Plasma CO2 Removal Due to CRRT and Its Influence on Indirect Calorimetry

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.

Inclusion criteria

• AKI requiring CRRT
• Patient on CRRT who's filter you want to change
• Expected stable patient during the test ( +- 2h) evaluated at discretion of physician :

• No alteration in medication
• Stable respiratory settings where no change in conditions is expected. If possible, controlled mode ventilation is preferred.
• Expected stable pH and lactate
• no intervention will be made on patient (transport/washing/physiotherapy/…)
• no alterations on settings of CRRT is expected to be made.
• Maximal respiratory settings: max FiO2: 60% / max inspiratory plateau pressure 30 mmHg/max tidal volumes 8ml/kg
• pH between 7,30-7,50, lactate levels <2,0
• starting settings CRRT with citrate:

• Blood pump flow: 150 ml/min
• Predilution ( citrate): 1500-2300ml/h
• Dialysate dose: 25-40 ml/kg/h
• ultrafiltration: 0-300 ml /h
• Substitution: NaCl 300-800 ml/h or B22: 400-2000 ml/h

Exclusion criteria

• Pregnancy / lactation
• Contra-indications for the use of indirect calorimetry as stated by the AARC (FiO2>60%, chest tubes)
• Severe hemodynamic or ventilator instability.
• CRRT modalities unusual to daily clinical ICU practice