A randomised comparative crossover study to assess the affect on circuit life of varying pre-dilution volumes associated with continuous veno-venous haemofiltration (CVVH) and continuous veno-venous haemodiafiltration (CVVHDf)

Abstract

Continuous renal replacement therapy (CRRT) is an established treatment option in Australia for critically ill patients with acute renal failure (ARF). Critical care nurses play a primary role in the set-up of equipment, monitoring and care of patients receiving CRRT. Although described as a continuous therapy, delays or interruptions in CRRT can interfere with treatment efficiency. A review of the literature identified how optimal circuit function is an important factor in determining the effectiveness of treatment and patient outcomes.The aim of this research was to evaluate treatment efficiency in terms of circuit life between two widely used forms of CRRT, continuous veno-venous haemofiltration (CVVH) versus continuous veno-venous haemodiafiltration (CVVHDf). The investigation focused attention on the influence higher pre-dilution volumes and convective clearance of CVVH may have on circuit life when compared to the lower pre-dilution volumes and diffusive clearance required for CVVHDf.This thesis describes how the impact of CVVH versus CVVHDf on circuit life was investigated using a randomised comparative crossover study design. Once institutional ethics committee approval had been received, 45 patients were recruited to the study who were 18 years or older and required the commencement of CRRT as part of their Intensive Care treatment. Of the 45 patients who were randomised to receive CVVH or CVVHDf, 31 patients achieved a successful crossover to the alternative technique. Failure to achieve a „natural‟ circuit life – that is one which terminated due to clotting, in a CVVH and CVVHDf circuit accounted for the large drop out rate. Blood flow rate, vascular access device and insertion site, haemofilter, anticoagulation and machine hardware were standardised. An ultrafiltrate dose 35 millilitres (ml) per kilogram (kg) per hour (hr) delivered pre-filter was used for CVVH and a fixed pre-dilution volume of 600ml per hr with a dialysate dose of 1litre (L) was used for CVVHDf. Patients were excluded if coagulopathic, thrombocytopenic or unable to receive heparin.Of the 31 paired comparisons there was a significant difference in circuit life measurements between CVVH and CVVHDf after a paired-sample t-test was performed following natural logarithm base-e (ln) dataset transformation (CVVH 6.101 versus CVVHDf 6.779, P-value = 0.001). A Wilcoxon signed ranks test used raw dataset values of circuit life measurements as an alternative non-parametric comparison (Z = -4.076, P-value < 0.001).The probability of circuit survival for each treatment mode was estimated using the Kaplan-Meir method from the 93 circuits which had survived to clotting (50 CVVH circuits and 43 CVVHDf circuits). Using the truncation point of 16 hr as a measure of expected minimum survival, 50 percent (%) of CVVHDf circuits remained in operation when compared with only a 5% for CVVH circuits. The same 93 circuits were also used in a linear multiple regression analysis. None of the independent variables (activated prothrombin time, platelet count, heparin dose, patient haematocrit, urea) had a coefficient partial correlation > 0.09 (coefficient of determination = 0.117) or a linear relationship which could be associated with circuit life (P-value = 0.228).The evaluation of treatment efficiency in terms of circuit life between the different techniques of CVVH and CVVHDF is of clinical importance, since each treatment mode depends upon a measure of circuit longevity to achieve adequate replacement of renal function. Numerous factors have been described which influence circuit life in the delivery of CRRT including circuit and filter design, anticoagulation and staff training and expertise. In this study a longer circuit life was reported using CVVHDf which incorporated lower pre-dilution volumes when compared with the higher pre-dilution volumes associated with CVVH. This could possibly be explained by the physical processes involved in fluid and solute transport across the filter membrane. The choice of CRRT mode is a factor which may be an important independent determinant of circuit life using the techniques of CVVH and CVVHDf. This information may influence intensive care nursing practice in respect of mode selection for CRRT in collaboration with medical colleagues.