Page 1364 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

CHAPTER 98: Renal Replacement Therapy in the Intensive Care Unit 937

delivery fell in the intermittent versus continuous therapy arm. After the To better understand the effect of RRT in treating acid-base distur-
initiation of therapy, intracranial pressure fell in the CRRT group prior bances, investigators have specifically examined the effect of RRT on
to returning back to the baseline value. Conversely, intracranial pressure lactate clearance. Two hundred patients with AKI and lactic acidosis
increased in the intermittent therapy group and remained increased treated with CVVH were examined retrospectively. While lactic acidosis
throughout the duration of the treatment. No difference in hospital resolved in 45% of individuals while they were receiving CRRT, resolu-
outcome or mortality was reported. The results of the, albeit limited, tion of lactic acidosis had no effect on survival. Further, the authors
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literature on RRT in the setting of acute liver failure suggest that inter- attributed most of the improvement in the lactic acidosis to spontane-
mittent therapies increase ICP and, thus, increase the risk for cerebral ous improvement in metabolism and improved endogenous lactate
herniation. Further, similar to other states of circulatory dysfunction, clearance rather than attributable to CVVH therapy. A prospective
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there is greater hemodynamic stability with CRRT versus intermittent study designed to examine lactate clearance by CRRT confirmed this
therapy. Individuals with both AKI and fulminant hepatic failure may conclusion. Ten individuals with AKI and stable plasma lactate levels
represent a specialized group of patients where, in attempts to “do no treated with CVVH were infused with sodium lactate. Serial blood and
harm,” CRRT is the preferred modality of treatment. effluent lactate concentrations were measured to determine total lactate
■ ACUTE NEUROLOGIC INJURY clearance and the proportion of lactate clearance attributable to the
CRRT. Total plasma lactate clearance had a mean of 1379 mL/min with
The data demonstrating the superior cerebral perfusion of CRRT over a mean clearance of 24.2 mL/min attributable to extracorporeal therapy.
Fractional lactate clearance achieved by CRRT ranged between 0.5%
IHD in patients with fulminant hepatic failure may also apply to other and 3.2%. While lactate clearance achieved by CRRT was equivalent
forms of acute neurologic injury where cerebral perfusion pressure is to urea clearance, the extracorporeal circuit played a very small role in
already compromised such as intracranial hemorrhage, postneurosurgi- total lactate clearance. The available data on RRT in treating acid-based
27
cal, etc. While explicit data comparing cerebral perfusion in patients disorders provide limited guidance regarding the superiority of one
receiving CRRT or IHD do not exist, the same principles outlined above modality. The available data suggest that endogenous pathways of
apply. IHD does induce increased in brain water, thereby increasing clearing intermediate metabolites (eg, lactate) are far superior to extra-
ICP. In clinical scenarios where acute neurologic injury has occurred corporeal removal even with continuous therapies. Thus, while there is
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and lowering of CPP may exacerbate the injury, CRRT may be prefer- often much enthusiasm regarding the use of CRRT to correct acid-based
able as a modality. If CRRT is not available, IHD with reduced strategies disorders in the critically ill with AKI, available data do not suggest it
to minimize solute shift—reduced blood flow rates, reduced dialysate significantly affects patient outcome.
flow rates and dialysate sodium modeling (keeping dialysate sodium
>145 mmol/L)—should be employed. ■ HYPERPHOSPHATEMIA
■ ACID-BASE DISTURBANCE Increased total body phosphate is near-universally observed in the setting

RRT is an effective means of treating acid-base disturbances. Large of significant renal dysfunction. Concomitant tissue injury from tumor
lysis, rhabdomyolysis, or visceral organ ischemia observed in the setting
amounts of a buffered solution (bicarbonate or lactate based) can be of critical illness as well as concomitant respiratory acidosis can all com-
administered with concomitant removal of excess sodium, water as well pound impaired renal clearance and lead to severe hyperphosphatemia.
as the removal of organic acids that accumulate in renal dysfunction. The large volume of distribution of phosphate limits the efficiency of
The critically ill represent a special population with regard to acid-base intermittent renal replacement therapies in phosphate clearance. In a
disturbance. Often, they have mixed acid-based disorders (concomitant study by Ratanarat et al comparing phosphate clearance in IHD, sus-
metabolic and respiratory disturbances) and the metabolic acidosis tained low-efficiency dialysis (SLED), and CVVH, investigators found
results not only from the retention of the organic acids that accumu- that duration of therapy was the only variable that correlated to phos-
late in renal dysfunction, but also the generation of organic acids from phate clearance and, accordingly, CVVH achieved the greatest phosphate
anaerobic metabolism (ie, lactate, pyruvate, etc) and the other effects clearance. The mean phosphate removal per week was 90 mmol in IHD
of systemic inflammation. Given the increased complexity and, often, group, 235 mmol in the SLED group, and 397 mmol in the CVVH group
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more severe form of acid-base disturbance in the setting of AKI and (p < 0.0001). The superiority of continuous modalities in phosphate
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critical illness, the question of modality of RRT arises. clearance suggests that in clinical scenarios where there is severe hyper-
Unfortunately, little data exist to address this question. The previous phosphatemia or continued intracellular phosphate release (eg, tumor
studies outlined did not look at acid-base status as an entry criterion lysis, rhabdomyolysis, etc), CRRT is the preferred modality.
or as an outcome. The results of a retrospective, observational study
bances. Forty-seven individuals with AKI in ICU treated with IHD were ■ SEVERE SEPSIS
suggest a potential benefit of CRRT for the control of acid-base distur-
compared with 49 patients with AKI in the ICU treated with CVVHDF The overall prognosis of critically ill patients with AKI remains poor with
with regard to the effect of RRT on electrolyte and acid-base status. The mortality rates exceeding 50% in the most recent multicenter epidemio-
two groups were comparable with regard to baseline arterial bicarbon- logic study. A proposed theory is that in sepsis, and especially in sepsis-
1
ate concentration and proportion of individuals with decreased arte- associated AKI, the marked increase in proinflammatory cytokines
rial bicarbonate concentration. Patients treated with CVVHDF had a creates the clinical milieu of hypotension, volume overload, protein-
significant increase in the arterial bicarbonate concentration in the first energy wasting, and neurologic dysfunction that collectively contribute
48 hours, whereas the individuals treated with IHD did not. Further, to overall high degree of morbidity and mortality. Investigators have
individuals treated with CVVHDF were more likely to maintain normal demonstrated that increased levels of proinflammatory cytokines (TNF-
arterial bicarbonate concentration throughout the observation period α, IL-1β, IL-6) are associated with increased mortality in the setting
than their counterparts receiving IHD (71.5% vs 59.2%; p = 0.007). The of generalized critical illness with and without AKI. 29,30 Extracorporeal
study did not examine the clinical significance of these differences. therapy, in the form of renal replacement therapy, represents a potential
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Given that the patients were well matched and the patients treated with means of removing these substances. A prospective study of 33 patients
IHD were supported with the only modality available at the time (rather receiving CVVH demonstrated removal of TNF-α, IL-1β, and IL-6
than individuals selected as “less sick”), the study findings support the via their presence in the ultrafiltrate. However, plasma levels of cyto-
superiority of CRRT in ameliorating acid-base disturbance. However, kines were not significantly affected. Similarly, a prospective study
31
whether the improved laboratory parameters translate to better patient of 15 patients receiving CVVH demonstrated that both proinflamma-
outcomes cannot be determined from this study. tory cytokines (TNF-α, IL-1β, IL-6) and anti-inflammatory cytokines

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