Subspecialty Management |
|Chapter 53:||Anesthesia and the Renal and Genitourinary Systems|
ANESTHESIA FOR THE PATIENT WITH RENAL DISEASE
Evaluation of Renal Function
The best indication that a patient has renal disease is usually obtained from the medical history (Ch. 34). Physical findings often are minimal until renal disease is far advanced unless hypertension is present. Urinalysis is sufficient laboratory screening for identification of kidney disease if the patient does not have a history of genitourinary abnormalities. If renal disease is thought to be present, more precise methods of assessing renal function are necessary. Laboratory tests useful in evaluating renal function are described next (Table 53–2).
TABLE 53–2. Commonly Ordered Renal Function Tests
Gross and microscopic observation of the urine and its sediment, with determination of urinary pH, specific gravity, protein content, and sugar content, is one of the most readily available, inexpensive, and informative laboratory tests.
The gross appearance of the urine may indicate the presence of bleeding or infection in the genitourinary tract. Microscopic examination of urinary sediment may reveal the presence of casts, bacteria, and various cell forms, supplying diagnostic information in patients with renal disease.
Urinary pH is a measure of the ability of the kidneys to acidify urine. The kidneys share regulation of acid-base balance with the lungs and provide the sole pathway of excretion for the 60 mEq of hydrogen ion (nonvolatile acid) produced each day by normal metabolism. 5 The three renal mechanisms that prevent the development of acidemia are the reabsorption of filtered bicarbonate, the acidification of buffers in the tubular urine (i.e., the excretion of titratable acid) and the production of ammonia in tubular cells and its excretion as ammonium ion. The inability to excrete an acid urine in the presence of systemic acidosis is indicative of renal insufficiency.
Urinary specific gravity is an index of concentrating ability, specifically, renal tubular function. Determination of urinary osmolality, that is, measurement of the number of moles of solute (osmoles) per kilogram of solvent, is a similar, more specific test. Excretion of concentrated urine (specific gravity 1.030, 1050 mOsm/kg) is indicative of excellent tubular function, whereas urinary osmolality fixed at that of plasma (specific gravity 1.010, 290 mOsm/kg) is indicative of renal disease. The urinary dilution mechanism persists after concentrating defects are present, so that a urinary osmolality of 50 to 100 mOsm/kg may still be consistent with advanced renal disease.
Patients without renal disease may excrete up to 150 mg of protein per day; greater amounts may be present after strenuous exercise or after standing for several hours. Massive proteinuria (i.e., >750 mg/d) is always abnormal and is usually indicative of severe glomerular damage. However, proteinuria also may be due to (1) failure of tubular reabsorption of the small amount of protein that is normally filtered, (2) abnormally increased concentrations of normal plasma proteins, or (3) the presence of abnormal plasma proteins, which are then excreted in the urine.
Glucose is freely filtered at the glomerulus and subsequently is reabsorbed in the proximal tubule. Glycosuria signifies that the ability of the renal tubules to reabsorb glucose has been exceeded by an abnormally heavy glucose load and is usually indicative of diabetes mellitus. However, glycosuria also may be present in hospitalized patients without diabetes who are receiving intravenous glucose infusions.
Complete Blood Count
Anemia may be present in patients with renal disease because of abnormalities in production of erythropoietin (erythropoiesis-stimulating factor [ESF].) The exact mechanism of ESF formation is unknown. 6, 7, 8 One view is that in response to hypoxia, the kidney elaborates a precursor of ESF, which combines with a plasma protein to form active ESF. 7 Another theory is that the kidney produces an enzyme, renal erythropoietic factor, which converts a precursor in plasma to ESF. 7 In advanced renal disease there appears to be decreased ESF activity and hence anemia. 8 The absence of ESF, as may occur in the anephric patient, results in hemoglobin levels of 6 to 8 g/100 mL. Recently available commercial preparation of erythropoietin has been effective in alleviating the chronic anemia associated with end-stage renal disease. Hemoglobin concentrations of 10 g/100 mL or greater are not uncommon in patients with end-stage renal disease treated with recombinant erythropoietin. White blood cell and platelet counts are of particular importance in patients who have a transplanted kidney, as immunosuppressive therapy may cause bone marrow suppression.
Creatinine and Urea Concentrations and Clearances
Measurements of creatinine and urea concentrations and clearances provide valuable information regarding general kidney function. Creatinine in the serum results from the turnover of muscle tissue and is dependent on daily dietary intake of protein; normal values are in the range of 0.5 to 1.5 mg/100 mL, with values of 0.5 to 1.0 mg/100 mL present during pregnancy. Creatinine is freely filtered at the glomerulus and apart from an almost negligible increase in content due to secretion in the distal nephron; it is neither reabsorbed nor secreted. Therefore, serum creatinine measurements reflect glomerular function 9 (Fig. 534), and creatinine clearance is a specific measurement of GFR. Because there is such a wide range in normal values, a 50 percent increase in serum creatinine concentration, indicative of a 50 percent reduction in GFR, may go undetected unless baseline values are known. It also should be apparent that the excretion of drugs dependent on glomerular filtration may be significantly decreased despite what might seem to be only slightly elevated serum creatinine values (1.52.5 mg/100 mL). Serum creatinine concentration and clearance are better indicators of general kidney function and GFR than similar measurements of urea nitrogen. Urea nitrogen concentration and clearance are subject to wide intraindividual variations secondary to changes in hydration, rate of urine flow, and dietary protein intake.
|FIGURE 534 Theoretical relationship between blood urea nitrogen (BUN) and creatinine versus glomer-ular filtration rate (GFR). (From Kassirer JP: Clinical evaluation of kidney functionglomerular function. N Engl J Med 285:385, 1971. Copyright 1971, Massachusetts Medical Society. All rights reserved.)|
Creatinine clearance measurement is made over a 24-hour period and calculated as follows:
A 24-hour clearance is more accurate than a 2-hour creatinine clearance test, which is frequently used because it is more convenient. Normal values are 85 to 125 mL/min in women and 95 to 140 mL/min in men. Creatinine clearance decreases with age and the value approaches 70 at age 70.
Sodium, potassium, chloride, and bicarbonate concentrations should be determined if impairment of renal function is suspected. However, these tests usually remain normal until frank renal failure is present, and hyperkalemia does not occur until patients are uremic. 10
pH and Blood Gases
If significant renal disease is present, patients consuming a diet high in animal protein may have metabolic acidosis due to the inability to excrete nonvolatile acid metabolites. Arterial blood pH, bicarbonate, and PCO2 should be determined to measure the extent of acid-base imbalance (Ch. 38).
Standard posteroanterior and lateral radiographic exposures (Ch. 23) of the chest may be of value in determining the presence and extent of hypertensive cardiovascular disease, pericardial effusion, and uremic pneumonitis.
The electrocardiogram (ECG) (Ch. 32) reflects the toxic effects of potassium excess more closely than does determination of serum potassium concentration. As hyperkalemia progresses, tall peaked T waves, depression of the ST segment, and widening of the QRS complex are seen. When potassium values increase above 8.0 mEq/L, ventricular standstill and bizarre arrhythmias may occur. Digitalis toxicity is a real danger in patients with advanced renal disease, particularly if electrolyte imbalance is present; it is best detected with the ECG. Digitalis will shorten the QT interval, depress the ST segment, and cause ventricular premature contractions that may be coupled or tripled, producing bigeminal or trigeminal rhythms. Hypocalcemia is associated with prolongation of the QT interval on the ECG. Hypocalcemia, hyperkalemia, and digitalis excess are the most likely causes of arrhythmias during anesthesia in uremic patients. Finally, the ECG may be of value in diagnosing hypertensive and ischemic heart disease.
These laboratory tests will define the degree of acute or chronic renal impairment that is present. Until approximately 50 percent of renal function is lost, the usual biochemical indices of renal function, except for creatinine clearance, are within normal limits; other signs and symptoms of renal impairment are absent. Patients with this degree of impairment are said to have decreased renal reserve 10 ; their anesthetic management is no different from that of patients with normal renal function.
Renal insufficiency is said to be present when patients have mild azotemia, nocturia, decreased maximum urinary concentrating ability, and slight anemia. Patients in this category require special attention, as further small decrements in renal function could lead to significant deterioration of their condition. Drugs excreted primarily by the kidney may have a clinically discernible, prolonged duration of action.
Renal failure is characterized by progressive anemia, hypocalcemia, hyperphosphatemia, and loss of urinary concentrating and diluting ability. Polyuria, hyponatremia, and hyperchloremia may be present, but hyperkalemia is rare. Creatinine levels in the range of 3.5 to 4.0 mg/100 mL are common, as are creatinine clearance values of 15 to 20 mL/min. The clinical condition of these patients is precarious, and management must be directed at avoiding further loss of renal function. They are unable to adapt to rapid and significant changes in fluid balance, and they run the risk of becoming acutely hypovolemic or fluid overloaded. Further deterioration in renal function will cause them to become grossly uremic and to require hemodialysis to control the signs and symptoms of renal failure. Drugs excreted primarily by the kidney should be avoided, or if they must be given, they should be administered in decreased dosage and/or at prolonged intervals.
Effects of Drugs in Patients With Reduced Renal Function
Drugs Used for Premedication
Acute Renal Failure and Hemodialysis
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