Complications in Equine Surgery. Группа авторов

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 or discomfort. In a recent retrospective study of postoperative abdominal hemorrhage, clinical signs included tachycardia, decreasing PCV/TP, abdominal discomfort, and incisional drainage. The hemoabdomen was confirmed by ultrasound or abdominocentesis [42]. Swirling, echogenic fluid is characteristic of hemoabdomen, and abdominocentesis will confirm the diagnosis (Figure 7.3). Blood loss into the intestinal lumen can be more difficult to detect until it is passed in the feces. Intraluminal blood loss should be suspected in horses that have had an enterotomy or large colon resection, and that have an acute, severe decrease in PCV along with tachycardia and melena within 72 hours of surgery [43].

       Treatment

      See “Fluid therapy and blood transfusion” and “Adjunctive systemic treatment” sections above.

       Reoperation

      Reoperation is often the last resort for postoperative hemorrhage, but should be considered early if there is unexpected postoperative hemorrhage and if there is a chance that a ligature may have slipped. A return to surgery may be needed if the patient is deteriorating despite medical therapy, although these patients are likely to be unstable under anesthesia [44]. If bleeding was detected at surgery but was inaccessible, or if the source of bleeding is unlikely to be accessible through the same surgical approach, an alternate approach is indicated. For example, a hemoabdomen post‐castration may be best treated through a standing laparoscopic approach [38].

Figure 7.3 Transabdominal ultrasound image showing cellular echogenic free fluid consistent with hemoabdomen.

      Source: Courtesy of Teresa Burns.

      In a case series at a level 1 human trauma center, reoperation for bleeding in trauma patients was prompted by direct signs, such as external bleeding or bleeding from drains, in 74% of patients. Indirect signs that led to reoperation included hemodynamic instability, decrease in hematocrit, and abdominal distention [44].