of the surgery until bleeding is controlled. In the case of paranasal sinus surgery, the packing can be removed during a standing procedure 24 to 48 hours later, with reevaluation of the sinus and completion of the procedure under better visualization.
Fluid therapy and blood transfusion
Initial stabilization for acute blood loss is accomplished with intravenous crystalloid fluids. A starting point for resuscitation should be an initial bolus of approximately 20 ml/kg. Overzealous resuscitation can result in further bleeding due to an increase in blood pressure and dilution of clotting factors. The goals of fluid therapy should be to bring the mean arterial blood pressure to within a range of 60–70 mmHg, and maintain tissue perfusion [25]. Blood lactate can be used to help determine response to fluid therapy, with the aim to normalize lactate (<2 mmol/l) within 24 hours [32]. Synthetic colloids have been shown to cause platelet dysfunction and reduced von Willebrand factor and factor VIII, and have been associated with increased blood loss during surgery in human patients [33]. In healthy ponies treated with gelatin and hydroxyethyl starch, hemodilution occurred but there were no clinically significant effects on hemostasis [34]. Despite the lack of evidence of adverse effects in horses, the use of synthetic colloids is not recommended in the acutely bleeding patient.
Blood transfusion is recommended for acute blood loss of greater than 20% blood volume, especially if there are concerns about continued blood loss. Additional parameters that indicate a need for transfusion include signs of shock (heart rate >60/min, CRT >3 sec, cold extremities, depressed mentation) despite adequate volume resuscitation, oxygen extraction ratio greater than 40%, lactate greater than 4 mmol/l, and acute hemorrhage with a PCV less than 20%.
In acute blood loss situations, the volume of blood lost can be estimated based on the severity of shock. For example, a horse that is severely tachycardic with decreased pulse pressure, pale mucous membranes, and altered mentation can be estimated to have lost approximately 30% of its blood volume [25]. Up to half of the volume lost should be replaced by a whole blood transfusion. In cases of normovolemic anemia, the following formula can be used to estimate transfusion volume:
Blood transfusion volume (ml):
The target PCV will depend on whether the horse is at risk of continued bleeding and whether there are any comorbidities that might decrease perfusion. This author will typically target a PCV of 25%, although the total blood transfusion volume will often be limited by how much blood the donor horse can give.
Donor horses are the most common source of blood for transfusion, but autologous salvaged blood should also be considered. Cell salvage devices can be used to collect blood from surgical sites or drains. Blood is suctioned from the surgical site, filtered, centrifuged, washed, and returned to a bag for reinfusion into the patient [35]. This technique has been reported in canine patients, and could be used in equine patients if the equipment is available [36]. Blood can be collected and transfused directly into the patient without processing, but the cell salvage system reduces contaminants. A leukocyte depletion filter is needed when there may be contamination of blood with neoplastic cells or bacteria.
When blood is lost into a body cavity (hemothorax or hemoabdomen), it can also be left to be reabsorbed by the patient. The immediate hypovolemia must be addressed with IV fluids, but the majority of shed blood may be reabsorbed via lymphatics within 48 hours [37]. If PCV falls below 20% or the horse continues to have signs of shock despite fluid resuscitation, a blood transfusion may still be needed. Allogeneic transfusion from a donor horse is most common, but collection of blood from the abdominal cavity and reinfusion has also been reported [38].
Adjunctive systemic treatment
The mainstays of systemic treatment for acute hemorrhage are fluid therapy and blood transfusion. There are a number of procoagulant medications that can also be used to enhance hemostasis in the horse [39]:
Formalin – Proposed to enhance endothelial or platelet activation, reported dose of 10–100 ml of 10% formalin in 1 L isotonic saline
Aminocaproic acid – Lysine derivative that inhibits fibrinolysis by binding plasminogen activators and enhancing antiplasmin activity. The previously reported doses are 10–40 mg/kg IV q6h slow in saline or 3.5 mg/kg/min for 15 min then 0.25 mg/kg/min constant rate infusion.
Tranexamic acid – Similar mechanism of action as aminocaproic acid; 5 g IV every 12 hours or 10 g PO every 6 hoursNew research suggests that as little as 1/20 of the published doses of aminocaproic acid and tranexamic acid may be effective in horses [40].
Conjugated estrogens – May polymerize mucopolysaccharides in vessel walls or decrease antithrombin activity, 0.6 mg/kg IV every 24 hours
Yunnan baiyao – Chinese herbal medication with demonstrated hemostatic efficacy, possibly due to activation of platelets, enhanced expression of surface glycoproteins on platelets [41].
Expected outcome
The acute risks of intraoperative hemorrhage include rapid shock and death, particularly if a large vessel is ruptured, such as a portal vein rupture during reduction of an epiploic foramen entrapment. There are no specific reports on intraoperative mortality due to hemorrhage in equine patients.
Postoperative Hemorrhage
Definition
Postoperative hemorrhage can occur immediately after surgery or can be delayed by several days after surgery. Hemorrhage is most commonly from the surgical site, but can occur in distant areas if a coagulopathy has developed.
Risk factors
Same as for intraoperative hemorrhage (see above)
Pathogenesis
The pathogenesis of postoperative hemorrhage is the same as for intraoperative hemorrhage. Inadequate hemostasis may not be recognized at the time of surgery, possibly due to lower blood pressure under anesthesia, positioning (e.g. lower pressure in the distal limb of a horse in dorsal recumbency), or a temporary clot that becomes dislodged after surgery.
Prevention
Same as for intraoperative hemorrhage (see above)
Diagnosis and monitoring
Acute blood loss of 30% of blood volume will result in cardiovascular shock due to hypovolemia and reduced oxygen delivery to the tissues. Signs of shock include tachycardia, tachypnea, prolonged capillary refill time, cool extremities, depressed or anxious mentation, and hypotension. In horses, splenic contraction will increase the PCV, so the decrease in PCV will typically lag behind the decrease in total solids (TS).
Postoperative hemorrhage may be apparent if there is blood leaking from the surgical drain, incision, or nasal passage. Tachycardia, tachypnea, and pale mucous membranes may signal ongoing blood loss, and serial PCV/TS can help to determine the severity of blood loss. TS should decrease within minutes to hours of blood loss, but PCV may remain normal even during terminal blood loss, due to the effects of splenic contraction [24]. Internal bleeding into the abdomen or thorax may not be apparent until the horse begins to show signs of shock
