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Background  Low hemoglobin concentration (Hb) and low mean arterial blood pressure (MAP) impact outcomes in critically ill patients. We utilized an experimental model of “normotensive” vs. “hypotensive” acute hemodilutional anemia to test whether optimal tissue perfusion is dependent on both Hb and MAP during acute blood loss and fluid resuscitation, and to assess the value of direct measurements of the partial pressure of oxygen in tissue (P t O 2 ).    Methods  Twenty-nine anesthetized rats underwent 40% isovolemic hemodilution (1:1) (or sham-hemodilution control,  n  = 4) with either hydroxyethyl starch (HES) ( n  = 14, normotensive anemia) or saline ( n  = 11, hypotensive anemia) to reach a target Hb value near 70 g/L. The partial pressure of oxygen in the brain and skeletal muscle tissue (P t O 2 ) were measured by phosphorescence quenching of oxygen using G4 Oxyphor. Mean arterial pressure (MAP), heart rate, temperature, arterial and venous co-oximetry, blood gases, and lactate were assessed at baseline and for 60 min after hemodilution. Cardiac output (CO) was measured at baseline and immediately after hemodilution. Data were analyzed by repeated measures two-way ANOVA.    Results  Following “normotensive” hemodilution with HES, Hb was reduced to 66 ± 6 g/L, CO increased ( p  < 0.05), and MAP was maintained. These conditions resulted in a reduction in brain P t O 2  (22.1 ± 5.6 mmHg to 17.5 ± 4.4 mmHg,  p  < 0.05), unchanged muscle PO 2 , and an increase in venous oxygen extraction. Following “hypotensive” hemodilution with saline, Hb was reduced to 79 ± 5 g/L and both CO and MAP were decreased ( P  < 0.05). These conditions resulted in a more severe reduction in brain P t O 2  (23.2 ± 8.2 to 10.7 ± 3.6 mmHg ( p  < 0.05), a reduction in muscle P t O 2  (44.5 ± 11.0 to 19.9 ± 12.4 mmHg,  p  < 0.05), a further increase in venous oxygen extraction, and a threefold increase in systemic lactate levels ( p  < 0.05).    Conclusions  Acute normotensive anemia (HES hemodilution) was associated with a subtle decrease in brain tissue P t O 2  without clear evidence of global tissue hypoperfusion. By contrast, acute hypotensive anemia (saline hemodilution) resulted in a profound decrease in both brain and muscle tissue P t O 2  and evidence of inadequate global perfusion (lactic acidosis). These data emphasize the importance of maintaining CO and MAP to ensure adequacy of vital organ oxygen delivery during acute anemia. Improved methods of assessing P t O 2  may provide an earlier warning signal of vital organ hypoperfusion.

Experimental assessment of oxygen homeostasis during acute hemodilution: the integrated role of hemoglobin concentration and blood pressure