Comparison of direct and indirect methods of intra-abdominal pressure measurement in normal horses.

Objectives: To develop a direct method for measuring intra-abdominal pressures in the standing horse, identify a reference interval for direct intra-abdominal pressures, compare these pressures to indirect intra-abdominal pressures measured from the bladder, and determine the optimal bladder infusion volume for indirect pressure measurement.
Design: Prospective, experimental study.
Setting: A university-based equine research facility.
Animals: Ten healthy adult horses, 5 males and 5 females.
Interventions: Direct intra-abdominal pressures were measured through an intraperitoneal cannula and zeroed at the height midway between the height of the tuber ishii and point of the shoulder. Indirect measurements of intra-abdominal pressure were performed by measuring intravesicular pressures through a transurethral catheter zeroed at the tuber ishii.
Measurements and Main Results: Direct pressure measurements obtained in the standing horse were subatmospheric (mean, -1.80 cm H(2)O; SD, 1.61 cm H(2)O; 95% CI, -2.80 to -0.80) and were shown to decrease as the horse's weight increased (Pearson's r=-0.67, P=0.04), with no effect of head position (P=0.15). Mean baseline indirect pressure measurements (mean, -8.63 cm H(2)O; SD, 4.37 cm H(2)O; 95% CI, -13.05 to -4.21) were significantly different from the pressures measured directly from the abdomen (P<0.001). Indirect pressure measurements were noted to increase with increasing volumes infused into the bladder, and were statistically different at a volume of 100 mL (P=0.004). There was low to moderate correlation between direct and indirect pressure measurements of intra-abdominal pressure over a range of fluid volumes infused into the bladder (Pearson's correlation range -0.38 to 0.58).
Conclusion: Pressures measured directly in the standing horse were subatmospheric, and increased as the horse's weight increased. Indirect pressures measured were altered by increasing volumes infused in the bladder. There was no significant correlation between the 2 methods of intra-abdominal pressure measurement.