Pressure rise, reduced venous return, release, then overshoot
Mnemonic
Four phases, best remembered by what happens to blood pressure:
- Phase 1 - onset of straining. Intrathoracic pressure squeezes the aorta, so blood pressure rises briefly
- Phase 2 - continued straining. Venous return falls, so blood pressure falls, then partially recovers as the baroreflex causes tachycardia and vasoconstriction
- Phase 3 - release. Intrathoracic pressure drops, so blood pressure falls briefly
- Phase 4 - overshoot. Venous return floods back into a vasoconstricted circulation, so blood pressure rises above baseline and there is a reflex bradycardia
“Up, down, down, up.”
The absent overshoot is the abnormal finding: a square wave response, with no phase 2 fall and no phase 4 overshoot, indicates heart failure; absence of the phase 4 bradycardia indicates autonomic neuropathy.
It is also used to alter murmurs: strain reduces venous return, so it increases the murmurs of hypertrophic cardiomyopathy and mitral valve prolapse and reduces all others.
Expansion
- Phase 1 - onset of straining: raised intrathoracic pressure is transmitted to the aorta, so blood pressure rises briefly
- Phase 2 - continued straining: venous return falls, so stroke volume and pressure fall; baroreceptor unloading causes tachycardia and vasoconstriction, which partly restores pressure
- Phase 3 - release: intrathoracic pressure falls suddenly, so blood pressure drops transiently
- Phase 4 - overshoot: venous return surges into a vasoconstricted circulation, so pressure rises above baseline, and baroreceptor activation produces reflex bradycardia
Clinical uses: terminating supraventricular tachycardia through the phase 4 vagal surge; testing autonomic function; and distinguishing murmurs, since reduced venous return in phase 2 softens most murmurs but increases those of hypertrophic obstructive cardiomyopathy and mitral valve prolapse.