Blood – thicker than water?

Anaesthesia Blog, May 2016.

The first use of this phrase was probably in a German proverb (originally: Blut ist dicker als Wasser), Reinhart Fuchs by Heinrich der Glîchezære. By 1670, the modern version was included in John Ray’s collected Proverbs, and later appeared in Sir Walter Scott’s novel Guy Mannering (1815) and in Thomas Hughes’s Tom Brown’s School Days (1857). The June edition of Anaesthesia contains a number of articles that look at how thick blood actually needs to be and why this is important in the peri-operative period.

Anaemia is an increasingly prevalent condition, and has actually been described as an epidemic by the WHO. The Association of Cardiothoracic Anaesthetists (ACTA) undertook their first National Audit which is published here and describes how the rate of anaemia varies in different regions of the UK, from 23% to nearly double at 45%. The major finding of this study was confirmation of the association between anaemia and worse outcomes, and specifically that the lower the haemoglobin concentration is before surgery, the greater the chance of death is after cardiac surgery. In other words, thicker blood is good (except it seems if Hb > 150 g.l-1, which is also associated with increased mortality – is the blood too thick?).

Predicting which patients are more likely to bleed is one of the holy grails of peri-operative care, so that these high-risk patients can be targeted and actively managed. A group of authors used two point-of-care analysers to measure platelet function and found that a measurement performed soon after coming off bypass was best at predicting excessive bleeding. Tests performed at the start of surgery were not predictive, which is important as many manufacturers do recommend measuring at this point as a ‘baseline’, but this study suggests this may not be important. Also of interest was that patients taking pre-operative aspirin or clopidogrel were not more likely to bleed excessively, but anaemic patients were. Back to thicker blood is better!

Postpartum haemorrhage is the leading cause of maternal mortality globally, and this important study looked at the effect of platelets on major haemorrhage. Out of over 6000 deliveries over a one-year period, 356 (6%) women experienced moderate or severe haemorrhage, most commonly due to uterine atony, surgical bleeding or genital tract trauma. However, only 12 women required transfusion of platelets, which is much lower than in major haemorrhage due to trauma or following cardiac surgery, and this is a very interesting finding. Indeed, if the women had a normal platelet count before delivery and did not have placental abruption or other cause of consumptive coagulopathy, they had to bleed 5000 ml before requiring platelet transfusion. All this means that a fixed protocol including platelets is unnecessary for obstetric haemorrhage, and this is certainly relevant to clinical practice in every obstetric unit.

The final study that looked at bleeding was undertaken following major liver resection.  Not many anaesthetists look after these patients in the peri-operative period, but the authors finding that most patients were in fact less likely to bleed (were hypercoagulable – thicker blood?) after surgery, as measured using point-of-care tests, is relevant to all anaesthetists. This implies that instead of bleeding, most patients are at increased risk of thrombosis postoperatively, despite major surgery/transfusion, and that laboratory tests are in fact misleading during the immediate postoperative period. This would mean that prophylactic anti-thrombotic agents such as low-molecular weight heparin should be started earlier, and that laboratory tests should be replaced by point-of-care tests, a fact which is emphasised in this editorial. It really seems to me that the time has come to use point-of-care tests more widely, even once surgery has finished.

To finish off this blog on blood and bleeding, the new AAGBI Guidelines on transfusion have just been published, and they replace three previous guidelines on blood component therapy, massive haemorrhage and red cell transfusion. They will appear in the journal next month and will be sent round to every anaesthetic department as well as the Chief Executive of every NHS Trust in the UK. Read them yourself and see if blood really is thicker than water, or at least how important it is in the peri-operative period.



Andrew Klein