The impact of kidney replacement therapy (KRT) on nutritional requirements
For patients receiving RRT as a treatment in AKI, nutritional requirements will be affected. All patients with AKI requiring RRT should be referred to the dietitian (Kanagasundaram et al, 2019).
Intermittent haemodialysis (IHD) may be used with stable patients. Fluid removal may be limited and therefore it can be challenging to fully meet nutritional requirements within fluid allowances. Nutrient-dense and low electrolyte feed can be useful. Alternatively, discussion with the medical team to adapt dialysis prescriptions may be necessary to allow a greater volume of feed to be delivered.
Sustained low-efficiency diafiltration (SLED-F) may be used with some patients. Preliminary data showed that unadjusted losses of free amino acids and 18 trace elements in IHD and SLED-F appear to be similar, despite theoretical concerns that the extended hybrid treatment might confer increased risk of micronutrient loss (Oh et al, 2019).
Continuous Renal Replacement Therapy (CRRT) can be used with patients who have cardio-vascular instability, and therefore is the method of choice for critically ill patients. Care is needed to examine in detail the types and volumes of replacement and dialysis solutions that are used, since these may contain ‘hidden’ calories. The anticoagulant citrate contains calories whereas heparin does not contain calories.
Where lactate-containing solutions are being used, energy will be derived from its metabolism. Each millimole of lactate provides 0.32kcal (Casaer et al, 2008). In practice it is often difficult to estimate exact net energy gains but it should be taken into account. In newer replacement solutions, bicarbonate has replaced lactate as a buffer and consequently there will be no calorie gain.
Citrate is an anticoagulant commonly used in CRRT to prevent clotting (sometimes used instead of heparin) within the circuit by binding with calcium, providing a calorific value is 0.59 kcal/mmol (Oudemans et al, 2012; Balik et al, 2013). Citrate absorption during CRRT depends on the citrate dose, pump rate of the filter and the amount filtered out and lost in effluent. In practice, opinions differ whether to include the calorie provision of citrate due to uncertainty around it’s utilisation within the krebs cycle. The theoretical calorie provision (figure 1) is likely to be most significant and should be borne in mind with individuals where there is particular concern around overfeeding and or those at risk of refeeding syndrome.
Calorie gains will be significantly higher if using glucose and/or lactate containing dialysate solution and as such caution should be taken to avoid calories gained during CRRT leading to overfeeding in critically ill patients (Rogers et al, 2021). Similarly it is important to account for calorie provision from other medications, primarily propofol, used as a sedative in the critical care setting. Each millilitre of propofol provides 1.1 kcal/ml, which is usually taken into consideration when a patient is receiving at least 10ml/hr.
CRRT also has a negative influence on nutrient balance. Extra-corporal losses of amino acids are significant with estimated amino acid losses between 10 and 15g a day (Davies et al, 1991, Frankenfield et al, 1993, Patel et al, 2017 Oh et al 2019). Total amino acid losses have been shown to be lower in IHD vs SLED-F vs CRRT (Oh et al 2019). In CRRT protein provision should be increased to compensate for these losses and catabolism (Brown & Compher 2010) with caution (Heyland et al 2023; Stoppe et al 2023). To achieve these high protein intakes in enterally fed patients without providing excessive amounts of energy, the use of high protein, moderate energy feeds or the addition of modular protein supplements is recommended to increase the nitrogen to calorie ratio (authors opinion).
CRRT allows feed volumes and electrolyte intake to be liberalised. Hyperphosphataemia is rapidly corrected, and since most current replacement solutions currently contain no phosphate, intravenous supplementation with phosphate will usually be required if hypophosphatemia is to be avoided (Bellomo & Boyce 1993).