Reciprocal splanchnic-thoracic blood volume changes during the Valsalva maneuver

Abstract

The Valsalva maneuver is frequently used to test autonomic function. Previous work demonstrated that the blood pressure decrease during the Valsalva maneuver relates to thoracic hypovolemia, which may preclude pressure recovery during phase II, even with normal resting peripheral vasoconstriction. We hypothesized that increased regional blood volume, specifically splanchnic hypervolemia, accounts for the degree of thoracic hypovolemia during the Valsalva maneuver. We studied 17 healthy volunteers aged 15-22 yr. All had normal blood volumes by dye dilution. Subjects also had normal vascular resistance while supine as well as normal vasoconstrictor responses during 35 degrees upright tilt. We assessed changes in estimated splanchnic, pelvic-thigh, and lower leg blood volume, along with thoracic blood volume shifts, by impedance plethysmography before and during the Valsalva maneuver performed in the supine position. Early increases in splanchnic blood volume dominated the regional vascular changes during the Valsalva maneuver. The increase in splanchnic blood volume correlated well (r2 = 0.65, P < 0.00001) with the decrease in thoracic blood volume, there was less correlation of the increase in pelvic blood volume (r2 = 0.21, P < 0.03), and there was no correlation of the increase in leg blood volume (r2 = 0.001, P = 0.9). There was no relation of thoracic hypovolemia with blood volume or peripheral resistance in supine or upright positions. Thoracic hypovolemia during the Valsalva maneuver is closely related to splanchnic hyperemia and weakly related to regional changes in blood volume elsewhere. Changes in baseline splanchnic vascular properties may account for variability in thoracic blood volume changes during the Valsalva maneuver.

Fig. 1

Time course of blood pressure and thoracic, splanchnic, pelvic, and leg impedances during a representative Valsalva maneuver. Increase in thoracic impedance precedes phase I blood pressure change. Decreases in splanchnic, pelvic, and leg impedances occur at successively later times. Splanchnic impedance falls while thoracic impedance rises initially; thereafter, splanchnic impedance rises while thoracic impedance falls. Pelvic and leg impedance changes remain relatively stable throughout phase II.

Fig. 2

Time course of blood pressure (BP), thoracic and splanchnic impedance, and calculated segmental blood volume during a typical representative Valsalva maneuver in a representative subject. A reciprocal relation between thoracic and splanchnic impedances is reflected in blood volume changes. Again, initial rise in splanchnic blood volume and fall in thoracic blood volume are followed by opposite changes.

Fig. 3

Relation between fractional calculated splanchnic blood volume and fraction of thoracic blood volume, fractional calculated pelvic blood volume and fraction of thoracic blood volume, and fractional calculated leg blood volume and fraction of thoracic blood volume during the Valsalva maneuver. Splanchnic volume changes are highly and inversely correlated to thoracic volume changes. Pelvic blood volume changes correlate less well, and leg volume changes do not correlate with thoracic blood volume decreases.

Fig. 4

Relation between fractional change in segmental thoracic blood volume calculated from impedance plethysmography and blood volume, assessed in the supine position, using indocyanine green dye dilution. There is no significant correlation between blood volume and thoracic blood volume change.