Nitric oxide and its role in the management of hypoxaemia in the anaesthetised horse-a narrative review.

Nitric oxide (NO), particularly in its inhaled form (iNO), plays a crucial role in managing low blood oxygen levels (hypoxaemia) during anesthesia in horses. This review discusses NO’s discovery, its medical applications in humans, and the promising evidence supporting pulsed inhaled nitric oxide (PiNO) for improving oxygenation in anesthetized horses.

Introduction and Historical Background

• Discovery of nitric oxide and its identification as a biological molecule involved in various physiological processes.
• Initial research highlighting NO’s role as a vasodilator and signaling molecule in the cardiovascular system.
• Evolution of NO research leading to therapeutic applications, especially in respiratory medicine.

Use of Inhaled Nitric Oxide (iNO) in Human Healthcare

• iNO is licensed for treatment of hypoxic respiratory failure, particularly in human neonates.
• It has been effectively employed in adults for refractory hypoxaemia, such as cases linked to acute respiratory distress syndrome (ARDS) and viral infections like COVID-19 caused by SARS-CoV-2.
• iNO acts by selectively dilating pulmonary blood vessels in well-ventilated areas of the lung, improving ventilation-perfusion matching and oxygenation.

Pulsed Inhaled Nitric Oxide (PiNO) in Equine Anaesthesia

• PiNO refers to the delivery of nitric oxide in pulses synchronized with ventilation, targeting better distribution in the lungs.
• Studies demonstrate that administering PiNO to anesthetized horses improves oxygenation by redirecting blood flow from poorly ventilated (dependent) lung areas to better ventilated (nondependent) regions.
• This mechanism reduces intrapulmonary shunting, whereby blood bypasses oxygenation sites, minimizing hypoxaemia during anesthesia.
• Evidence includes both experimental data and clinical case reports involving client-owned horses undergoing surgeries such as colic repair and arthroscopic procedures.

Technological Advances and Clinical Implications

• The original PiNO delivery device (NOrse, Datex-Ohmeda Research Department, Helsinki, Finland) has been redeveloped to enhance practicality and accessibility in clinical settings.
• With technological improvements, PiNO administration is becoming a feasible treatment for managing hypoxaemia in anesthetized horses during surgery.
• This represents a potential paradigm shift in equine anesthesia management, improving safety and outcomes.
• Further clinical studies and wider adoption may establish PiNO as a standard adjunct treatment in equine veterinary practice.

Conclusions

• Inhaled nitric oxide has a well-established role in human medicine for managing hypoxic respiratory conditions.
• Emerging evidence supports the effectiveness of pulsed iNO in improving oxygenation during general anesthesia in horses by optimizing pulmonary blood flow and reducing shunting.
• The clinical application of PiNO in equine anesthesia is promising, with advancing technology facilitating its use.
• Adoption of PiNO could enhance perioperative care and reduce hypoxaemia-related complications in equine surgery.