Generally blood loss in all species is separated into 4 categories, based on if the blood loss is internal or external to the animal, and whether it is acute (sudden) or chronic.
Acute external blood loss is likely the most common category, and occurs as a consequence to accidental trauma (eg., wire cuts), surgical trauma (eg., castration, sinus surgery, revision of proud flesh), or erosion of a major blood vessel in a guttural pouch due to fungal lesions.
Acute internal blood loss examples include peri-parturient rupture of a middle uterine artery, external trauma to the spleen, thoracic trauma (haemothorax), neoplasia (eg., splenic haemangiosarcoma) or surgical trauma (eg., overiectomy, cryptorchid castration).
Red blood cells in horses have an average lifespan of 155 days, whereas platelets only last for around 7 days, and neutrophils only 10.5 hours! After haemorrhage red blood cell production (erythropoiesis) in the bone marrow is evident by 3 days and is maximal by 7 days. The lag is due to the production and release of erythropoietin (EPO) from the kidneys, in response to low levels of oxygen. Erythropoietin acts on derivatives of undifferentiated cells that are committed to becoming red blood cells, stimulating the proliferation and maturation of these cells into mature red blood cells. As part of this maturation process the red blood cell loses it’s nucleus, residual organelles, and becomes smaller. In most species, but not the horse, the rush to replenish red blood cells leads to an increased number of characteristic immature red blood cells being released called reticulocytes. These do not contain a nucleus, but do have a reticular network of ribosomal RNA that is visible under a microscope. This is rare in the horse, but immature red blood cells in the horse circulation are larger (macrocytes) than normal erythrocytes, reflected by an increased mean corpuscular volume (MCV). This indirect method provides the clue for red blood cell regeneration in horses.
Blood volume is estimated at 8% of the horse’s bodyweight, i.e., 40 litres for a 500 kg adult horse. Horses can lose around 1/3 of this blood volume (eg, 13 litres) without dying. Horses have a huge splenic reserve, and the acute loss of blood evokes a catecholamine release that causes splenic contraction. In the initial stages of blood loss there are very little changes in red blood parameters (PCV, Total protein) because all of the components of blood are lost in equal volumes. In order to maintain an adequate circulating blood volume there is movement of interstitial fluid into the vascular space. This will slowly dilute out the remaining red blood cells and protein causing reductions in both the haematocrit (Packed Cell Volume) and total protein. This fluid redistribution takes around 24 hours after the acute blood loss.
There are differences in responses to external versus internal blood loss. Two thirds of red cells lost into the thoracic or abdominal cavity will be ‘autotransfused’ via the lymphatics within 72 hours.
Signs of blood loss
Acute blood loss if severe will produce hypovolaemic shock. Signs will include elevations in heart rate (tachycardia) and respiratory rate (tachypnoea), sweating, trembling, pale mucous membranes with prolongation of the CRT, cold extremities (feet, ears), weakness and collapse, and death. Signs of internal haemorrhage into thoracic cavity will additional produce rapid, shallow chest excursions. Intra-abdominal haemorrhage will cause colic.
The signs of chronic blood loss will be more obscure as an anaemia will develop without hypovolaemia. A low red blood cell count and PCV can occur very slowly without any obvious signs until they become critically low. Assuming normal kidneys and bone marrow it is expected that the MCV will be in the high normal range reflecting regeneration.
Therapy
The first important act is to discover the underlying source of blood loss and stop it! Potentially no additional therapy may be required if the volume of blood loss is not too large. For larger volumes lost a blood transfusion may be necessary.
There are clinical and laboratory triggers for initiating a transfusion. The clinical triggers include pale to white membranes and a CRT beyond 3 seconds, poor peripheral pulses, difficulty in distending the jugular veins, or a persistently elevated heart rate (greater than 60 beats per minute). Laboratory triggers include an elevated blood lactate (> 4.0 mmol/L), a PCV <20% in the first 12 hours after haemorrhage, or a PCV < 12% during the initial 48 hours.
Transfusions are usually necessary if 20-30% of the estimated total blood volume is lost. This equates to a minimum volume of 8-12 litres. Estimating blood volume loss is difficult, as it always seems a lot worse than it typically is. To quote a pathologist colleague of mine: ‘a little bit of blood and a lot of water makes a lot of blood!‘
Emergency resuscitation typically involves hypertonic saline (6-7.2% sodium chloride at 4-5 ml/Kg bodyweight) and crystalloid solutions. This is to provide expansion of the plasma volume. If you don’t have access to hypertonic saline then you can use 40-80 ml/kg bodyweight of an isotonic solution intravenously. An alternative is to give a synthetic colloid, such as Hetastarch. This is an expensive alternate as the recommended dose is 10 mL/kg bodyweight or 10x500mL bags for a 500 kg horse. Consequently, this not often used in practice for this role despite it being superior to hypertonic saline.
The obvious problem with rapid expansion of plasma volume is that it will cause further dilution of the red blood cells and plasma proteins. Therefore it is usually a stop-gap until a transfusion can be organised.
How much blood do you need? This is calculated using either one of the following equations, noting that the equations do not provide identical answers to the total blood deficit. We also should intend to replace 20-40% of these calculated volumes to stabilize the horse.
EQUATION 1:
[(PCV of the donor – PCV of the patient) X Bodyweight(kg) X .08]
_____________________________________________________________________
PCV of the donor
For example, if the PCV of the donor is 38% and the PCV of the patient is 11% in a 500 kg horse results in:
(38 – 11) x 500 x 0.08
______________________ = 28.4 litre deficit; aiming to replace 20-40% of this => 5.5-11 litres desired
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EQUATION 2:
[PCV desired – PCV observed] X Bodyweight(kg) X .08
_________________________________________________________
PCV of the donor
[34-11] x 500 x 0.08
____________________ = 24.2 litre deficit; aiming to replace 20-40% of this => 5-10 litres required
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Selection of the donor
There are similar but not identical considerations given to selection of a donor for foals with neonatal isoerythrolysis. Ideally a hospital has access to Universal Blood Donors. These are Aa and Qa antigen negative (ideally also Ae and Ca negative) and anti-Aa and anti-Qa antibody negative horses. If these aren’t available then ideally select a Standardbred gelding that hasn’t had a transfusion as a donor. Ideally there would be the ability to perform a major cross-match using the donors red blood cells added to the patients serum and checking for haemolysis and agglutination. Checking for haemolysis requires the addition of rabbit complement. Strictly speaking a cross match shouldn’t be required for the 1st transfusion.
Blood is collected into containers pre-filled with anti-coagulant citrate dextrose (ACD) at 100-125 ml/L. Up to 8 litres can be collected from a healthy donor every 8 weeks – the donor should have a PCV greater than 34%.
Giving the transfusion
Blood needs to given using an inline blood filter set. Administer the blood slowly initially, and if physical examination remains stable then increase. Adverse reactions are usually characterized by elevated heart and respiratory rates, piloerection and fever. Transfused red blood cells do not last up to 155 days! By 4 days post-transfusion 60% are lost, and by 7 days this increases to 90%. If you have to transfuse a second time then by 48 hours 80% of the cells are destroyed.
Assuming no more blood loss complete recovery takes between 4 and 12 weeks, with the PCV increasing between 0.5% and 1% a day.
Other therapies that have been used include naloxone, aminocaproic acid and formalin.
Dosage
Dosing information is intended for use by registered veterinarians or veterinary nurses. Equiimed assumes no responsibility for the information detailed below. Equiimed shall not be liable for any damages resulting from reliance on any information provided below, or by reason of any misstatement or typographical errors. Ultimately veterinarians should consult information provided by the manufacturer prior to use.
Tags: Haemopoietic system