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Vents and ABGs2/05/04
Hi all – here’s an update of the article on vents, etc. As usual, please remember that this material is not supposed to be the final word on anything – instead, it’s supposed to represent what gets passed from a preceptor to a new orientee. Please let us know when you find errors, and we’ll fix them right away.
1-What are ventilators all about?
2-What is intubation?
2-1 How is intubation done?
2-2- How long can a person stay intubated?
3-What is extubation?
3-1- How is a person extubated?
3-1- What are mechanics?
3-1- How do I know if my patient can be extubated?
4-What is an endotracheal tube?
5-What is FiO2?
6- What is O2 saturation?
7- What is the oxygen-hemoglobin saturation curve, and why do I have to care about it?
8-What is a “tidal volume”?
9-What is meant by “rate”?
10- What is “minute ventilation”?
10-1- A very cool thing…
11-What is “oxygenation”?
12-What is “ventilation”?
13-What is PEEP?
13-1- What is a recruitment maneuver?
14-What is “pressure-limited” ventilation?
15-What is volume ventilation used for nowadays?
16- How do I interpret my patient’s ABGs?
17-What are the ideal ABG values, and why do we use them?
17-1-Respiratory effects on blood gases
17-2-Metabolic effects on blood gases.
18-An ABG scenario…
19-Another, related scenario…
20-How should I suction my patient?
20-1- Should I use saline?
20-2- How high should I set the suction control?
20-3- How often should I suction my patient?
21-What is non-invasive ventilation?
22-What is mask CPAP?
23-What is bi-pap?
24-What are “I”-pap and “E-pap”?
25-How well does non-invasive ventilation work?
1- What are ventilators all about?
“All right. He’s on pressure control at 25, on 100%, with a rate of 30, and 10 of PEEP, and look, his last gas was 65 – 72 - 7.15; I think you’re in for a busy night. I’ve suctioned him twice with saline – he has no secretions at all, I guess that goes with the ARDS, but I don’t see how he can go through all this without getting a pneumonia at some point…”
(Note to people who know about ABGs already: apparently I learned to write gases down backwards – unlike the rest of the known universe, we write pO2 – pCO2 – pH. Hey, just celebrating diversity, y’know.)
Here’s a subject that could go on for ever, almost. Vents have been around for a while now – thirty years?, and they can do all sorts of things that they couldn’t in years past. In the old days, vents did only two things – they’d push a given volume of air into a patient, at a given concentration of oxygen, and then let that volume come back out again. (Even that is a lot better than nothing: one of our RRTs came back from volunteering in post-earthquake Iran recently where, since they had no vent, they manually bagged a baby for 16-odd hours until he could be med-evacked out.)
Nowadays you can vary not only the size of the breath you give, but how hard that breath gets pushed in – this is called “pressure-limited” ventilation, and it’s made all the difference in things like ARDS: everybody died of ARDS in the old days, and now most of them live. Ventilators can let the patient initiate her own breaths if she can, and support her more, or less, as desired, through the whole inspiration - expiration cycle. Or the vent can provide all the breaths for the patient. All these choices are tools in the kit, and have their own acronymic names: PSV, PCV, SIMV – you’ll learn what these mean as you work with them, and which ones are used in different situations.
Oh my, how the time she does go by… I thought I’d find an image or two of the vents we used to use: this is a picture of an old Emerson vent, a “washing machine”, which back in the Upper Cretaceous Period (middle 1980’s) was all that we had. So off I go onto Google, looking around – well, look at the image link.
It’s official – I’m a museum piece. They’ll put me in a glass case next to one of these babies somewhere, with a little sign, something like “Nursus Criticalis, Earlious Ignoramus Extremius”.
2- What is intubation?
Intubation is the placement of a clear silicone-plastic tube in a patient’s trachea. “Call anesthesia and give them a heads-up – I think we’re going to have to intubate Ralph’s patient.”
Uh, Ralph? You sure you got the right size tube, there?
2-1- How is intubation done?
Intubation is done in our hospital by the anesthesia resident on call. Intubation has a whole FAQ to itself, and you can look there for lots more information.
2-2- How long can a person stay intubated?
We usually put a limit of about two to three weeks. We try to stretch the time limit a bit if we think that the patient will extubate soon, but after three weeks they usually wind up with a trach and a g-tube.
3- What is extubation?
Extubation is the removal of the tube, which ought to be a planned event: “That’s the second time that Ralph’s patient has made it to extubation. Except the first one was a self-extubation, right? I hope he flies this time.”
3-1- How is a person extubated?
The patient’s mouth and trachea are suctioned, the cuff is deflated, and the patient is briefly bagged to make sure there’s a “leak” after the cuff is down. Then the tapes are removed, and the tube is taken out smoothly in one motion, on exhalation. We usually put patients on a 100% corrugated face mask setup and then wean the oxygen, keeping saturations greater than 95-96%. Would you use that setup if your patient had severe COPD?
What if there was no leak when you deflated the cuff? Would you extubate that patient?
3-2- What are mechanics?
Mechanics are a set of numbers that tell you how ready your patient is to be extubated. We measure three things:
- Tidal volume – should be greater than ~ 500cc for a 75 kg patient.
- Vital capacity – the maximum exhaled volume after a maximal inspiratory effort – should be about 1500cc.
- Negative inspiratory force; “Ask respiratory, what’s the patient’s NIF?” - should be greater than - 50cm.
Acceptable numbers for all three of these means that your patient is strong enough to breathe on her own. What if “I’ve been suctioning her at least every hour for tons of secretions.” Would you extubate her then?
3-3- How do I know if my patient can be extubated?
Basically because they’ll look ready – comfortable on minimal vent settings, manageable amounts of secretions, good mechanics numbers, adequate oxygenation on near-room-air FiO2 – you’ll know. One other important thing to keep in mind is the state of the trachea. A traumatic intubation, or other scenarios can provoke tracheal swelling, and once in a while it’s severe enough that the ET tube is the only thing keeping the airway open.
The trick to assessing this is the presence of an air leak when the ET tube cuff is deflated. Dropping the cuff and bagging the patient should produce enough of an airflow back up through the patients’ mouth that they could actually speak – this is a useful trick for communicating with intubated folks who are desperately trying to tell you something, but who can’t write it out. Be sure to preoxygenate them, and suction them first, and watch for any signs of desaturation or distress when you try this.
4- What is an endotracheal tube?
This refers to the tube itself. “What size tube do you think they want? They’ve been putting in a lot of eights lately, because they’re having to do so many bronchoscopies. ”
Not usually green…see the inflatable cuff at the end near the top of the picture? The thing floating on a string in the middle of the image is the “pilot balloon”, which is inflated with air, and then passes that air along to the cuff.
5- What is FiO2?
“Fraction of Inspired Oxygen” – how much oxygen this patient is on. “What’s his Fi02? 80%? But he’s oxygenating really poorly – you think he’s having a PE?”
6- What is O2 saturation?
The saturation number tells you how much of the hemoglobin in your patient’s blood is saturated with oxygen, and is read by a probe that the patient wears on a finger. The probe actually looks at the pulsatile flow in the finger, and measures changes in the blood color as the saturation changes – better is brighter red. “You’d better call the team, this patient’s sat is going down.”
One problem comes from the fact that other things besides oxygen can saturate a hemoglobin molecule: carbon monoxide is the big one. Binds tightly, too – very hard to get off without a trip to the hyperbaric chamber. A patient with CO poisoning will have a lovely sat by finger probe, nice and high, around 100 – except that it’s not oxygen his blood is carrying!
7- What is the oxygen-hemoglobin saturation curve, and why do I have to care about it?
(Uh –oh, a graph! This is getting scary… mommm!…Okay. Deep breath. Re-saturate yourself, there.
Y’okay? It’s gonna be okay – really, it is! Just stick with me here a minute.)
The left shift and right shift stuff has to do with the fact that changes in the patient’s physiology will change the way the hemoglobin binds up the O’s. Holy O’s, Batman - can I remember this?
Or let’s do it the smart way and figure it out from the chart. If your patient becomes, uh, alkalotic, pH up, and the curve moves to the left, then what happens? A sat of 90, instead of representing a pO2 of 60, represents what? Draw a line down to the pO2 scale: about 50. Oooh – a left shift is not a good thing.! Harder for the hemoglobin to saturate?
How about a right shift, say, for a patient with a temp spike?
This time the sat reads lower than it ought to – a sat of about 70 means a pO2 of 60. Easier to saturate? So a right shift isn’t such a bad thing?
(Jayne – is this even remotely right?)
Now. See that dark curved line up there? See how it goes up rapidly, going from the left to the right? That means something very important. Here it comes: what it means is that even with a saturation that seems fairly high, like 90%, your patient’s pO2 may be pretty stinky, like 60. See that? – that’s what those straight lines are telling you.
That’s pretty important. That steep curve means that your hemoglobin saturates quickly as you go upwards, and desaturates just as quickly on the way down. If you draw your own intersecting lines across the graph, you’ll see that a sat of 80 actually means that your patient has a pO2 of something like – what? Stinky.
There are always exceptions, and stinkiness is no exception: some people walk around with a pO2 of about 50 – group, who are they? Y’all heard of the 50-50 club? Who are those people? What? Oh, sorry! I mean, “What subset of the general population admitted to the critical-care pulmonary facility comprises the sub-moiety which fits this physiologic demographic?” (Arrgh!)
8- What is tidal volume?
Tidal volume is a number that describes how much air a patient moves in and out of his lungs, both lungs together at the same time, with each breath, either in, or out, measured in cc of air. Bigger (within the normal range of roughly 300-450cc) is generally better. For example, an asthmatic may only “move” about 100 cc or less with each breath until they “break” – so if they start moving larger tidal volumes, that would represent improvement. But too big could be bad – pushing too much air into stiffened lungs could cause a barotrauma (“pressure injury”). This subject is complex , and the goals are often different in different situations– be patient, and spend lots of time observing at the bedside.
9- What is rate?
This one is easy: rate means the number of breaths delivered in a minute. Sometimes you want to control the patient, and control the rate – sometimes you want to let the patient initiate their own breaths. It depends on whether they’re getting better, or worse, and what was wrong in the first place.
10- What is “minute ventilation”?
Minute ventilation is another number. Multiplying the rate, times the tidal volume, gives you the total air “moved” – that went in and out of the patient – measured in liters, in a minute. If you measured mine – say I was breathing 16 times per minute, with a tidal volume of about 400cc – my minute ventilation would be, 6400cc, or 6.4 liters. This goes up and down in different conditions – for example, an asthmatic in a flare might initially be able to keep her minute ventilation up near normal by breathing with a tidal volume of about 100cc at a rate of 60 breaths per minute, but she’d get tired pretty fast. Time for nebs, and heliox.
10-1- A very cool thing…
Just as an aside – this is so cool. Helium is lighter than air, right? Meaning that it’s thinner, less dense than normal air, which is mostly nitrogen anyway, and rather thick and heavy by comparison. So, in asthma, when the bronchi are tightened up, the problem is that the thick, heavy normal air doesn’t get in and out of those tight tubes very well – what to do? Try this: mix your normal amount of oxygen, say 21%, with helium instead of nitrogen: “heliox”. This thinner mix actually slips in and out of those tight tubes more easily than that nasty heavy nitrogen-type air we usually breathe, and can often help asthmatics avoid being intubated. So cool. Anybody know – do the patients talk funny on heliox? Divers do, I know, although they use heliox for different reasons.
11- What is “oxygenation”?
Oxygenation is how well oxygen is getting into the blood from the lungs, measured by P02. Clearly, if the alveoli are full of water, then the oxygen is going to have a hard time getting into the little exchange capillaries in the alveolar walls- which is why people oxygenate poorly in CHF. A normal PO2 is roughly 80-100. Suppose you had a patient who was on 100% oxygen, and they had a PO2 of about 85. An inexperienced person might look at that PO2 and say: “Well, they’re oxygenating okay.” – Nuh-uh!
Think about this for a second. What FiO2 are you breathing right now, at room air: 21%, right? And your P02 by blood gas is roughly 80 to 100, right? Okay, so, just by the numbers alone, the ratio of “21” to “80 to 100” is what, roughly 1 to 4, or 1 to 5? In other words, your P02 should be roughly 4 or 5 times your Fi02, given normal lungs. So, if you intubated me, and put me on 100%, what should my P02 be? – roughly 400-500! So our friend in the paragraph above, who is on 100% oxygen, with a P02 of 85 – is he oxygenating well? No, he is not. He is oxygenating very poorly indeed. “Wow, this oxygenation is really awful.”; “Yeah, I know – you think we should diurese him?”; “Probably, this is mostly CHF – he had a really wet looking chest x-ray.”
12- What is “ventilation”?
Ventilation: how well CO2 is being removed from the blood, measured by pC02: a higher pC02 means that less C02 is being cleared – that ventilation is worse. With better ventilation, (with a higher “minute ventilation” number), the pC02 goes down. (It always helped me to think of C02 as ‘exhaust gas’- it’s the byproduct of aerobic respiration, like ‘engine emissions’ coming out of a tailpipe, it’s what you want to get rid of.) The asthmatic who moves a small tidal volume will not clear pC02 very well – she will “retain” it. “Oooh, look at her pC02, it’s up to 90.” – “Yeah, she’s retaining C02.” “What should we do?” “Well, we need to ventilate her better. We probably need to turn the rate up on the vent.”
Remember, oxygenation and ventilation are different, and mean specific things.
13- What is “PEEP”?
PEEP: this stands for Positive End-Expiratory Pressure. This means that at the end of an expiration, the vent doesn’t let the lungs empty completely – instead, it uses a fixed amount of pressure, measured in cm of water (as though it were water in a column, exerting weight downwards), to hold the lungs open at the end of the breath. This does two things – it keeps the alveoli from collapsing (“atelectasis”), and it pushes the air forcibly, if gently, forwards into the lungs. This forward pressure helps oxygen diffuse into the alveolar capillaries, and raises the P02 – so if your patient is on high levels of oxygen, this can let you reduce the FiO2.
Did that make sense? You don’t want to keep a patient on really high levels of oxygen – anything greater than 60% is considered “toxic FiO2”, so if you can do things to reduce that level, you try to do them. (Too much oxygen for too long produces fibrotic lung damage.) PEEP is one of the maneuvers you can make – they call this “trading PEEP for FiO2” – the idea being that as you apply more PEEP, you can reduce the amount of oxygen you have to give through the vent. So PEEP is applied in increments of 2.5 cm, up to about 15 or 17.5. That’s a lot of PEEP. Another way to think of PEEP is as “forward pressure” – the pressure the vent is using to push air into the patient’s lungs. There are other forms of “forward pressure” that are used in different “modes” of ventilation, that we’ll look at below.
13-1- What is a recruitment maneuver?
A creative application of the PEEP concept. Simple idea: you apply a really high level of PEEP in a long steady forward push – 20 seconds maybe, at a level of 30cm. (That’s a whole lot of PEEP.) This hopefully blows open, or pops open all the little collapsed bunches of alveolar grapes that are closed on themselves. Often very helpful, sometimes not.
14- What is ‘pressure-limited’ ventilation?
We looked at this briefly up on page 2. In the olden days, at least back when I started in the units in the early 1980’s, there was only one kind of ventilation – “volume ventilation” – the thought was that you wanted the machine to push a given volume of air into the patient for a tidal volume – say, 500cc. You would dial in a volume of 500cc, and turn on the vent, the old Emerson vents, and the machine would push that volume into that patient, no matter how hard it had to push to do it. So the volume was fixed, and the pressure varied with how stiff or flexible the patient’s lungs were: if they had nice soft normal lungs, only a nice low pressure was needed to push the air volume in. Stiff lungs, and the machine had to push lots harder, and would! The problem was, you can only push so hard through the airways, until – blam!, a pneumothorax. We used to measure the ‘PIP’ – the Peak Inspiratory Pressure – the point at which the machine pushed the hardest, always measuring in cm of water pressure. 40 cm was a lot – 50 was scary, and at 60 you’d get out your chest tube insertion kit and pleurevac and set up, because you knew what was coming – you’d see ARDS patients with six chest tubes. And they almost all died.