SummaryControlled mechanical ventilation (CMV) is routinely used in equine anaesthesia, with many different options available to mechanically deliver breaths. The complexity of respiratory pathophysiology in anaesthetised horses and the wide range of devices available is described in this scoping review. The first part of the review outlines basic equine respiratory physiology and pathophysiology during anaesthesia to illustrate what makes horses prone to inefficient gas exchange and ventilation when they are recumbent. The difference between spontaneous ventilation and CMV is reviewed and basic considerations of CMV are explored in more detail.

Equine respiratory physiology might be influenced by the presence of an endotracheal tube (ETT). This experimental, randomized cross-over study aimed to compare breathing pattern (BrP) and ventilation distribution in anesthetized horses spontaneously breathing room air via ETT or facemask (MASK). Six healthy adult horses were anesthetized with total intravenous anesthesia (TIVA; xylazine, ketamine, guaiphenesin), breathing spontaneously in right lateral recumbency, and randomly assigned to ETT or MASK for 30 min, followed by the other treatment for an additional 30 min. During a second anesthesia 1 month later, the treatment order was inversed. Electrical impedance tomography (EIT) using a thoracic electrode belt, spirometry, volumetric capnography, esophageal pressure difference (ΔPoes), venous admixture, and laryngoscopy data were recorded over 2 min every 15 min. Breaths were classified as normal or alternate (sigh or crown-like) according to the EIT impedance curve. A mixed linear model was used to test the effect of treatment on continuous outcomes. Cochran-Mantel-Haenszel analysis was used to test for associations between global BrP and treatment. Global BrP was associated with treatment (p = 0.012) with more alternate breaths during ETT. The center of ventilation right-to-left (CoVRL) showed more ventilation in the non-dependent lung during ETT (p = 0.025). The I:E ratio (p = 0.017) and ΔPoes (p < 0.001) were smaller, and peak expiratory flow (p = 0.009) and physiologic dead space (p = 0.034) were larger with ETT. The presence of an ETT alters BrP and shifts ventilation toward the non-dependent lung in spontaneously breathing horses anesthetized with TIVA.

Summary Controlled mechanical ventilation (CMV) is routinely used in equine anaesthesia, with many different options available to facilitate the delivery of mechanical breaths. The complexity of equine respiratory pathophysiology and the wide range of devices available is described in the two parts of this scoping review. The second part of this review provides an overview of commercially available and hand‐made devices that can be used to deliver CMV in large animals. A short overview of basic physics for mechanical ventilation is given. Working principles are discussed and, where relevant, connected to the pathophysiological background of equine ventilation. The objective of this review is to demystify ventilation modes and ventilators used in equine anaesthesia over the last two decades. In addition, this review aims to provide a practical guide to what the anaesthetist needs to consider before using CMV, how CMV can be delivered and monitored, and which principles need to be applied are included. Comparison of the clinical effects between spontaneous breathing and CMV is included.