Farman, Colin; (1994) Plasma viscosity changes during haemodilution therapy of cerebral ischaemia. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The use of low molecular weight dextran (LMWD) as a haemodiluent can lead to a significant increase in blood plasma viscosity. This may be detrimental to the residual collateral blood flow through the ischaemic brain penumbra. In an experimental primate model of focal cerebral ischaemia, isovolaemic and hypervolaemic haemodilution with two infusates, LMWD and Haemaccel, were compared. Their influence on blood and plasma viscosity and haematocrit were studied along with their effect on regional cerebral blood flow (rCBF), oxygen delivery and tissue oxygen tension. Plasma viscosity was significantly elevated following either isovolaemic or hypervolaemic haemodilution with LMWD, but was not increased following haemodilution with Haemaccel. The raised plasma viscosity was associated with elevated whole blood viscosity. Both infusates lowered erythrocyte aggregation, but the LMWD induced increase in plasma viscosity compromised any rheological benefit. Hypervolaemic or isovolaemic haemodilution with either LMWD or Haemaccel significantly increased rCBF in all areas of the brain. The increase in blood flow to ischaemic areas with lost autoregulation to hypercapnia demonstrated the critical role of blood viscosity in determining blood flow, but these ischaemic regions also became more sensitive to hypercapnia induced intracerebral steal. The increases in rCBF were significantly correlated with changes in haematocrit and blood viscosity and an optimum haematocrit for maximal oxygen delivery to ischaemic brain was calculated (32%). Haemodilution induced changes in oxygen delivery were also directly linked with changes in brain tissue oxygen tension. The optimum haematocrit value for maximum oxygen delivery was found to be dependant upon the depth of ischaemia and could be influenced by changes in plasma viscosity. It was concluded that haemodilution therapy during normocapnia could improve rCBF and oxygen delivery to ischaemic brain, and that the deleterious effects of LMWD elevated plasma viscosity could be avoided by the use of a comparable haemodiluent; Haemaccel.

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