Objective—To image the spatial distribution of pulmonary blood flow by means of scintigraphy, evaluate ventilation-perfusion (VA/Q) matching and pulmonary blood shunting (Qs/Qt) by means of the multiple inert gas elimination technique (MIGET), and measure arterial oxygenation and plasma endothelin-1 concentrations before, during, and after pulse-delivered inhaled nitric oxide (PiNO) administration to isoflurane-anesthetized horses in dorsal recumbency.
Animals—3 healthy adult Standardbreds.
Procedures—Nitric oxide was pulsed into the inspired gases in dorsally recumbent isoflurane-anesthetized horses. Assessment of VA/Q matching, Qs/Qt, and Pao2 content was performed by use of the MIGET, and spatial distribution of pulmonary blood flow was measured by perfusion scintigraphy following IV injection of technetium Tc 99m–labeled macroaggregated human albumin before, during, and 30 minutes after cessation of PiNO administration.
Results—During PiNO administration, significant redistribution of blood flow from the dependent regions to the nondependent regions of the lungs was found and was reflected by improvements in VA/Q matching, decreases in Qs/Qt, and increases in Pao2 content, all of which reverted to baseline values at 30 minutes after PiNO administration.
Conclusions and Clinical Relevance—Administration of PiNO in anesthetized dorsally recumbent horses resulted in redistribution of pulmonary blood flow from dependent atelectatic lung regions to nondependent aerated lung regions. Because hypoxemia is commonly the result of atelectasis in anesthetized dorsally recumbent horses, the addition of nitric oxide to inhaled gases could be used clinically to alleviate hypoxemia in horses during anesthesia.
Objective—To assess physiologic responses and plasma endothelin (ET)-1 concentrations associated with abrupt cessation of nitric oxide (NO) inhalation in isoflurane-anesthetized horses.
Animals—6 healthy adult Standardbreds.
Procedures—Horses were anesthetized with isoflurane in oxygen and placed in dorsal recumbency. Nitric oxide was pulsed into the respiratory tract for 2.5 hours, and then administration was abruptly discontinued. Just prior to commencement and at cessation of NO administration, and at intervals during a 30-minute period following cessation of NO inhalation, several variables including PaO2, mean pulmonary artery pressure, venous admixture or pulmonary shunt fraction (Qs/Qt), and plasma ET-1 concentration were recorded or calculated.
Results—After cessation of NO inhalation, PaO2 decreased slowly but significantly (172.7 ± 29.8 mm Hg to 84.6 ± 10.9 mm Hg) and Qs/Qt increased slowly but significantly (25 ± 2% to 40 ± 3%) over a 30-minute period. Mean pulmonary artery pressure increased slightly (14.0 ± 1.3 mm Hg to 16.8 ± 1 mm Hg) over the same time period. No change in serum ET-1 concentration was detected, and other variables did not change or underwent minor changes.
Conclusions and Clinical Relevance—The improvement in arterial oxygenation during pulsed inhalation of NO to healthy isoflurane-anesthetized horses decreased only gradually during a 30-minute period following cessation of NO inhalation, and serum ET-1 concentration was not affected. Because a rapid rebound response did not develop, inhalation of NO might be clinically useful in the treatment of hypoxemia in healthy isoflurane-anesthetized horses.