Disposable ETT cuff manometer

[MSDeltaFlt]

The maximum airflow that can pass through a tube is an inverse function of the radius to the power of four. In English, that means that if you reduce the radius of a tube by half, the flow that can pass through the tube is cut 16 times (1/16). For patients that have respiratory issues to begin with (e.g. COPD), trying to get them to breathe spontaneously through the tube prior to extubation can be a non-starter as the resistance it way too high. I don't know how many times I had to explain to a doctor that if they could breathe through the pea shooter they put in, the patient wouldn't be in the hospital to begin with.

That's what SBT with Pressure Support is for....

Yes, with the advancement of modern microprocessor ventilators, T-pieces are no longer needed prior to extubation. However, with the smaller tubes the PIP's are unnecessarily high and give false readings as to what is going on in the pulmonary tissue. The lower the pressures + the easier the flow = the less resistance = the less chance for barotrauma.

You choose you ETT size just like you choose your IV cath size. Get as big as you can get for very specific reasons.

It is never a "go big or go home" philosophy. It's more like a "if it'll fit it stick it" but just one size smaller that the absolute largest that will fit. After SBT's they sometimes like to deflate the cuff to make sure move enough air around the tube just prior to extubation. If after a few breaths they're still doing good, have them cough as you pull it. But while they're intubated bigger is better.

 

[Brandon O]

However, with the smaller tubes the PIP's are unnecessarily high and give false readings as to what is going on in the pulmonary tissue. The lower the pressures + the easier the flow = the less resistance = the less chance for barotrauma.

I would put this a little differently. Elevated peak pressures due to a small tube have very little relevance to the lungs at all; it is mostly annoying because your alarms will need adjusting (and the same people who get freaked out by high blood pressures will call you a lot). You can force flow through a pretty teeny tube by generating very high pressures, which our vents can do.

However, you can't force the patient to exhale, which is a passive process. So a smaller tube may limit your achievable respiratory rate, lest you cause breath stacking. It also makes pulmonary toilet more difficult (harder to fit down a bronch, more likely to get something gnarly like a mucus plug).

 

[MSDeltaFlt]

Elevated peak pressures due to a small tube have very little relevance to the lungs at all; it is mostly annoying because your alarms will need adjusting (and the same people who get freaked out by high blood pressures will call you a lot). You can force flow through a pretty teeny tube by generating very high pressures, which our vents can do.

You'll have to adjust alarms unnecessarily. You CAN force flow through a teeny tube by generating very high pressures by modifying your vents' flow rates... UNNECESSARILY. Thus masking what is truly going on within the pulmonary system. Increasing flow rates and generating higher PIP's increases barotrauma. Choosing to do this increases ventilator hours and hospital stay when it could have come to that merely by choosing a larger ETT.

An ounce of prevention is worth a pound of cure.

 

[Brandon O]

You'll have to adjust alarms unnecessarily. You CAN force flow through a teeny tube by generating very high pressures by modifying your vents' flow rates... UNNECESSARILY. Thus masking what is truly going on within the pulmonary system. Increasing flow rates and generating higher PIP's increases barotrauma. Choosing to do this increases ventilator hours and hospital stay when it could have come to that merely by choosing a larger ETT.

An ounce of prevention is worth a pound of cure.

Of course you're right that there's no sense in making trouble if it can be avoided. But I'm not sure I agree that elevated peak pressures due to a small tube increase the risk of barotrauma. Those pressures are not occurring in the lungs; they are solely upstream of the tube.

 

[MSDeltaFlt]

But the increased flow does.

Think of it like this. When you wash your car, can you get more dirt off (do more damage to the dirt) with the water gently flowing out of the water hose or when you hold your thumb over the end increasing the back pressure to increased the flow?

Your lungs are just as pink as the inside of your mouth. They don't get that pink by being tough. They get that pink by being that delicate.

 

[Brandon O]

But the increased flow does.

This is an interesting idea, and not one that I've seen supported in the literature (i.e. that higher flows are associated with more lung injury). I suppose one could argue that the more rapidly you expand alveoli, the more damage you cause them, but I don't know. Most "atelectrauma" is probably caused at the moment of recruitment (the shear force induced when you snap them open from fully collapsed), not when you expand them from small to large. If you have some data on this I'd be curious to see it.

But anyway, I think it's immaterial. A smaller tube does not dictate higher flow rates.

- In a volume mode, flow is fixed and set by the user. The vent will generate whatever peak pressure is needed to maintain the desired flow. Higher peaks will be needed to achieve the same flow, if the tube is smaller, but as discussed, this pressure is not reflected in the lungs, so it is mostly immaterial.

- In a pressure mode, flow is dynamic and adjusted by the vent to achieve the target pressure. With a very tight tube, this would actually mean that you will reach your pressure at a LOWER flow (since it's bottlenecking against the small conduit), which I suppose could mean your inspiratory cycle could be delayed and (if inspiratory time is not increased to accommodate it), may mean that flow has not dropped to zero by the end of expiration. This is a bit theoretical as I'm not sure if I've ever seen this happen.

 

[MSDeltaFlt]

https://www.sciencedirect.com/topics/nursing-and-health-professions/peak-inspiratory-pressure

Barotrauma
Elevated peak inspiratory pressures and mean airway pressures have been implicated as being traumatic to the lung parenchyma. High peak inspiratory pressures are associated with pneumothorax, whereas elevated mean airway pressures are associated with pneumothorax and reduction in cardiac output.73 It is not clear whether high peak inspiratory pressures are a primary or secondary phenomenon associated with the generation of pneumothorax. It is possible that nonhomogeneous lung ventilation (areas of poorly ventilated and well-ventilated alveoli in close proximity) results in pressure gradients across the interstitium and alveoli and the potential for rupture. However, it is a common clinical strategy to try to limit peak inspiratory pressure and mean airway pressure as much as possible.

http://journal.chestnet.org/article/S0012-3692(16)34065-X/fulltext

In our study, increased levels of PIP and PEEP were associated with the development of all forms of barotrauma. The association of PIP with PTX has been reported previously, with levels of greater than 35 to 50 cm H2O being associated with higher risk of both PTX and ME.2, 11, 12, 13

http://www.frca.co.uk/Documents/100308 Physics of flowLR.pdf

Ventilation
The principles here are similar to those with the intravenous cannulae. Flow through
a tracheal tube is laminar so the Hagan-Pouseuille formula applies. If a smaller
diameter tracheal tube is used, then flow will be significantly reduced as it is
proportional to the forth power of the diameter, unless the pressure gradient is
increased (changing the tube from an 8mm to a 4mm may reduce flow by up to
sixteen-fold!)

I've been knowing this for almost 30 years. When you use smaller tubes you get used to seeing higher PIP's. These new microprocessor mechanical ventilators may be able to compensate to an extent flows and some PIP's. But increased PIP's are related to barotrauma. And when you have higher PIP's at the onset due to smaller tubes you mask increased PIP's that can occur with barotrauma.

What I'm saying is go big at the beginning. A patient's trach is larger than you might think. And don't get me wrong. I don't have a "Go big or go home" philosophy. I fully understand that you can get what you can get. But what I'm saying is, "If it'll fit it stick it".

If you have an elective intubation set out three ETT's. One you know you can usually get, one you'd like to get, and one a size smaller "just in case" their trachs are smaller than expected.

My personal philosophy is this. If you have to use more than a 5cc syringe to obtain minimal leak then odds are you could have used a larger ETT.

 

[MSDeltaFlt]

This is an interesting idea, and not one that I've seen supported in the literature (i.e. that higher flows are associated with more lung injury). I suppose one could argue that the more rapidly you expand alveoli, the more damage you cause them, but I don't know. Most "atelectrauma" is probably caused at the moment of recruitment (the shear force induced when you snap them open from fully collapsed), not when you expand them from small to large. If you have some data on this I'd be curious to see

I'll grant you that one. The amount of expansion from increased flow rates are a problem.