Do intravascular hypo- and hypervolaemia result in changes in central blood volumes?

Background: Hypovolaemia is generally believed to induce centralization of blood volume. Therefore, we evaluated whether induced hypo-and hypervolaemia result in changes in central blood volumes (pulmonary blood volume (PBV), intrathoracic blood volume (ITBV)) and we explored the effects on the distribution between these central blood volumes and circulating blood volume (V-d circ).Methods: Six anaesthetized, spontaneously breathing Foxhound dogs underwent random blood volume alterations in steps of 150 ml (mild) to 450 ml (moderate), either by haemorrhage, retransfusion of blood, or colloid infusion. PBV, ITBV and V-d circ were measured using (transpulmonary) dye dilution. The PBV/V-d circ ratio and the ITBV/V-d circ ratio were used as an assessment of blood volume distribution.Results: 68 blood volume alterations resulted in changes in Vd(circ) ranging from -33 to + 31%. PBV and ITBV decreased during mild and moderate haemorrhage, while during retransfusion, PBV and ITBV increased during moderate hypervolaemia only. The PBV/V-d circ ratio remained constant during all stages of hypo-and hypervolaemia (mean values between 0.20-0.22). This was also true for the ITBV/V-d circ ratio, which remained between 0.31 and 0.32, except for moderate hypervolaemia, where it increased slightly to 0.33 (0.02), PConclusions: Mild to moderate blood volume alterations result in changes of V-d circ, PBV and ITBV. The ratio between the central blood volumes and V-d circ generally remained unaltered. Therefore, it could be suggested that in anaesthetized spontaneously breathing dogs, the cardiovascular system maintains the distribution of blood between central and circulating blood volume.