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| 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.
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| 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.
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| 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. |
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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.
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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.
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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.
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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 the Valsalva maneuver