Mechanical ventilation of the horse

Equine anatomy and physiology

Horses anaesthetised and placed in dorsal recumbency often develop venous congestion and nasal oedema that can lead to partial or complete obstruction of the nasal passages and inadequate pulmonary gas (PaCO2 , PaO2 ) exchange. Placement of an endotracheal tube is recommended in order to maintain airway patency and facilitate the delivery of oxygen and ventilatory support.
Normally PaO2 is approximated as 5 times the inspired oxygen concentration (%: ex: 100% FiO2 = 5 x 100 = 500 mm Hg). A standing awake horse breathing 21% oxygen (room air) has a measured PaO2 of approximately 90–100 mm Hg (5 x 20%). During general anaesthesia oxygen tension in horses breathing room may be 55–70 mm Hg. PaO2 values in the horse may be as low as 50–60 mm Hg during general anaesthesia even with inspired oxygen concentrations near 100%. Hypoventilation (evidenced by increasing PaCO2 ), ventilation/perfusion mismatching with lung collapse, alveolar atelectasis (compression and absorption) and intrapulmonary vascular shunting are the primary causes responsible for the lower than predicted PaO2 in the anaesthetised horse. Diffusion impairment (i.e. pneumonia) will also contribute to low PaO2 .
Ventilatory support should be considered when PaCO2 levels exceed 70–80 mm Hg (pH<7.25–7.20) and/or PaO2 falls below 100 mm Hg. Blood gas analysis is helpful in defining clinical end points. […]