New Contrast Dosing Method Helps Avoid Nephropathy in PCI Patients

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Determining the appropriate dosage for contrast media based on renal function in patients undergoing percutaneous coronary intervention (PCI) is an effective way to avoid contrast-induced nephropathy (CIN), according to results of a large registry study published in the August 23, 2011, issue of the Journal of the American College of Cardiology.

Researchers led by Hitinder S. Gurm, MD, of the University of Michigan Medical Center (Ann Arbor, MI), compared different contrast dosing methods in 58,957 patients undergoing PCI enrolled in the Blue Cross Blue Shield of Michigan Cardiovascular Consortium registry from 2007 to 2008. The newer method uses the ratio of contrast volume to calculated creatinine clearance (CCC), a surrogate for glomerular filtration rate (GFR), to determine the appropriate contrast dose threshold. The comparison method, called the maximal acceptable contrast dose (MACD), is determined by calculating 5 ml of contrast per kg of body weight per mg/dL of baseline serum creatinine.

In the study, 1,470 patients developed the primary endpoint of CIN (≥ 0.5 mg/dL absolute increase in serum creatinine), while 142 developed nephropathy requiring dialysis. The mean and median contrast doses were 205 ml and 200 ml, respectively. Patients who developed CIN were more likely to be older and female, and to have numerous comorbidities. They were also more likely to have worse renal function at baseline, to undergo PCI as an emergency procedure, and to have more extensive CAD.

Risk Rises at 2, Skyrockets at 3

A comparison of area under the receiver-operator characteristic curve demonstrated that contrast volume adjusted for renal function (the ratio of contrast volume to CCC) was a better predictor of both CIN (c = 0.667 vs. c = 0.632; P < 0.05) and dialysis (c = 0.729 vs. 0.583; P < 0.05) compared with the MACD method.

The risk for CIN and need for dialysis increased when the ratio of contrast dose to CCC was 2 or more and rose drastically when the ratio was 3 or higher (P < 0.0001). Similar relationships were observed in patient subsets defined by cardiogenic shock, elective PCI, STEMI, and NSTEMI, though the absolute incidence of CIN was lower in more stable patients.

After adjusting for baseline clinical and other procedural variables, the contrast dose to CCC ratio remained an independent predictor of CIN and dialysis, with the risk approaching significance when the ratio was 2 or more and dramatically increasing when the ratio was 3 or higher (table 1).

Table 1. Risk of CIN and Dialysis Based on Contrast/CCC Ratio

“The key finding of our study is that the [contrast volume]/CCC ratio is a simple tool that can help guide contrast dosing in patients undergoing PCI,” Dr. Gurm and colleagues write. The . . .  ratio was superior to MACD in discriminating between patients most likely to develop CIN.”

Moreover, “[b]ecause creatinine clearance is routinely calculated for patients undergoing invasive cardiac procedures, use of the [contrast volume]/CCC ratio can be easily incorporated into clinical practice and has the possible implication of impacting patient outcome,” the authors write.

James A. Tumlin, MD, of the University of Tennessee College of Medicine (Chattanooga, TN), cautioned that the study has all the normal limitations of a retrospective analysis and is unable to prove causation. Nevertheless, “a 50,000-plus patient study has a certain amount of statistical ‘oomph’ that should really catch your attention,” he told TCTMD in a telephone interview, adding that he hopes the easy-to-use ratio will, in fact, impact practice.

Translation, Please . . .

“In English, if you have a creatinine clearance of 25 cc per minute, and they give you 75 ml of contrast, you are in that 3-to-1 ratio that puts you in the range for developing dialysis dependent contrast nephropathy,” Dr. Tumlin said. “That’s astonishing data.”

In a telephone interview with TCTMD, Dr. Gurm explained that the new ratio should be useful for most patients. “The whole idea is this is a number (GFR) we already know for every patient who comes into the cath lab,” he said. “The only time we don’t know this is if somebody comes in with acute MI or cardiogenic shock.”

As to whether that will translate to actual clinical use, Dr. Tumlin noted that “it kind of comes down to human nature and how much math I can do in my head. If I have to do body weight times kilograms divided by creatinine clearance, I can’t do that in my head. But I can divide 100 by 3.”

Decision Making in the Cath Lab

At Dr. Gurm’s institution, they have already started using the method. “In the cath lab, we will write down the patient’s GFR on the white board, and our nurses will warn us when we’re at 2 times GFR,” he said.

Dr. Gurm explained that just knowing the ratio up front helps them adjust during the case in terms of whether they perform certain tests like a left ventriculogram or biplane angiography. “It’s like anything else in life, when you have a goal, you can always meet that target,” he said. “So we try and set ourselves a threshold that we try and avoid [exceeding], and we’re able to do that in most cases.”

An alternative, Dr. Gurm added, is to stage a case.

He cautioned, though, that this approach is not always possible. “Sometimes you have a complication and you will exceed the threshold,” Dr. Gurm said. “You can’t do much about the patient coming in with shock because the goal is to get the artery open and you just may have to use more contrast. And you shouldn’t worry about that because the goal should be to save the patient’s life.”

Making It Simple

Drs. Gurm and Tumlin agreed that in current practice, clinicians mainly operate by the rule ‘use as little contrast as possible,’ and methods like the MACD formula are seldom used.

“We always tried to use less,” Dr. Gurm said. “That’s a good goal, but this [ratio] gives us a better target. For instance, if GFR is 30, we’re going to use 60 cc of contrast and not exceed that. That makes it very simple.”

Dr. Tumlin agreed, and put the paper’s contribution in even broader terms. “What the findings really add is for any of those people who were harboring incorrect thinking that a small change in creatinine is not clinically significant, nobody in his right mind would argue that dialysis dependent acute renal failure is not of substantial mortality,” he said. “So now we have the observation of a bump in creatinine leading all the way to need for renal replacement therapy, and your mortality rates go from maybe 15-20% to 55-60%. That’s a world of difference.”

 

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Source:
Gurm HS, Dixon SR, Smith DE, et al. Renal function-based contrast dosing to define safe limits of radiographic contrast media in patients undergoing percutaneous coronary interventions. J Am Coll Cardiol. 2011;58:907-914.

 

 

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