Breath-holding in horses.

Firstly is breath holding normal or abnormal , physical or psychological?


Human animals  - might hold their breath momentarily when startled, or for longer periods if one was concerned at what might enter the lungs if we kept breathing. e.g.. smoke or water (diving). These are of course regarded as normal events.

There are also instances where children and others have been know to breath hold when in stressful situations. Unless they can be persuaded to begin breathing they may even 'pass out' or feint! In this case breath holding is abnormal but psychological.

An individual may also ‘breath hold’ when asleep. This is usually a symptom of a condition referred to as ‘sleep apnoea’. Apnoea is a period when normal breathing ceases. There are several types of sleep apnoea including ‘obstructive sleep apnoea’ where the physical cause is the collapse or obstruction of the upper airway or throat! Here breath holding is abnormal and also has a physical origin.


Horses  - can selectively breath hold whilst walking or trotting as breathing and stride are not linked during these gaits. So to breathe once whilst taking three steps or to not breathe when distracted by a passing vehicle is quite normal when walking or trotting.

A horse may also breath hold momentarily whilst jumping an obstacle or leaving starting barriers / gates and this is regarded as being a normal event.

However once the horse breaks into a canter or a gallop, stride and breath become coupled. i.e. one stride one breath! The reason for this is that as the fore legs are extended (forward stride) the thorax (rib cage) expands and the intestines move backwards (are left behind) which together facilitate the inflow of air into the lungs (inspiration). Conversely as the the fore limbs come back and under the body the rib cage contracts to allow the shoulder to move back freely and the intestines move forward, together helping to expel air from the lungs (expiration).

If then a horse is seen to breath hold (often noted during a misty mornings gallop) or said to be breath holding (by its rider) is this a physical or psychological problem ?

If breath holding is evident only for a short period and associated with events such as the momentary contact between two horses or a sudden distraction or missed stride then it is probably a normal event.

If however breath holding occurs over a longer distance (50 to 200 meters) and particularly when associated with periods of maximum effort then it is most probably abnormal. It may also be noted that during these periods the horses head may assume a more ‘up& back’ position and some shortening of stride can occur.

So why is this occurring and is this a psychological or physical problem? 

Firstly the answer to the later is that it is both psychological and physical!

To answer the first part of the question we need to understand a little about upper airways collapse. And yes it is at least similar in some respects to ‘obstructive sleep apnoea’ but in this case we are not talking about a sleep state but one of intense exercise.

The similarity between these differing circumstances is that  upper airways collapse in humans ( sleep) and horses (intense exercise) is often associated with a decrease in muscular tone in the muscles of the upper airway or throat. It is of course this muscular effort that maintains the patency of the airway (Larynx & nasopharynx) and allows proper flow of air to the lungs. During sleep most muscles are of course in a more relaxed state ( worsened if alcohol is involved) whilst during intense exercise the relaxation is more often associated with the onset of muscular fatigue! (muscles running out of energy )

The other factor that increases the chance of airways obstruction in horses is an increase in the suction or negative pressures that occur as the horse breathes in (inspiration). As a horse ‘up tempos’ / increases effort towards the end of a race or gallop both length of stride and inspiratory effort increase leading to increases of these potentially airways collapsing negative pressures.

Breath holding is most often evident in horses that have previously experienced  upper airways collapse during periods of maximal exercise either on the training or race track.

Then why are they now holding their breath? Doesn’t this make things worse?

The ‘upside’ of breath holding is that during these periods two positive factors come into play. Firstly the tone in the horses throat is increased and secondly inspiratory efforts are reduced. This has been referred to as Reduced Respiratory Effort (RRE). Both of these will temporarily reduce the likelihood of upper airways collapse.

Unfortunately the ‘downside’ to breath holding is that ultimately ( usually after a maximum of 200 meters) this reduce respiratory effort (RRE) will result in oxygen deprivation (anoxia) and the onset of muscular fatigue. When this occurs the horse must ‘go for air’ i.e. relax muscle tone and breathe in. At this point a large breath may be followed by rattling or raspy breathing and a slowing of pace. The horse may also take an abnormal time to recover after exercise and exhibit associated increases in heart rate and respiratory rate recovery times.


Can I do anything about this problem?


Ans. Quite obviously if you can reduce the chance of upper airways collapse during maximal exercise then you should reduce the likelihood or necessity for the horse to hold its breath!


References.


Ahern T J  Acquired pharyngeal dysfunction (APD), J of Equine Vet Sci 1993  13:

                   125-128.


Ahern T J  Oral palatopharyngoplasty. A survey of one hundred post operative

                    raced horses, J of Equine Vet Sci  1993  13: 670-672.