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KEY POINTS
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.
Knowledge of normal kidney function is particularly important to interpret the physiology of the neonate, the parturient, and the elderly patient, for whom differentiating normal from abnormal may be challenging and even counterintuitive.
Serum creatinine and creatinine clearance remain the current clinical standard to assess steady-state glomerular filtration, although the search continues for substances with more “ideal” properties (ie, steady production, complete filtration, no secretion or absorption, convenient inexpensive measurement) and other “early biomarkers” that more rapidly identify sudden changes in kidney function.
Kidney-mediated acid–base, electrolyte, or fluid disorders are common preoperatively and may be sufficiently important to require correction before surgery can proceed.
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.
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.
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.
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.
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“The composition of blood is determined not by what the mouth ingests but by what the kidneys keep.”1 This well-known quotation of renal physiologist Dr Homer Smith highlights not only the kidneys’ central role in homeostasis, but also why even minor renal perturbations can have systemic implications. So important is the recognition of abnormal kidney function that its review should be part of even the most abbreviated perioperative assessment. Although the kidneys are routinely evaluated for their ability to filter creatinine from plasma, also relevant is appreciation that this task is but one of numerous roles the kidneys play in maintaining homeostasis. Normal renal physiology and the consequences of impairment are reviewed here in the context of their implications for care of the perioperative patient.
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THE NORMAL KIDNEY: CORRELATES OF STRUCTURE AND FUNCTION
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The kidneys are paired, mesoderm-derived retroperitoneal organs that weigh approximately 150 g each. Although constituting only 0.4% of body weight, together they receive 25% of cardiac output, with a blood flow that exceeds heavily exercised muscle by 8-fold, the most perfused by weight of any major organ. However, unlike muscle, additional primary factors ...