Sucking or Blowing – What’s the Difference?

Sometimes it’s worth wondering if the things we hear, see and feel are quite as we thought they were. Dr Alan Garner has a look at your senses when you get into the chest and wonders whether it’s all as straight forward as we like to think? 

Let’s start this post by stating right upfront that this is about chest wounds.  If that is not what you were thinking then time to look elsewhere.

What I want to discuss is the clinical diagnosis of tension pneumothorax in the field.  The reason for the discussion is that I believe it is way over-diagnosed.  When I worked in the UK 6 years ago it seemed tension was being diagnosed frequently and the reason given was the sound as they breached the pleura with the forceps.  As the patient was positive pressure ventilated at the time then the sound must have been air rushing out of the pleural space as their intrathoracic pressure was positive throughout the respiratory cycle right?

Remember how we can’t rely on the sounds involved in clinical examination in the prehospital environment because they’re too unreliable? Well I was being told this one was always right. ‘Always’ is a big word in medicine

I’m also aware of at least one case where a patient with a single epigastric gunshot wound from a low velocity weapon had intubation and then bilateral finger thoracostomies.  The comment at the time was that the prehospital doctor, who no doubt went into it all in good faith, stated that at the time of the thoracostomies they found a pneumothorax on one side and a tension on the other.

However on imaging and surgery the projectile went straight back into the pancreas and nowhere near either hemithorax or the diaphragm.  Indeed the only injuries identified to any part of the chest were the thoracostomy wounds themselves.  Again an intubated patient so the intrathoracic pressure must have been positive right?  If the lung felt down then it had to be a pneumothorax?  And if there was a sound on breaching pleura it must have been a tension?

Clearly in the second case the signs were misleading so what is happening here? Let’s put aside for a second the challenges of the initial diagnosis of pneumothorax and focus on the feel with the finger and the sound to the ears. Could it be that some of the evidence we’ve been lead to believe tells us we’re dealing with a pneumothorax can mislead experienced, well trained clinicians?

Diving In

Perhaps I have done a few more chest drains than most.  Partly that is due to more than 20 years in the prehospital space but I probably did even more when I was a registrar 25 year ago.  I spent 6 months working for a couple of respiratory physicians and I put lots of drains (mainly for malignant effusions) in patients who certainly did not have a pneumothorax before I started.  It was common to hear a noise as the pleura was breached as the air rushed in.  But this of course was in spontaneously ventilating patients and that is different right?

Obviously we need to go back to the physiology to see what is driving the movement of air either into or out of the hole we have made to determine whether the sound we are hearing is air going in, or air going out.

Back to Basics

Transpulmonary pressure is the pressure gradient that drives normal ventilation.  It is the difference between the alveolar pressure and the intrapleural pressure in the lungs.

Ptp = Palv – Pip. Where Ptp is transpulmonary pressure, Palv is alveolar pressure, and Pip is intrapleural pressure.

(If you’d like a little more on this the excellent Life in the Fast Lane has a bit on transpulmonary pressure here.)

Also it turns out that you can get a google preview of John West’s classic textbook on respiratory physiology. Take a moment to go and enjoy Figure 4-9 on page 59. 

You can see from panel B (I meant it, go and have a look) that intrapleural pressure varies between about -5 and -8 cmH2O at the mid-lung level during normal respiration. It is always negative and that’s due to elastic recoil of the lung which is being opposed by the chest wall. It is less negative at the dependent regions of the lung (reducing alveolar size) and more negative at the apex (increasing alveolar size).

Let’s Add Air

In the situation of a small pneumothorax the air in the pleural space makes the intrapleural pressure less negative and the driving pressure difference for ventilation is therefore reduced.  If the pneumothorax is completely open to the air such as with an open thoracostomy wound the intrapleural pressure is equal to atmospheric pressure, the elastic recoil of the lung causes complete collapse and ventilation by chest expansion is impossible – positive airway pressure has to be applied.

It is not the situation of the pneumothorax that particularly concerns me.  If they are hypoxic or hypotensive and the patient has a pneumothorax the chest should be decompressed – a complete no-brainer.  The question is why are good clinicians decompressing normal chests and thinking there was a pneumothorax or even a tension when there was not? Does the physiology lead us there?

Patient One

First let’s consider the non-intubated patient with normal respiration and no pneumothorax.  This is the situation with the patients with malignant effusions I was putting drains in years ago.  Here the alveolar pressure is never more than a cmH2O or two positive or negative.  The intrapleural pressure however is -5 to -8 cmH2O.  Therefore it does not matter what phase of respiration you breach the pleura, the pressure gradient between the pleural space and atmosphere is negative and air will rush in.

The gradient is bigger in inspiration when alveolar pressure is negative (and therefore the total pressure is around -8 cmH2O) and less negative during expiration when it is more like -5 cmH2O.  It is however always negative.  It does not matter which part of the respiratory cycle you breach the pleura, air is going to flow into the pleural space and the elastic recoil of the lung will drive it to collapse.  If you hear a noise as I often did, it is air rushing in, the classic sucking chest wound. An iatrogenic one.

Patient Two

I don’t think anyone would have an issue with things so far.  So let’s move on to the intubated patient who does not have a pneumothorax.  I am going to assume here that there is not a lot of airway resistance in our trauma patient (which is not to say they don’t have underlying obstructive pulmonary disease, anaphylaxis to the induction drugs you gave or a clot sitting in a big bronchus/ETT) as it makes the discussion a bit easier to assume that resistance is minimal (futile according to the Daleks) and the pressure you are seeing on your ventilator gauge is largely transmitted directly to the alveoli.

Looking at our transpulmonary pressure equation, unless the airway pressure and hence alveolar pressure is higher than about 5 cmH2O then the gradient at the time you open the pleura means air is going to enter the pleural cavity.  (If they have significant airway resistance this could happen with much higher airway pressures).

Just have a quick eyeball of this time pressure chart of a standard volume cycled ventilator with no PEEP (and a self-inflating bag will provide a similar though more variable trace).  And I deliberately have no PEEP in this chart.  PEEP is not likely to be the first thing we reach for in the hypotensive trauma patient we have just intubated where we are concerned about the possibility of a pneumothorax.

chart

With normal lungs the peak pressure here is probably about 20 cmH2O.  What proportion of the total respiratory cycle is the airway pressure (and hence the alveolar pressure in our patient with low airway resistance) likely to be below 5 cmH2O?  If your little prehospital ventilator has a roughly 1:2 I:E ratio as most do, then the answer is most of it.

In other words unless you have PEEP of at least 5 cmH2O even in your intubated patient the transpulmonary pressure is negative for a good half of the respiratory cycle.  During at least half the respiratory cycle, if you hear a noise as you breach the pleura you are hearing air rushing IN.

The elastic recoil of the lung is the reason that you feel the lung has collapsed by the time you pull the forceps out and put your finger in unless you have some PEEP in play.

Now I’m not saying there has never been a time when the air wasn’t rushing in. I don’t think much of the word “always” in medicine, remember? I’m just suggesting that what we know of physiology would argue that there is at least a solid proportion of the time where that transpulmonary pressure gradient is negative when you breach the pleura, which means that there’s likely to be a good proportion of cases where those “certain” clinical signs become less reliable.

For a demonstration of this with the mother of all open thoracotomies (in a cadaver) check out this video.

The cadaver is intubated, a “generous” pleural decompression wound has been created, and on each expiration the lung collapses right down unless PEEP is applied.  And note the collapse is complete on each expiration.

As long as the thoracostomy is big enough to freely communicate with the air (and if you are relying on the open “finger” technique rather than putting in a drain it needs to be large or they may re-tension), when you put your finger in during expiration the lung will be collapsed unless there is a reasonable amount of PEEP splinting things open pretty impressively.

It will be collapsed whether it already was before you made the wound or whether it happened as you spread the forceps and made the communicating hole.  The time between making the hole and getting that sense of lung up or lung down with the finger is ample time for the lung to collapse down. It seems like this particular clinical sign probably tells you nothing about the state of play prior to the wound being made.

So noises can be deceptive and feeling a collapsed lung just means that the lung recoiled as the pleura was opened.  Can you even guarantee which phase of the respiratory cycle the patient was in when you made that hole? Unless you had at least 5 cmH2O (and maybe more) PEEP on at the time you breached the pleura neither of these signs necessarily means anything.

allan-henderson
Maybe none of us can trust our big ears?

Now, what?

Again, I’m not really into saying things like “always” or “never”. What I’m suggesting is that there might be a lot more grey around these clinical signs than might first seem to be the case.

So how do you know if they had a pneumothorax? For me that is almost always by ultrasound now.  I don’t know how I managed for 15 of those 20+ years of prehospital care without one.  Sometimes of course the scan is equivocal and you need to make a call based on the signs you see and the condition of the patient but I find this to be very infrequent with a good high frequency linear probe.

And as for tension the hallmark is abnormal physiology, particularly blood pressure.  If decompressing the chest fixes the physiology then they had a tension.  If it does not then they had a simple pneumothorax – or none at all.  Because the noise you heard as you breached the pleura may have been air either entering or leaving the building, hearing a noise does not help you either way.  Was Elvis ever in the building at all?

 

Notes:

I had the brilliant Dr Blair Munford review a heap of the physiology here to make sure it matched up.

After that link to the LITFL bit on transpulmonary pressure again? Then go right here.

And John West’s masterpiece (well at least the page mentioned) is here.

That image of Nahni with the big ears was posted to the Creative Commons part of flickr by Allan Henderson and is unaltered here.

Oh, and in case you didn’t know the truly amazing John West, Adelaide boy made good, has recorded his whole lecture series for you to go and watch. Because when you’re in your 80s you’ll probably be contributing to medical education like that too, right?

6 thoughts on “Sucking or Blowing – What’s the Difference?”

  1. Dear Doctor Garner,

    Thank you very much for sharing this very interesting piece.

    I am commenting as a UK student paramedic nearing graduation.

    As you worked in the UK you may be aware that paramedics here are trained to perform thoracocentisis using a 14G IV cannula inserted into the 2nd ICS miclavicularly (often called ‘needle thoracostomy’ or ‘needle chest’), if a tension pneumothorax is suspected. One of the often heard tips we receive during our training is that hearing a rush of air confirms the diagnosis of tension pneumothorax.

    Do you believe the principle you describe in your article may apply to this procedure as well? Is an air rush an equally unreliable sign in cannula thoracocentisis?

    I look forward to reading more of your work.

    Most sincerely,

    Ben Stevens

    Like

    1. Hi Ben

      It may hiss if it was under tension when you insert the needle but it is probably not a guarantee unfortunately.

      If you insert a needle in a positive pressure ventilated patient at the end of inspiration where there is no pneumothorax the needle will enter the lung parenchyma where there may be a pressure of 20cm or so of water. I think it is possible that you could hear a hiss with this kind of pressure gradient. The hiss should occur with each inspiration though which would be a clue the needle is in the lung rather than pleural space.

      Like

  2. Thanks for the thought provoking dogmalysis. As you say, it was probably a tension if the physiology improved following decompression, and the ‘lung up’ vs ‘lung down’ comment is probably not that useful. In our service a PEEP valve is fitted as a default to the self-inflating bag, which may increase the relevance of the finding, but may not.

    In an otherwise excellent post, I have to strongly take issue with one of your points which undermines the credibility of your case. ‘Resistance is futile’ is a statement commonly made by the Borg in Star Trek, not the Daleks in Doctor Who. Hence only 9/10.

    Cliff

    Like

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s