Hypoxia results in an increase in total systemic shunt flow in dogs

There is considerable evidence to indicate that blood flow to many organs changes in response to hypoxia. However, since blood may traverse nutrient capillaries or be shunted through precapillary arteriovenous channels, measurements of total organ blood flow alone may not accurately reflect microcirculatory adaptations to hypoxia. To test this hypothesis, we determined the effect of controlled normoxic ventilation (PO2 = 86.4 +/- 2.8 Torr; N = 10) and controlled hypoxic ventilation (PO2 = 45.9 +/- 1.0 Torr; N = 5) on cardiac output, nutrient blood flow and total systemic shunt flow in dogs using radiolabelled 15 micrometers microspheres. The microspheres were injected from left heart catheters in each animal and total systemic shunt flow was estimated from the radioactivity present in pulmonary artery reference samples. Cardiac output, as well as nutrient blood flow to the heart and brain, was similar in both hypoxic animals and normoxic controls. Although we could detect no significant changes in nutrient blood flow to the remaining organs in response to hypoxia, total systemic shunt flow increased from a baseline value of 4.4% of cardiac output to a maximum value of 7.9% of cardiac output in hypoxic animals compared to 2.3% of cardiac output in normoxic animals at a similar time point (P less than 0.01). This hypoxia-induced increase in total systemic shunt flow may contribute to the overall decline in organ function often observed in this setting.