The effects of pulse-delivered inhaled nitric oxide on arterial oxygenation, ventilation-perfusion distribution and plasma endothelin-1 concentration in laterally recumbent isoflurane-anaesthetized horses.

OBJECTIVES

Anaesthetized horses commonly become hypoxaemic due to ventilation/perfusion (V·A/Q·) mismatch and increased pulmonary shunt fraction (Qs·/Qt·). Pulse-delivered inhaled nitric oxide may improve oxygenation but may increase plasma concentration of the potent vasoconstrictor, endothelin-1 (ET-1). Objectives: Study 1) compare arterial oxygen concentration (PaO2) and saturation (SaO2), calculated Qs·/Qt· and ET-1 concentration; and Study 2) assess V·A/Q· matching and measured Qs·/Qt· in isoflurane-anaesthetized horses in left lateral recumbency receiving pulse-delivered inhaled nitric oxide (PiNO group) or inhalant gas only (C group).

STUDY DESIGN

Prospective research trial.

ANIMALS

Ten Healthy adult Standardbred horses. Two horses were anaesthestized in both groups in a random cross-over design with >4 weeks between studies.

METHODS

Study 1) Cardiopulmonary data including PaO2, SaO2, Qs·/Qt· and ET-1 concentration were measured or calculated prior to and at various points during PiNO administration in 6PiNO and 6C horses. Two-way repeated measures anova with Bonferroni significant difference test was used for data analysis with p < 0.05 considered significant. Study 2) V·A/Q· matching and Qs·/Qt· were determined using the multiple inert gas elimination technique in 3 horses. Data were collected after 60 minutes of anaesthesia without PiNO (baseline) and 15 minutes after PiNO was pulsed during the first 30%, and then the first 60%, of inspiration. Data were descriptive only.

RESULTS

Study 1) PaO2 and SaO2 were higher and calculated Qs·/Qt· was lower in the PiNO group than the C group at most time points. ET-1 was not different over time or between groups. Study 2) V·A/Q· matching and measured Qs·/Qt· were improved from baseline in all horses but PiNO60% provided no improvement when compared to PiNO30%.

CONCLUSIONS AND CLINICAL RELEVANCE

PiNO delivered in the initial portion of the inspiration effectively relieves hypoxaemia in anaesthetized horses by improving V·A/Q· matching and decreasing Qs·/Qt· without affecting ET-1.