Understanding Hemodynamic's I'm confused please help

nurseprnRN, BSN, RN · Feb 2, 2013

It's helpful if you can step back first and think of what the anatomy of the circulatory system is supposed to accomplish. It's supposed to move a fluid around in a bunch of blood vessels, pumped out at high pressure from the left side of the heart, returned to the heart by passive squeezing in the veins and kept from sloshing backwards by valves in the vessels. Then the right side of the heart is supposed to push it through the lungs (at a lower pressure, because it only has to perfuse the lungs right next door, not all the way down to the toes like the arterial system) to do the gas-exchange thing. Then the fluid goes back to the left side of the heart and out to the body again.

Ventricular filling pressure is just the pressure that is in the ventricles at the end of diastole (LVEDP, left ventricular end-diastolic presssure). For a given volume delivered to a ventricle, pressure can be lower if the ventricle is nice and soft and flexible and empty, ready to accept a new load, than if it's hard and scarred up or has leftover blood in it from the last systole because the AV is hard to open OR because its contractility was so lousy that it didn't empty well. Another term that is used could be "preload," pre- meaning "before systole," and load, well, being the load of blood delivered to the ventricle that it is gonna have to move out in systole. You can measure load as weight or volume, but the way we look at it is by measuring the pressure that occurs there. Pressure changes tell us what's going on in there. Think about a soft balloon (low pressure) and a hard one (high pressure). Which has more air in it?

Let's look at the blood flow in a linear fashion. I regret that I cannot give these in color so you can see the blue of venous, the red of arterial. But hey. Draw them on a piece of paper in color. The lungs are pink :)

Body > Veins > Vena Cava > Right Atrium > tricuspid valve > Right Ventricle > pulmonic valve > Pulmonary Artery > LUNGS >Pulmonary Vein > Left Atrium > mitral valve > Left ventricle > aortic valve > Arteries > Body

Think about when the valves between two chambers are OPEN. By definition, each chamber must be at the same pressure, right? So, at the end of diastole, just before systole, the pressure in the LV is the same as LA pressure is the same as the pressure in the pulmonary vein (no valve in the way there) and in the pulmonary capillary bed. And since there are no valves in the pulmonary capillary bed, tracking backwards, you can see that LV end diastolic pressure equals end-diastolic PULMONARY ARTERY PRESSURE, which is, conveniently, what we look at when we are wondering what's going on in the left heart. You can even follow it back all the way to the right atrium, and the vena cava-- central venous pressure! Wow!

OK. Now, why do we care about LV end-diastolic (filling) pressure? It's because that's where the work of supplying the whole body goes. For that, I wish I could draw you a nice little curve here. I can't, so I will describe it and YOU will draw it on a piece of paper to look at while we chat. Trust me on this. It WILL help you.

Horizontal axis: label this "preload" or any other term you like. Filling pressure, PA diastolic pressure is the same thing (see above) and you can even extrapolate all the way back to central venous pressure, for a rough trend-setting bit of data.

The vertical axis you will call "cardiac output," or "blood pressure," because the line we are going to draw is going to explain something really cool.

Start lowish on the left, near the vertical axis-- low filling pressure means low BP. Think: hemorrhage, hypovolemia, makes your BP low, right?

Slant the line upwards to the right, showing that blood pressure (cardiac output) increases the more blood you put into the heart. (Tank up that hypovolemic guy, and BP improves.) But at some point, that upward-going curve peaks, flattens out...and then it DROPS as the preload keeps increasing. This is because cardiac muscle is like a rubber band-- the more you stretch it, the harder it contracts...to a point, at which point it gets too stretched out and actually contracts less well. Draw a little asterisk at the top of that curve, where it starts to fall, then let it fall a little bit. That asterisk marks the best cardiac output you can get-- preload and output are optimal for that heart. Beyond that point, where the line slopes downwards, lies congestive heart failure- the heart is too full, has more than it can handle, and it fails. (This is, BTW, called the Frank-Starling Law of the heart, and you just drew the Frank-Starling curve, ta-da!) Pressure backs up into the pulmonary capillary bed making the lungs get wet and heavy. This is when people get diuretics (to decrease that excessive preload) AND drugs to improve their contractility.

Of course, if contractility is lousy because of coronary artery disease, previous MI, or whatever, this whole curvy line thing will kinda slide over to the left-- the myocardium will fail with lower pressures than it would if it had better contractility. Better contractility (a right shift) means it will handle more preload (higher filling pressures) and make better BP out of it. Draw a second curve to the right of the first one, parallel to it, to see that. With me so far?

I think you can see how coronary artery disease will give you higher filling pressures-- when the heart is failing a bit, it goes past the top of its curve more easily because its contractility is diminished.

Mitral STENOSIS will, in fact, decrease your LV preload, but it will increase pressures back into the lungs and, eventually, the right heart, because of the resistance to flow from the right side to the LV. Mitral REGURGITATION, on the other hand, will result in higher filling pressures because when the ventricle contracts in systole, some of the blood goes backwards, leaving excess sloshing around between the atrium and ventricle; the ventricle will have to accept a higher preload at diastole, and it doesn't like it. Over the top of the curve again.

Well, I hope this hasn't confused you. I used to tell my students they had to know this because we saw lots of people with all sorts of deficits, but if they didn't have hearts and lungs, they were dead and we didn't have to take care of them anymore. Works in every possible area you could work, except pathology. Please ask me if I've confused you anywhere. Also, please get the Anatomy Coloring Book and the Physiology Coloring Book (real books, and very helpful to students; free 2-day shipping from Amazon for students). These will be much more helpful than your average textbook.

javapearl · Feb 3, 2013

Ok so I had the lecture on this yesterday and my test is on Monday, I tape recorded the lecture and have listened to it over again but I'm so confused. When they go over the different pressures cvp, rap, rvp, pam, pawp etc... I can't keep all the meanings together and understand this at all. This is my second test and I didn't do so well on my first one so if I can't grasp this I think I'll be done for. I'm trying my hardest and have put hours into researching this topic but I feel so stupid because I can not grasp it. Any help would be appreciated.

I just had this lecture and exam. My friend and I each made studyguides for this. I would love to pass them on to you, but not sure how. I see we are not allowed to give emails... and I am not sure I can copy and paste them here. So doesn't seem like I am very helpful. If you know of a way I can get them to let me know... otherwise I will see if I can highlight the important things.

Esme12, ASN, BSN, RN · Feb 3, 2013

You can post your study guides as word attachments when you post.....when you reply go to the "go advanced" option and word attachment is there.

This site is for beginning ICU nurses but I think you will find it helpful.....icufaq's.org look at it all but look at PA-Lines, and Central Lines.

Kaysmom8 · Feb 3, 2013

Thanks GrnTea, so the pulmonary artery cath measures the left side of the heart for heart disease, shock ards, conditions that alter co and fluid status as well and cvp monitors the right side mainly fluid volume status? The pawp measures the left atria and left ventricle and the right atria are measured by the cvp and the right ventricles are measured by the pap?

Do I have this right:

Arterial lines and they are put in arteries mainly the wrist to measure bp and blood draws for abg's

Central venous lines mainly for right side of the heart, blood volume and fluid status

Pulmonary artery catheters for the left side and they are used to diagnose and evaluate the heart

If the pulm cath goes through the right side of the heart then why do they use cvp lines?

Sorry I'm trying to sort all this out and this is all I have concentrated on for my test tomorrow and I still have a ton of other things to study I think I'm in trouble...

hodgieRN · Feb 3, 2013

To answer your question, not all pts are able to have a pulm caths. It has to be maintained by nurses who are trained to use them. However, you can get a CVP reading from other things (not just pulm catheters). Most pts who have a central line (non-valved) can have CVP readings. So, if you have a jugular central line, a subclavian central line, or even a non-valved PICC line, you can get CVP readings. A CVP reading equals your right atrial pressure. PICCs, subclavians, and jugular lines terminate in the superior vena cava, or the junction of the vena cava and the atrium, so all you do is hook-up a CVP transducer and you can get the reading. (Sometimes, a CVP reading from a non-valved PICC isn't really that accurate, but it's better than nothing!). So, if a pt has a central line and they become septic, go into shock, or have issues with volume overload or depletion, you can get a reading without inserting a pulm cath. It can give you a good idea of their volume status or give some hints to right side of the heart.

And remember that pulmonary artery catheters not only look at the left side of the heart, but the right side as well (where as a CVP only looks at the right). PA caths give readings for the right side of the heart and the pulmonary artery. After you inflate the balloon and "wedge" it, you are able to get a reading for the left side of the heart b/c the balloon occludes the pressure behind it (basically). So, you get the right and the left. And, if you have any pulmonary issue like COPD or a PE, it can through off some of the readings. That one of the things the nurses are trained to do. They have to be able to pick out, interpret, and trouble shoot issues that can happen with PA caths.

Arterial lines not only have the ability to get real time B/P's and abg blood draws, they can also be hooked up to certain monitors that can read cardiac output, stroke volume, and systemic vascular resistance. It's a lot of great information for just a simple peripheral line. So, if a pt can't have a pulm cath, they can have a central line for CVPs and an a-line for b/p and cardiac output, which are much more minimally invasive compared to a PA cath.

nurseprnRN, BSN, RN · Feb 3, 2013

Thanks GrnTea, so the pulmonary artery cath measures the left side of the heart for heart disease, shock ards, conditions that alter co and fluid status as well and cvp monitors the right side mainly fluid volume status? The pawp measures the left atria and left ventricle and the right atria are measured by the cvp and the right ventricles are measured by the pap?
Do I have this right:
Arterial lines and they are put in arteries mainly the wrist to measure bp and blood draws for abg's
Central venous lines mainly for right side of the heart, blood volume and fluid status
Pulmonary artery catheters for the left side and they are used to diagnose and evaluate the heart
If the pulm cath goes through the right side of the heart then why do they use cvp lines?
Sorry I'm trying to sort all this out and this is all I have concentrated on for my test tomorrow and I still have a ton of other things to study I think I'm in trouble...

You're doing pretty well.

The PA cath has more than one lumen. One is at the end, to measure PA pressures and, when the balloon is inflated to block PA pressures, to look through the pulmonary capillary bed and tell you left sided pressures. The other is a CVP port for CVP and fluids. Some have another lumen for fluids.

Cardiac outputs are measured with PA catheters. A known volume of cool fluid is bolused through the CVP port and a temp sensor at the far end in the PA looks at the temp curve as it goes past, and calculates the liters per minute of cardiac output from that. It's reading right heart cardiac output, but that has to be the same as left heart cardiac output (if it isn't, where'd the extra blood go? :) )

Esme12, ASN, BSN, RN · Feb 4, 2013

This is a great power point on PA catheters...

Kaysmom8 · Feb 4, 2013

Thank you all for taking the time to help me out, I passed my test today but not with the grade I'm used to getting the past two tests I just made it part of me want's to through in the towel. I'm just so exhausted and the learning/struggling to comprehend all of this info is really making me feel kind of depressed...

nurseprnRN, BSN, RN · Feb 4, 2013

::looks around:: Did people tell this nice student that nursing school was easy and all nurses do is "follow doctor's orders"? Hmmm, that was unkind of them. We have to know a LOT of physiology, because everyone we take care of has some disturbance in it (else we don't have to take care of them). On the other hand, look around-- we're nurses, and we learned it! You can too. Figuring out systems and how they all fit together is pretty entertaining once you get the hang of it.

hodgieRN · Feb 7, 2013

Congrats on passing! Lots of students don't pass tests, class, and clinicals. In other posts, I've mentioned the fact that overachieving can often be the down fall of students. As GrnTea said, nursing school isn't easy. Throwing an A here and there says nothing about your capabilities for the future. Getting depressed and wanting to throw in the towel will be your downfall; not the fact that didn't get an A. Self-doubt is dangerous. It is a lot of information and when you have been a nurse for 5, 10, or even 20 years down the road, there will be more information for you to learn. It never stops...but that's a good thing. Part of you needs to live in the now. Learn what you need to pass exams today and be realistic. The other part of you needs to live in the future. Learn what you need to be a good nurse and dream of greatness. But, you have to balance the two. Many students just look at the future. They only imagine themselves as great nurses and immediately expect all the basic stuff the fall in line. As if nursing was a destiny that can't be taken away, so how could they get anything other than an A. Or, they have achieved everything else in life or always been a star. But realistic achievements and failures today will mold you for the future. In nursing and medicine, nothing comes easy. So, if you are depressed, get over it and figure out what you need to do today to improve. Other students come on here and ask us to complete their homework assignments, so you are way ahead of the game b/c you are giving effort at learning the vast world of medicine. Who cares if your grades aren't perfect. You passed. Set realistic goals and obtain them. If you don't succeed at something, learn from it, then over come it. Overachieving will stress you to the point of giving up. If you can get all A's, do it. Go for it. If you don't get all A's, get as close as you can, but it doesn't matter if you get a B or even a C here or there. Once you are nurse, no one cares what your GPA was in school. What people do care about is whether or not you are good at your job. No one thought I'd make it. Some dude who wore board shorts to class and hated getting up before 9 am. I was going up against others who said "everyone in my family are nurses and I just understand how the body works, blah, blah. I volunteered at the hospital all summer. What are you doing here?" Half of them didn't make while I got to cross the stage in my cap and gown. Get rid of your depression and exchange it for motivation. Stick it out. Be harder on yourself when you are a nurse....when you have been in the game. If you do it now, you won't survive. As corny as it sounds, it's all about belief, motivation, goals, and getting back on your feet while you are learning school. Dust yourself off and keep grinding away.