Hemofiltration or hemodialysis for acute kidney injury?

The optimal choice of modality for acute renal replacement therapy is unclear at present. Diffusive therapy (hemodialysis) removes small solutes mainly, whereas convective therapies (hemofiltration and hemodiafiltration) may also eliminate larger molecules such as myoglobin or cytokines. Conversely, convective therapies might predispose patients to filter clotting and thus increased costs. A systematic review and meta-analysis of clinical trials could not find evidence for clinical benefits of either modality. Thus, the decision on renal replacement therapy modality still is based on the clinical status of the individual patient, the expertise of the medical and nursing staff, and local circumstances and availability.

Th e strategy for renal replacement therapy (RRT) in patients with acute kidney injury (AKI) remains controversial. To date, crucial questions such as the optimal timing, treatment intensity, and modality for acute RRT have not found defi nitive answers from adequately powered prospective randomized controlled trials (RCTs). An area of particular uncertainty is the choice between hemodialysis, hemofi ltration, or hemodiafi ltration (that is, between diff usive and convective treatment or a combination of the two).
Although all three modalities can easily be performed on a modern continuous RRT machine with a few simple modifi cations of the setup, some major technical diff erences do exist. With convection, the elimination of solutes across a membrane is carried by water fl ux, resulting in similar removal of small, middle-sized, and larger molecules for which the membrane is permeable. In contrast, diff usive clearance is highly effi cient in removing small solutes but less so in eliminating larger compounds (such as uremic 'middle molecules' , myoglobin, or cytokines). However, convec tion requires large rates of ultrafi ltration in order to be eff ective. If the necessary replacement fl uid is adminis tered post-fi lter, hemoconcentration within the hemo fi lter will result and patients potentially will be predis posed to clotting. On the other hand, if replacement fl uid is added pre-fi lter, the treatment effi cacy is reduced since solutes are diluted before elimination.
Whether and how these fundamental diff erences translate into clinical outcomes are unclear at present. In this issue of Critical Care, Friedrich and colleagues [1] present a systematic review and meta-analysis of clinical trials that compare hemofi ltration and hemodialysis for the treatment of AKI. Th e authors found no diff erence in mortality or other clinical outcomes such as RRT dependence in survivors, organ dysfunction, or vasopressor use. Not unexpectedly, the data suggested an increased clearance of larger molecules with hemofi ltration but also a shorter fi lter life. Th e essential conclusions from the meta-analysis are that we do not have a suffi cient database at present to recommend one procedure over the other, a fact refl ected in the current AKI guidelines of the KDIGO (Kidney Disease Improving Global Outcomes) group [2] and the UK Renal Association [3], and that we will require larger clinical trials before defi nitive recommendations can be made.
However, the question is whether a 'defi nitive' prospective RCT in unselected populations with AKI will actually help to resolve this issue. As with the choice of intermittent versus continuous versus 'hybrid' RRT as the initial treatment for AKI, one size likely will not fi t all. More likely, future studies will have to address the question of whether there are specifi c subgroups of patients who might benefi t from convective therapies (for example, myoglobinuric or septic AKI patients in whom the enhanced removal of myoglobin or cytokines by hemofi ltration might help to improve clinical course and renal recovery). Moreover, the question of RRT 'dose' is inextricably linked with the choice of modality. If replacement fl uid is added pre-fi lter in order to limit

Abstract
The optimal choice of modality for acute renal replacement therapy is unclear at present. Diff usive therapy (hemodialysis) removes small solutes mainly, whereas convective therapies (hemofi ltration and hemodiafi ltration) may also eliminate larger molecules such as myoglobin or cytokines. Conversely, convective therapies might predispose patients to fi lter clotting and thus increased costs. A systematic review and meta-analysis of clinical trials could not fi nd evidence for clinical benefi ts of either modality. Thus, the decision on renal replacement therapy modality still is based on the clinical status of the individual patient, the expertise of the medical and nursing staff , and local circumstances and availability.
hemoconcentration and clotting risk, total treatment volumes must be increased by 20% to 30% to achieve equivalent clearance of small solutes. Th is, together with a potentially reduced fi lter life, may lead to increased costs of convective therapies in comparison with continuous hemodialysis.
In clinical practice, the intensivist will have to balance the desired intensity of treatment, in particular with regard to the removal of larger molecules, against clotting risk, fi lter life, and costs. Inevitably, local experience and circumstances will also infl uence the choice of modality. As the current UK Renal Association guidelines [3] put it, 'choice of RRT modality should be guided by the individual patient's clinical status, medical and nursing expertise, and availability of modality' .