Just over a fortnight ago I had a two vessel coronary artery bypass graft. I was back home on the sixth day after the operation, and the community cardiac nurse visited six days later. Among other things, she took a blood sample because she thought I looked anaemic. This was not a surprise, because my wife had commented for a few days on my pallor, and I had that very morning checked my conjunctivas and found them pale.
My haemoglobin concentration came back at 86 g/l, while red cell volume was in the normal range. The community nurse duly asked my general practitioner to prescribe iron supplementation, my GP did so (as I would have done in her place), and I happily started taking ferrous sulphate 200 mg twice daily. However, having time on my hands, I started thinking about the use of iron in this context. I now question the clinical logic.
Consider. The usual mean lifespan for red blood cells in the circulation is 120 days. On average then, 0.83% (1/120) of the red blood cell population dies every 24 hours and, to maintain the normal steady state, is replaced from bone marrow haemopoiesis. The cardiopulmonary bypass pump is a traumatic environment for red blood cells. Some are destroyed, and the mean survival time of the surviving population is reduced. Cell lifespan is reduced by a mean of 33-50% (personal communication, G Bertoni, vascular surgery research fellow, Oxford). Therefore, when I came off the pump, my red blood cells had a mean survival of 60-80 days and were thus dying off at the rate of 1.25-1.67% of the total every 24 hours.
For this argument, let's take a mean of 70 days, or 1.43% loss per 24 hours. Intraoperative blood loss was replaced by transfusion (also red blood cells with reduced survival time), and let's ignore loss of cells during the pump period. Also, set aside for now any compensatory haemopoietic response, and assume marrow replacement continues at 0.83% of total per day. Therefore, by 70 days after I came off the pump, 100% of my surviving red blood cell population will have been cleared from my circulation. During this time, my bone marrow will have generated 0.83% of the total cell population a day, leaving me with 58.3% (70/120) of my “usual” cell population. No mystery as to how normocytic anaemia arises after coronary artery bypass grafting.
In reality, of course, there is a compensatory erythropoietin and bone marrow response, which increases the red blood cell replacement rate. This shortens the duration of postoperative anaemia and lessens the dip in cell population. By the way, can anyone say whether this crude mathematical model of my convalescent musings comes anywhere near the observed dynamics of anaemia after coronary artery bypass grafting?
And now to iron. In the postoperative anaemia there must be plenty of recycled iron from the prematurely senescent red blood cells. As far as I recall, in the absence of iron deficiency the absorption ceiling for dietary iron does not change. Oral iron supplements remain unabsorbed and do not affect haemopoiesis. There is no evidence that my preoperative iron stores were other than normal, so there was no benefit to me in taking ferrous sulphate. There was a disbenefit, because it made my piles hurt. I stopped taking the ferrous sulphate after two days, and have continued to convalesce well.
What I have not yet done is find the courage to tell this to my GP, the community cardiac nurse, or indeed my wife.