CARDIOVASCULAR JOURNAL OF AFRICA • Vol 21, No 1, January/February 2010
AFRICA
9
Cardiovascular Topics
The simplified modification of diet in renal disease
equation as a predictor of renal function after coronary
artery bypass graft surgery
MJ SWART, AM BEKKER, JJ MALAN, A MEIRING, Z SWART, G JOUBERT
Summary
Background:
After open-heart surgery, a percentage of
patients have impaired renal function. This deterioration is
even seen in patients with serum creatinine (s-creatinine)
values that fall within the normal laboratory range, there-
fore s-creatinine is not an accurate reflection of renal func-
tion. Glomerular filtration rate (GFR) is a better indication
of renal status. GFR can be calculated with the simplified
modification of diet in renal disease (MDRD) equation – a
formula that takes age, gender, race and s-creatinine level
into account. The purpose of this study was to investigate
the relationship between estimated GFR pre-operatively and
renal impairment postoperatively.
Methods:
All patients who had an isolated coronary artery
bypass graft (CABG) done by one surgeon in one hospital
between January 2005 and October 2007 had their s-creat-
inine levels determined pre-operatively. Using a computer
desktop calculator, the patient’s age, gender and race were
used together with the s-creatinine value to estimate the
GFR. Prior to CABG, all patients were grouped into the five
stages of chronic kidney disease. Renal outcome postopera-
tively was compared with the estimated pre-operative GFR.
Results:
Nineteen per cent of the 451 patients had chronic
kidney disease pre-operatively, as defined by the National
Kidney Foundation, according to their estimated GFR.
Twenty-three per cent of these patients had renal impair-
ment after surgery. Of the patients with reasonable renal
function pre-operatively only 4% had further deterioration
of renal function. Mortality did not differ significantly, but
patients with postoperative renal impairment stayed in
hospital on average 2.4 days longer than those who had no
renal impairment postoperatively.
Conclusions:
Patients with chronic kidney disease before
CABG have a six times greater chance of developing further
renal impairment postoperatively than those with reasonable
renal function beforehand. There is therefore a significant
relationship between estimated GFR before CABG and
deterioration of kidney function after surgery. The GFR, as
calculated with the simplified MDRD, is a predictor of the
risk of having renal dysfunction after CABG.
Keywords:
CABG, kidney, renal function
Submitted 12/1/09, accepted 14/5/09
Cardiovasc J Afr
2010;
21
: 9–12
Organ dysfunction after open-heart surgery impairs the rehabili-
tation of these patients. The establishment of co-morbidities is
therefore part of the pre-operative workup of patients for surgery.
Special investigations have cost implications, but knowing the
risk factors beforehand makes interventions more timely and
treatment more accurate. Renal dysfunction in particular has
an influence on mortality and morbidity.
1
Kidneys are subject
to damage due to heart failure, emergency surgery, intra-aortic
balloon pump use, low haematocrit during cardio-pulmonary
bypass (CPB), and low cardiac output. The systemic inflamma-
tory process associated with CPB itself negatively influences
kidney performance.
Serum (s) creatinine is no longer considered an accurate test
to evaluate renal function. An elderly woman with little muscle
mass may have impaired renal function although her s-creatinine
levels may fall within the laboratory’s normal range.
2
Glomerular
filtration rate (GFR) is now considered the most accurate way of
establishing renal function. However, to determine the clearance
of an exogenous marker is expensive and time consuming. Even
determining creatinine clearance from a 24-hour urine sample is
not always practical before a cardiac operation.
An alternative to the inaccurate s-creatinine test, and the
problems associated with urine creatinine clearance, or costly
radio-isotope determination of GFR is the prediction of GFR
from simple and readily available values. As far back as 1976,
Cockcroft and Gault suggested a method to estimate the GFR
based on the patient’s age, gender, body mass and s-creatinine
value.
3
More recently, the GFR is calculated from age, gender,
race, and s-creatinine, s-urea and s-albumin values. This is the
calculation suggested by the Modification of Diet in Renal
Disease (MDRD) study group. It is considered more accurate
than the method by Cockcroft and Gault and is even more precise
Bloemfontein Medi-Clinic, Bloemfontein, South Africa
MARIUS J SWART, MB ChB, FCS (SA),
Medical student, School of Medicine, University of the Free
State, Bloemfontein, South Africa
ARLETTE M BEKKER
JANNES J MALAN
ANTON MEIRING
ZORADA SWART
Department Biostatistics, Faculty Health Sciences,
University of the Free State, Bloemfontein, South Africa
GINA JOUBERT, BA, MSc
