Can Systolic BP ever be less than Diastolic?

[ChrisHoStuart]

My background is physics; not biology, but I am doing a course to be a nursing assistant. We’ve been learning to take blood pressure, and what it means.

Systolic is the higher number; pressure when the heart beats. Diastolic is the lower pressure, when the heart rests between beats.

Our lecturer said that in some really unusual cases, diastolic can be greater than systolic.

This makes no sense to me. I can’t see how that could possibly ever happen. Anyone know of any kind of trauma or bizarre issue that could mean diastolic pressure being greater than systolic? I’m wondering if it could be from an instrument error, or perhaps if there’s a huge difference in pressure in parts of the body from trauma or the like so that diastolic in one part is more than systolic in another. But I’m guessing.

Will be chasing it up with the lecturer again, but in the meantime; anyone ever heard of such a thing? Thanks for any suggestions!

[Darby]

How would you know? A blood pressure reading is just a reading of high pressure and low – it’s assumed the high is systolic…watching the gauge, the high pressure matches the pulse, but that would happen if some weird reverse was true also.

One might imagine a condition where osmotic movement increases base pressure, but I don’t know how ineffectual a heartbeat might have to be to decrease it rather than increase it. It doesn’t seem likely…

[daniel.kurz]

It is possible, kinda. Remember that systolic pressure is the pressure measured during ventricular contraction pushing blood out into the system. Diastolic pressure is the pressure when the ventricle is not pushing. Now, if you have a region of the arm where you are measuring the pressure the diastolic you could have localized diastolic pressure higher than systolic. It would not be systemic though. So when the heart goes through systolic phase and pushes blood out, the blood hits that region and cannot go further and instead finds another way to go around.

I would say in response though that this is purely theoretical and not likely a problem to think about. In the true sense of systolic and diastolic pressure at the level of the heart, if the systolic pressure was lower than the diastolic then the blood wouldn’t leave the heart and the heart would fail. You can have localized diastolic pressure being higher (such as vasoconstriction) but don’t let your lecturer confuse you.

[JackBean]

Daniel: could you explain more, how can that happen?

[teju85]

I don’t think that’s possible. I mean, strange things happen – people have a heart on the right side, instead of left – but systolic blood pressure being lower than diastolic blood pressure just doesn’t make sense.
Systolic blood pressure – When the heart shrinks and pushes the blood into main arteries (aortas), blood pressure rises for a moment – this is known as systolic blood pressure.
Diastolic blood pressure – After each shrinkage the heart expands again and is filled with new blood – at that moment blood pressure in the arteries drops for a moment – this is known as diastolic blood pressure.
It’s also not possible according to this blood pressure diagram. Systolic blood pressure is always higher than diastolic blood pressure. It is possible however that systolic is only a bit higher than diastolic blood pressure, but never lower!
When heart expands it pushes the blood out into vein/artery system, so the pressure rises as the blood must move from one location to another and since arteries are elastic, they expand to compensate the pressure. When heart shrinks, it "sucks" the blood back and makes more room for blood, so the pressure drops.
I am no doctor, but I think this is impossible. It’s like asking if air pressure inside a balloon is higher when you fill the balloon with air, than when you let the air out. This is very similar to how heart works, just that instead of air, there’s blood, and instead of air pressure, there’s blood pressure.

[teju85]

quote Darby:

How would you know? A blood pressure reading is just a reading of high pressure and low – it’s assumed the high is systolic…watching the gauge, the high pressure matches the pulse, but that would happen if some weird reverse was true also.

One might imagine a condition where osmotic movement increases base pressure, but I don’t know how ineffectual a heartbeat might have to be to decrease it rather than increase it. It doesn’t seem likely…

Because it’s not possible. In terms of physics.

[Al]

I have 0 medical training to be clear. This was about three and a half years ago. I had an ambulance called on me and ended up in hospital for mental health crisis and self harm back while I was active in addiction. I heard one of my monitor alarms going off and looked up to see my blood pressure numbers quickly loosing range until they had flipped. as the systolic and diastolic began to approach eachother a nurse prepped a syringe and quickly inserted it into the spare spot in my second IV. I woke up some time later. I’m not sure what happened in my body.

[Author]

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