MEtabolic Consequences of Continuous Renal Replacement Therapy and Impact on Indirect cAlorimetry Study

Acute kidney injury (AKI) complicates a critical illness from 13% up to 78%, needing renal replacement therapy (RRT) up to10 % of all patients in the intensive care unit (ICU). Both intermittent (IRRT) and continuous renal replacement therapy (CRRT) are used. The advantage of the latter is that it has lesser influence on hemodynamics and is better tolerated in critical ill patients.

Another complication during their stay is the inability to feed themselves. Nutrition is a cornerstone in the care for the critical ill and should be started within 3 days of admission to the intensive care unit. To optimize a nutritional prescription, protein and energy targets need to be defined. Predicting formulae based on anthropometric measures and other parameters can be used to calculate the caloric need but indirect calorimetry (IC) remains the gold standard. Caloric need can be derived from Energy expenditure which is calculated with the Weir's equation using carbon dioxide (CO2) production (VCO2) and oxygen (O2) consumption (VO2). Therefore, it is underestimated if CO2 is lost through other means than the normal respiratory route. Hence one of the contra-indications of IC is CRRT.

The totalCO2 (tCO2) travels through the vascular structures within the red blood cells or inside plasma. There, most of the content has 3 different forms: as physically dissolved CO2, bicarbonate, and carbamino compounds. These compounds are in equilibrium with each other. During RRT, a potential loss of CO2 and its different forms may occur due to ultrafiltration in the dialysate. No large trials were conducted trying to quantify this loss nor identifying the determining factors which can be used to predict this loss. Indeed, one author even found a gain in tCO2 of the blood during dialysis with acetate. Trisodiumcitrate is used as an anticoagulant during CRRT. It is a weak base and due to pH change may alter the equilibrium of the Henderson-Hasselbalch equation and thus influence the balance between CO2 and HCO3- and its extraction through CRRT.

Although indirect calorimetry in the intensive care unit has been evaluated during CRRT, the loss of tCO2was not considered. The investigators explored the possibility to predict and easily calculate this CO2 exchange so IC can be used during CRRT.