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Carbon dioxide added late in inspiration reduces ventilation-perfusion heterogeneity without causing respiratory acidosis

¹Department of Pediatrics, Children's Hospital and Regional Medical Center; ²Departments of Physiology and Biophysics and of Medicine; ³Department of Anesthesiology and ⁴Medical Service, Veterans Affairs Puget Sound Health Care System, University of Washington, Seattle, Washington 98105

Submitted 19 February 2003 ; accepted in final form 2 December 2003

We have shown previously that inspired CO₂ (3-5%) improves ventilation-perfusion (A/) matching but with the consequence of mild arterial hypercapnia and respiratory acidosis. We hypothesized that adding CO₂ only late in inspiration to limit its effects to the conducting airways would enhance A/ matching and improve oxygenation without arterial hypercapnia. CO₂ was added in the latter half of inspiration in a volume aimed to reach a concentration of 5% in the conducting airways throughout the respiratory cycle. Ten mixed-breed dogs were anesthetized and, in a randomized order, ventilated with room air, 5% CO₂ throughout inspiration, and CO₂ added only to the latter half of inspiration. The multiple inert-gas elimination technique was used to assess A/ heterogeneity. Late-inspired CO₂ produced only very small changes in arterial pH (7.38 vs. 7.40) and arterial CO₂ (40.6 vs. 39.4 Torr). Compared with baseline, late-inspired CO₂ significantly improved arterial oxygenation (97.5 vs. 94.2 Torr), decreased the alveolar-arterial PO₂ difference (10.4 vs. 15.7 Torr) and decreased the multiple inert-gas elimination technique-derived arterial-alveolar inert gas area difference, a global measurement of A/ heterogeneity (0.36 vs. 0.22). These changes were equal to those with 5% CO₂ throughout inspiration (arterial PO₂, 102.5 Torr; alveolar-arterial PO₂ difference, 10.1 Torr; and arterial-alveolar inert gas area difference, 0.21). In conclusion, we have established that the majority of the improvement in gas exchange efficiency with inspired CO₂ can be achieved by limiting its application to the conducting airways and does not require systemic acidosis.

hypercapnia; respiratory acidosis; oxygen; dog; multiple inert-gas elimination technique

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