Effects of Ultrafiltration, Dialysis, and Temperature on Gas Exchange During Hemodiafiltration: A Laboratory Experiment

Abstract

To study gas exchange in the filter during continuous venovenous hemodiafiltration (CVVHDF), an air-tight heated mixing chamber with adjustable CO2 supply was constructed and connected to a CVVHDF monitor. Bicarbonate-free crystalloid (Part 1) and packed red blood cell (Part 2) solutions were circulated at 150 ml x min(-1). Gas exchange expressed as pre-postfilter difference in CO2 and O2 contents was measured at different CVVHDF settings and temperatures of circulating and dialysis solutions. Ultrafiltration was most efficacious for CO2 removal (at 1,000 ml x h(-1) ultrafiltration CO2 losses reached 13% of prefilter CO2 content). Addition of dialysis (1,000 ml x h(-1)) increased CO2 loss to 17% and at maximal parameters (filtration 3,000 ml x h(-1), dialysis 2,500 ml x h(-1)), the loss of CO2 amounted to 35% of prefilter content. Temperature changes of circulating and/or dialysis fluids had no significant impact on CO2 losses. The O2 exchange during CVVHDF was negligible. Currently used CVVHDF is only marginally effective in CO2 removal. Higher volume ultrafiltration combined with dialysis can be expected to reach clinical significance.

pCO₂ Reveals Arteriovenous Fistula Recirculation in Bicarbonate Hemodialysis (RecirCO₂lation Test)

“…In particular, Sombolos et al [24] proposed that an increase of HCO 3 >1.8 mmol/l in AL after 5 min of high-flux HD with respect to predialysis values is predictive of VAR. However, according to our findings, HCO 3 -increase is less efficient to discover VAR, when compared with pCO 2 -increase and with the single pCO 2 value at 5 min too; furthermore, HCO 3 concentration is affected by dialysis treatment, whereas pCO 2 remains largely unchanged throughout the entire dialysis session [28,32,33,[35][36][37] . Finally, pCO 2 is directly measured, whereas HCO 3 is computed by internal Gas Analyzer software from pCO 2 , with consequent lower accuracy.…”

“…This large difference is the driving force to CO 2 diffusion across filter membrane, allowing CO 2 dialysance 3 folds higher than bicarbonate [23] , so that blood pCO 2 at the exit of the filter is higher than pCO 2 entering into the filter [23,33,36] . This CO 2 load 77 from dialysate enters into the extracorporeal circuit, impermeable to gas [37] , and flows to the systemic circulation where it is finally cleared away by the lungs ( fig. 1 a).…”

Nutritional management in the critically ill child with acute kidney injury: a review

“…There are concerns regarding the validity of measuring REE in patients receiving CRRT. These concerns are related to bicarbonate fluxes that occur at the level of the HD membrane, namely, that the use of bicarbonate-based dialysis solutions may lead to Bbicarbonate enrichment^in the patient, which converts to CO 2 , leading to a false elevation of expired CO 2 , with a consequent overestimation of REE [42,43]. Suggested criteria for selecting pediatric patients who might benefit from indirect calorimetry in the PICU is provided in Table 3.…”