Effects of oxygen, positive end-expiratory pressure, and carbon dioxide on oxygen delivery in an animal model of the univentricular heart.
OBJECTIVE
Respiratory manipulations are a mainstay of therapy for infants with a univentricular heart, but until recently little experimental information has been available to guide their use. We used an animal model of a univentricular heart to characterize the physiologic effects of a number of commonly used ventilatory treatments, including altering inspired oxygen tension, adding positive end-expiratory pressure, and adding supplemental carbon dioxide to the ventilator circuit.


RESULTS
Lowering inspired oxygen tension decreased the ratio of pulmonary to systemic flow. This ratio was 1.29 +/- 0.08 at an inspired oxygen tension of 100%, 0.61 +/- 0.09 at an inspired oxygen tension of 21%, and 0.42 +/- 0.09 at an inspired oxygen tension of 15% (p < 0.05 compared with an inspired oxygen tension of 100% and a positive end-expiratory pressure of 0 cm H2O). High-concentration supplemental carbon dioxide (carbon dioxide tension of 80 to 90 mm Hg) added to the ventilator circuit decreased inspired oxygen tension from 1.29 +/- 0.11 to 0.42 +/- 0.12 (p < 0.05 compared with baseline). A mixture of 95% oxygen and 5% carbon dioxide (carbon dioxide tension of 50 to 60 mm Hg) did not decrease the pulmonary/systemic flow ratio significantly. All three types of interventions influenced systemic oxygen delivery, which was a function of the pulmonary/systemic flow ratio. As the pulmonary/systemic flow ratio decreased from initially high levels (greater than 1), oxygen delivery first increased and reached an optimum at a flow ratio slightly less than 1. As the pulmonary/systemic flow ratio decreased further, below 0.7, oxygen delivery decreased. The ability of systemic arterial and venous oxygen saturations to predict the pulmonary/systemic flow ratio was examined. Venous oxygen saturation correlated well with both pulmonary/systemic flow ratio and systemic oxygen delivery, whereas arterial oxygen saturation did not accurately predict either pulmonary/systemic flow ratio or oxygen delivery.


CONCLUSIONS
This model demonstrated the value of estimating the pulmonary/systemic flow ratio before initiating therapy. When the initial ratio was greater than about 0.7, interventions that decreased the ratio increased oxygen delivery and were beneficial. When the initial pulmonary/systemic flow ratio was below 0.7, interventions that decreased the ratio decreased oxygen delivery and were detrimental. We conclude by presenting a framework to guide therapy based on the combination of arterial and venous oxygen saturations and the estimate of the pulmonary/systemic flow ratio that they provide.
Riordan
C J
CJ
Division of Thoracic and Cardiovascular Surgery, University of Louisville School of Medicine, Ky 40292, USA.
Randsbeck
F
F
Storey
J H
JH
Montgomery
W D
WD
Santamore
W P
WP
Austin
E H
EH
D000818:Animals; D000831:Animals, Newborn; D001158:Arteries; D001775:Blood Circulation; D002245:Carbon Dioxide / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D004195:Disease Models, Animal; D006330:Heart Defects, Congenital / Q000628:therapy*; D006352:Heart Ventricles / Q000002:abnormalities; D010100:Oxygen / Q000008:administration & dosage / Q000097:blood; D010102:Oxygen Inhalation Therapy; D010313:Partial Pressure; D011175:Positive-Pressure Respiration; D011652:Pulmonary Circulation; D012121:Respiration, Artificial; D013552:Swine; D014655:Vascular Resistance; D014680:Veins
D002245:Carbon Dioxide; D010100:Oxygen