Renal oxygen delivery during cardiopulmonary bypass
Wolverhampton, UK
* E-mail: david_morrice{at}mac.com
Editor—I read with interest the study by Loef and colleagues,1 on renal function in selected patients undergoing cardiac surgery with cardiopulmonary bypass (CPB). It is reassuring to discover that there was no significant evidence of deterioration in glomerular filtration rate (GFR). A deleterious patient or treatment characteristic could theoretically have caused a significantly lower GFR after operation and there was an attempt to select out such patients. I am nevertheless concerned for several reasons about the omission of data on oxygen delivery/DO2 (e.g. haemoglobin level, pump flow) before, and at the time of CPB. First, the medullary zone of the kidneys has for some time been assumed to be prone to hypoperfusion because it is an area of high metabolic rate ‘on the verge of hypoxia’. Recent research on rats has suggested that this may be more specifically the juxta medullary zone.2 Reduced oxygen delivery (calculated by the oxygen delivery equation) therefore gives a physiological basis for potential renal damage. Low intraoperative haematocrit occurs as a result of blood loss, haemodilution, and low starting haemoglobin and directly affects oxygen delivery during cardiac surgery. Ranucci and colleagues3 demonstrated that lowered haematocrit is linked to development of renal dysfunction. Blood transfusion data have not been declared, but red cell transfusion has also been identified as a risk factor for adverse outcome after cardiac surgery.4 In addition, Karkouti and colleagues5 identified that a preoperative haemoglobin level below 9 g dl–1 is an independent risk factor for adverse outcome after cardiac surgery. I would also like to comment that although pump flow is stated at 2.2 litre min–1 kg–1 in the paper, in our department it is customary to reduce this by approximately 5% per °C of cooling according to mixed venous oxygen estimation and I wonder if this was the case in this study. Ranucci has commented that increasing pump flow when haematocrit falls can counteract the effect of reduced O2 carrying capacity, so if pump flow was not reduced I wonder if this has had a beneficial effect. It certainly would be of interest in the future to have a similarly conducted study which includes a group with anaemia to permit comparison of outcome, as anaemic patients do present for cardiac surgery relatively commonly.
Groningen, The Netherlands
* E-mail: b.loef{at}hccnet.nl
Editor—We thank Dr Morrice for addressing several issues that may explain the maintenance of renal function after CPB in our patient population. At our institution, pump flow is routinely maintained at a fixed level during mild hypothermia. Further, haematocrit levels during CPB are targeted at 20–25%; if necessary because of haemodilution, blood donation is instituted at the start of bypass. Because of our procedure, we agree with Dr Morrice that the fixed pump flow during CPB, providing increased DO2, may offer an explanation for the protection of renal function. However, our study was not designed to establish the effects of oxygen delivery on perioperative renal function and the data requested are unavailable. Indeed, it would be of interest in the future to have a similar study conducted to evaluate the influence of oxygen delivery on renal function during CPB. In view of the ease of recruiting patients and the duration of the trial, it would be worthwhile to explore whether such a study would benefit from the use of sensitive markers for glomerular and tubular function.6
References
1 Loef BG, Henning RH, Navis G, et al. Changes in glomerular filtration rate after cardiac surgery with cardiopulmonary bypass in patients with mild preoperative renal dysfunction. Br J Anaesth (2008) 100:759–64.
2 Whitehouse T, Stotz M, Taylor V, Stidwill R, Singer M. Tissue oxygen and hemodynamics in renal medulla, cortex & cortico-medullary junction during hemorrhage–reperfusion. Am J Physiol Renal Physiol (2006) doi:10.1152/ajprenal.00475.2005.
3 Ranucci M, Romitti F, Isgro G, et al. Oxygen delivery during cardiopulmonary bypass and acute renal failure after coronary operations. Ann Thorac Surg (2005) 80:2213–20.
4 Kuduvalli M, Oo AY, Newall N, et al. Effect of peri-operative red blood cell transfusion on 30-day and 1-year mortality following coronary artery bypass surgery. Eur J Cardiothorac Surg (2005) 27:592–8.
5 Karkouti K, Wijeysundera DN, Beattie WS, for the Reducing Bleeding in Cardiac Surgery (RBC). Risk associated with preoperative anemia in cardiac surgery: a multicenter cohort study. Circulation (2008) 117:478–84.
6 Loef BG, Henning RH, Epema AH, et al. Effect of dexamethasone on perioperative renal function impairment during cardiac surgery with cardiopulmonary bypass. Br J Anaesth (2004) 93:793–8.
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