18
Pressors and Vasoactives
Here’s an update of the update of our first-ever article. As usual, please remember that these articles do not mean to be the final opinion on anything! They are only meant to reflect our own experience and knowledge, which is – scary – getting up to about 50 years combined. Always check with your own references and authorities! And when you find mistakes, let us know? Thanks!
1- What is a pressor?
2- What is shock?
3- Are there different kinds of shock?
4- What are the three parts of a blood pressure?
5- What does “pump” mean?
6- What is “inotropy”?
7- What about “volume”?
8- What’s “crystalloid”?
9- What is “squeeze”?
10- How does this relate to shock?
10-1- PA Lines
10-2- Something that will make you look really smart!
(Or really antiquated :))
11- Which shock state reflects a “pump” problem?
12- What is “ejection fraction”?
13- Which shock state reflects volume?
14- Which shock state reflects arterial squeeze?
15- What measurements do we use at the bedside for treating shock states?
16- How do pressors fit into the treatment of shock states?
17- How do pressors work on receptors?
17-1 - Agonizing receptors
17-2 - Antagonizing receptors
17-3 - SVR
18- How are other shock states treated?
19- Are pressors used to treat hypovolemic shock? An important point. Another one.
20- What about cardiogenic shock?
21- What other pressors are there?
22- What basic considerations should I keep in mind when using these drugs?
22-1- Setting up the drips.
22-2- Drug rates.
23- Are there other vasoactives that I need to know about?
24- How do we use vasopressin?
25- Why don’t we use the Trendelenburg position for hypotension anymore?
26- A Chart Thing for those who like them…
The Quiz!
1- What is a pressor?
“Blood pressure medicines” come in a couple of varieties: there are some that make blood pressure go up, and there are those that make it go down. The word “pressor” is usually used to mean the first kind. Another word that describes these drugs (both kinds) is “vasoactives”, which is to say: affecting blood pressure, or heart rate, or both. The major use for pressors is in the treatment of one kind of shock or another.
2- What is shock?
n. (noun)
1. “Something that jars the mind or emotions as if with a violent unexpected blow.” http://www.thefreedictionary.com/shock
2. The realization that you are working in the MICU…
http://media.photobucket.com/image/shocked%20face/unclewulf/hayden_shocked_face_resized.jpg?o=11
Shock is usually described as a state in which the body’s tissues aren’t getting enough blood flow for one reason or another. The peripheral tissues – way away from the major vessels, and supplied by smaller vessels whose perfusion suffers when blood pressure drops – lose much of the blood supply that they depend on for oxygen and nutrient delivery. So they switch gears at the cellular level: they change from aerobic respiration, in which they use delivered oxygen to make energy, to anaerobic respiration, which works, but poorly. The byproduct, or “engine emission” of aerobic respiration is carbon dioxide, which we get rid of by breathing. But the emission from anaerobic respiration is unfortunately lactic acid, and since the blood vessels are not carrying wastes away effectively – being underperfused – the lactic acid builds up, creating a metabolic acidosis. The acidosis makes blood pressure even harder to maintain, since most pressors like adrenaline (epinephrine) and norepinephrine (levophed) depend on the blood pH – if the pH is too low, they won’t work very well.
3- Are there different kinds of shock?
Yes – three main ones, but to understand them, we need to talk about how exactly a blood pressure is maintained. It turns out that there are three major components of a blood pressure.
4- What are the three components of a blood pressure?
We think of them as: “pump”, “volume”, and “squeeze”. Of course, it’s lots more complicated than that, and as always, most of the information in all of these articles is written “with a lot of lies thrown in” – there are shelves of textbooks that have been written on each subject that we try to cover in a few pages. But the point is: how can you organize the ideas in your head to figure things out at the bedside? Quick-and-dirty is often what will help most…
Keep in mind as we go along that each of these components can be measured, and that many of the tools we use in the unit are designed to do just that.
5- What is “pump”?
Pump is the heart. Anything interfering with inotropy, heart rate, or cardiac output, be it an MI, an arrhythmia, ischemia – is a pump problem.
How might you measure your patient’s ability to pump? Numerically, I mean?
http://www.klangundkleid.ch/img/moebel/sofina/08510_double-stroke-hand-pump.jpg
6- What is “inotropy”?
Inotropy means: “how hard the left ventricle is working to pump, to empty itself”.
That’s this one. Why do we worry about the left ventricle so much, in relation to blood pressure? Actually, we worry about the right ventricle too – but for sort of different reasons. Take a
look at the article on PE’s for more about this. (www.icufaqs.org/PulmonaryEmbolism.doc)
Hmm – think we could measure this “LV inotropy”?
http://www.everyschool.org/u/wcms/tanaka/Heart.jpg
7- What about “volume”?
Easy enough: this is the circulating volume in the blood vessels. You have to include the relative volumes of red cells and plasma to this idea though – there may be plenty of red cells, but if a patient’s plasma volume is low – which is to say she’s dehydrated, hypovolemic, but not from bleeding – you wouldn’t give that person blood, would you? Or the other way around – you wouldn’t give just crystalloid to a person with a low crit from bleeding, would you?
8- No. What’s crystalloid?
Any “clear-as-crystal” IV fluid is “crystalloid” – it’s a word used for a kind of IV volume replacement - as opposed to “colloid”, meaning anything protein-based such as albumin of one kind or another, or plasma – but as I understand it, not red cells. Anyhow, right – you would correct volume loss with what the person needed, based on what they needed: red cells, or the ‘”water” component of the circulating volume.
How might you measure your patient’s volume status?
http://www.fluvaccine.com/Stat/images/itemslarge/2U4.jpg
9- What is “squeeze”?
“Squeeze” has actually been used around ICUs for long time to mean two different things – some people use it to describe how tight the arterial bed is - which is to say how tight, or constricted the entire system of arterial vessels is. Other people use “squeeze” to mean inotropy. I use it the first way, because it helps me to think visually about what’s happening to the patient – it’s a useful concept when you’re faced with a hypotensive situation that you’re trying to sort out.
We need to measure this too…
http://www.mikkis.co.uk/admin/images/Stressball-Home.jpg
10- How does this relate to shock?
The three components of a blood pressure actually reflect the three kinds of shock that you’re likely to see in intensive care. The trick in treating each of these correctly comes from our ability to measure each of the components precisely. Any idea how we might do that?
10-1- The tool you need in this situation is a PA line – a pulmonary artery catheter, also known as a Swan-Ganz line, or just a “Swan”.
That’s the long yellow thing, wrapped in the clear plastic thing, going into the blue thing, there at the patient’s ear, sort of? Which is connected to the white thing, going into his neck?
In proper terms: the PA line, inside its clear sheath, is advanced into the patient through a white introducer, which has a clear side port :)
Dude, put a tegaderm on that site! (Why?)
http://www.castenholz.org/ptguide/special.htm
PA lines tell you everything you want to know:
- how well the pump is pumping (cardiac output, cardiac index)
- how full the right side of the heart is (CVP), and how full the left side is (wedge pressure) – that’s the volume…
- and how well your patient’s arteries can squeeze : that’s the SVR – the “systemic vascular resistance”…
PA lines are serious juju – they’re invasive, they’re tricky to place, they need very serious care and feeding – in fact, they’ve got a whole enormous FAQ all to themselves, and they need one! But understanding how pump, volume and squeeze all go together is important to understanding how pressors work. Go take a look! (www.icufaqs.org/PALinesApril04.doc)
10-2- Something that will make you look really smart.
(Or really antiquated :))
An alternative – if your patient has no PA, but does have a central line and a radial a-line, you can call the in-house IABP tech to come and do a “green dye” cardiac output. They hook up a little color-measuring thing to the arterial line, and they inject some form of dye - (probably green!) - through the distal port of the CVP. Then they measure how long it takes for the dye to show up at the a-line, multiplied by this, divided by that, aligned with the coefficient of Hammerschmidt, over the square of the patient’s shoe size… and out come the numbers. Cool!
11- Which shock state reflects a “pump” problem?
The kind of shock that reflects “pump failure” is “cardiogenic” shock, which is to say: “originating in the heart”. Simple idea: the blood pressure is low because the pump isn’t pumping. This is usually because of a sizable MI, but people with end-stage heart disease of one kind or another, such as cardiomyopathy (“heart-muscle-disease”), or people who have had multiple MI’s - leaving them with a very low ejection fraction - can live on the edge of cardiogenic shock much of the time.
12- What is ejection fraction?
“EF” is the amount of blood ejected from the left ventricle into the arterial circulation with every systolic contraction, expressed as per cent. Normal is something like 50-70%. Impressively low is usually said to be less than 30%, and “cardiac cripples” who can’t get up from the chair without shortness of breath sometimes run in the low teens.
Here’s the LV at the end of diastole – all full, ready to go.
http://www.heart1.com/images/content/ejection1.jpg
Here’s the LV at the end of systole – the LV is contracted. 45% of the blood in the LV is left, so 55% has been ejected into the aorta.
See? That’s “pump”. So what happens if the pump can’t pump?
http://www.heart1.com/images/content/ejection2.jpg
13- Which shock state reflects volume?
“Hypovolemic” shock reflects low volume – and again, the fix depends on which component of circulating volume the patient has lost. You probably wouldn’t give red cells to a patient with heat stroke, whose crit might be up around 60%. And you would try not to give crystalloid to a person with a big blood loss. Would you give this patient a pressor?
14- Which shock state reflects arterial squeeze?
“Septic” shock reflects “squeeze”. (Cardiogenic shock affects squeeze too, but we get into that in the FAQ articles on PA lines and balloon pumping (www.icufaqs.org/IABPFAQ.doc) – take a look at those for more than you ever wanted to know on the subject! J)
It turns out that the arteries are contractile – they can be made to open up (“dilate”), or tighten up (“constrict”). The whole system of arterial vessels is sometimes called the “arterial bed” – and it helps to think of the whole bed, the whole system, loosening or tightening up in response to various states.
In septic shock, the germs floating about in the systemic circulation produce a set of unpleasant chemicals called endotoxins. These specifically affect the arterial vessels - they loosen up, causing the blood pressure to drop. An analogy would be a garden hose turned on full – if you squeeze the hose, the pressure rises, and the water squirts across the yard. If you release the squeeze, the water pressure drops, and the water runs all over your shoes. Similarly, if the arterial system as a whole tightens up, the patient’s blood pressure rises, and if the system loosens up, the pressure falls – which is the cause of hypotension in septic shock.
http://www.medicalillustration.net/Museum_images/images/arteries_runner.jpg
So the trick in diagnosing hypotension is to figure out: which of the three components is the problem? There’s lots more on this subject in the PA-line FAQ.
15- What measurements do we use at the bedside in the ICU for treating shock states?
Well, we start with blood pressure, but you probably knew that part. Use the tools at hand. A blood pressure cuff is a good start. A seriously hypotensive patient on pressor drips, really ought to have an arterial line. www.Icufaqs.org/ArterialLines.doc
Here’s one. It’s the same catheter as an IV, usually a 20 gauge angiocath, inch and a quarter, in the radial artery, hooked up to a transducer.
http://www.zefon.com/medical/aline.htm
A “labile” blood pressure – an unstable one – needs constant monitoring, because… well, because it’s unstable! The need for pressor titration (dialling the dose up or down) is ongoing – you want to wean these drips down whenever you can, while still keeping the blood pressure in the range your patient needs. An actively septic patient, or a cardiogenic one, can require pressor titration every few minutes! You didn’t think you were going to get to sit down in the ICU, did you? If you don’t have an a-line – use the non-invasive BP cuff, and set it to cycle frequently. How frequently? What if the patient is coagulopathic? What is coagulopathic? What if she has a low platelet count? What is a low platelet count?
Then there are the central line numbers: CVP, wedge pressure, and the ones we get from “shooting numbers”: cardiac output/ index (CO/CI), stroke volume (SV), and systemic vascular resistance (SVR).
Each of these measurements corresponds to one or another of the three parts of the blood pressure, and each kind of shock has a characteristic pattern of these that is often immediately obvious one you shoot your first set of numbers after a PA line goes in.
16- How do pressors fit into the treatment of shock states?
The choice of pressor depends on the nature of the problem. To explain this, a quick review of adrenergic receptors will help. There are three adrenergic receptor sets that we worry about in the ICU: the alpha receptors which are located in the arteries, and the two kinds of beta receptors: beta-1’s (you have one heart, that’s where they are), and beta-2’s, (you have two lungs, that’s where those are.)