Discussion
blood loss, hemoconcentration, critically ill, hemodilution
BLOOD LOSS, MINOR, CRITICALLY ILL
In this quasi-randomized pragmatic cluster trial of critically ill patients requiring CKRT, we found that filter life in pre-filter CVVH was 79% of filter life in CVVHD, a result that was statistically significant. The median absolute difference was 4.8 hours, and the mean absolute difference was 7.9 hours. The percentage of filters changed due to clotting was higher (26.7%) for the pre-filter CVVH group than for CVVHD group (17.5%). Finally, a total of 11.8% of pre-filter CVVH vs 21.2% in CVVHD reached 72 hours, suggesting that the differences in filter life were driven in part by the number of filters reaching maximum life.Filter loss is a major problem in CKRT delivery which results in a decrease in time on treatment, reductions in daily delivered effluent dose, and blood loss. In addition, more frequent filter changes increase the cost of CKRT delivery [Circuit clotting can occur for a variety of reasons, including suboptimal vascular access, inadequate anticoagulation, or underlying patient factors. One possible contributing factor that has been relatively understudied is CKRT modality, and in fact, few previous studies have evaluated CKRT modality as a risk factor for filter loss. In one study from 2006, Ricci et al. evaluated differences in solute removal between CVVH and CVVHD in 15 patients, and found that CVVHD had a significantly longer half-life (37 hours vs 19 hours) [Our finding of longer filter life in CVVHD can potentially be explained by modality-specific differences in hemoconcentration and concentration shifts within the filter. One major difference between convective (CVVH) modalities using post-filter replacement fluid and diffusive (CVVHD) modalities in seen in filtration fraction (FF), which is the ratio of effluent flow rate divided by plasma flow rate, is a measure of the degree of hemoconcentration that occurs as blood passes through a CKRT filter [Prefilter administration of replacement fluid dilutes blood entering the filter, and therefore prolongs filter life [In CVVHD, there is no movement of fluid across the filter other than minor contributions from ultrafiltration, because clearance is achieved through diffusion rather than convection. As a result, there is neither hemoconcentration nor rapid shifting in hematocrit concentration in CVVHD, which may theoretically reduce the rate of filter clotting. This prospective study did show prolonged filter life and reduced filter clotting with CVVHD rather than pre-filter CVVH, as we hypothesized given differences in FF and concentration shifts between the two modalities. Further research into the possible mechanisms of filter clotting by modality is needed.Our findings have potential implications for CKRT modality selection. Historically, convective clearance or a combination of convection and diffusion has been preferred over diffusion alone. Convection is known to have better middle-molecule clearance than diffusion [Pre-filter CVVH is known to have less efficient clearance compared to either CVVHD or post-filter CVVH because blood is diluted prior to entering the circuit. This disadvantage was thought to be off-set by better filter life due to hemodilution of blood before entering the filter [Our trial has multiple limitations that must be acknowledged. The median filter lives in this study of 21.8 and 26.6 hours (32.1 and 40.0 mean hours) for pre-filter CVVH and CVVHD respectively are lower than many, though not all, other published studies using citrate [In addition, COVID-19 affected nearly a third of the AKI-CKRT population in this trial. COVID-19 is known to increase filter clotting [Another limitation is that patients were able to change modality during the study at the discretion of the attending on service, which was deemed necessary to ensure that patient risk would be minimal in the study. Modality changes from pre-filter CVVH to CVVHD, and vice-versa, were rare, without a clear directional bias. The most common modality change was from CVVH to CVVHDF, which affected 47 filters, and may have been driven by higher filter pressures in the pre-filter CVVH group. Conversely, changing from CVVHD to CVVHDF was seen in only 17 filters. The per-protocol analysis, however, showed results that were similar to the ITT analysis. Finally, an accidental block repeat at blocks 9 and 10 resulted in 11 pre-filter CVVH blocks compared to 9 CVVHD blocks.