10.7 Acute Kidney Injury and Contrast-Associated Nephropathy

Why AKI Is a Cardiac Outcome Measure

Test-plan item II.E.1 is examined from the angle that matters clinically: acute kidney injury (AKI) in a cardiac patient is a marker of hemodynamic failure and a driver of mortality, not an incidental lab abnormality. A creatinine rise of only 0.3 mg/dL after admission for decompensated heart failure roughly doubles in-hospital mortality. AKI forces reduction or withdrawal of the guideline-directed medical therapy that keeps the patient alive, restricts contrast for the angiogram they need, contraindicates the anticoagulant they were on, and lengthens stay. The nurse who reports a falling hourly urine output at hour 6 rather than a rising creatinine at hour 48 is the one who changes the trajectory.

KDIGO Definition and Staging

AKI is present when any one of these is met: a serum creatinine rise of 0.3 mg/dL or more within 48 hours; a rise to 1.5 times baseline known or presumed to have occurred within the prior 7 days; or urine output under 0.5 mL/kg/h for 6 hours.

Two testable subtleties. First, the patient is staged by whichever criterion is worse — a patient making 0.2 mL/kg/h for 24 hours is stage 3 even if the creatinine has barely moved, because creatinine lags injury by 24–48 hours. Second, in a cachectic patient with low muscle mass or in a patient who has received large-volume resuscitation, creatinine is diluted and falsely reassuring; urine output and trajectory are more honest.

The Differential, Written in Cardiac Terms

Prerenal (functional) — reduced effective renal perfusion, tubules intact:

• Low cardiac output from cardiogenic shock, severe HFrEF, or a post-cardiotomy low-output state.
• Venous congestion — the most under-recognized cause. A CVP above 12–15 mmHg raises renal interstitial and venous pressure, collapsing the transrenal perfusion gradient. In decompensated heart failure, elevated CVP predicts AKI more strongly than a low cardiac index. This is why aggressive decongestion often improves creatinine.
• Overdiuresis, hypotension, vasodilator or sedation-related hypotension, RAAS inhibitors and NSAIDs impairing autoregulation.

Intrinsic — tubular, interstitial, glomerular or vascular injury:

• Acute tubular necrosis (ATN) from prolonged ischemia or sepsis.
• Iodinated contrast after catheterization or CT angiography.
• Atheroembolic (cholesterol embolization) disease after aortic instrumentation — a subacute stepwise creatinine rise days to weeks after catheterization, with livedo reticularis, blue toes, eosinophilia and low complement. It does not respond to hydration.
• Pigment nephropathy from hemolysis — the signature of mechanical circulatory support (Impella, VA-ECMO, LVAD pump thrombosis) with rising LDH and plasma free hemoglobin, and from rhabdomyolysis after prolonged down time or limb ischemia.
• Cardiac-surgery-associated AKI (CSA-AKI), affecting roughly 20–30% of bypass patients, driven by cardiopulmonary bypass duration, non-pulsatile low perfusion pressure, hemolysis and inflammation.
• Interstitial nephritis from antibiotics, proton pump inhibitors or diuretics; endocarditis-related glomerulonephritis.

Postrenal — obstruction: clot retention after anticoagulated hematuria, a kinked or clogged urinary catheter (always flush or exchange the catheter before declaring anuria), prostatic obstruction, and abdominal compartment syndrome with a bladder pressure above 20 mmHg plus new organ dysfunction — a genuine risk in massively resuscitated post-arrest and post-surgical patients.

Discriminating Prerenal From Intrinsic at the Bedside

Adjuncts: bladder pressure for compartment syndrome, point-of-care renal and IVC ultrasound for hydronephrosis and volume assessment (with venous excess ultrasound patterns describing hepatic, portal and intrarenal vein Doppler as congestion markers), and damage biomarkers such as urinary TIMP-2 x IGFBP7, NGAL and cystatin C that flag tubular stress before creatinine rises.

Contrast-Associated Acute Kidney Injury

Contrast-associated AKI (CA-AKI) is defined as a creatinine rise of 0.3 mg/dL or 1.5 times baseline within 48–72 hours of iodinated contrast with no alternative explanation. Modern data — including matched cohorts using CT scans without contrast — show the attributable risk of contrast is smaller than once believed, and much of what was labelled contrast nephropathy in the cath lab is really hemodynamic injury and atheroembolism. That said, contrast still matters in the patient with pre-existing risk, and the CMC tests the prevention bundle.

Risk Factors and Risk Quantification

Pre-existing CKD with eGFR under 60 mL/min/1.73 m2 (and especially under 30) is the dominant risk factor. Add diabetic nephropathy, age over 75, hypotension or shock, intra-aortic balloon pump, anemia, heart failure with low EF, hypovolemia, high contrast volume and nephrotoxin co-exposure. The Mehran risk score aggregates these. Two practical volume rules: keep the contrast volume-to-eGFR ratio under about 3.0–3.7, and keep total contrast under the classic maximum allowable dose = 5 x body weight in kg divided by serum creatinine in mL. Staging procedures — diagnostic angiography today, PCI in 48–72 hours — is a legitimate risk-reduction strategy the nurse should expect to see.

The Evidence-Based Prevention Bundle

Metformin, told honestly. Metformin does not cause contrast nephropathy and does not damage the kidney. It is renally cleared, so if AKI develops after contrast the drug accumulates and can precipitate lactic acidosis. Current practice: continue metformin in patients with eGFR 60 or above; hold it at the time of contrast and for 48 hours afterward, restarting only after renal function is confirmed stable, when eGFR is under 30, when the patient has AKI, or when contrast is given intra-arterially with first-pass renal exposure. That distinction — held for lactic acidosis risk, not for nephrotoxicity — is the exam point.

Nursing Management of Established AKI

1. Hourly urine output on every at-risk patient and a running mL/kg/h calculation. Verify catheter patency before reporting anuria.
2. Daily weight on the same scale, at the same time, in the same clothing, and reconcile it against the intake/output record — a 1 kg change is about 1 L.
3. Renal medication review every day. Enoxaparin, DOACs, digoxin, sotalol, dofetilide, gabapentin, famotidine, many antibiotics and most opioids need dose or interval adjustment or discontinuation. Nurses catch the missed adjustment because they give the drug.
4. Eliminate avoidable nephrotoxins — NSAIDs and ketorolac, aminoglycosides, and unnecessary contrast studies.
5. Hemodynamic optimization from both directions: a MAP of 65 mmHg or more (a higher target of 75–80 mmHg is often reasonable in chronic hypertension or CKD) for forward perfusion, and relief of venous congestion. Track CVP, jugular venous distension, hepatic congestion, and diuretic responsiveness — a spot urine sodium above 50–70 mEq/L 1–2 hours after an IV loop dose signals a good response; a lower value predicts diuretic resistance and prompts dose escalation or sequential nephron blockade.
6. Electrolyte and acid-base surveillance: potassium, magnesium, phosphate, bicarbonate, and daily ECG review for hyperkalemic changes.
7. Avoid hypotonic and potassium-containing fluids in oliguric AKI; nutrition support without excessive protein restriction.

When Renal Replacement Therapy Is Triggered — AEIOU

A rising creatinine alone is not an indication. Trials of accelerated versus standard initiation showed no survival benefit to starting early in the absence of these triggers, and early initiation exposes patients to catheter and hemodynamic complications. In the hemodynamically unstable cardiac patient, continuous renal replacement therapy (CRRT) is preferred over intermittent hemodialysis because slow continuous fluid removal avoids the abrupt preload drop and myocardial stunning that intermittent sessions cause.