The Effect of a Somatostatin Analog on Splanchnic Blood Volume During Head-Up Tilt

Abstract

Blood volume shifts are an important factor contributing to the physiological responses to head-up tilt. In particular, capacitance organs such as the liver store large volumes of blood that may be redistributed during orthostatic stress. PURPOSE: In this investigation we attempted to measure blood volume changes in the abdominal region during 70° head-up tilt. Furthermore, we quantified changes in abdominal blood volume distribution in response to selective splanchnic vasoconstriction induced by a somatostatin analog (125 mg octreotide acetate iv). METHODS: Thirteen men (24±5 yrs; mean ± SEM) were outfitted with band electrodes bracketing the abdominal region (xiphoid to iliac crest) for measurement of electrical impedance shifts (Minnesota Impedance Cardiograph). Abdominal blood volume changes were calculated from the change in impedance using the Nyboer equation. Blood resistivity was estimated from hematocrit. On separate days, measurements were recorded before, during and after tilt, one with octreotide and one with placebo (double-blind, randomized order). RESULTS: Analysis of individual data revealed both increases and decreases in abdominal blood volume in response to tilt with a mean change of +34.2±6ml (P <0.005). No significant effect of octreotide administration was noted. CONCLUSIONS: The change in abdominal electrical impedance between baseline and tilt reflects the collection of blood in the abdomen during orthostatic stress. Furthermore, selective constriction of the splanchnic circulation by octreotide infusion is unable to mitigate this response.