Chapter 14

1. Kidney functions are centrally involved in whole-body homeostasis and normally keep body fluid volume, osmolarity, electrolyte content and concentration, and acidity within narrow limits.

2. Knowledge of normal kidney function is particularly important to interpret the physiology of the neonate, the parturient, and the elderly patient, where differentiating normal from abnormal may be challenging and even counterintuitive.

3. Although the search continues for a substance with "ideal" properties to assess glomerular filtration (ie, steady production, complete filtration, no secretion or absorption, convenient inexpensive measurement) through its clearance from the circulation, serum creatinine and creatinine clearance are the current clinical standard.

4. Kidney-mediated acid–base, electrolyte, and/or fluid disorders are common preoperatively and may be sufficiently important to require correction before surgery can proceed.

5. Familiarity with the spectrum of acute and chronic renal disorders that may be encountered in the perioperative patient is essential to logically anticipate perioperative problems and design rational therapeutic strategies.

6. Prevention is the most important tool in the approach to perioperative acute kidney injury; this requires knowledge of potential insults, including the renoprotective value of meticulous attention to minimizing hemodilution and transfusion.

7. Major acute kidney injuries significantly impair the kidneys' ability to maintain the internal environment; in these situations adherence to guidelines aimed at preserving volume, electrolyte, acid–base, and nutrition balance within the limits of the remaining renal homeostatic reserve may be sufficiently effective that dialysis can be avoided.

8. Patients with impaired renal filtration have altered responses to normal medication dosing; a simple prescribing approach for water-soluble agents involves a calculated percentage reduction in drug dosage to match the reduction in glomerular filtration. However, drug-level measurement or algorithms for a specific drug dosing may be recommended.

"The composition of blood is determined not by what the mouth ingests but by what the kidneys keep…"1 This well-known quote of renal physiologist Dr Homer Smith highlights not only the kidneys' domain of influence, but also why even minor renal perturbations have widespread effects. The kidneys play a key role in homeostasis, making review of their function essential for even the most abbreviated perioperative assessment. Although consideration is often limited to issues of filtration and clearance, these tasks comprise only a part of the kidneys' involvement in homeostasis. Normal renal physiology and the consequences of impairment are reviewed here in context of their implications for the perioperative physician.

The kidneys are paired mesoderm-derived retroperitoneal organs that weigh approximately 150 g each. Notably, despite the fact that by body weight kidneys make up only 0.4%, they receive 25% of cardiac output; in relative terms, this exceeds muscle blood flow with heavy exercise by 8-fold and makes them the most highly perfused major organ in the body. Partial explanation for this remarkable blood flow comes from unusually high-normal renal vein oxygen levels, which evidence a "luxury perfusion" component to renal blood flow (RBF) that serves plasma filtration at rates as high as 125 to 140 mL/min in adults. Metabolic demands are not the primary determinant of ...

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