Cerebral O2 and CO2 transport in isovolumic haemodilution: Compensation of cerebral delivery of O2 and maintenance of cerebrovascular reactivity to CO2

This study investigated the influence of acute reductions in arterial O-2 content (CaO2) via isovolumic haemodilution on global cerebral blood flow (gCBF) and cerebrovascular CO2 reactivity (CVR) in 11 healthy males (age; 28 +/- 7 years: body mass index; 23 +/- 2 kg/m(2)). Radial artery and internal jugular vein catheters provided measurement of blood pressure and gases, quantification of cerebral metabolism, cerebral CO2 washout, and trans-cerebral nitrite exchange (ozone based chemiluminescence). Prior to and following haemodilution, the partial pressure of arterial CO2 (PaCO2) was elevated with dynamic end-tidal forcing while gCBF was measured with duplex ultrasound. CVR was determined as the slope of the gCBF response and PaCO2. Replacement of similar to 20% of blood volume with an equal volume of 5% human serum albumin (Alburex (R) 5%) reduced haemoglobin (13.8 +/- 0.8 vs. 11.3 +/- 0.6 g/dL; P < 0.001) and CaO2 (18.9 +/- 1.0 vs 15.0 +/- 0.8 mL/dL P < 0.001), elevated gCBF (+18 +/- 11%; P = 0.002), preserved cerebral oxygen delivery (P = 0.49), and elevated CO2 washout (+11%; P = 0.01). The net cerebral uptake of nitrite (11.6 +/- 14.0 nmol/min; P = 0.027) at baseline was abolished following haemodilution (-3.6 +/- 17.9 nmol/min; P = 0.54), perhaps underpinning the conservation of CVR (61.7 +/- 19.0 vs. 69.0 +/- 19.2 mL/min/mmHg; P = 0.23). These findings demonstrate that the cerebrovascular responses to acute anaemia in healthy humans are sufficient to support the maintenance of CVR.