Page 71 - Clinical Application of Mechanical Ventilation

Effects of Positive Pressure Ventilation 37

TABLE 2-6 Serum Indicators of Renal Failure

Serum Measurements Normal Renal Failure

Blood urea nitrogen (BUN) 10 to 20 mg/dL Increased

Creatinine 0.7 to 1.5 mg/dL Increased
BUN to creatinine ratio 10:1 Normal or increased

Creatinine clearance (male) 97 to 137 ml/min Decreased
Glomerular filtration rate 90 to 120 mL/min/1.73 m 2 ,15 mL/min/1.73 m 2

Potassium 3 to 5 mEq/L Usually increased
Sodium 138 to 142 mEq/L Usually decreased

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2 days (Hansen-Flaschen et al., 1993). The possibility of prolonged neuromuscular
Hypoperfusion of the
kidneys may affect the rate blockade can reduce a patient’s lung mechanics and ability to be weaned from me-
of drug clearance leading to a chanical ventilation.
higher drug concentration in
the circulation. Glomerular Filtration Rate (GFR). Decreased renal function caused by positive pres-
sure ventilation may also affect other drugs whose clearance is mainly dependent on
the GFR of the kidneys. Glomerular filtration results from high pressure within the
glomerulus or renal capillary. This is caused by differences in the tone of the affer-
ent and efferent arterioles, the vessels that lead into and out of the glomerulus. The
afferent arteriole is maintained in a somewhat dilated state relative to the efferent
arteriole, which is always somewhat constricted. When blood flow to the kidney is
normal, a hydrostatic pressure head causes the high rate of renal perfusion seen in the
normovolemic state. When coupled with back pressure from the partially constricted
efferent arteriole, pressures within the glomerulus are maintained at an elevated state
and are responsible for its ultrafiltration function. When renal perfusion drops, the
pressure causing glomerular filtration decreases, leading to a decrease in filtration.
Examples of drugs that are eliminated by this mechanism include digoxin, vanco-
mycin, beta-lactam antibiotics, and the aminoglycosides (e.g., gentamycin, tobra-
mycin) (Perkins et al., 1989). A decreased GFR or decreased creatinine clearance
may lead to a higher concentration of these drugs in the circulation.

Tubular Secretion. Another group of drugs whose elimination could be reduced by a
lower renal blood flow are drugs undergoing tubular secretion. Tubular secretion is
the mechanism whereby substances are secreted from the blood via the peritubular
capillaries into the renal tubule to become a part of the urine. Examples of drugs
that are eliminated by this mechanism include digoxin, furosemide, procainamide,
and some penicillins (Perkins et al., 1989). Decrease of renal tubular secretion
causes a relatively higher concentration of these drugs in the circulation.
Reabsorption. The third group of drugs whose elimination could be decreased are
those being reabsorbed at a higher rate. Reabsorption in the renal tubules is the

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