- The combined blood flow through both kidneys
normally accounts for 20–25% of total cardiac output.
- Autoregulation of renal blood flow normally occurs
between mean arterial blood pressures of 80 and 180 mm Hg.
- Renal synthesis of vasodilating prostaglandins
(PGD2, PGE2, and PGI2) is an important
protective mechanism during periods of systemic hypotension and
- Dopamine and fenoldopam dilate afferent and efferent
arterioles via D1-receptor activation. Fenoldopam and low-dose
dopamine infusion can at least partially reverse norepinephrine-induced
- Reversible decreases in renal blood flow, glomerular
filtration rate, urinary flow, and sodium excretion occur during
both regional and general anesthesia. These effects can be at least
partially overcome by maintenance of an adequate intravascular volume
and a normal blood pressure.
- The endocrine response to surgery and anesthesia
is probably at least partly responsible for the transient postoperative
fluid retention that is seen in many patients.
- Methoxyflurane has been associated with a syndrome
of polyuric renal failure. Its nephrotoxicity is dose related and
is the result of release of fluoride ions from its metabolic degradation.
- High plasma fluoride concentrations following
prolonged enflurane anesthesia may also occur in obese patients
and those receiving isoniazid therapy,
- Compound A, a breakdown product of sevoflurane
that is formed at low flows, can cause renal damage in laboratory
animals. Clinical studies have not detected significant renal injury
in humans during sevoflurane anesthesia.
- Certain surgical procedures can significantly
alter renal physiology. The pneumoperitoneum produced during laparoscopy
produces an abdominal compartment syndrome–like state.
The increase in intraabdominal pressure typically produces oliguria
(or anuria). Other surgical procedures that can significantly compromise
renal function include cardiopulmonary bypass, cross-clamping of the
aorta, and dissection near the renal arteries.
The kidneys play a vital role in regulating the volume and composition
of body fluids, eliminating toxins, and elaborating hormones such
as renin, erythropoietin, and the active form of vitamin D. Surgery
and anesthesia can have important effects on renal function. Failure
to take these effects into consideration could result in serious errors in
patient management. Fluid overload, hypovolemia, and postoperative
renal failure are major causes of postoperative morbidity and mortality.
Diuretics are an important class of drugs that is frequently
employed in the perioperative period. Preoperative diuretic therapy
is common in patients with hypertension and with cardiac, hepatic, and
renal disease. Diuretics are also used intraoperatively, particularly
during neurosurgical, cardiac, major vascular, ophthalmic, and urological
procedures. Familiarity with the various types of diuretics, their
mechanisms of action, side effects, and potential anesthetic interactions
is therefore essential.
Each kidney is made up of approximately 1 million functional
units called nephrons. Anatomically, a nephron consists of a tortuous
tubule with at least six specialized segments. At its proximal end
(Bowman’s capsule), an ultrafiltrate of blood is formed,
and as this fluid passes through the nephron, its volume and composition
are modified by both the reabsorption and the secretion of solutes.
The final product is eliminated as urine.
The six major anatomic and functional divisions of the nephron
include the glomerular capillaries, the proximal convoluted tubule,
the loop of Henle, the distal renal tubule, the collecting tubule, and
the juxtaglomerular apparatus (Figure 31–1 and Table 31–1).
31–1. Functional Divisions of a Nephron.1
| Save Table
31–1. Functional Divisions of a Nephron.1
|Glomerulus||Ultrafiltration of blood|
Glucose, protein, amino acids|
|Potassium, magnesium, calcium|
|Phosphates,3 uric acid, urea|
|Loop of Henle||Reabsorption|
|Potassium, calcium, magnesium|
|Juxtaglomerular apparatus||Secretion of renin|
Major anatomic divisions of the nephron.
(Modified and reproduced, with permission, from
Ganong WF: Review of Medical Physiology, 20th
ed. McGraw-Hill, 2001.)
The glomerulus is composed of tufts of capillaries that jut into
Bowman’s capsule, providing a large surface area for the
filtration of blood. Blood flow is provided by a single afferent
arteriole and is drained by a single efferent arteriole (see below).
Endothelial cells in glomeruli are separated from the epithelial
cells of Bowman’s capsule only by their fused basement
membranes. The endothelial cells are perforated with relatively
large fenestrae (70–100 nm), but the epithelial cells interdigitate
tightly with one another, leaving relatively small filtration slits
(about 25 nm). The two cell types with their basement membranes
provide an effective filtration barrier for cells and large-molecular-weight
substances. This barrier appears to have multiple anionic sites
that give it a net negative charge, which favors the filtration
of cations but somewhat hinders filtration of anions. A third cell
type, mesangial cells, are located between the basement membrane
and epithelial cells near adjacent capillaries. Mesangial cells
are thought to play a significant role in the regulation of glomerular
filtration. They contain contractile proteins that respond to vasoactive substance,
secrete various substances, and take up immune complexes. Mesangial
cells contract, reducing glomerular filtration, in response to angiotensin
II, vasopressin, norepinephrine, histamine, endothelins, thromboxane
A2, leukotrienes (C4 and D4), prostaglandin
F2, and platelet-activating factor. They relax, increasing
filtration, in response to atrial natriuretic peptide (ANP), prostaglandin
E2, and dopamine.
Glomerular filtration pressure (about 60 mm ...