was 8% better than by SLED, and 60% better than by IHD. This can be illustrated using graphs which were stolen shamelessly from the Liao paper. For large and middle molecules, CVVH is even better - twice as effective as SLED and four times as effective as IHD. This is mainly because the therapy is sustained continuously.
** Though the college answer presents intermittent haemodialysis as a technique purely dependent on diffusion, in practice this is not the case, even though it's called "haemodialysis". Some ultrafiltration needs to occur in order to control the fluid balance, and this is the norm for maintenance dialysis - that's why people discuss the "dry weight" of chronic IHD patients. In case this baremetal fact needs to be varnished with peer-reviewed literature, one could pull out just about anything from the Am J Kid Dis or Nephrology and Transplantation. Literally anything. Here's a 2016 study by Assimon et al correlating ultrafiltration rate to mortality, for example. For interests' sake, its worth noting that in that study the average ultrafiltration rate was about 6.6ml/hr/kr, or about 1.8L total fluid removal after a 4hr session for an average 70kg patient. Since probably the late 1980s straight HD has been replaced by HDF (haemodiafiltration), because the latter was a significant improvement. Canaud et al (1989) described it as "the new standard".
*** The college suggests that fluid management is inherently more "flexible" in CVVH. This is an interesting statement. An article by Murugan (2016) also suggests that CVVH and CRRT in general allows more precision in the control of fluid balance. Practically, it is difficult to determine how the same 100ml/hr fluid removal by SLED or IHD is any less easy or less flexible. Arguably, it is actually easier to achieve a high negative fluid balance with SLED or IHD because the nurses do not need to laboriously exchange full bags of effluent. The main "flexibility"is really the ability to run the circuit continuously, which means you remove as much fluid as you want - you just need to keep it running for longer.
Assimon, Magdalene M., et al. "Ultrafiltration rate and mortality in maintenance hemodialysis patients." American Journal of Kidney Diseases 68.6 (2016): 911-922.
Jean, Guillaume, et al. "Online-haemodiafiltration vs. conventional haemodialysis: a cross-over study." BMC nephrology 16.1 (2015): 70.
Canaud, B., et al. "Hemodiafiltration with on-line production of bicarbonate infusate." Improvements in Dialysis Therapy. Vol. 74. Karger Publishers, 1989. 91-100.
Liao, Zhijie, et al. "Kinetic comparison of different acute dialysis therapies." Artificial organs 27.9 (2003): 802-807.
Murugan, Raghavan, et al. "Precision fluid management in continuous renal replacement therapy." Blood purification 42.3 (2016): 266-278.
O'Reilly, Philip, and Ashita Tolwani. "Renal replacement therapy iii: IHD, CRRT, SLED." Critical care clinics 21.2 (2005): 367-378.