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Eur J Cardiothorac Surg 1999;15:717-722
© 1999 Elsevier Science NL
a Department of Cardiothoracic Surgery, Wythenshawe Hospital, Manchester, M23 9LT, UK
b Department of Biochemistry, Wythenshawe Hospital, Manchester, M23 9LT, UK
Received 23 September 1998; received in revised form 1 February 1999; accepted 10 February 1999.
Corresponding author. Wessex Regional Cardiac and Thoracic Unit, Southampton General Hospital, Tremona Road, Southampton, SO16 6YD UK. Tel.: +44-1703-777-222; fax: +44-1703-796-614; email: gustmtang@aol.com
| Abstract |
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Key Words: Dopamine Coronary artery bypass grafting Acute renal failure Retinol-binding protein Renal tubular injury
| Introduction |
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| Materials and methods |
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Surgical procedure
CABG was performed in each case with the institution of CPB. The latter was accomplished by the use of a single 2-stage venous cannula (Medtronic dlp, Medtronic UK) inserted into the right atrium/inferior vena cava and an arterial cannula (Medtronic dlp, Medtronic UK) placed in the ascending aorta. The procedure was conducted under moderate hypothermia (32°C) with myocardial protection provided by intermittent antegrade cold-blood cardioplegia and topical cooling. Flow was controlled at 2.5 l/min per m2 during CPB with judicious use of metaraminol to maintain a perfusion pressure of 65 mmHg (14 patients received 110 mg during bypass and after administration of protamine, patients were distributed adequately between the two groups).
Infusions and medications
Antibiotics
All patients received three doses of netilmicin 3 mg/kg and 2 g of flucloxacillin for 48 h in divided doses.
Infusions
Maintenance fluid until oral fluid intake is satisfactory: 1 ml/kg of Dextrose 5% in 1/2 normal saline. Central venous pressure was maintained between 8 and 12 mmHg with the use of Gelofusin. Donor blood transfusion was only indicated when haematocrit is below 28%.
Assessment of renal function
Daily measurements were taken during the first postoperative week of the following parameters: (i) weight-adjusted urine output (ml/kg) (ii) fluid balance (input/output) (iii) serum creatinine (µmol/l) (iv) blood urea (mmol/l) (v) urinary RBP (µg/mmolCr). Measurements of creatinine and urea were made in venous blood samples. Twenty-millilitre aliquots of urine were collected in sterile tubes and stored in a refrigerator until analysis.
Analysis of RBP
Urinary excretion of RBP was assessed using a commercial reagent kit (Randox Laboratories, Antrim, UK) employing an ELISA technique.
The calibrators and diluted samples were added to microtitre plate wells precoated with antibody specific for RBP. After incubation for 2 h the plate was washed and antibody conjugated to horse radish peroxidase was added for a further 1 h incubation. Following a final wash step, substrate solution was incubated in the wells resulting in a coloured product which was measured at 450 nm after quenching with dilute sulphuric acid. The colour intensity is proportional to the amount of RBP present. All samples and calibrators were analyzed in duplicate and the mean absorbance results for the calibrators were plotted against calibrator concentration, using semi-logarithmic graph paper. Any samples with an absorbance greater than the top calibrator were re-assayed using a greater dilution. The within batch coefficient of variation was less than 9% and the between batch coefficient of variation was less than 10%, across the assay range [8].
Statistical analyses
Data are presented as the mean±SD. For each parameter comparisons between the two groups were made using MannWhitney U-test, and a statistically significant difference was considered when P<0.05.
| Results |
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| Discussion |
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The incidence of renal dysfunction following open-heart surgery varies between 0.1 and 39% according to the criteria employed [9]. When it does occur, the extent of dysfunction may vary from subclinical [10] to full-blown acute renal failure requiring replacement therapy. The aetiology is believed to be multifactorial but primarily related to techniques of cardiopulmonary bypass [11], peri- and postoperative renal insult [12] and postoperative cardiac dysfunction [13]. Some of these factors such as the conduct of cardiopulmonary bypass have been standardized in the present study. Furthermore, the perioperative haemodynamic status remained comparable between the two groups. It is unlikely, therefore, that any of these factors could have accounted for the impressive difference in the extent of renal dysfunction observed.
A review of the literature suggests that acute renal dysfunction secondary to cardiac surgery may have both glomerular and tubular components
[9]
[14]. Traditional tests of renal function such as serum creatinine and creatinine clearance used in clinical practice focus on glomerular filtration and are relatively insensitive indicators of early damage because of the existence of a large `renal functional reserve'
[15]. Consequently the incidence of impaired glomerular function reported following open-heart surgery varied considerably and a raised serum creatinine may be observed in up to 30% of cases (12). The subjects in the present study were recruited on the basis that they were unlikely to compromise their renal reserve postoperatively as demonstrated by the maintenance of normal levels of serum creatinine following surgery. Until recently early injury to the renal tubule, on the contrary, has not been studied to the same detail since mild damage to that part of the nephron is difficult to assess using conventional indices. Accurate and sensitive methods of detecting tubular injury were developed during the 1980s which depend either on the ability of the proximal tubular cells to reabsorb low-molecular-mass proteins (e.g. ß2-microglobulin and RBP) or on the leakage of intracellular proteins of high-molecular-mass from the damaged tubules (e.g. N-acetyl-ß-D-glucosaminidase and
-glutamyltranspeptidase). RBP is synthesized in the liver, freely filtered through the glomeruli and normally fully reabsorbed by the proximal tubules so that only trace amounts appear in the urine even in the presence of massive proteinuria. Consequently urinary RBP is considered an excellent marker of early tubular damage and has been shown to be a more sensitive and accurate index than measurements of other tubular enzymes because of its greater stability in acidic urine
[7]. An increase in the urinary excretion of renal tubular enzymes indicating proximal tubular injury has been observed soon after CPB
[14]
[16]
[17]. This is in agreement with our own observations in the control group. Such injury may increase tubular permeability and promote leakage of fluid from the tubular lumen into the peritubular circulation. Significant transtubular back-leak is strongly believed to contribute towards more severe forms of acute renal failure by compromising glomerular filtration rate
[4]. However, this phenomenon is probably unimportant in the mild to moderate forms of renal dysfunction
[18], such as that observed in the present study.
Different approaches have been tried in order to prevent renal dysfunction following cardiac surgery with variable outcome. These primarily involve modifying aspects of cardiopulmonary bypass such as the temperature employed, pump-priming with crystalloid to achieve haemodilution and the use of pulsatile flow [17]. Among the pharmacological agents on trial, dopamine infused at low to intermediate rates of 2.54.0 µg/kg per min has gained popularity over the years. The perceived benefits include raised cardiac output, improved renal perfusion from receptor-mediated renal vasodilation [3], reduced renal metabolic activity, diuresis and natriuresis [19]. However, no reliable data exists to claim preservation of renal function or improved patient survival as a result of dopamine treatment following cardiac, aortic and liver transplantation surgery [4] [5] [6]. A recent review concluded that dopamine should not be used as a prophylactic agent except in specific situations in which there is some evidence of benefit, such as when its diuretic and inotropic properties might be of use [19]. More importantly, if adverse effects are experienced, or no beneficial effects are derived, the infusion should be stopped [20]. In this context our findings have conclusively demonstrated for the first time that dopamine used at the `renal dose' not only afforded no prophylactic benefit to the nephrons but was indeed grossly harmful to renal tubular function. This observation could partially account for the diuretic action of dopamine although the precise mechanism of enhanced tubular injury remains to be elucidated.
There has been a distinct lack of objective evidence from randomized controlled trials to confirm the purported benefits of using low dose dopamine for renal prophylaxis in the variety of settings in which it is commonly indicated. Consequently advocates of `renal dose' dopamine inevitably base their recommendations more on opinions than facts. The present study has gone some way to clarify this issue in a well defined population of cardiac surgical patients. Given the wide spectrum of potential adverse effects associated with the use of dopamine [21] [22] our findings fundamentally altered the benefit to risk ratio of this drug in a group of patients who are otherwise at low risk of developing renal dysfunction. Based on such principles, we do not recommend the use of dopamine for routine renal prophylaxis in this group of cardiac surgical candidates. It remains to be seen however, whether dopamine could still be an effective prophylactic agent in patients who may be at higher risk of acquiring acute renal failure following cardiac surgery, such as those with pre-existing renal impairment [23] and cardiac dysfunction [9].
| Footnotes |
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| Appendix A. Conference discussion |
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Mr. Tang: The answer to your first question is, no, the level of retinol binding protein in the urine is not dependent on the actual volume urine output. It is very difficult to see, but in fact if you look at the unit used on the y-axis when I showed you the difference between two curves, it is actually corrected for the amount of creatinine excreted in the urine as well. So it's all standardized. And the answer to your second question is, yes, we are in the process of doing a study, which we call phase II, looking at whether dopamine is an effective prophylactic agent in patients who are considered at higher risk of developing acute renal dysfunction following coronary surgery.
Dr L. von Segesser (Lausanne, Switzerland): Can you tell us something about hemolysis; was this evenly distributed in the two groups?
Mr Tang: You mean in terms of the serological markers of hemolysis?
Dr von Segesser: One could imagine that a lot of hemoglobin, which is free in the plasma, could saturate the kidney from one group and less in the other, because there were differences there.
Mr Tang: Right. I think what you are trying to get at is that some of these toxins can affect renal tubular function. We haven't actually specifically looked at the level of microglobulin or haptoglobin between the two groups, but effectively there were no differences in any of the other parameters we looked at, such as perioperative hemodynamic status and the requirement for transfusion, etc.
Mr J. Kuo (Plymouth, UK): I enjoyed your study because I have a similar interest and I did an identical study 2 years ago looking at a similar group of patients, but we came to a different conclusion. We actually monitored renal function during bypass, measuring RBP levels at intervals during bypass, and we found that the RBP levels are lower in the dopamine group compared with the nondopamine group, and we back up our data with a second renal marker, the alpha-1-microglobulin.
My question to you is, did you actually measure retinol binding protein during bypass, and if you haven't, can you actually draw any conclusion as to whether or not dopamine offers any advantage to this group of patients during bypass?
Mr Tang: No. Retinol binding protein was basically measured on a daily basis in 20-ml aliquots of urine which is collected from day zero onward during the 1st postoperative week. We didn't actually collect specimens while the patient was on bypass. And I think you mentioned that you assess renal function using beta-2-microglobulin.
Dr Kuo: Alpha-1-microglobulin, not beta.
Mr Tang: Okay. As far as I am aware, studies which compared the use of different renal tubular enzymes as markers of early renal tubular injury have indicated that RBP is the most reliable and accurate marker of them all, partly because it is a stable protein, even in acidic urine, and I think partly also because of the dynamics of its reabsorption. And I don't think simply by altering the marker and using other tubular enzymes as our end point would actually alter our conclusion, if that is the right answer to your question.
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