As we refine our examination technique for assessing the metabolic status of the a horse in an competitive event like endurance, some of the parameters we look for in accessing metabolic fitness take on greater significance as the ride progresses. When accessing a horse for metabolics no one parameter is used in isolation. Unlike pulse or gait where the rule sets a standard i.e. below 60bpm or consistent abnormal gait, metabolic failure assessment is result of a number of problems which include gait, pulse as well as demeanour, mucous membranes, capillary refill time, gut sounds, hydration status, injuries and muscle tone. All these parameters put together tell a story of the horse's fitness to continue. Understanding the story depends on how well you are interpreting what you are reading.

By far the key indicators used to assess metabolic fitness of the horse are mucous membranes and capillary refill time. The oral mucous membranes of a horse are normally salmon or pale pink. This colour reflects the number of red blood cells in the horse's blood stream. If a blood sample is taken from a horse and allowed to settle, it will divide into two components - the red blood cells (which contain haemoglobin) and plasma (which contains water, electrolytes and protein). The normal ratio of red cells to plasma in the endurance horse is about 40:60. If a horse is anaemic, the red blood cell to plasma ratio changes down to 20:80. This means that the oral mucous membranes will be very pale and even white if the anaemia is extreme. When a horse is competing in an endurance ride it will lose large amounts of water and electrolytes from the plasma portion - altering the ratio in favour of the red blood cells, ie. 70:30. This means that the mucous membranes will be much darker in colour due to the higher ratio of red cells to water. As a result of the large water loss from sweat, the membranes will also feel dry and sticky. Interpreting oral mucous membranes is subjective - ie. not easily quantified and is subject to the opinion of the person examining.

When examined, oral mucous membrane colour and assessed as A, B, C, or D. With A being clear, salmon pink and moist and D being muddy/blue/purple in colour and extremely dry. B and C represent the in between states with B being more red than pink and no longer moist and C being dark red/injected and dry. A horse with C and D mucous membranes is clearly in trouble and showing clinical signs of dehydration and toxaemia (accumulation of toxic compounds in the blood).

Capillary refill time (CRT), on the other hand, is more objective and can be quantified. This means that even the most inexperienced person can understand the importance of the ratings given to this important parameter. CRT represents the ability of the horse to pump blood through the small blood vessels in the mucous membranes of the mouth. CRT is the time (in seconds) taken for the pinkness to reappear in the mucous membranes of the mouth after applying finger or thumb pressure to the gum. In a normal horse, once the pressure of the thumb is removed, the time for the pinkness to reappear is 1 to 1 ½ seconds. A horse that is dehydrated, that is, loss of water and electrolytes where the ratio of red cells to plasma is now in favour of the red cells (70:30), the CRT can stretch out as far as 3 - 4 seconds. In these cases the horse is going into survival mode and directing the blood to important organs and away from the mouth. Also, this means that the blood is thicker (less water) making it harder for the heart to pump the red blood cells through the small blood vessels or capillaries. Counting the seconds for the pinkness to come back is a reliable and repeatable observation, reflecting the deterioration of the horse.

When recording the observations of capillary refill time, 1 to 1 ½ seconds is normal. 2 seconds reflects moderate dehydration; 3 seconds reflects substantial dehydration; and 4 or more seconds means the horse is in a critical condition. Horses with a CRT of 3 or more seconds and mucous membrane colour rating of C and D will require intravenous fluid and electrolyte therapy to repair the fluid imbalance.

Putting all this together, a horse with mucous membrane colour of C, a CRT of 3 seconds, a skin turgor rating of 3+ with a depressed demeanour and barely audible gut sounds would be eliminated and sent straight to the hospital. A horse with mucous membrane colour of B, a CRT of 2 seconds, skin turgor rating of 2 - 3 with low but audible gut sounds and eating and drinking would be expected to return to normal at the end of the hold time. A sure sign that this horse is in trouble and unlikely to recover after the hold time would be a poor appetite and disinterest in drinking. The warning signs are obvious and this horse is telling you a story.

Brian Sheahan
Chairman
State Veterinary Panel