Theory 01/Cardiology & Haemodynamics /Cardiovascular physiology/3 min
The Reflection Sets the Pressure
Stiffness is the usual culprit. The clock on the reflected wave may matter more than the spring.
Systolic blood pressure is set by the timing of the wave reflected from the periphery rather than by arterial stiffness alone, so a stiff artery with late reflection can produce a lower systolic pressure than a compliant one with early reflection.
01The evidence
- Pulse pressure analysis distinguishes the forward wave from the reflected wave, and the augmentation index, which measures how much the reflection adds to systolic pressure, depends on when the reflection returns rather than on how large it is.Pulse pressure overview
- The reflected wave is known to boost aortic root pressure during diastole, which is why its timing, not just its amplitude, determines both systolic load and coronary perfusion.NIDDK reference document
02The tension
Arterial stiffness is treated as the master variable: stiffer vessels mean higher pressure, and stiffness reduction is a therapeutic goal. The relation is real but loose, and the exceptions are systematic. People with the same pulse wave velocity can differ substantially in systolic pressure, and the difference tracks where the reflection lands.
03The theory
Each heartbeat sends a pressure wave down the aorta. It reflects at branch points and at the high-resistance arterioles and returns. If it returns during systole, it adds to the peak. If it returns after the aortic valve closes, it adds to diastolic pressure instead, which is where it helps coronary flow.
The timing depends on the distance to the reflection site and on the wave speed. A stiffer artery conducts faster, so the reflection returns earlier and adds to systole. But a shorter body, a lower reflection site, or a change in arteriolar tone can do the same thing at constant stiffness.
That makes the augmentation index a timing measure, and it predicts that interventions which move the reflection site, such as changing posture, vasodilating the periphery, or altering heart rate, should change systolic pressure more than interventions that change stiffness alone.
The clinical consequence is that stiffness-reducing drugs may underperform where the reflection is early for anatomical reasons, and that a therapy targeting arteriolar tone could be more effective in those patients.
04What this is not
The stiffness account makes pulse wave velocity the determinant of systolic pressure. This makes the arrival time the determinant, which predicts that two people with equal stiffness can differ because their reflection geometry differs.
05Predictions
- P1Augmentation index should predict systolic pressure better than pulse wave velocity in a population with varied body size.
- P2Changing posture or peripheral resistance should shift systolic pressure without changing stiffness.
- P3Patients with early reflection for anatomical reasons should respond poorly to stiffness-reducing therapy.
06What would refute it
Systolic pressure tracking stiffness at fixed reflection timing would restore the master-variable account.
07What it would change
It would make reflection timing a treatment target, and explain non-response to stiffness-lowering drugs.
08Evidence base
- Pulse pressure — chapters and articleshttps://www.sciencedirect.com/topics/nursing-and-health-professions/pulse-pressure
- Reflected wave and aortic root pressure — NIDDK referencehttps://repository.niddk.nih.gov/public/study_document/CKiD_V7/Documents/CKiD_MOP_04-01-18.pdf