Vascular Access in Continuous Renal Replacement Therapy - Essay Example

Vascular Access in Continuous Renal Replacement Therapy Acute renal failure (ARF) also referred to as acute kidney injury alludes to sudden slowing of the kidney’s function resulting in elevation of plasma creatinine, blood urea nitrogen and reduced urine output (Rabindranath, Adams, MacLeod and Muirhead, 2007, p. 3). Correction of the causes of AKI in most cases leads to full recovery however; some cases may require renal replacement therapy (RRT). One form of RRT is continuous renal replacement therapy (CRRT) which is used for a period of 24 hrs in a day. CRRT describes different techniques of purifying blood which differ in solute transport mechanism, membrane type, use or non-use of dialysate solution and form of vascular access (Soni, Nagarik, Adikey and Raman, 2009, p. 24). CRRT uses diffusion (haemodialysis), convection (haemofiltration) or combines these two methods (heamodiafiltration) in order to achieve solute removal from the blood. Small molecular weight substances like potassium, urea and creatinine are efficiently removed through haemodialysis (Kellum, Mehta, Angus, Palevsky and Ronco, 2002, p.1858). For larger solute molecules, heamofiltration is the most efficient method of their removal in comparison with dialysis. In haemofiltration, filtration of plasma through the semi-permeable membrane is caused by hydrostatic pressure. In addition, solutes cross the membrane alongside the plasma which results in convective solute transport flowing in the same direction as water (Medve, Preda and Gondos, 2010, p. 104). Therefore, haemofiltration needs the use of replacement fluid to avert excessive removal of fluid, electrolyte depletion as well as iatrogenic acidosis. Since the plasma solute concentration is the same as that of the removed filtrate, concentration of the solutes in the blood plasma remaining need to be diluted using substitution fluid. The use of combined convective and diffusive clearance as well as haemofiltration is effective for removal of large and small molecular weight solutes (Joannidis and Oudemans-van Straaten, 2007, p. 219). Modalities The several modalities of CRRT available are continuous veno-venous haemodiafiltration (CVVHDF), continuous veno-venous haemofiltration (CVVH) and continuous veno-venous haemofiltration (CVVHD). In terms of solute clearance rate, CRRT is lower in comparison with IHD (Intermittent haemodialysis treatments). However, balance clearance within the 24 hr period that CRRT is undertaken is much higher. Moreover, fluid elimination during CRRT is much slower and requires continuous use of anticoagulants which risks bleeding (Uchino, Bellomo, Morimatsu, Morgera, et al. 2007, p. 1567). Continuous veno-venous haemodiafiltration (CVVHDF) There is counter-flow of blood and dialysis solution within the dialysis filter Blood flows at a speed of between 100 and 200ml/min Dialysis solution speed is between 1 and 2 l/h Optimisation of the ultrafiltration speed is dictated by convective transport of dissolved substances and volume loss. The removal of solutes is done simultaneously by both diffusion and convection Substitution fluid is used to replace lost fluid Continuous veno-venous haemofiltration (CVVH) Removal of solutes occurs via convective transport Ultrafiltrate which is produced must be replaced using a substitution solution Removal of ultrafiltrate may cause patient’s volume loss Continuous veno-venous haemofiltration (CVVHD) There is counter-flow of blood and dialysis solution within the dialysis filter Blood flows at a speed of between 100 and 200ml/min Dialysis solution speed is between 1 and 2 l/h The administration of fluid is not routine Solute removal occurs through diffusion Principles and Indications Membrane characteristics must be considered when choosing treatment modalities of CRRT. These characteristics include; biocompatibility, solute removal and water permeability (Fall and Szerlip, 2010, p. 583). In general, efficiency of small molecules in CRRT is largely depended on ultrafiltration/dialysate flow rate. It follows then that characteristics of solute removal are not usually considered in the selection of a dialysis membrane (Kraus, 2009, p. 139). High flux membranes designed to allow high volume water permeability are highly recommended in procedures for haemofiltration. Although there is lack of conclusive evidence that patient outcome is affected by membrane biocompatibility, there is consensus that synthetic membranes are more popularly used than cellulose based membranes (Overberger, Pesacreta and Palevsky, 2007, p. 625). Replacement/Dialysate Solutions Most of the CRRT modalities usually require sterile replacement fluids to replenish the removed ultrafiltrate. There is no direct contact between dialysate and blood due low blood-side pressures (Ricci, Ronco, D'amico, De Felice, R., et al. 2006, p.696). However, back filtration is possible especially when high permeability membranes are used. Optimal replacement solution augments normal water composition of plasma by replacing minerals and electrolytes and not metabolic solutes which are in plenty due to renal failure. Composition of these solutions is usually balanced differently in order to attain different metabolic goals (Hladik, Tymonova, Zaoral, Kadlcik and Adamkova, 2001, p. 10002). There is a limitation of using these sterile solutions because they are lactate or acetate based and the efficiency of converting them into bicarbonates is limited especially in multiple organ failure patients. In the recent past, bicarbonate based sterile solutions have been made available commercially and are highly tolerated as opposed to acetate or lactic based solutions. Modifications in dialysate and replacement fluids are usually required when citrate anticoagulation is used especially in CRRT. The use of customised solutions (locally prepared) is often necessary because commercial solutions are rare. The liver metabolises citrate into bicarbonates therefore negating the need of a buffer in the dialysate. In addition, citrate regional anticoagulation requires a dialysate that is hyponatremic so that it can avert hypernatremia (Santiago, Lopez-Herce1, Urbano, Solana, Castillo1, Ballestero, Botran and Bellon, 2009, p. 4). Moreover, it is highly recommended that the fluids be void of calcium. Until recently, there were few commercially available bicarbonate based calcium free CRRT fluids. Many medical centres have resulted into manufacturing these fluids in their facilities. The commercially available solution is usually availed as a concentrate and is supposed to be added to three litres of sterile water. Past failure by bedside nurses to mix sterile water with the concentrate has resulted into the death of patients. Such tragedies underscore the use of commercially available solutions because mixing of sterile water and the concentrate should be done at the time of manufacture (Pannu and Gibney, 2005, p. 148). Vascular Access There is variation in the way blood supply is accessed for extra corporeal circuit in CRRT. Arteriovenous techniques which are rarely used currently require cannulation of both the femoral vein and artery. The variance in blood flow rates usually exacerbates chances of thrombosis. Generally, small lengths as well as large bore catheters are usually used for arteriovenous techniques in order to allow a high flow rate of blood. On the other hand, double lumen venous catheters are usually used for venovenous (pumped) systems. Size is usually selected at site insertion on the basis of the need to optimise flow rate. When choosing the insertion site, many clinicians do not consider risk of infection despite the fact that infections are usually observed after femoral catheters have been used. Rules of hygiene must be complied with during catheter insertion. In double lumen catheters, both the input and output gates are in close proximity and they is recirculation of instrumental amounts of filtered blood into the extracorporeal system. This recirculation can however be minimized through proper positioning of the catheter. When a femoral catheter is used, then its tip should be dipped in inferior cava vein (Vijayan, 2009, p. 134). Management and Complications Anticoagulation forms an essential and important component of RRT including CRRT. Platelets are activated due blood passing through the extracorporeal circuit which induces prothrombotic and inflammatory mediators leading to deposition of fibrin on the filter membranes. This affects longevity of filters and decreases the efficacy of dialysers in their solute and water removal. On the other hand, insufficient anticoagulation leads to deterioration of filtration and clotting of the filter leading to blood loss and increased costs in terms of filter replacement. Unfractionated heparin is highly recommended for CRRT and is administered in a bolus form in order for the maintenance of (PTT) partial thromboplastin time. Systemic anticoagulants are not recommended for patients predisposed to bleeding although modification of heparin can be undertaken in such circumstances. However, heparin use is often linked with incidences of bleeding and in other cases thrombocytopenia induced by heparin. In dialysis undertaken without heparin, blood flows are maintained at 250-500mL per minute and saline is injected into the dialysis circuit (arterial limb) every 15-30 minutes in order to minimise haemoconcentration flush fibrin strands into the bubble trap (Foot and Fraser, 2005, p.325). Regionalised anticoagulation of the circuit is often preferred for recently postoperative patients and those that are not allowed to have systemic anticoagulation. Heparinisation is usually undertaken through regional, post filter infusion, regionalisation and pre-filter administration. Regional anticoagulation is actualised through continuously infusing citrate into the circuit (arterial limb). This leads to chelating of calcium and prevention of activation of the coagulation cascade. Removal of the citrate-calcium complex is through combining dialysate without calcium against dialysis clearance and endogenous processes. With patients with normally functioning livers have normalisation of their ionised calcium and citrate levels in thirty minutes after citrate infusion has been stopped. The levels of plasma calcium can also be restored by continuous calcium infusion when blood is being returned to the patient. The rate of citrate infusion is often adjusted to maintain ACT (activated clotting time) way above 160 seconds. Regional citrate anticoagulation necessitates the use of customised dialysis solution with continued monitoring of the levels of ionised calcium. Some of the potential complications that can arise from regional citrate anticoagulation are hyponatremia, hypocalcaemia, citrate toxicity and metabolic alkalosis especially in patients with minimally functioning livers. However, proper monitoring of this technique can lead to low rate of complications. Its use in CRRT techniques leads to increase of filter longevity of up to 96 hours while heparin regionalization has a filter longevity of 36-48 hours (Ricci, Ronco, D'amico, De Felice, R., et al. 2006, p.693). References Fall, P., and Szerlip, H. M. 2010, ‘Continuous renal replacement therapy: cause and treatment of electrolyte complications’, Seminars in dialysis, vol. 23, no. 6, pp. 581-585 Foot, C.L. and Fraser, J.F. 2005, ‘So you need to start renal replacement therapy on your ICU patient?’ Current Anaesthesia & Critical Care, vol. 16, pp.321–329 Hladik, M., Tymonova, J., Zaoral, T., Kadlcik, M., and Adamkova, M. 2001, ‘Treatment by continuous renal replacement therapy in patients with burn injuries’, Complement, vol. 3, pp. 10000. Joannidis, M., and Oudemans-van Straaten, H. M. 2007, ‘Clinical review: Patency of the circuit in continuous renal replacement therapy’, Critical Care, vol. 11, no. 4, pp. 218-21 Kraus, M. A. 2009, ‘THE CLINICAL APPLICATION OF CRRT—CURRENT STATUS: selection of dialysate and replacement fluids and management of electrolyte and acid?base disturbances, Seminars in Dialysis, vol. 22, no. 2, pp. 137-140 Kellum, J. A., Mehta, R. L., Angus, D. C., Palevsky, P., and Ronco, C. 2002, ‘The first international consensus conference on continuous renal replacement therapy’, Kidney international, vol. 62, no.5, pp.1853-1863. Medve, L., Preda, E. and Gondos, T. 2010, ‘The practice of renal replacement therapy in the intensive care unit’, © Borgis - New Medicine, vol. 3, pp. 102-106 Overberger, P., Pesacreta, M. and Palevsky, P. M. 2007, ‘Management of renal replacement therapy in acute kidney injury: a survey of practitioner prescribing practices’, Clinical Journal of the American Society of Nephrology, vol. 2, no. 4, pp. 623-630. Pannu, N. and Gibney, R.T.N 2005, ‘Renal replacement therapy in the intensive care unit’, Therapeutics and Clinical Risk Management, vol. 1, no. 2, pp. 141–150. Rabindranath, K.S., Adams, J., MacLeod, A.M., Muirhead, N. 2007, ‘Intermittent versus continuous renal replacement therapy for acute renal failure in adults. Cochrane Database of Systematic Reviews, no.3. Art. No.: CD003773. DOI: 10.1002/14651858.CD003773.pub3. Ricci, Z., Ronco, C., D'amico, G., De Felice, R., Rossi, S., Bolgan, I., ... & Piccinni, P. 2006, ‘Practice patterns in the management of acute renal failure in the critically ill patient: an international survey’, Nephrology Dialysis Transplantation, vol. 21, no. 3, pp.690-696. Santiago, M.J., Lopez-Herce1, J., Urbano, J., Solana, M.J., Castillo1, J., Ballestero, Y., Botran, M and Bellon, J.M., 2009, Complications of continuous renal replacement therapy in critically ill children: a prospective observational evaluation study’, Critical Care, vol. 13, no. 6, pp. 1-11 Soni, S.S., Nagarik, A.P., Adikey, G.K. and Raman, A. 2009, ‘Using continuous renal replacement therapy to manage patients of shock and acute renal failure’, Journal of Emergencies Trauma and Shock, vol. 2, no. 1, pp. 19-22 Uchino, S., Bellomo, R., Morimatsu, H., Morgera, S., Schetz, M., Tan, I., ... & Kellum, J. A. 2007, ‘Continuous renal replacement therapy: A worldwide practice survey’, Intensive care medicine, vol. 33, no.9, pp. 1563-1570. Vijayan, A. 2009, ‘Vascular access for continuous renal replacement therapy’, Seminars in Dialysis, vol. 22, no. 2, pp. 133-136 Read More