Physical consequences

STRUCTURAL AND 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 sacs.

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 intolerable

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 ž 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 working muscles.

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 to months.

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 challenge.

Arterial blood gases are usually normal until the disease is extensive and the breathing pattern changes.