BACKGROUND There is limited knowledge of the breathing strategy and impact on the patency of the upper respiratory tract (URT) in swimming horses. OBJECTIVES To describe the respiratory responses and endoscopic appearance of the URT during tethered swimming in horses. STUDY DESIGN Prospective descriptive study. METHODS Ten race-fit horses, with no history of URT obstruction, were examined during tethered swimming. Endoscopic examination, heart rate, sound recordings and above and below water video recordings were obtained. Plasma lactate concentration was measured before and 5 min after swimming and tracheal endoscopy was performed 30 min after exercise to assess for presence of blood or mucus. Four horses also underwent endoscopy during exercise on the track. RESULTS Mean (±s.d.) breathing frequency was 28 ± 5 breaths/min during swimming, with a brief inspiration (mean ± s.d. TI = 0.51 ± 0.08 s), followed by a period of apnoea (1.59 ± 0.53 s) and then a short, forced expiration (TE = 0.42 ± 0.5 s). During apnoea all horses exhibited complete collapse of the URT including closure of the external nares, nasopharynx and rima glottidis (with bilateral adduction of the arytenoid cartilages and vocal folds) and, in two horses, epiglottic retroversion. No horses had URT collapse during overground exercise. Locomotor-respiratory coupling was not observed during swimming. Median (IQR) plasma lactate post swim was 4.71 mmol/L (2.08-8.09 mmol/L) vs 0.68 mmol/L (0.65-0.71 mmol/L) preswim. Post swim endoscopy revealed grade 1 exercise-induced pulmonary haemorrhage (EIPH) in 2 horses. Median mucus grade was 1 (range 0-3). MAIN LIMITATIONS Overground endoscopy was not performed in all horses. CONCLUSIONS Horses experienced complete URT collapse associated with post inspiratory apnoea when swimming. The reason for this is unknown but may be to aid buoyancy or associated with the mammalian dive response - a survival reflex to preserve oxygen stores and prevent water entering the lungs.

中文翻译：

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在拴马游泳期间完全上气道塌陷和呼吸暂停。

背景对于游泳马的呼吸策略和对上呼吸道（URT）通畅的影响的知识有限。目的 描述在马系绳游泳过程中 URT 的呼吸反应和内窥镜外观。研究设计 前瞻性描述性研究。方法 10 匹适合比赛的马，没有 URT 阻塞史，在系留游泳期间进行了检查。获得内窥镜检查、心率、声音记录和水上和水下视频记录。游泳前和游泳后 5 分钟测量血浆乳酸浓度，运动后 30 分钟进行气管内窥镜检查以评估是否存在血液或粘液。四匹马在赛道上运动时也接受了内窥镜检查。结果游泳期间平均 (±sd) 呼吸频率为 28 ± 5 次/分钟，短暂吸气（平均 ± sd TI = 0.51 ± 0.08 s），然后是一段时间的呼吸暂停（1.59 ± 0.53 s），然后是短暂的强制呼气（TE = 0.42 ± 0.5 s）。在呼吸暂停期间，所有马都表现出 URT 完全塌陷，包括外鼻孔、鼻咽和声门边缘（双侧杓状软骨和声带内收）和两匹马会厌后倾。没有马在地上运动中出现 URT 塌陷。游泳期间未观察到运动-呼吸耦合。游泳后血浆乳酸中位数 (IQR) 为 4.71 mmol/L (2.08-8.09 mmol/L)，而游泳前为 0.68 mmol/L (0.65-0.71 mmol/L)。游泳后内窥镜检查显示 2 匹马出现 1 级运动性肺出血 (EIPH)。中位粘液等级为 1（范围 0-3）。主要局限 并非所有马都进行了地面内窥镜检查。结论 马在游泳时经历了与吸气后呼吸暂停相关的完全 URT 塌陷。其原因尚不清楚，但可能是为了帮助浮力或与哺乳动物的潜水反应有关——一种保存氧气储存和防止水进入肺部的生存反射。