Continuous Renal Replacement Therapy in Critically Ill Patients: Monitoring Circuit Function

Abstract

There is currently no universally accepted method to monitor circuit function or guidelines for circuit replacement during continuous renal replacement therapies (CRRT). The objectives of this study were to diagnose the causes of circuit failure, identify factors responsible for circuit clotting and determine a predictive monitor of circuit function. The CRRT technique used in this study was continuous venovenous haemodialysis (CVVHD). Continuous monitoring of circuit pressures (pre- and post-haemofilter and their difference: the transfilter pressure gradient) was used to diagnose the causes of circuit failure. In circuits ceasing due to clotting, the factors thought to contribute, anticoagulation, haematocrit and platelet count, were measured at the commencement of CVVHD and every eight hours thereafter until circuit failure. Monitors of circuit function, creatinine clearance and plasma to diafiltrate urea ratio were measured every eight hours and compared to the transfilter pressure gradient. During a three-month period data was collected on five consecutive patients (41 consecutive haemofilters). Clotting of the haemofilter (63%) and air detection chamber (7.5%) were the most common identifiable causes of circuit failure. The duration of their circuit life was described using multiple regression analysis, i.e. hours of filter life = -82.8 + (Δ platelet count x 0.25) + (Δ haematocrit x 3.6) + (circuit flow [ml/min] x 4) R2 = 0.77. A rise in transfilter pressure gradient and a fall in haemofilter function discriminated clotted filters with falling function (decrease in creatinine clearance and urea ratio) from unclotted filters. In any circuit an increase of 26 mmHg or more in the transfilter pressure gradient accurately predicted circuit failure due to clotting and imminent cessation of function. Increases in platelet count, haematocrit, and low circuit flows are important determinants of haemofilter life. The measurement of transfilter pressure gradient across the haemofilter is an accurate bedside monitor of circuit function.

Conventional coagulation and thromboelastograph parameters and longevity of continuous renal replacement circuits

“…However, the effect the total platelet number has on circuit longevity appears weak. We were only able to show that a rising platelet count whilst on a CRRT circuit was predictive of shorter circuit life [10]. Independent of platelet number, platelet function can be an important determinant of coagulation status in CIPs.…”

“…Various authors have made recommendations as to maximum circuit duration; these have varied from 48 to 72 h or until there is visual evidence of clot [8,9]. We have found visual evidence of clot, particularly at the ends of the haemofilter, to be unreliable in predicting circuit failure due to clotting [10]. In our experience when circuit duration falls below 18 h CRRT efficacy and cost effectiveness are increasingly adversely affected.…”

“…These pressures were pulsatile and an electronic mean was used to calculate the pressure gradient across the haemofilter (P-grad). The P-grad rises as the haemofilter clots over a period of time due to increased resistance and we have previously shown, with an identical circuit, that a rise in P-grad above the baseline of greater than 26 mmHg identifies haemofilter clotting [10].…”

“…In the case of heparin there is no demonstrated relationship between degree of heparinisation, as measured by the activated partial thromboplastin time (APTT) and duration of circuits [1]. We have shown that, in a small group of CIPs receiving CRRT, there was no significant relation between the starting, mean, highest and lowest APTT and circuit life [10]. Furthermore, there appears to be no reliable way to predict which CIP needs heparin at all.…”

“…By monitoring pre-and post-haemofilter circuit pressures continuously it is possible to determine whether circuits are lost due to haemofilter clotting, air detector chamber clotting or dialysis catheter problems [10]. Circuits are also replaced electively for patient procedures or movement, while others fail without any characteristic pressure changes of circuit clotting, presumably due to inadequate correction of transient flow problems.…”