Phases of BP variation during a Valsalva maneuver with intact autonomic nervous system
What are the four phases of blood pressure variation during a Valsalva maneuver in a person with intact autonomic nervous system?
During phase I, increased intra-abdominal and intrathoracic pressure results in compression of the large vessels and aorta and therefore transient increase in the blood pressure, accompanied by a reflex bradycardia.
Shortly after, reduction in the venous return to the heart leads to reduced stroke volume and blood pressure (early phase II).
This decline in blood pressure triggers the sympathetic nervous system with resultant increase in Norepinephrine level, peripheral vascular resistance, and blood pressure (late phase II).
When the Valsalva maneuver is released, the sudden decline in intrathoracic pressure causes a concomitant decrease in blood pressure and tachycardia (phase III).
Finally, in phase IV in normal subjects, despite the return of the venous return and cardiac output to the baseline values, blood pressure continues to rise as a result of the persistent high peripheral vascular resistance (residual from late phase II).
Therefore, blood pressure usually overshoots the baseline in this phase