Flow redistribution during progressive hemorrhage is a determinant of critical O<inf>2</inf> delivery

O2 consumption (V̇O2) of anesthetized whole mammals is independent of O2 delivery (ḊO2) until ḊO2 declines to a critical value (ḊO(2c)). Below this value, V̇O2 becomes O2 supply dependent. We assessed the influence of whole body ḊO2 redistribution among organs with respect to the commencement of O2 supply dependency. We measured ḊO2, V̇O2, and ḊO(2c) of whole body, liver, intestine, kidney, and remaining carcass in eight mongrel dogs during graded progressive hemorrhage. Whole body ḊO2 was redistributed such that the organ-to-whole body ḊO2 ratio declined for liver and kidney and increased for carcass. We then created a mathematical model wherein each organ-to-whole body ḊO2 ratio remained approximately constant at all values of whole body ḊO2 and assigned organ V̇O2 to predicted organ ḊO2 by interpolation and extrapolation of observed V̇O2-ḊO2 plots. The model predicted that O2 supply dependency without redistribution would have commenced at a higher value of whole body ḊO2 for whole body (8.11 ± 0.89 vs. 6.98 ± 1.16 ml·kg-1·min-1, P < 0.05) and carcass (6.83 ± 1.16 vs. 5.06 ± 1.15 ml·kg-1·min-1, P < 0.01 ) and at a lower value of whole body ḊO2 for liver (6.33 ± 1.86 vs. 7.59 ± 1.95, ml·kg-1·min-1, P < 0.02) and kidney (1.25 ± 0.64 vs. 4.54 ± 1.29 ml·kg-1·min-1, P < 0.01). We conclude that redistribution of whole body ḊO2 among organs facilitates whole body O2 regulation

Renal O<inf>2</inf> consumption during progressive hemorrhage

Most mammalian tissues regulate O2 utilization such that O2 consumption (V̇O2) is relatively constant at O2 delivery (ḊO2) higher than a critical value (ḊO(2 c)). We studied the relationship between V̇O2 and ḊO2 of kidney and whole body during graded progressive exsanguination. The relationship between whole body V̇O2 and ḊO2 was biphasic, and whole body V̇O2 decreased by 5.6 ± 14.4% (P = NS) from the initial value to the value nearest whole body ḊO(2 c). Kidney ḊO2 decreased in direct proportion to whole body ḊO2 such that the average R2 value describing the linear regression of kidney ḊO2 vs. whole body ḊO2 was 0.94 ± 0.02. The relationship between kidney, like whole body, V̇O2 and ḊO2 appeared biphasic; however, kidney V̇O2 decreased by 63.3 ± 10.4% (P < 0.0001) from the initial value to the value nearest kidney ḊO(2 c). Renal O2 extraction ratio was relatively constant over a wide range of kidney ḊO2, whereas whole body O2 extraction ratio increased progressively at all whole body ḊO2 values as whole body ḊO2 decreased. However, final values of O2 extraction ratio were indistinguishable for whole body (0.86 ± 0.1) and kidney (0.86 ± 0.06) (P = NS). We conclude that the pattern of kidney and whole body V̇O2 response to decreasing ḊO2 differs during hemorrhage, particularly in the range of ḊO2 normally associated with tissue wellness

Regional oxygen delivery in oxygen supply-dependent states

Assessment of the adequacy of systemic O2 delivery (DO2) is central in the evaluation of critically ill patients, but estimates of systemic DO2 do not assess the effectiveness of regional DO2 to all vascular beds whose functions may require different degrees of blood flow depending on their metabolic and functional demands. The oxygen supply-consumption curve includes a supply-in-dependent portion, which represents the reserve capacity of the body to maintain oxygen consumption (VO2) despite inadequate increases in DO2, and a supply-dependent portion, which represents the physiologic adaptation that occurs once DO2 is unable to meet the metabolic demands of the body. Experiments in dogs revealed that when systemic DO2 was progressively reduced, blood flow was maintained in the vital organs (heart and brain) and redistributed away from the kidneys and liver, enhancing the ability of the whole organism to use oxygen efficiently. Disease states and iatrogenic conditions that alter this vasoregulatory process may directly impair organ system function. © 1990 Springer-Verlag