Evaluation of the predilution technique in reducing the occurrence of bleeding during continuous venovenous haemofiltration in critically ill patients. Efficacy of predilution in reducing the amount of anticoagulation during CVVH

Background and objectives

Anticoagulation during haemofiltration is one of the major issues. We have to take into account three parameters: bleeding occurrence, filter life, and filter efficiency. The purpose of the study was to assess the efficacy of the predilution technique in reducing the incidence of symptomatic bleeding (by cutting down the amount of heparin) but without avoiding a shorter filter life.

Setting

Intensive care unit of a secondary hospital.

Haemofiltration technique

All these patients were put on continuous venovenous haemofiltration using a Hospal-Prisma-Pump. The membrane was in polyacrylonitrylc (1 m² of surface-capillary filter). We exchange 1 l/h without using dialysis.

Protocol

Twenty consecutive critically ill patients were randomised to have either a predilution set or a post-dilution one. Full circuit heparinisation was used to achieve an APTT twice baseline. The vascath site was restricted to the right internal jugular vein. The pump speed was kept at 100 ml/min. Filter efficiency was assessed by the haemopermeability index (HPI). The HPI represent the ultrafiltrate flow divided by the transmembrane pressure. The filter efficiency is lost when the HPI has dropped by 70%. Filter life was measured in hours until the circuit clotted off. Data were analysed using non-parametric statistical methods.

Results

• The median filter life was 38.5 h for the predilution group (n = 10) and 39.3 h for the post-dilution one (n = 10). The difference was not statistically significant (P > 0.05).

• Symptomatic bleeding happened in one patient in the predilution group and in four patients in the post-dilution one (P < 0.05, highly significant).

• The median amount of heparin used in the predilution group was about 450 U/h and 750 U/h in the post-dilution group (P < 0.05, highly significant).

• The median filter efficiency was 32.3 h for the predilution group and 33.9 for the post-dilution group (P > 0.05, no statistical difference).

• We have also measured the haematocrit, the platelet count, the oncotic pressure and the total calcium in the circuit prior to the filter (after the dilution).

• Median haematocrit was 25.1% in the predilution group and 28.5% in the other group (P < 0.05).

• The median platelet count was 116 × 10⁹/l in the predilution group and 131 × 10⁹/l in the other group (P < 0.05).

• The median oncotic pressure was 31.3 mmHg in the predilution group and 37.2 mmHg in the other group (P < 0.05).

• No difference was seen in measuring the total calcium between groups.

• Obviously, we measured haematocrit, platelet count, pancreatic pressure and total calcium in the serum and we could not find any statistical difference between groups.

Conclusion

The predilution technique is a good tool for clinicians in the setting of CVVH in critically ill patients at risk for bleeding. It allows the physician to reduce drastically the amount of heparin used without impairing the filter life and the filter efficiency. The mechanism involved seems to be the reduction of the oncotic pressure rather than other systems involving the calcium.