Optimising pulmonary gas exchange in anaesthetised horses : unravelling the role of pulsed inhaled nitric oxide using computed tomography angiography of the lung

Mortality rates in healthy, anaesthetised horses are higher than in most other species. Hypotension, hypoxaemia and hypoventilation are implicated as risk factors, which develop due to the combined effects of general anaesthesia and recumbency. Hypoxaemia is largely a consequence of ventilation perfusion (V̇ /Q̇ ) mismatch. When horses are recumbent, dependent areas of lung become compressed and collapse. These atelectatic lung regions receive a large proportion of pulmonary blood, do not participate in gas exchange and contribute significantly to venous admixture. Whilst the hypercapnia associated with hypoventilation is easy to manage, treatment of hypoxaemia is more challenging. Mechanical ventilation (MV) is often employed, but can have detrimental effects, and the response to it is unpredictable. Pulsed inhaled nitric oxide (PiNO), has been used successfully to manage hypoxaemia in anaesthetised horses. The presumptive mechanism of action is via redistribution of pulmonary perfusion, from dependent areas of lung, to better ventilated, non-dependent lung regions. This movement of blood occurs due to the selective, pulmonary vasodilatory effect of PiNO. However, it necessary to further elucidate the mechanism of action of PiNO. The aims of these studies were to: develop a CT method to quantify regional pulmonary perfusion in the equine lung; measure changes in regional pulmonary perfusion when PiNO is administered, during spontaneous breathing (SB) and MV and; measure changes in pulmonary perfusion in response to PiNO during SB and MV in hypotensive and normotensive horses. The CT method could reliably measure changes in aerated and atelectatic regions of lung, and compared well to previously reported values measured using microspheres. During SB and MV in the normotensive horse, PiNO caused a redistribution of blood to non-dependent lung regions which led to improvements in gas exchange. Unexpectedly, PiNO was ineffective during MV if horses were hypotensive. However, during SB, the response to PiNO was similar regardless of blood pressure. By developing a new CT method with angiography for studies of the distribution of pulmonary perfusion, these experiments have shown that PiNO is an effective and safe treatment option for hypoxaemic horses, but blood flow and blood pressure must be supported if horses are mechanically ventilated.