Anesthesia for Patients with Mild to Moderate Renal Impairment
The kidney normally possesses large functional reserve. GFR, as determined by creatinine clearance, can decrease from 120 to 60 mL/min without any clinically perceptible change in renal function. Even patients with creatinine clearances of 40–60 mL/min usually are asymptomatic. These patients have only mild renal impairment but should still be thought of as having decreased renal reserve. The emphasis in the care of these patients is preservation of the remaining renal function, which is best accom-plished by maintaining normovolemia and normal renal perfusion.
When creatinine clearance decreases to 25–40 mL/min, renal impairment is moderate, and patients are said to have renal insufficiency. Azote-mia is always present, and hypertension and ane-8 mia are common. Correct anesthetic management of this group of patients is as critical as management of those with frank kidney failure, especially during procedures associated with a relatively high incidence of postoperative kidney failure, such as cardiac and aortic recon-9 structive surgery. Intravascular volume depletion, sepsis, obstructive jaundice, crush injuries, and renal toxins such as radiocontrast agents, certain antibiotics, angiotensin-converting enzyme inhibitors, and NSAIDs (see Table 29–4) are additional major risk factors for acute deterio-ration in renal function. Hypovolemia and decreased renal perfusion are particularly impor-tant causative factors in the development of acute postoperative kidney failure. The emphasis in man-agement of these patients is on prevention, because the mortality rate of postoperative kidney failure may surpass 50%. The combination of diabetes and preexisting kidney disease markedly increases the perioperative risk of renal function deterioration and of kidney failure. Renal protection with adequate hydration and maintenance of renal blood flow is indicatedfor patients at high risk for kidney injury and kidney failure undergoing cardiac, major aortic reconstruc-tive, and other surgical procedures associated with significant physiological trespass. The use of man-nitol, low-dose dopamine infusion, loop diuretics, or fenoldopam for renal protection is controversial and without conclusive proof of efficacy.
The American Society of Anesthesiologists’ basic monitoring standards are used for procedures involving minimal fluid losses. For procedures asso-ciated with significant blood or fluid loss, close mon-itoring of hemodynamic performance and urinary output is useful . Although mainte-nance of urinary output does not ensure preserva-tion of renal function, urinary outputs greater than 0.5 mL/kg/h are preferable. Continuous intraarterial blood pressure monitoring is also important if rapid changes in blood pressure are anticipated, such as in patients with poorly controlled hypertension and in those undergoing procedures associated with abrupt changes in sympathetic stimulation or in cardiac preload or afterload.
Selection of an induction agent is not as impor-tant as ensuring an adequate intravascular vol-ume prior to induction; induction of anesthesia in hypovolemic patients with renal insufficiency frequently results in hypotension. Unless a vaso-pressor is administered, such hypotension typi-cally resolves only following intubation or surgical stimulation. Renal perfusion, which may already be compromised by preexisting hypovolemia, may then deteriorate further, first as a result of hypo-tension, and subsequently from sympathetically or pharmacologically mediated renal vasoconstric-tion. If sustained, the decrease in renal perfusion may contribute to postoperative renal impairment or failure. Preoperative hydration usually prevents this sequence of events.
All anesthetic maintenance agents are accept-able, with the possible exception of sevoflurane administered with low gas flows over a prolonged time period. Intraoperative deterioration in renal function may result from adverse effects of the operative procedure (hemorrhage, vascular occlu-sion, abdominal compartment syndrome, arterial emboli) or anesthetic (hypotension secondary to myocardial depression or vasodilation), from indi-rect hormonal effects (sympathoadrenal activa-tion or antidiuretic hormone secretion), or from impeded venous return secondary to positive-pressure ventilation. Many of these effects are almost completely avoidable or reversible when adequate intravenous fluids are given to maintain a normal or slightly expanded intravascular volume. The administration of large doses of predomi-nantly α-adrenergic vasopressors (phenylephrine and norepinephrine) may also be detrimental to preservation of renal function. Small, intermittent doses, or brief infusions, of vasoconstrictors may be useful in maintaining renal blood flow until other measures (eg, transfusion) are undertaken to correct hypotension.
As reviewed above, appropriate fluid administration is important in managing patients with impaired renal function. Concern over fluid overload is jus-tified, but problems are rarely encountered in such patients with normal urinary outputs if rational fluid administration guidelines and appropriate moni-toring are employed . The adverse consequences of excessive fluid overload—namely, pulmonary congestion or edema—are far easier to treat than those of AKI and kidney failure.
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