What are the physics of intravenous drips?
October 28, 2010 5:48 PM Subscribe
What are the physics of intravenous drips?
I visited my friend in the hospital today and she sent me away with homework. "Find out how these damn intravenous things actually work, would you?" Neither of us is a scientist or an engineer, but we can both handle a fair amount of technical information. Anybody have any links to suggest? I haven't been able to find anything that lays it out.
I visited my friend in the hospital today and she sent me away with homework. "Find out how these damn intravenous things actually work, would you?" Neither of us is a scientist or an engineer, but we can both handle a fair amount of technical information. Anybody have any links to suggest? I haven't been able to find anything that lays it out.
If you are talking the actually dripping, gravity. Once into the blood stream, it gets carried with, well, the blood through out the body, getting absorbed.
An i.v. pump is used to calculate the drip rate, or in the old days we had to stand there with our watch behind the drip chamber and count the drops. Ugh.
If you are looking for something more specific, please clarify.
posted by 6:1 at 5:58 PM on October 28, 2010
An i.v. pump is used to calculate the drip rate, or in the old days we had to stand there with our watch behind the drip chamber and count the drops. Ugh.
If you are looking for something more specific, please clarify.
posted by 6:1 at 5:58 PM on October 28, 2010
An *injection* refers to various routes of medication by needle, (subcutaneous, intra-musclar). Some medications can be given via "IV Push", but not all medications. Some medications can be given "piggy back", meaning in a little bag going in along side the maintenance i.v. solution.
posted by 6:1 at 6:03 PM on October 28, 2010
posted by 6:1 at 6:03 PM on October 28, 2010
Best answer: Here is a link that talks about hydrostatic pressure in the human circulatory system. Basically, the higher the elevation of a liquid, the more hydrostatic pressure it has. So the blood pressure in your neck is higher than the blood pressure in your legs when you're standing up, and the reason they always measure the blood pressure in your upper arm is that it's about the same level as your aorta, which is the pressure they'd really like to measure if they could.
So the drip bags full of drugs/fluid have to be higher than your veins to give the IV fluid greater hydrostatic than your blood. I guess medical people just figured out empirically what height worked, and was also convenient.
Is this what you were asking about? If not, please clarify.
posted by Quietgal at 6:06 PM on October 28, 2010 [1 favorite]
So the drip bags full of drugs/fluid have to be higher than your veins to give the IV fluid greater hydrostatic than your blood. I guess medical people just figured out empirically what height worked, and was also convenient.
Is this what you were asking about? If not, please clarify.
posted by Quietgal at 6:06 PM on October 28, 2010 [1 favorite]
Normal peripheral venous pressures. 25 mmHg is equivalent to about 13 inches of H2O. However, on some patients the IV bag can be disconnected and the saline lock left open, and no blood will reflux back into the tubing. On other patients, blood will flow back into the IV tubing even with the IV bag connected, if it becomes positioned incorrectly.
With the exception of ERs, it's rare to see an IV being infused by gravity alone any longer.
posted by 517 at 6:14 PM on October 28, 2010
With the exception of ERs, it's rare to see an IV being infused by gravity alone any longer.
posted by 517 at 6:14 PM on October 28, 2010
Response by poster: Hydrostatic pressure...thank you! That is extremely helpful. I realise now that I would also love some kind of schematic or diagram describing mechanics of all the various components and what they do. For example, how, exactly, does the pump regulates the drip?
posted by aunt_winnifred at 6:18 PM on October 28, 2010
posted by aunt_winnifred at 6:18 PM on October 28, 2010
Best answer: I will attempt to lay this out in as vivid of a mental picture as possible, using the electronic IV pumps in my example.
1) An IV catheter is placed in a vein. Veins carry blood and fluid back to the heart and have one-ways valves that are opened by the pressures of your working muscles against them. This is why it's important to move, even in bed, when you're in the hospital getting fluids--so fluid doesn't sit around and create dependent edema in those non-moving limbs. The "needle" that punctures the vein is removed and a small-gauge plastic catheter floats in the vein. The pressure of the fluid upstream in the bag is always higher than the pressure of venous blood against the catheter, so the blood stays in your vein. Even if your orders state that you don't need any fluids or medicine, the rate will be set around 20-30 mL (per hospital protocol-could be less) per hour, or the "KVO" (keep vein open) rate. Without the upstream pressure of fluid, the catheter will clot and no fluid can get to you. If you need fluids it's usually around 100 ml/hour (unless it's an emergency and you need fluids "wide open"). Your medicine is also adjusted according to the dynamics of the med you're getting--and those vary.
2) "Upstream" is your IV bag hanging on a pole. This creates downstream pressure. However, the tubing is threaded through an electronic pump that maintains your rate. Inside the pump is a roller that "milks" the tubing with calibrated pressures to achieve the right rate, which is programed into the pump. The pump also keeps track of how much fluid you've received, how long it will take for a bag of fluid or medicine to finish, and has various alarms to alert your RN to these events.
3) "Downstream" are one way ports with "luer lock" valves. These all tubing connections and needleless syringes are fitted with these "luer locks" so that tubing can be fitted together and short-acting or one-time medicine can be "pushed" into your "line." The luer ports on your tubing is female and is cleaned with alcohol and then another tube or needless syringe with a male part pushes the valve in, gaining access to your line. This is how several bags can run at the same time into your line (like saline plus a medicine bag). Normally, you have one bag of fluid and then other medicines are "piggybacked" onto that bag. The medicine bag is hung higher than the fluid and the pump is programmed so the medicine "runs in first" and then when it's empty, the fluid takes over.
I hope this helps. Also, ask at the beginning of a shift if at some point in the RN's shift you could have an education session on how IVs and IV pumps work with a little demo--especially if there are nursing students on the floor.
(I am in final clinical rotations to be a nurse practitioner and love messing around with the IV pumps. I also have a great analogy of cardiovascular preload, afterload, and heart failure using the IV pump as the illustrative analogy).
I hope your friend is feeling better!
posted by rumposinc at 6:18 PM on October 28, 2010 [11 favorites]
1) An IV catheter is placed in a vein. Veins carry blood and fluid back to the heart and have one-ways valves that are opened by the pressures of your working muscles against them. This is why it's important to move, even in bed, when you're in the hospital getting fluids--so fluid doesn't sit around and create dependent edema in those non-moving limbs. The "needle" that punctures the vein is removed and a small-gauge plastic catheter floats in the vein. The pressure of the fluid upstream in the bag is always higher than the pressure of venous blood against the catheter, so the blood stays in your vein. Even if your orders state that you don't need any fluids or medicine, the rate will be set around 20-30 mL (per hospital protocol-could be less) per hour, or the "KVO" (keep vein open) rate. Without the upstream pressure of fluid, the catheter will clot and no fluid can get to you. If you need fluids it's usually around 100 ml/hour (unless it's an emergency and you need fluids "wide open"). Your medicine is also adjusted according to the dynamics of the med you're getting--and those vary.
2) "Upstream" is your IV bag hanging on a pole. This creates downstream pressure. However, the tubing is threaded through an electronic pump that maintains your rate. Inside the pump is a roller that "milks" the tubing with calibrated pressures to achieve the right rate, which is programed into the pump. The pump also keeps track of how much fluid you've received, how long it will take for a bag of fluid or medicine to finish, and has various alarms to alert your RN to these events.
3) "Downstream" are one way ports with "luer lock" valves. These all tubing connections and needleless syringes are fitted with these "luer locks" so that tubing can be fitted together and short-acting or one-time medicine can be "pushed" into your "line." The luer ports on your tubing is female and is cleaned with alcohol and then another tube or needless syringe with a male part pushes the valve in, gaining access to your line. This is how several bags can run at the same time into your line (like saline plus a medicine bag). Normally, you have one bag of fluid and then other medicines are "piggybacked" onto that bag. The medicine bag is hung higher than the fluid and the pump is programmed so the medicine "runs in first" and then when it's empty, the fluid takes over.
I hope this helps. Also, ask at the beginning of a shift if at some point in the RN's shift you could have an education session on how IVs and IV pumps work with a little demo--especially if there are nursing students on the floor.
(I am in final clinical rotations to be a nurse practitioner and love messing around with the IV pumps. I also have a great analogy of cardiovascular preload, afterload, and heart failure using the IV pump as the illustrative analogy).
I hope your friend is feeling better!
posted by rumposinc at 6:18 PM on October 28, 2010 [11 favorites]
Response by poster: Thank you rumposinc! that is exactly what we need.
posted by aunt_winnifred at 6:35 PM on October 28, 2010
posted by aunt_winnifred at 6:35 PM on October 28, 2010
Ugh, I got myself confused between pressure head and hydrostatic pressure. Ignore what I said about pressure in neck vs legs, but you can test the effect of elevation for yourself. Attach a hose to a reservoir, put water in the reservoir, and vary the vertical distance between reservoir and open end of hose. The water will flow out faster when the distance is greater, because the greater pressure differential pushes the water harder.
Or just fill a piece of tubing with water and pinch both ends closed with your fingers, then hold the two ends at different heights and open your fingers and watch how forcefully the water flows out. When the ends are about the same height, the water will drain out relatively slowly. When the tube is vertical (i.e., the ends are at maximum height difference) the water will shoot out quickly. The speed of fluid flow is a reflection of the pressure it's under.
Ignoring the use of IV pumps for the moment, the higher the IV drip bag above the outlet (= the needle), the faster the fluid will flow. If it's connected to a person, the more it can "fight" the body's blood pressure. If the bag is too low, blood can flow back into the tubing because the body's blood pressure overcomes the pressure in the drip line.
I hope your friend gets well soon, despite my efforts to confuse her and myself. sorry 'bout that!
posted by Quietgal at 7:29 PM on October 28, 2010
Or just fill a piece of tubing with water and pinch both ends closed with your fingers, then hold the two ends at different heights and open your fingers and watch how forcefully the water flows out. When the ends are about the same height, the water will drain out relatively slowly. When the tube is vertical (i.e., the ends are at maximum height difference) the water will shoot out quickly. The speed of fluid flow is a reflection of the pressure it's under.
Ignoring the use of IV pumps for the moment, the higher the IV drip bag above the outlet (= the needle), the faster the fluid will flow. If it's connected to a person, the more it can "fight" the body's blood pressure. If the bag is too low, blood can flow back into the tubing because the body's blood pressure overcomes the pressure in the drip line.
I hope your friend gets well soon, despite my efforts to confuse her and myself. sorry 'bout that!
posted by Quietgal at 7:29 PM on October 28, 2010
Quietgal and Aunt Winnifred--
If your friend is just on a fluid drip (vs. meds or blood or something), you can just ask your nurse if s/he can unhook your line for a minute, leave it unclamped, and drip the end into a wastepaper basket as s/he holds the fluid bag up higher and lower. I would totally do this for a patient curious about the whole thing. Alternately, the next time she changes tubing or needs to prime a line (prefill it with fluids or meds), tell her/him that you want to watch and see how pressures work. She can prime over a wastepaper basket moving the bag up and down so you can see how the rate varies.
Also, when doing this, look at the unhooked end of the line to see the make luer lock parts, and when s/he hooks it up, watch how it depresses into the female part on your catheter. It's a pretty rad system and an important development in IV tech.
posted by rumposinc at 8:00 PM on October 28, 2010
If your friend is just on a fluid drip (vs. meds or blood or something), you can just ask your nurse if s/he can unhook your line for a minute, leave it unclamped, and drip the end into a wastepaper basket as s/he holds the fluid bag up higher and lower. I would totally do this for a patient curious about the whole thing. Alternately, the next time she changes tubing or needs to prime a line (prefill it with fluids or meds), tell her/him that you want to watch and see how pressures work. She can prime over a wastepaper basket moving the bag up and down so you can see how the rate varies.
Also, when doing this, look at the unhooked end of the line to see the make luer lock parts, and when s/he hooks it up, watch how it depresses into the female part on your catheter. It's a pretty rad system and an important development in IV tech.
posted by rumposinc at 8:00 PM on October 28, 2010
The chamber is a another downstream control of rate--to regulate the pressures from the bag before that pressure hits the line. It is also a visual double-check of drip rate (gtt)--you can calculate ml/hr based on the drips per minute in the chamber. Additionally, it can contain filters to filter the contents of the fluid (especially in the cases of blood products or intravenous nutrition), and it is made of a flexible material so that you can squeeze it, forcing fluid into the lines to prime them.
posted by rumposinc at 6:59 AM on October 29, 2010
posted by rumposinc at 6:59 AM on October 29, 2010
Response by poster: Thanks so much everyone. I took your information in today and we used it to make a good thorough analysis of all the bits and pieces. My friend was very pleased. It's nice to have some understanding of the machines that are plugged into you.
posted by aunt_winnifred at 3:33 PM on October 29, 2010
posted by aunt_winnifred at 3:33 PM on October 29, 2010
Aunt Winni--
Totally! I think we can forget in the hospital that it's not normal to attach machines and precisely engineered plastic to people's bodies and when patients don't say anything about it, we assume they're cool with it, when really they're just overwhelmed. Plus, people in general are interested in how things work, and equipment explanation is a great place to start a conversation with a patient. I'll be thinking about your friend and hope her time with Mr. IV Pole is a very brief affair. He's interesting, and all, but not all that cuddly, it turns out.
posted by rumposinc at 4:20 PM on October 29, 2010
Totally! I think we can forget in the hospital that it's not normal to attach machines and precisely engineered plastic to people's bodies and when patients don't say anything about it, we assume they're cool with it, when really they're just overwhelmed. Plus, people in general are interested in how things work, and equipment explanation is a great place to start a conversation with a patient. I'll be thinking about your friend and hope her time with Mr. IV Pole is a very brief affair. He's interesting, and all, but not all that cuddly, it turns out.
posted by rumposinc at 4:20 PM on October 29, 2010
This thread is closed to new comments.
I'm hypothesizing here, but as it's a small vein, the pressure is low (it's not an artery - which would squirt all over the place) and the reservoir thingy that the bag drips into is kept elevated above the patient, which creates some pressure to gently force the liquid into the bloodstream. It goes in slowly.... drop by drop, not like an injection for this reason - low pressure.
posted by TravellingDen at 5:57 PM on October 28, 2010