Ultrasonographic measurement of spleen volume in horses

Ante-mortem measurement of spleen size may be valuable to understand and study hemodynamic changes during exercise and in clinical determination of splenomegaly. The hypotheses were that spleen volume can be calculated from transcutaneous ultrasonographic measurements in live standing horses; and that the spleen volume is proportional to blood volume, maximal under the influence of detomidine and minimal under the influence of epinephrine. Eight donated horses free of splenic disease were studied. Ultrasonographic spleen volume and Evans blue dye-dilution blood volume were measured before (baseline) and after intravenous administration of detomidine and epinephrine (separately). Spleen volume was calculated using a standard ellipsoid formula (VOL_standard) and with the formula modified for the unique shape of the equine spleen (VOL_modified). Horses were euthanised using pentobarbital sodium, spleens removed immediately and measurements repeated on the removed spleen. Descriptive statistics, correlation and regression analysis were performed. Mean VOL_standard was 31.4±9.8 litre at baseline, 34.0±10.5 litre after detomidine, and 5.4±3.1 litre after epinephrine. VOL_modified was 25.1±9.1 litre at baseline, 27.4±9.6 litre after detomidine, and 4.2±2.4 litre after epinephrine. Post-mortem spleen volume was 14.1±4.6 litre and was correlated with VOL_standard (r=0.86, P=0.006) and VOL_modified (r=0.87, P=0.004). Blood volume was correlated with VOL_standard (r=0.81, P=0.014) and VOL_modified (r=0.78, P=0.02). Packed cell volume after epinephrine was correlated with VOL_standard (r=0.95, P < 0.001) and VOL_modified (r=0.96, P < 0.001). When compared to baseline, spleen volume was smaller after epinephrine (P=0.009) but not larger after detomidine (P=0.12). Blood volume after epinephrine was larger than after detomidine (P=0.009). The study demonstrated that splenic size, under various pharmacological influences, can be calculated from ultrasonographic measurements in the live standing horse. Packed cell volume after epinephrine was the best indicator of spleen volume.