Definition & Mechanism
CI-AKI: a rise in serum creatinine of 0.3 mg/dL or more, or 25% or more, within 48 to 72 hours after intravascular contrast, with no alternative explanation. The third most common cause of hospital-acquired AKI.
Two preventable mechanisms:
Medullary hypoxia: contrast causes direct renal vasoconstriction, reducing blood flow to the metabolically demanding outer medulla, the primary mechanism.
Direct tubular toxicity: iodinated contrast is directly toxic to tubular epithelial cells, causing oxidative stress and cell death. High-osmolar agents cause more direct injury; iso-osmolar agents are significantly safer.
Risk factors to know before picking up a catheter: CKD with eGFR under 60 (the single strongest predictor), diabetes, heart failure, high contrast volume, age over 75, anemia, intra-arterial delivery, and pre-existing AKI.
Mehran Risk Score
Built for PCI patients, takes about 90 seconds to calculate.
| Risk factor | Points |
|---|---|
| Hypotension (SBP <80 for 1+ hour, or requiring pressors/IABP within 24h) | +5 |
| Intra-aortic balloon pump | +5 |
| Congestive heart failure (NYHA III-IV or pulmonary edema) | +5 |
| Age over 75 | +4 |
| Anemia (Hct <39% men / <36% women) | +3 |
| Diabetes mellitus | +3 |
| Contrast volume | +1 per 100 mL |
| Creatinine >1.5 mg/dL, or eGFR 40-60 / 20-40 / <20 | +4 / +2 / +4 / +6 |
| Total score | CI-AKI risk |
|---|---|
| ≤5 | 7.5% |
| 6-10 | 14.0% |
| 11-15 | 26.1% |
| >16 | 57.3% |
Every point value and every risk-category percentage above matches Mehran et al's original 2004 JACC derivation exactly.
The Three Trials
PRESERVE (NEJM 2018, N=5,177): bicarbonate vs saline, NAC vs placebo, 2×2 factorial in high-risk pre-angiography patients. Bicarbonate was not superior to saline (4.4% vs 4.7% major adverse kidney events, P=NS). NAC was not superior to placebo. The trial that settled routine bicarb and NAC use.
AMACING (Lancet 2017, N=660, eGFR 30-59): the first RCT of no hydration at all versus prophylactic IV saline. No prehydration was non-inferior to prehydration (2.7% vs 2.6% CI-AKI, P=NS), and less fluid overload occurred in the no-prophylaxis arm. This challenged universal hydration dogma in moderate-risk patients specifically.
POSEIDON (Lancet 2014, N=396, CKD plus at least one additional risk factor): LVEDP-guided hydration versus a standard 1.5 mL/kg/hr saline rate. LVEDP-guided hydration cut CI-AKI from 16.3% to 6.7%, a 59% relative risk reduction, NNT 11 (P=0.005).
Hydration Protocol
IV 0.9% isotonic saline is the cornerstone of CI-AKI prevention. Bicarbonate is not superior (PRESERVE). Oral hydration is not equivalent to IV.
Standard protocol: 1.0-1.5 mL/kg/hr 0.9% NaCl, starting 3-6 hours before contrast, continued through the procedure, then 1.0-1.5 mL/kg/hr for 6-24 hours after the last contrast injection. Target urine output over 150 mL/hr during and after the procedure as a marker of adequate hydration.
POSEIDON LVEDP-guided protocol (preferred in the cath lab): 3 mL/kg 0.9% NaCl over 1 hour before the procedure regardless of LVEDP, then:
| LVEDP | Post-procedure rate |
|---|---|
| Under 13 mmHg | 5 mL/kg/hr |
| 13-18 mmHg | 3 mL/kg/hr |
| Over 18 mmHg | 1.5 mL/kg/hr |
Continue the LVEDP-guided rate for 4 hours post-procedure.
Drugs: What Works and What Doesn't
| Agent | Verdict |
|---|---|
| IV 0.9% saline | Works. Class I evidence, the only proven intervention |
| N-acetylcysteine | Does not work. PRESERVE settled this |
| Sodium bicarbonate over saline | Not superior. May help for convenience in emergent procedures with short prep time |
| Statins (high-dose loading) | Probably beneficial, roughly 30-40% relative risk reduction across meta-analyses |
| Ascorbic acid (vitamin C) | Uncertain, conflicting data |
| Furosemide, high-dose, unmatched | Harmful. Forced diuresis without matched fluid replacement worsens outcomes |
| RenalGuard-style matched hydration with furosemide | Promising, an emerging option |
| Theophylline / adenosine antagonists | Insufficient evidence, not in major guidelines |
Contrast Volume, Corrected
The concept behind this slide is sound and well sourced: a contrast volume-to-safe-limit ratio under 1.0 is genuinely the target that tracks with risk (Brown et al, Circ Cardiovasc Interv, 2010). The formula itself, as stated in the original post, has an error.
As stated in the post: MACD = 5 × weight (kg) × eGFR ÷ serum creatinine (mg/dL). Worked example given: 80 kg patient, eGFR 35, creatinine 1.7 → MACD = 824 mL.
The actual published formula (Cigarroa et al): MACD = 5 × body weight (kg) ÷ serum creatinine (mg/dL). No eGFR term. Applying it to the same patient: 5 × 80 ÷ 1.7 = 235 mL, traditionally capped at 300 mL regardless of the calculated value.
The post's own worked example doesn't reconcile with its own stated formula either; 5 × 80 × 35 ÷ 1.7 works out to roughly 8,235 mL, not 824 mL. The eGFR term shouldn't be in the formula at all.
The underlying clinical point, minimize contrast in patients with reduced eGFR and keep the volume-to-limit ratio under 1.0, is correct and well supported. Use the classic MACD formula (weight over creatinine, no eGFR) to actually calculate it.
Seven Rules
1. Identify risk before the case, not after. Mehran score plus contrast/eGFR ratio, 90 seconds, no excuses.
2. IV saline is the only proven intervention. PRESERVE settled bicarb and NAC. Use the LVEDP-guided protocol where available.
3. Keep the contrast-to-safe-limit ratio under 1.0. Use the corrected MACD formula from Section 6 to calculate it.
4. IVUS first, contrast second. Every mL you don't inject is a mL that can't damage a kidney.
5. Hold metformin 48 hours post-contrast if eGFR is under 30-45, due to lactic acidosis risk if post-contrast AKI develops. Not pre-contrast, post.
6. Never give furosemide to force diuresis without matched replacement. Volume depletion kills the kidney faster than the contrast does.
7. Load with a high-dose statin if the patient isn't already on one.
Quick Reference Card & Primary Sources
| Question | Answer |
|---|---|
| What actually prevents CI-AKI? | IV isotonic saline, ideally LVEDP-guided |
| What doesn't help? | Bicarbonate over saline, NAC, unmatched forced diuresis |
| How do I calculate MACD? | 5 × weight (kg) ÷ creatinine (mg/dL); no eGFR term |
| What ratio should I target? | Contrast volume ÷ MACD under 1.0 |
| Fastest risk stratification? | Mehran score, roughly 90 seconds |
Primary sources checked for this guide:
- Mehran R, Aymong ED, Nikolsky E, et al. A Simple Risk Score for Prediction of Contrast-Induced Nephropathy After Percutaneous Coronary Intervention. J Am Coll Cardiol. 2004;44(7):1393-1399.
- Weisbord SD, Gallagher M, Jneid H, et al. Outcomes after Angiography with Sodium Bicarbonate and Acetylcysteine (PRESERVE). N Engl J Med. 2018;378(7):603-614.
- Nijssen EC, Rennenberg RJ, Nelemans PJ, et al. Prophylactic Hydration to Protect Renal Function (AMACING). Lancet. 2017;389(10076):1312-1322. PMID 28233565.
- Brar SS, Aharonian V, Mansukhani P, et al. Haemodynamic-Guided Fluid Administration for the Prevention of Contrast-Induced Acute Kidney Injury (POSEIDON). Lancet. 2014;383(9931):1814-1823.
- Brown JR, Robb JF, Block CA, et al. Does Safe Dosing of Iodinated Contrast Prevent Contrast-Induced Acute Kidney Injury? Circ Cardiovasc Interv. 2010;3(4):346-350. PMC4843793 (source of the MACD ratio concept and the correct formula).
- Cigarroa RG, Lange RA, Williams RH, Hillis LD. Dosing of contrast material to prevent contrast nephropathy in patients with renal disease. Am J Med. 1989;86(6 Pt 1):649-652 (original MACD formula).