Colic management often depends on interpretation of abdominal fluid to determine the benefit of going to surgery and the chances of survival.
A recent study looked at using abdominal fluid as a sensitive indicator of intestinal injury and how that can inform next steps in treatment [Radcliffe, R.M.; Liu, S.Y.; Cook, V.L.; et al. Interpreting abdominal fluid in colic horses: Understanding and applying peritoneal fluid evidence. Journal of Veterinary Emergency and Critical Care 2022, vol 32(S1), pp 81–96; doi: 10.1111/vec.13117].
Combining a rectal examination with abdominal fluid analysis yields excellent diagnostic information for colic management. Abdominal fluid analysis includes looking at its gross appearance, nucleated and RBC counts, total protein and lactate concentration. It is also recommended that veterinarians conduct a cytology and culture in some instances.
The authors described normal characteristics of healthy abdominal fluid of adult horses:
- An odorless, clear to light yellow color and transparent fluid
- Total nucleated cells < 5 x 109 cells/L
- 2:1 distribution of neutrophils:macrophages
- Total protein <2.0 g/dL
- No red blood cells unless secondary to needle puncture during collection
- L-Lactate <1.2 mmol/L, which is less than systemic lactate concentrations
- SAA <0.5 mg/L
Turbidity increases as protein, WBC and RBC levels rise, and it is especially dense following intestinal rupture. Strangulating intestinal lesions, especially of the small intestine, are associated with progressive transformation of color from golden to orange to red depending on the degree of injury and release of red blood cells into the fluid. Diseases associated with intestinal ischemia might begin with serosanguinous fluid that becomes more turbid and orange to bloody with progression.
The authors noted that a case of a non-strangulating obstruction might have normal peritoneal fluid in the initial one to two hours. Over time, fluid in a simple obstruction might slightly darken its light yellow color. Turbidity increases with more protein leakage along with a mild increase in total nucleated cells, especially neutrophils. In this case, L-lactate might increase to 3 mmol/L due to reduced intestinal wall perfusion from intestinal distention.
A strangulating intestinal obstruction leads to more obvious changes from leakage of RBCs and protein (3.5 – 6 g/DL). With ongoing damage complicated by bacteria and endotoxin elevations, total nucleated cell counts elevate, and turbidity increases. As intestinal compromise advances, L- lactate levels increase to >15 mmol/L. It can be challenging to differentiate between iatrogenic hemorrhage from vessel puncture during collection versus hemorrhage from strangulating intestinal lesions, but clinical progression or deterioration provide additional information.
Changes in peritoneal fluid from enteritis and colitis depend on the degree of intestinal inflammation. These might start with normal-appearing fluid that changes in color and turbidity as the diseased bowel persists. In general, peritoneal lactate won’t exceed systemic lactate in these cases unless severe disease ensues—then peritoneal lactate can be twice as much as systemic levels.
Septic peritonitis abdominal fluid is thick, turbid, dark yellow to orange, and has a milky appearance due to high increases in total protein and total nucleated cells. L-lactate might elevate to ≥12 mmol/L. Cytology is important to identify bacteria and degenerative neutrophils. The pH is often <8.3, glucose <30 mg/dL, and fibrinogen >200 mg/dL in a septic abdominal fluid sample. Ultrasound examination of the abdomen can yield definitive information that corroborates findings on fluid analysis, especially in the case of abdominal rupture or a strangulating obstruction. All this is corroborated to clinical signs and response (or lack of) to medical treatment.
Early in the course of serious intestinal lesions, peritoneal fluid might appear normal until intestinal compromise worsens, so serial acquisition of abdominal fluid samples might be appropriate.
The authors stressed that abdominal fluid analysis is an invaluable diagnostic tool in directing treatment. The fluid results coupled with other available diagnostic measures—physical exam, rectal exam, abdominal ultrasonography, complete blood count, serum chemistry—inform practitioners about the next steps for a horse with colic: to either continue medical management, send the horse to surgery or elect euthanasia.