Clinical Effects of Continuous Renal Replacement Therapies

Fluid Removal

Continuous RRT (CRRT) slowly and continuously removes fluid, approximating ongoing urinary output, whereas intermittent hemodialysis must extract up to 2 days' worth of administered fluid plus excess body water, which may be present in the anuric patient as a result of a pathologic process, in one relatively brief session. The intravascular volume depletion associated with intermittent hemodialysis (IHD) is due to the high rate of fluid removal required and the transcellular and interstitial fluid shifts caused by the rapid dialytic loss of solute. The major consequence of rapid fluid removal is hemodynamic instability. Critically ill patients need continuous volume infusions: blood and fresh frozen plasma, vasopressors and other continuous infusions, and parenteral and enteral nutrition, which must be delivered without restriction or interruption even in hypercatabolic patients. In the clinical setting of anuria, providing such infusions carries a constant risk for fluid overload and high daily ultrafiltration requirements. Examples of patients in whom sudden intravascular volume shifts may be catastrophic are the patient with acute respiratory distress syndrome (ARDS), the septic patient who is becoming refractory to vasopressors, and the patient with cerebral edema. Furthermore, all critically ill patients tolerate hypotension poorly, with a definite risk of cardiac arrest, particularly if they are already inotrope dependent. Indeed, the damaged kidneys, which have temporarily lost pressure-flow autoregulation, also may be threatened with fresh ischemic lesions occurring with each hemodialysis session, leading to a delay in renal recovery. Patients should be assessed actively for the final target of fluid removal and must be reassessed carefully and frequently, whichever method is used to achieve this. Setting the rate of removal requires consideration of the severity of complications of fluid overload, anticipated fluid intake, expected rate of vascular refilling, and cardiovascular tolerance to transient reduction in intravascular volume resulting from ultrafiltration. Although many tools can be used to predict the response to fluid administration (such as pulse pressure variations or passive leg raising), there are no good indicators to predict tolerance to fluid removal. A fluid removal trial (reverse fluid challenge) is therefore often the only option while assessing cardiovascular tolerance with the available hemodynamic tools.

The importance of fluid balance management is enhanced in the specific category of patients with decompensated heart failure. In fact, it is just these patients who may well respond positively to continuous ultrafiltration with a rise in cardiac index, while avoiding a fall in arterial pressure, owing to a beneficial change in preload optimizing myocardial contractility on the Starling curve. In many instances, congestive heart failure not responding to conventional therapy now can be treated successfully in this way.

In critically ill children, the correction of water overload is considered a priority. It has been shown that restoring adequate water content in small children is the main independent variable for outcome prediction. This concept is much more important in critically ill neonates, in whom a relatively larger volume of fluid must be administered to deliver an adequate amount of drug infusion, parenteral or enteral nutrition, and blood derivatives. Several retrospective and observational studies also have confirmed the importance of fluid overload as an independent variable affecting adult critically ill patients' mortality. It is possible that starting ultrafiltration when a lower degree of fluid accumulation has been reached and targeting a negative fluid balance in the first treatment hours may improve outcome. However, provided no prospective data, it is impossible to recommend a priori at what level of fluid gain RRT should be started or the net ultrafiltration rate that should be prescribed. These factors should be tailored according to individual patient requirements.