FUNCTIONAL CONSEQUENCES OF SAID
- Airflow through small airways is markedly reduced due to mechancal
narrowing and active consriction. Take for example the two airways below. The branching
airway on the left is from a normal horse. The airway walls are thin, and the internal
lumen diameter is wide. On the right, is an airway from an 11 year old working
Thoroughbred gelding with cough and exercise intolerance of several weeks duration.
400 x, Courtesy of Dr. Laurent Viel,
Univ. of Guelph
- Notice that in the SAID lung, there are a lot of cells (dots) around the
airway and within the airway wall, leaving little room for air passage. The airway looks
contracted. In general, the spongey -looking tissue around the airways (air sacs,
alveoli) looks relatively normal, as SAID is an airway disease, not a disease of the air
- Because airflow is restricted, air travels to unobstructed airways--the path of least
resistance. Fewer air sacs are filled with each breath, and insufficient oxygen can reach
its final destination in the airsacs. Horses "max out" their aerobic capacity,
since they are unable to deliver enough oxygen to their massive working muscles.
- Horses have to work harder to move air in and out of those narrowed
spaces. Forceful breathing may be necessary to keep pace with the need for oxygen,
especially during exercise. Forceful breathing requires more oxygen as well, and it
becomes a viscious cycle of insuficient oxygen delivery and too much oxygen consumption.
As blood levels of oxygen drop, horses might just slow down or stop. Some horses are so
severely affected, that oxygen is low without exercise, making even light exercise
- The narrow, inflamed airways become very sensitive and
"twitchy" to environmental challenges, including allergens, irritants,
pollutants, viruses and inert particles. Their tendency to constrict is exaggerated. The
airways become "hyper-reactive". In fact, hyper-reactivity is a trait that can
be used to identify horses with SAID by a controlled exposure to an irritant such as
histamine (see Lung Function Testing).
- Anything that taxes the respiratory system can exacerbate the signs of
SAID. For example, manipulation of head position ("collection") increases the
impedance to airflow. This normal activity may seem uncomfortable to horses with airflow
limitation (i.e. SAID). They are already on the fringe of 'starving' for air. Head tossing
and refusal to do various forms of work can result.
- Performance suffers. Typical complaints are that "racehorses slow
down at the ¾ pole", or sport horses are "sluggish" or "unwilling to
move forward in their training." Dressage horses refuse more difficult work. This is
because oxygen levels in the blood are dropping, and cannot keep pace with the demand of
- The level of fitness can then begin to drop, since horses can not work as
hard. The lower level of work to which they resort does not challenge the heart and
muscles, so their level of conditioning decreases. this is why recovery from SAID involves
both alleviating the airway obstruction and improving fitness. This can take several weeks
- Exercise induced pulmonary hemorrhage (EIPH, "bleeding")
may be linked to SAID. Horses with EIPH have been found with small airway inflammatory
disease at post-mortem examination. The bleeding itself can further inflammation. Flow
limitation and hypoxemia (low blood oxygen levels) might be a common link between EIPH,
SAID, and poor performance in individual horses.
- Lung function testing shows increased baseline respiratory system
resistance, frequency dependence of resistance, and airway hyper-reactivity to histamine
- Ventilation / perfusion mismatching.
- Arterial blood gases are usually normal until the disease is extensive
and the breathing pattern changes.