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Renal disease and the surgical patient: Minimizing the impact

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WHAT ARE THE RISK FACTORS FOR ACUTE KIDNEY INJURY?

Risk factors for acute kidney injury in surgical patients
The etiology of AKI is complex and multifactorial. Risk factors can be divided into patient- and surgery-associated risk factors (Table 3).

Cardiac surgery

CKD is a risk factor not only after cardiac surgery but also after percutaneous procedures. In a meta-analysis of 4,992 patients with CKD who underwent transcatheter aortic valve replacement, both moderate and severe CKD increased the odds of AKI, early stroke, the need for dialysis, and all-cause and cardiovascular mortality at 1 year.22,23 Increased rates of AKI also have been found in patients with CKD undergoing CABG surgery.24 These results point to a synergistic effect between AKI and CKD, with outcomes much worse in combination than alone.

In cardiac surgery, the most important patient risk factors associated with a higher incidence of postoperative AKI are age older than 75, CKD, preoperative heart failure, and prior myocardial infarction.19,25 Diabetes is an additional independent risk factor, with type 1 conferring higher risk than type 2.26 Preoperative use of angiotensin-converting enzyme (ACE) inhibitors may or may not be a risk factor for cardiac surgery-associated AKI, with some studies finding increased risk and others finding reduced rates.27,28

Anemia, which may be related to either patient or surgical risk factors (eg, intraoperative blood loss), also increases the risk of AKI in cardiac surgery.29,30 A retrospective study of CABG surgery patients found that intraoperative hemoglobin levels below 8 g/dL were associated with a 25% to 30% incidence of AKI, compared with 15% to 20% with hemoglobin levels above 9 g/dL.29 Additionally, having severe hypotension (mean arterial pressure < 50 mm Hg) significantly increased the AKI rates in the low-hemoglobin group.29 Similar results were reported in a later study.30

Among surgical factors, several randomized controlled trials have shown that off-pump CABG is associated with a significantly lower risk of postoperative AKI than on-pump CABG; however, this difference did not translate into any long-term difference in mortality rates.31,32 Longer cardiopulmonary bypass time is strongly associated with a higher incidence of AKI and postoperative death.33

Noncardiac surgery

AKI is less common after noncardiac surgery; however, outcomes are severe in patients in whom it occurs. In a study of 15,102 noncardiac surgery patients, only 0.8% developed AKI and 0.1% required renal replacement therapy.34

Risk factors after noncardiac surgery are similar to those after cardiac surgery (Table 3).34–36 Factors with the greatest impact are older age, peripheral vascular occlusive disease, chronic obstructive pulmonary disease necessitating chronic bronchodilator therapy, high-risk surgery, hepatic disease, emergent or urgent surgery, and high body mass index.

Surgical risk factors include total vasopressor dose administered, use of a vasopressor infusion, and diuretic administration.34 In addition, intraoperative hypotension is associated with a higher risk of AKI, major adverse cardiac events, and 30-day mortality.37

Noncardiac surgery patients with postoperative AKI have significantly higher rates of 30-day readmissions, 1-year progression to end-stage renal disease, and mortality than patients who do not develop AKI.35 Additionally, patients with AKI have significantly higher rates of cardiovascular complications (33.3% vs 11.3%) and death (6.1% vs 0.9%), as well as a significantly longer length of hospital stay.34,36

CAN WE DECREASE THE IMPACT OF RENAL DISEASE IN SURGERY?

Before surgery, practitioners need to identify patients at risk of AKI, implement possible risk-reduction measures, and, afterward, treat it early in its course if it occurs.

The preoperative visit is the ideal time to assess a patient’s risk of postoperative renal dysfunction. Laboratory tests can identify risks based on surgery type, age, hypertension, the presence of CKD, and medications that affect renal function. However, the basic chemistry panel is abnormal in only 8.2% of patients and affects management in just 2.6%, requiring the clinician to target testing to patients at high risk.38

Patients with a significant degree of renal dysfunction, particularly those previously undiagnosed, may benefit from additional preoperative testing and medication management. Perioperative management of medications that could adversely affect renal function should be carefully considered during the preoperative visit. In addition, the postoperative inpatient team needs to be informed about potentially nephrotoxic medications and medications that are renally cleared. Attention needs to be given to the renal impact of common perioperative medications such as nonsteroidal anti-inflammatory drugs, antibiotics, intravenous contrast, low-molecular-weight heparins, diuretics, ACE inhibitors, and angiotensin II receptor blockers. With the emphasis on opioid-sparing analgesics, it is particularly important to assess the risk of AKI if nonsteroidal anti-inflammatory drugs are part of the pain control plan.

Nephrology referral may help, especially for patients with a GFR less than 45 mL/min. This information enables more informed decision-making regarding the risks of adverse outcomes related to kidney disease.

WHAT TOOLS DO WE HAVE TO DIAGNOSE RENAL INJURY?

Several risk-prediction models have been developed to assess the postoperative risk of AKI in both cardiac and major noncardiac surgery patients. Although these models can identify risk factors, their clinical accuracy and utility have been questioned.

Biomarkers

Early diagnosis is the first step in managing AKI, allowing time to implement measures to minimize its impact.

Serum creatinine testing is widely used to measure renal function and diagnose AKI; however, it does not detect small reductions in renal function, and there is a time lag between renal insult and a rise in creatinine. The result is a delay to diagnosis of AKI.

Biomarkers other than creatinine have been studied for early detection of intraoperative and postoperative renal insult. These novel renal injury markers include the following:

Neutrophil gelatinase-associated lipocalin (NGAL). Two studies looked at plasma NGAL as an early marker of AKI in patients with CKD who were undergoing cardiac surgery.39,40 One study found that by using NGAL instead of creatinine, postoperative AKI could be diagnosed an average of 20 hours earlier.39 In addition, NGAL helped detect renal recovery earlier than creatinine.40 The diagnostic cut-off values of NGAL were different for patients with CKD than for those without CKD.39,40

Other novel markers include:

  • Kidney injury marker 1
  • N-acetyl-beta-D-glucosaminidase
  • Cysteine C.

Although these biomarkers show some ability to detect renal injury, they provide only modest discrimination and are not widely available for clinical use.41 Current evidence does not support routine use of these markers in clinical settings.

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