The effects of continuous venovenous hemofiltration on coagulation activation

Acute renal failure requiring renal replacement therapy occurs in approximately 4% of patients admitted to the intensive care unit, and often these patients are treated with some form of continuous renal replacement therapy (CRRT) [1]. CRRT requires anticoagulation to allow the passage of blood through the extracorporeal circuit over a prolonged period. Maintenance of CRRT circuits for sufficient duration is important for efficacy, cost-effectiveness and minimization of blood component loss. On the other hand, the systemic anticoagulation techniques used to prevent clotting of the circuit are important causes of morbidity in CRRT. Understanding the mechanisms involved in premature clotting of the filtration circuit is mandatory to optimize anticoagulation and to maintain filter patency.

Several studies have addressed the pathophysiology of circuit thrombogenesis, but the exact mechanism by which it occurs has not yet been elucidated. Multiple factors may play a role: the extracorporeal circuit itself, treatment modalities, platelet factors, coagulation factors, natural anticoagulants and fibrinolysis [2, 3]. Clotting of CRRT circuits could be caused by increased activation of coagulation, initiated either by the (intrinsic) contact activation pathway or the (extrinsic) tissue factor/activated factor VII (FVIIa) pathway, or by low activity of the endogenous anticoagulant pathways, such as the antithrombin system, the protein C/protein S system and the tissue factor pathway inhibitor system. In addition, decreased fibrinolysis could also contribute to clotting of extracorporeal circuits.

Although much is known about the effect of a single hemodialysis treatment on the coagulation system, very few prospective studies have monitored the effects of repeated passage of blood through a CRRT circuit, and these studies were always performed with concurrent administration of anticoagulants, usually unfractionated heparin or low molecular weight heparin [4‐7]. As heparin influences tissue-factor-mediated coagulation, contact-activated coagulation [7] and fibrinolysis [8], however, studies on the activation of coagulation during CRRT should ideally be performed without anticoagulation.

In the present study in critically ill patients with acute renal failure, we studied the effects of continuous venovenous hemofiltration (CVVH) without the use of anticoagulation on the activation of coagulation and fibrinolysis.

In the present study in critically ill patients, we investigated the early effects of CVVH without anticoagulation on systemic markers of coagulation activation and fibrinolysis. During the first six hours of CVVH, increased thrombin generation was found in only four out of ten patients. An early increase in transmembrane pressure, indicating filter clotting, was exclusively seen in the four patients with thrombin generation. Premature clotting of the circuit was found in three of these four patients, necessitating replacement of the circuit. CVVH without anticoagulation did not change the systemic concentrations of markers of the intrinsic pathway or the extrinsic pathway, nor did CVVH affect the systemic concentrations of fibrinolysis markers.

Thrombin generation on an artificial surface, such as the filter membrane, has traditionally been attributed to contact activation of the intrinsic pathway of coagulation that starts upon exposure of contact factors (factor XII, high molecular weight kallikrein and prekallikrein) to a negatively charged surface and their subsequent activation. We did not find any change in plasma levels of the FXIIa–C1-inhibitor complex and the kallikrein-C1-inhibitor complex, making initiation of coagulation via this pathway less likely. This finding confirms the results of Salmon and colleagues, who did not find an increase in contact activation during CVVH using a polyacrilonitrile membrane and systemic heparinization [6]. Interestingly, in our study baseline levels of the FXIIa–C1-inhibitor complex and the kallikrein–C1-inhibitor complex were relatively lower in patients with early increased thrombin generation during CVVH. Several authors have described the role of FXIIa and kallikrein in the activation of fibrinolysis [11, 12]. Factor XII is able to activate fibrinolysis by three different pathways: it activates prekallikrein, which in turn activates urokinase-type plasminogen activator; following the activation of prekallikrein, the kallikrein generated can liberate t-PA; and factor XII activates plasminogen directly.

The role of contact activation-dependent fibrinolysis in vivo is unclear, but a relationship between contact activation-dependent fibrinolysis and thromboembolic complications has been described [13, 14]. Low baseline activation of the contact system may therefore be associated with lower fibrinolysis and an increased risk of filter clotting. In our study, however, fibrinolysis during CVVH was not decreased in group A. On the contrary, we observed a trend towards increased PAP levels during CVVH in patients with early clotting of the filter. This PAP level increase is most probably caused by activated coagulation leading to plasmin generation from plasminogen on the formed fibrin. In this respect, therefore, the PAP levels may be more an indication of coagulation than of fibrinolytic activity itself.

Alternatively, one could speculate that patients with higher baseline levels of FXIIa–C1-inhibitor complex and kallikrein–C1-inhibitor complex have higher baseline thrombin generation. Baseline F1+2 levels were higher in group B than in group A, although the difference was not statistically significant, possibly due to the small number of patients. Thrombin is required for activation of the endogenous anticoagulant protein C system [15, 16]. In patients with higher levels of FXIIa–C1-inhibitor complex and kallikrein-C1-inhibitor complex, it is conceivable that coagulation activation during CVVH is decreased following increased endogenous anticoagulant activity. Indeed, an anticoagulant effect of thrombin infusion has been reported in a dog model [16]. In the present study, the protein C levels were no different at baseline between the two groups; however, we did not measure the 'activated' protein C levels.

A contribution of the extrinsic pathway to thrombin generation on artificial surfaces is unexpected at first sight since tissue factor is normally not found on the surface of cells in contact with blood. Monocytes, however, can express tissue factor under certain pathophysiologic conditions, mostly associated with increased endotoxin and/or cytokine levels [17]. Based on the measurements of circulating FVIIa, soluble tissue factor and TFPI, we did not find signs of activation of coagulation via the extrinsic pathway. Our findings are in contrast with another study that concluded activation of tissue factor/FVIIa-mediated coagulation took place in critically ill patients treated with CVVH [4]. This conclusion was based on increased levels of thrombin–antithrombin complexes and FVIIa and on decreased levels of TFPI during CVVH. The change in circulating FVIIa and TFPI levels, however, was relative to values just after the start of CVVH with concurrent administration of heparin. No change in FVIIa and TFPI levels was found when they were compared with pre-CVVH values. The observed changes in TFPI and FVIIa in that study may represent the effects of heparin, rather than activation of the tissue factor/FVIIa-mediated pathway of coagulation, because the concentration of TFPI increases after administration of heparin [18], and because high TFPI levels may bind FVIIa. In our study CVVH was performed without administration of heparin, and no changes in markers of tissue factor/FVIIa-mediated coagulation were observed.

What is the mechanism of increased thrombin generation in the absence of detectable activation of the extrinsic coagulation system and intrinsic coagulation system? One explanation could be a lack of sensitivity of the systemic markers such as soluble tissue factor, FVIIa and TFPI. The total volume of blood in the extracorporeal circuit is only approximately 300 ml. The absolute amount of thrombin formation may therefore be too low to lead to detectable increases in plasma levels of precursor proteins, such as soluble tissue factor or FVIIa. In that case, different study designs are needed to show the pathophysiologic mechanism underlying coagulation during CVVH (for example, studies analyzing tissue factor expression on monocytes in prefilter and postfilter samples, or studies directly analyzing the clot formed in the hemofilter).

Alternatively, an increase in systemic coagulation markers could be prevented by the removal of markers across the filter membrane into the ultrafiltrate or secondary to adsorption to the membrane. The high molecular weight (≥ 35 kDa) and polarity of coagulation factors, however, should significantly prevent marker removal during hemofiltration [19]. In our previous in vitro hemofiltration study using the same cellulose triacetate membrane as in the present study, we found only minimal filtration of IL-6 (molecular weight, 23–30 kDa) and the calculated sieving coefficient was approximately 0.1 in the predilution mode [20]. In general, the process of adsorption to the membrane is rapidly saturated, but we cannot rule out some adsorption to the membrane during the first hour of CVVH.

Finally, it is also conceivable that alternative pathways of thrombin generation are responsible for filter clotting, including the direct activation of factor X, either on the surface of activated platelets or by the integrin receptor MAC-1 on leukocytes [3].

Low levels of natural anticoagulants have been suggested to contribute to early filter clotting. In the randomized CRRT study by Kutsogiannis and colleagues [21], comparing regional citrate anticoagulation with heparin anticoagulation, decreasing antithrombin levels were an independent predictor of an increased risk of filter failure. In the retrospective study by du Cheyron and colleagues [22] in sepsis patients requiring CRRT and with acquired antithrombin deficiency, anticoagulation with unfractionated heparin plus antithrombin supplementation prevented premature filter clotting. In our own experience, treatment with recombinant human activated protein C obviated additional anticoagulation during CVVH in patients with severe sepsis [23]. In the present study, however, baseline levels of antithrombin and protein C were not extremely low and no significant difference between patients with and without early thrombin generation was found.

Another natural defense mechanism against activated coagulation is the fibrinolytic system, and a disturbance of the normal balance between fibrinolysis and antifibrinolysis might play a role in thrombosis of the CVVH circuit. In our study the difference in PAP complex, t-PA and plasminogen activator inhibitor type 1 levels before and during CVVH were not statistically significant in patients with and without thrombin generation, but our study is limited by the small number of patients.

At baseline, the platelet count was significantly lower and the prothrombin time and activated partial thromboplastin time were significantly longer in those patients without subsequent coagulation activation. The association of a low platelet count with a decreased risk of filter clotting confirms our finding in earlier studies [24]. The association also confirms the findings of Holt and colleagues, who showed an association between the starting activated partial thromboplastin time and the time to circuit clotting [25].

We conclude that activation of coagulation can be detected in a minority of intensive care patients treated with CVVH without anticoagulation. Systemic concentrations of markers of the tissue factor/FVIIa system and the contact system did not change during CVVH. We suggest that different studies investigating the activation of coagulation directly at the site of the filter are needed to elucidate the mechanism of clot formation during CVVH.