Renal replacement therapy for ARF generally involves intermittent hemodialysis (IHD) or continuous renal replacement therapy (CRRT), e.g., continuous veno-veno-hemofiltration (CVVH). Since the hemodynamic stress is less with CVVH than with IHD, it is possible that any additional hemodynamic or nephrotoxic insult, which might prolong the course of ARF and thereby increase mortality, might be less with CRRT. The most recent metaanalysis, however, of randomized results comparing IHD with CRRT in ARF has not shown any difference in survival (72).
As with most disease conditions, the earlier an intervention can be instituted in acute renal ischemia, the more favorable the outcome. Thus, biomarkers more sensitive than the rise in serum creatinine concentration associated with ARF will be necessary to achieve early intervention. As previously discussed, there are several diagnostic markers under study. Presently, however, the determination of FENa using spot urine and blood sodium and creatinine measurements is the primary and most readily available early marker of established ARF.
Prolonged duration of the ARF clinical course and the need for dialysis are major factors projecting a poor prognosis. Patients with ARF who require dialysis have a 50–70% mortality rate. Infection and cardiopulmonary complications are the major causes of death in patients with ARF. Excessive fluid administration in patients with established ARF may lead to pulmonary congestion, hypoxia, the need for ventilatory support, pneumonia, and multiorgan dysfunction syndrome, which has an 80–90% mortality rate (9, 73, 74). Until means to reverse the diminished host defense mechanisms in azotemic patients with clinical ARF are available, every effort should be made to avoid invasive procedures such as the placement of bladder catheters, intravenous lines, and mechanical ventilation.
Over and above such supportive care, it may be that combination therapy will be necessary to prevent or attenuate the course of ARF. Such combination therapy must involve agents with potential beneficial effects on vascular tone, tubular obstruction, and inflammation. However, vasodilator agents such as calcium channel blockers and natriuretic peptides may induce unwanted side effects such as systemic vasodilation and hypotension (75, 76), which increase sympathetic tone and activity of the renin-angiotensin system (77). These compensatory neurohormonal responses support blood pressure but cause renal vasoconstriction (78), which may obscure beneficial effects of calcium channel blockers and natriuretic peptides on the kidney.
Since an improvement of GFR of only 10 ml/min from each kidney of a patient with ARF may circumvent the need for dialysis and potentially improve survival, the bilateral intrarenal infusion of a short-acting vasodilator and/or an impermeant solute such as mannitol is a potential approach that is less invasive than hemodialysis (79). Another approach is to combine a systemic vasodilator such as a natriuretic peptide, with dopamine (80) or mannitol (81), which not only will attenuate the systemic hypotensive effects of the vasodilator but can also increase tubular solute flow and thereby decrease tubular obstruction. This combined therapeutic approach has been shown to be effective in acute renal injury in experimental animals (81).
The effect of antiinflammatory agents, including reactive oxygen species scavengers, in the treatment of ARF should also be investigated. As discussed earlier, the use of inhibitors of NOS must be specific, since ARF is made more severe by nonspecific NOS inhibition (66). For example, a specific iNOS inhibitor, L-NIL, has been shown to afford protection in sepsis-related ARF in the rat (67). In this regard, cytokine-induced tubular damage may occur in sepsis in the absence of a decrease in renal blood flow. The antiinflammatory effect of and inhibition of iNOS by αMSH, which has been shown to be effective up to 8 hours after the insult, also needs to be studied as an approach to alter the course of clinical ARF (61). Inhibition of TNF-α has afforded renal protection in experimental endotoxemic ARF (82). However, the administration of antibodies to TNF-α in septic patients has not improved survival (83). In acute renal ischemia, the cellular breakdown of ATP occurs with leakage of the nucleotide products out of tubular cells. On this background, the administration of exogenous ATP has been shown to afford protection in experimental ischemic ATP. While ATP salvage has been shown to be protective against experimental tubular necrosis (84), recent experimental evidence has suggested that GTP salvage may be preferred in preventing apoptosis associated with acute renal injury (85). The effect of the administration of synthetic RGD peptides to attenuate tubular obstruction has yet to be examined in clinical ARF. However, since patients are generally seen after clinical ARF is established with a GFR less than 10% of normal, delivery of these synthetic RGD peptides to the tubular lumen would be difficult in the absence of accompanying intrarenal vasodilation. Experiments examining upregulation of heat shock proteins (57, 86) or the protective effects of preconditioning against a later insult (70) suggest intriguing renal-protective approaches in need of further study.
Since patients with clinical ARF are generally seen after the insult, with the exception of kidney transplant subjects (22) and recipients of radiocontrast media (87), ways to enhance recovery and thereby lessen the duration of clinical ARF have been sought. Unfortunately, the beneficial effects of insulin-derived growth factor observed in experimental animals were not duplicated in a randomized study of patients with clinical ARF (88). A recent exciting approach in this area has been the administration of stem cells or endothelial cells to enhance recovery from acute ischemic renal injury in experimental animals (89–92). There is ongoing research into the mechanisms whereby injured tubules are relined with new cells actively engaged in DNA synthesis. The pathways by which surviving cells reenter the cell cycle and replicate may involve the early immediate response genes (93).
In summary, while much has been learned about acute renal ischemic injury, which is frequent in hospitalized patients, the mortality of clinical ARF remains high. The future, however, holds substantial promise for earlier diagnosis and effective interventions that are able to prevent or shorten the course of acute renal injury and thus improve survival. An ARF network for clinical trials with adequate statistical power would certainly facilitate this process.