Renal replacement therapy blood flow

Some of the renal replacement therapies listed in Table 6.2 incorporate continuous hemodialysis or a combination of continuous hemofiltration and hemodialysis. Continuous hemodialysis differs importantly from conventional intermittent hemodialysis in that the flow rate of dialysate is much lower than is countercurrent blood flow through the dialyzer. As a result/ concentrations of many solutes in dialysate leaving the dialyzer (Cd) will have nearly equilibrated with their plasma concentrations in blood entering the dialyzer (Cp) (16/31). The extent to which this equilibration is complete is referred to as the dialysate saturation (Sd) and is calculated as the following ratio ... 

In contrast with intermittent hemodialysis in which dialyzer blood flow is rate limiting/ diffusive drug clearance during continuous renal replacement therapy is limited by dialysate flow (Qd)/ which typically is only 25 mL/min. Accordingly/ diffusive drug clearance (CLd) is calculated from the equation ... 

The utility of continuous renal replacement therapies (CRRT) such as continuous venous-venous hemodialysis (CWHD) in the treatment of poisoning is uncertain. As CRRT provides slower clearance than conventional hemodialysis it may not be appropriate for drug removal in acute intoxications [25]. However, the lower blood flow rates and longer treatment times of continuous modalities may be desirable for vulnerable, hemodynamically unstable, patients who are not candidates for conventional hemodialysis [7]. Unlike hemodialysis, CRRT can give effective clearances in hypotensive patients. If the clinical condition of the patient requires a low intensity treatment that will necessarily decrease diffusive clearance, slow extended dialysis (SLED) or continuous treatment times with additional convective clearance (CVVHF and CVVHDF) can likely provide adequate total drug clearance [24]. 

Supportive care goals for the critically ill patient with ARF include aggressive fluid management. Cardiac output and blood pressure must be supported to allow for adequate tissue perfusion. However, a fine balance must be struck in this regard. For example, fluids must be typically restricted in anuric and ohguric patients unless the patient is hypovolemic or is able to achieve fluid balance via renal replacement therapy. If fluid intake is not minimized, edema rapidly occurs, especially in hypoalbuminemic patients. In contrast, vasopressors like dopamine >2 mcg/kg per minute or norepinephrine are used to maintain adequate tissue perfusion, but may also induce kidney hypoxia via a reduction in renal blood flow. Consequently, S wan-Ganz monitoring is essential for critically ill patients. 

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