Subspecialty Management |
|Chapter 53:||Anesthesia and the Renal and Genitourinary Systems|
Renal Blood Flow
Kidneys receive approximately 15 to 25 percent of total cardiac output or 1.0 to 1.25 L blood/min via renal arteries depending on the state of body. Most of the blood is received by renal cortex, and only 5 percent of cardiac output flows through renal medulla, which makes renal papillae vulnerable to ischemic insults. Renal blood flow is regulated by a variety of mechanisms that control activity of vascular smooth muscles and thereby alter vascular resistance. The sympathetic tone of renal vessels increases during exercise to shunt renal blood flow to exercising skeletal muscle, similarly renal blood vessels relax during the resting condition of the body. Sympathetic stimulation due to surgery can increase the vascular resistance to reduce the renal blood flow, whereas anesthetic may reduce the renal blood flow by decreasing cardiac output. Glomerular capillaries separate afferent arterioles from efferent arterioles. The glomerular capillaries are high pressure systems, whereas peritubular capillaries are low pressure systems. So the glomerular capillaries are a fluid-filtering system, whereas peritubular capillaries are a fluid-absorbing system. Vasa recta are a specialized portion of peritubular capillaries that are formed from efferent arterioles and are important in the formation of concentrated urine by countercurrent mechanism. An intrinsic mechanism that causes vasodilation and vasoconstriction of renal afferent arterioles regulates the autoregulation of renal blood flow. A fall in mean arterial pressure will also decrease renal blood flow and eventually affect glomerular filtration rate (GFR) when the pressure decreases below 60 mm Hg. The persistent low mean arterial pressure above 60 mm Hg affects renal blood flow but does not affect the GFR due to the intrinsic mechanism of autoregulation (Fig. 533). This autoregulation is maintained between 60 and 160 mm Hg mean arterial pressure (MAP) in intact as well as denervated kidneys. 4
|FIGURE 533 Autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR). The relationships between RBF, GFR, and urine flow rate (UFR) and mean renal arterial pressure in dogs are shown as renal arterial pressure is varied from 20 to 280 mm Hg. Autoregulation of RBF and GFR is observed between about 80 and 180 mm Hg. (From Hemmings HC: Anesthetics, Adjuvants and Drugs and the Kidney. In Malhotra V [ed]: Anesthesia for Renal and Genitourinary Surgery. New York, McGraw-Hill, 1996, p 18.)|
Although knowledge of the neuroanatomy and renal blood flow is essential to provide adequate anesthesia, a thorough understanding of renal physiology and pharmacology is equally important. Frequently the genitourinary surgical patient has mechanical or functional renal disease. Anesthetics and surgery will significantly alter renal function. Conversely, renal dysfunction will significantly affect the pharmacokinetics and pharmacodynamics of anesthetics and adjuvant drugs. The evaluation of a patient with renal disease is discussed later.
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