Circulatory Effects of Hypoxia, Acute Normovolemic Hemodilution, and Their Combination in Anesthetized Pigs

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Abstract
Background: Because hemodilution decreases the oxygen-carrying capacity of blood, it was hypothesized that severe hemodilution would decrease the tolerance to alveolar hypoxia. Methods: Hemodynamics, oxygen transport, and blood lactate concentrations were compared in ten pigs with normal hematocrit (33 +/- 4%), and ten hemodiluted pigs (hematocrit 11 +/- 1%; mean +/- SD) anesthetized with ketamine-fentanyl-pancuronium during stepwise decreases in inspired oxygen fraction (FIO2; 1.0, 0.35, 0.21, 0.15, 0.10, 0.05). Results: Median systemic oxygen delivery (DO2SY) became critical (the DO2SY value when arterial lactate exceeded 2.0 mmol.l-1) at 10.4 ml.kg-1.min-1 (range 6.9-16.1) in hemodiluted animals and at 11.8 ml.kg-1.min-1 (5.9-32.2) in animals with normal hematocrits (NS). The relationship between mixed venous oxygen saturation and arterial lactate values was less consistent and median critical mixed venous oxygen saturation was higher (P < 0.05) in the hemodiluted group (35%, range 21-64), than in animals with normal hematocrits (21%, 7-68%). In animals with normal hematocrit, decreasing FIO2 from 1.0 to 0.10 resulted in a decrease in DO2SY from 26.3 +/- 9.1 to 9.3 +/- 3.9 ml.kg-1.min-1 (P < 0.01). Cardiac output did not change, systemic oxygen extraction ratio increased from 0.23 +/- 0.08 to 0.68 +/- 0.13 (P < 0.01), and arterial lactate from 0.9 +/- 0.2 to 3.4 +/- 3.0 mmol.l-1 (P < 0.05). Cardiac venous blood flow, as measured by retrograde thermodilution, increased from 5.7 +/- 2.9 to 12.6 +/- 5.7 ml.kg-1.min-1 (P < 0.01). When FIO2 was reduced to 0.05, three animals became hypotensive and died. In the second group, hemodilution increased cardiac output and systemic oxygen extraction ratio (P < 0.01). Cardiac venous blood flow increased from 4.1 +/- 1.7 to 9.8 +/- 5.1 ml.kg-1.min-1 (P < 0.01), and cardiac venous oxygen saturation from 22 +/- 5 to 41 +/- 10% (P < 0.01). During the subsequent hypoxia, cardiac output and DO2SY were maintained until FIO2 = 0.15 (DO2SY = 10.1 +/- 3.3 ml.kg-1.min-1). Cardiac venous blood flow was then 18.5 +/- 10.7 ml.kg-1.min-1 (P < 0.01), but in spite of this, myocardial lactate production occurred. At FIO2 = 0.10 (DO2SY = 7.7 +/- 3.0 ml.kg-1.min-1), arterial lactate concentration increased to 8.5 +/- 2.3 mmol.l-1 (P < 0.01), and most animals became hypotensive. All hemodiluted animals died when FIO2 was decreased to 0.05 (P < 0.01 when compared to animals with normal hematocrit). Conclusions: Systemic and myocardial lactate production occurred at similar systemic oxygen delivery rates in hemodiluted and nonhemodiluted animals. Mixed venous oxygen saturation may be a less reliable indicator of inadequate oxygen delivery during hemodilution.