The Valsalva maneuver (VM) is frequently used to test autonomic function. However, the VM is also affected by changes in blood volume and by blood volume redistribution. We hypothesized that even a standardized VM may produce a wide range of thoracic blood volume shifts. Larger blood volume shifts in some normovolemic individuals may be sufficient to induce decreases in blood pressure which preclude autonomic restoration of BP in phase II of VM. To test this hypothesis we studied 17 healthy volunteers aged 15-22 years. All had similar supine and upright vasoconstrictor responses and normal blood volume. We assessed changes in thoracic blood volume by impedance plethysmography before and during the VM performed supine. In some subjects, large decreases in BP were produced by thoracic hypovolemia. The maximum fractional decrease in BP correlated well (r2=0.64, p<.001) with thoracic hypovolemia and with systolic blood pressure at the end of phase II of the Valsalva maneuver (r2=0.67, p<.001). The blood pressure overshoot in phase IV of the maneuver was uncorrelated to phase II changes suggesting intact autonomic vasoconstriction. We conclude that the blood pressure decrease during the Valsalva maneuver is related to a variable decrease in thoracic blood volume which may be sufficient to preclude pressure recovery during phase II even with normal resting peripheral vasoconstriction. The Valsalva maneuver depends on vascular as well as autonomic activation, which broadens its utility but complicates its analysis.
The figure shows fractional thoracic volume (upper panel) and fractional blood pressure (lower panel) during the Valsalva maneuver. Minimum phase II blood pressure is respectively mildly, moderately, and markedly decreased in association with mildly, moderately, and markedly decreased thoracic blood volume. Blood pressure recovery in late phase II is complete for the mild subject, nearly complete for the moderate subject (a normal volunteer) and incomplete for the marked subject who fainted during this stage.
The figure depicts the relationship between the fraction change (decrease) in blood pressure and the fraction (decrease) intrathoracic blood volume calculated from impedance plethysmography. A linear fit to these data is constrained to pass through the origin since a zero volume change produces no pressure decrement.
One might ask where the thoracic blood volume goes to? We hypothesized that increased regional blood volume, specifically splanchnic hypervolemia, accounts for the degree of thoracic hypovolemia during the Valsalva maneuver. To accomplish this we measured changes in segmental blood volumes representing the regional circulations of the thorax (cardiac output), splanchnic circulation, pelvic and upper leg vascular beds, and the lower leg using impedance plethysmography methods.

The figure shows the time course of blood pressure (upper panel), and, from top down:  thoracic, splanchnic, pelvic, and leg impedances during a representative Valsalva maneuver. Onset of the increase in thoracic impedance precedes phase I blood pressure change. Onset of 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 of the maneuver.

 

 

The figure shows the time course of blood pressure (upper panel), thoracic and splanchnic impedance (two middle panels) and calculated segmental blood volume during a typical representative Valsalva maneuver in another patient. There is a reciprocal relation between thoracic and splanchnic impedances which is reflected in blood volume changes. Again the initial rise in splanchnic blood volume and fall in thoracic blood volume are followed by opposite changes.

 

The figure shows the relation between fractional calculated splanchnic blood volume and fraction thoracic blood volume (upper panel), between fractional calculated pelvic blood volume and fraction thoracic blood volume (middle panel), and between fractional calculated leg blood volume and fraction thoracic blood volume (lower panel) 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.

 

Thus, 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.

 

 


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New Circulatory Findings in Static Handgrip
New Circulatory Findings in the Valsalva maneuver