As an anaesthetist I often encounter patients with rare and unusual conditions. Last week was no different. My regular general surgery list had been cancelled and at the last minute I had been reassigned to another. Arriving at the Theatres Admission Unit one of the nurses made her way over to me and said with a glint in her eye, "Your first patient has HB-Malmo - do you know what that is?" Of course I didn't and my blank expression said it all. She faked a look of astonishment and eventually handed over a very useful document from the patient that clearly explained the condition. It turns out that HB-Malmo is a haemoglobin variant with an unusually high affinity for oxygen. Whilst this means that those with HB-Malmo eagerly pick up oxygen molecules from the lungs, they are slow to release them into the tissues. The physiologists out there will know that this is referred to as a leftward shift of the oxygen-haemoglobin dissociation curve.
The oxygen-haemoglobin dissociation curve shows the relationship between the partial pressure of oxygen (x axis) and the percentage of haemoglobin molecules saturated with oxygen (y axis). Those with HB-Malmo have a single mutation in chromosome 11 which slows down the release of oxygen from binding sites on haemoglobin. This leftward shift of the oxygen-haemoglobin dissociation curve can also occur in other physiological states such as a low partial pressure of carbon dioxide, a high pH and a low temperature. A very good video describing the oxygen dissociation curve can be found here
In practise, a leftward shift of the oxygen-haemoglobin dissociation curve means that those with HB-Malmo are unable to deliver oxygen molecules to their cells. To overcome this, the body responds in a number of different ways. Amongst the best known is the release of erythropoietin from the kidneys and liver. This triggers a sustained rise in red cell production and an increase in the concentration of haemoglobin in the blood. This is very similar to the acclimatisation process that takes place in a high altitude environment.
Haemoglobin 198 g/l (115 - 165)
Haematocrit 0.56 (0.37 - 0.47)
White Cell Count 11.3 109/L (4 - 11)
Platelets 245 109/L (150 - 450)
Mean Cell Volume 91.1 fl (80 - 100)
Mean Cell Haemoglobin 32.0 pg (27 - 32)
Mean Cell Haemoglobin Concentration 357 g/l (320 - 360)
A typical full blood count of a patient with HB-Malmo. There is a large number of red cells (HIGH haematocrit) that are of normal size (NORMAL mean cell volume) and haemoglobin concentration (NORMAL mean cell haemoglobin and mean cell haemoglobin concentration
Whilst the increase in circulating haemoglobin may go some way to improving the delivery of oxygen to the tissues there is a price to be paid - the thickened blood leads to an increase in the risk of stroke, pulmonary embolism and right heart failure. In some cases venesection is performed to reduce haematocrit and prevent complications from occurring. In discussions with the haematology team it was decided that such a procedure was not required in this case. Instead we agreed to give a generous amount of intravenous fluid and administer a "day case" general anaesthetic that would allow a return to normal activity shortly after the procedure had finished. Long acting opiates and muscle relaxant were very much "out", regional anaesthesia and simple analgesics were "in". This still left the oyxgen-haemoglobin dissociation curve. To stop it drifting further left we kept the patient's core temperature above 36.5 degrees C and set the ventilator to maintain an end tidal carbon dioxide of approximately 6 KPa.
All of this seemed to work very well and a follow up phone call a week later revealed no complications. A good outcome in an extraordinary patient!
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