Acute Renal Failure and Renal Support

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Chapter 33 Acute Renal Failure and Renal Support

Renal dysfunction is relatively common following cardiac surgery and is associated with a substantial increase in mortality rates. The incidence of postoperative renal dysfunction (PRD) varies according to the way in which it is defined and the population being studied. In unselected groups of cardiac surgery patients with PRD, mortality rates of 3.4% to 17% have been reported.14 The need for renal replacement therapy occurs in 1% to 4% of cases and is associated with mortality rates of 30% to 70%.1,2,47

In this chapter, the risk factors, diagnosis, and treatment of postoperative acute renal failure are discussed. Traditionally, renal replacement therapy for acute renal failure in cardiac surgery patients has involved peritoneal dialysis and intermittent hemodialysis. However, over the past decade, continuous and semicontinuous therapies that are better suited to the critically ill have become common. The indications, various types, and practicalities of these techniques are discussed. The physiology of normal and abnormal renal function is reviewed in Chapters 1 & 31.

ASSESSMENT OF RENAL FUNCTION

Tests of renal function evaluate either glomerular filtration rate (GFR) or tubular function. Severe renal hypoperfusion results in a reduction in the GFR and an increase in the plasma concentration of nitrogenous wastes (urea and creatinine). This is known as prerenal azotemia. When renal hypoperfusion is severe and sustained, acute tubular necrosis can occur, and it results in abnormal tubular function. The pathophysiologic mechanisms of prerenal azotemia and acute tubular necrosis are described in Chapter 1.

Glomerular Filtration

The two most widely used biochemical tests of GFR are plasma creatinine and blood urea nitrogen (BUN). The normal plasma creatinine concentration is 50 to 120 μmol/l (0.6 to 1.4 mg/dl) in males and 40 to 100 μmol/l (0.5 to 1.1 mg/dl) in females. The normal BUN concentration is 3.2 to 7.7 mmol/l (9 to 22 mg/dl). Of the two, creatinine is the more reflective of the GFR. Creatinine is released from muscle at a relatively constant rate, is freely filtered at the glomerulus and, unlike urea, is not reabsorbed in the tubules. Creatinine concentration is relatively constant despite changes in diet and metabolic rate. As people age, there is a gradual reduction in the GFR, which is matched by a decline in muscle mass. Thus, creatinine concentration remains relatively constant. The relationship between the creatinine concentration and the GFR is not linear (Fig. 33-1); with normal renal function, a small rise in creatinine concentration signals a large deterioration in the GFR; with severe renal dysfunction, large changes in creatinine reflect small changes in the GFR.

A more accurate estimate of the GFR, which is independent of age, is creatinine clearance. The normal range for creatinine clearance is 120 ± 25 ml/min for men and 95 ± 20 ml/min for women.8 Creatinine clearance (CCr) can be calculated on the basis of the plasma (PCr) and urinary (UCr) concentrations of creatinine and the urine volume (V):

(33-1) image

This requires 24-hour urine collection. A simpler method is to estimate creatinine clearance on the basis of the following empiric formula:

(33-2) image

This formula assumes that a steady state exists; it cannot be applied to estimate the GFR during the development of acute renal failure.

Measurement (or estimation) of creatinine clearance is particularly important in the elderly and in patients with abnormal body mass indexes.

As with creatinine, BUN levels are inversely related to the GFR. However, BUN levels are also influenced by the rate of urea production and the degree of tubular reabsorption. Increased urea production, and therefore increased BUN levels, occur with a high-protein diet, gastrointestinal bleeding, and increased protein catabolism (e.g., treatment with corticosteroids). Increased tubular reabsorption of urea, and therefore increased BUN levels, occur when tubular flow is low, as occurs in renal hypoperfusion. Thus, a greater increase in BUN levels relative to creatinine levels also occurs with hypovolemia and heart failure. One advantage of measuring BUN levels rather than creatinine levels is that they increase within a few hours of a reduction in the GFR, whereas creatinine takes at least 8 hours to become significantly elevated.

Urine Output

Oliguria (urine output <0.5 ml/kg/hr) is an important marker of renal hypoperfusion and evolving acute tubular necrosis. However, urine output must be interpreted with caution. With a normal diet, about 600 mOsm/day of solute waste products generated by metabolism must be excreted in the urine. With normal renal tubular function and maximal antidiuretic hormone stimulation, this requires a minimum of 500 ml/day of urine. In postoperative patients who have low dietary intake and high plasma levels of catecholamines, aldosterone, and antidiuretic hormone (see Chapter 32), oliguria may be a normal finding and not indicative of a reduced GFR or tubular dysfunction. Oliguria in association with normal plasma creatinine concentration and normal intravascular volume does not require treatment. However, oliguria in association with prerenal azotemia or acute tubular necrosis mandates a search for the cause of renal hypoperfusion, in particular, a careful assessment of intravascular volume status and cardiac function. Conversely, high urine output can occur despite intravascular volume depletion. Polyuria is common in the first few hours following cardiac surgery as a consequence of hypothermic cardiopulmonary bypass (CPB) and the administration of diuretics—particularly mannitol, which may be used in the CPB prime solution.

Oliguria is usually present with prerenal azotemia in the early period of acute tubular necrosis. If tubular necrosis is severe, anuria develops because of a low or absent GFR and tubular obstruction (see Chapter 1). If the GFR is then partially or fully restored, polyuria often develops due to the inability to concentrate urine.

ACUTE RENAL FAILURE

Definition

Various definitions of renal dysfunction are used; they are based on the plasma creatinine concentration, the percentage of rise in creatinine concentration, or the need for renal replacement therapy. Recently, the RIFLE (risk, injury, failure, loss, and end-stage kidney disease) classification has been proposed by the Acute Dialysis Quality Initiative Workgroup as a consensus definition of acute renal failure in critically ill patients.9 Renal dysfunction is divided into three categories (risk, injury, failure) on the basis of plasma creatinine concentration and urine output. In addition, there are two levels of outcome (loss, end-stage kidney disease), determined by the persistence of renal failure at 4 weeks or 3 months (Table 33-1). The RIFLE classification has recently been evaluated in cardiac surgery patients and has been shown to be an independent predictor of mortality: patients categorized as having risk, injury, or failure postoperatively had 90-day mortality rates of 8%, 22%, and 33%, respectively.10

Table 33-1 RIFLE Classification of Renal Dysfunction and Failure

Classification GFR Criteria Urine Output Criteria
Risk Increase in creatinine by 50% Urine volume <0.5 ml/kg/hr for 6 hr
Injury Increase in creatinine by 100% Urine volume <0.5 ml/kg/hr for 12 hr
Failure Increase in creatinine by 200% to more than 4 mg/dl (354 μmol/l), with a rise ≥0.5 mg/dl (44μmol/l Urine volume <0.3 ml/kg/hr for 24 hr or anuria for 12 hr
Loss Persistent acute renal failure or complete loss of renal function for >4 weeks  
End-stage kidney disease End-stage kidney disease (>3 months)  

GFR, glomerular filtration rate

From Bellomo R, Ronco C, Kellum JA, et al: Acute renal failure—definition, outcome measures, animal models, fluid therapy and i nformation technology needs: the Second International Consensus Conference of the Acute Dialysis Quality Initiative (ADQI) Group. Crit Care 8:R204-R212, 2004.

Risk Factors for Acute Renal Failure Following Cardiac Surgery

The causes of renal failure following cardiac surgery are multifactorial. Renal hypoperfusion may occur because of periods of hypovolemia, hypotension, or low cardiac output. Nonpulsatile CPB leads to reduced renal blood flow.11,12 Renal function may be adversely affected by the systemic inflammatory response to surgery and CPB and by free plasma hemoglobin generated by bypass-induced trauma to red blood cells. Atheroembolism may occur due to aortic manipulations such as cross-clamping, cannulation, and the use of an intraaortic balloon pump. Nephrotoxic drugs—in particular radiocontrast agents used for coronary angiography—may be administered during the perioperative period.

The risk factors for renal failure requiring renal replacement therapy following cardiac surgery are listed in Table 33-2. Of these, preexisting renal dysfunction is by far the most important; it is a major predictor of postoperative mortality.13

Table 33-2 Factors That are Independently Associated with the Need for Postoperative Renal Replacement Therapy Following Cardiac Surgery

Requirement for blood transfusion
Emergency operation
Preoperative renal dysfunction (creatinine >124 μmol/l or > 1.4 mg/dl)
Requirement for re-sternotomy
Mitral valve surgery
Low ejection fraction (< 40%)
Use of an intraaortic balloon pump
Prolonged cardiopulmonary bypass time

From Bove T, Calabro MG, Landoni G, et al: The incidence and risk of acute renal failure after cardiac surgery. J Cardiothorac Vasc Anesth 18:442-445, 2004.

Causes of chronic renal dysfunction include hypertension, diabetes, connective tissue disease, chronic use of nonsteroidal antiinflammatory drugs, glomerulonephritis, and polycystic kidney disease. Chronic renal dysfunction is very common in patients presenting for cardiac surgery. In one large series, 51% of patients had mild renal dysfunction (GFR 60 to 90 ml/min); 24% had moderate renal dysfunction (GFR 30 to 59 ml/min); 2% had severe renal dysfunction (GFR <30 ml/min); and 1.5% were dialysis dependent.13

Physiologic Consequences of Renal Failure

Acidosis and Hyperkalemia

In severe renal dysfunction, an elevated anion-gap metabolic acidosis develops due to the inability of the kidneys to excrete the acid load generated by metabolism (see Chapter 31). In chronic renal failure, metabolic acidosis becomes apparent when the GFR falls to about 25% of normal, but in acute tubular necrosis, acidosis is an early finding.

Potassium undergoes glomerular filtration and tubular secretion (see Chapter 32). Hyperkalemia can develop when renal blood flow is low—independent of acute tubular necrosis—as the result of reduced blood flow in the peritubular capillaries. Thus, hyperkalemia is closely linked to oliguria and is uncommon in polyuric renal failure.

NONDIALYTIC MANAGEMENT OF ACUTE RENAL FAILURE

Preventing Further Renal Injury

Optimizing Cardiovascular Performance

Maintaining an adequate circulating volume and hemodynamic state is of paramount importance in preventing further renal injury. Patients with evolving renal failure should undergo careful examinations of the cardiovascular and respiratory systems. If there is hypotension or evidence of low cardiac output, central venous and intraarterial pressure monitoring is indicated. Measurement of superior vena cava oxygen saturation (SSVCo2; see Chapter 20) is also valuable. Hypovolemia should be treated by a balanced salt solution. Normal (0.9%) saline can exacerbate acidosis (see Chapter 31) and Plasma-Lyte (which contains potassium; see Table 32-3) can exacerbate hyperkalemia. If, despite adequate volume resuscitation, the hemodynamic state remains inadequate, pulmonary artery catheterization and echocardiography should be considered to guide further treatment.

Table 33-3 Criteria for the Initiation of Renal Replacement Therapy

Oliguria (urine volume <500 ml/day)
Anuria (urine volume <50 ml/12 hr)
BUN >30 mmol/l (>85 mg/dl)
Creatinine >400 mol/l (>4.5 mg/dl)
Potassium >6 mmol/l
Pulmonary edema not responsive to diuretics
Severe metabolic acidemia (pH <7.1, base deficit >10 mmol/l)
Uremic encephalopathy
Uremic pericarditis
Uremic neuropathy

From Bellomo R, Ronco C: Indications and criteria for initiating renal replacement therapy in the intensive care unit. Kidney Int 66(suppl):S106-S109, 1998.

In normotensive patients, the lower limit of autoregulation of renal blood flow is 70 to 75 mmHg.19 This lower limit is likely to be higher in patients with chronic hypertension. Furthermore, in critically unwell patients, autoregulation may be lost entirely, in which case renal blood flow becomes pressure dependent.19

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