Flow

felixfelix · Dec 15, 2008

Don't know if this helps you, but I try to think in these terms.

flow is volume delivered in relation to time. pressure is force behind the flow.

electrical analogy- flow is amperage...pressure is voltage.

plumbing analogy- flow is r/t the size of the pipe...pressure is the push behind it.

joeyzstj, LPN · Dec 17, 2008

Hi
I always get confused between 'flow' and 'pressure' in relation to haemodynamics, what's really the difference please ?
Many thanks

I hope Im understaning what youre asking correctly. Flow and pressure are related, but quite different. If you think about SVR/PVR which is a measurement we derive from MAP-CVP/CO x 80. The normal for this is approximately 800-1200 dynes. The measurement of SVR tells us the resistance and/or relative size of a blood vessel. If you think about it, ONE reason we start vasopressors to increase SVR. When you take a a vessel, length of pipe or any tube for that matter and decrease the diameter of the tube by 1/2 you increase the resistance of flow 16 times. So, back to my orginal statement. We sometimes use certain vasopressors to increase SVR which increases the resistance of flow and contricts the peripheral vessels. There is a good saying that Pressure DOES NOT equal perfusion. As you can see in this example, this is why. Take for instance a vasopressor/Inotrope such as Dopamine which has a Beta-1 adrenergic effect. In addtion to and increase in SVR (at certain doses) you also get the increase inotropic effect which will also increas pressure through a different mechinism. NOW, Cardiac output or FLOW, and SVR have an inverse relationship. As SVR increases (vessel size gets amaller and flow resistance increases) Cardiac Output and Cardiac Index Decrease and vice versa. This is why its important to know hemodynamics and the drugs you are working with with new hearts. I still see people cranking up and Epi drip on a post-op heart who has an SVR of 1600 and a Cardiac Index of 1.8 trying to get more output, when in reality they need Nipride or to decrease the EPI to decrease the SVR and improve Output. I hope this doesnt confuse you. Good luck.