Hypoxia in Pulmonary Embolus

mechanism hypoxia featured

Hypoxia is almost ubiquitous in PE (In the comments Liam suggests otherwise. I honestly can’t find a clear answer. in PEITHO, a trial of submassive PE, 85% were receiving oxygen suggesting hypoxia. Certainly in the tiny PEs, lots of people don’t have hypoxia. If you have a better answer let us know in the comments). Yet it is not immediately clear why. You might think you know but certainly when I start to think about it too much it all becomes very muddy. This is mainly due to my poor understanding of respiratory physiology no doubt. I’ve tried to correct that somewhat with this post.

My basic thinking has always been. A PE is a big clot in the lung, this means part of the lung doesn’t work right, ipso facto then there must be hypoxia. That gets you through day to day existence in emergency medicine but it’s hardly a detailed description of the problem.

The terminology doesn’t really help here as V/Q mismatch technically seems to mean that there is an imbalance ventilation to perfusion. In the context of PE people state that the hypoxia is due to V/Q mismatch but don’t clearly state if it’s a high or low V/Q state.

The terms shunt and venous admixture are also used with some frequency which has a tendency to confuse the anatomist in me where the I can tell where the superior oblique is and what direction it runs in purely from the name…

I started with Rosen’s 8th Edition. The PE chapter is written of course by Jeff Kline.

A lodged clot can redistribute blood flow to areas of the lung with already high perfusion relative to ventilation and therefore cause more blue blood to pass through the lung without being fully oxygenated. This venous admixture is probably the primary cause of hypoxemia with PE and the increased alveolar-arterial oxygen difference

This actually is a pretty decent explanation of what seems to be happening. PE causes redistribution and instead of the left lung getting 50% of the cardiac output it suddenly gets 80% and the ventilation isn’t sufficient to oxygenate the blood.

But  figured I’d do a bit of further reading just to see what else is out there.

Paper #1

Huet Y, Lemaire F, Brun-Buisson C. Hypoxemia in acute pulmonary embolism. Chest. 88(6):829-36. 1985. [pubmed]


These guys studied a whopping 7 people,  all 1-9 days post PE, but 2 hrs post their formal angio that was used to diagnose it. Most had greater than 50% pulmonary vascular occlusion and most had some CXR changes too (something we don’t see very commonly). They all had Swan-Ganz catheters placed hence all the lovely data they got to play with.

They were all on heparin and were given urokinase after their “gas exchange test”. This involved infusing a bunch of inert gases dissolved in dextrose and measuring lots of things.

They suggest that the hypoxia had different causes depending on the patient and interestingly could be related to their CXR changes.

  • if they had atelectasis on CXR then there hypoxia was from shunt.
  • if the CXR was normal it was due to perfusion of lung units with low V/Q ratios (ie overperfusing a lung unit with no increase in ventilation)


In PE there is a degree of shifting of ventilation away from and around the underperfused lung, this presumably, is the lung autoregulating itself. This shifting is not particularly well done and after a while atelectasis occurs and as a result of that you now have shunt as an additional cause of hypoxia.

They conclude from their data that initial hypoxia is due to V/Q mismatch (in particular ,perfusion of lung units with low V/Q ratios) and later in the disease course it is likely shunting.

Paper #2

Burton GH, Seed WA, Vernon P. Observations on the mechanism of hypoxaemia in acute minor pulmonary embolism. British medical journal (Clinical research ed.). 289(6440):276-9. 1984. [pubmed]

These were probable PEs, all diagnosed on V/Q scans. All were tachy and breathless with normal CXRs and patients were identified through chart review. Mostly post op. They describe them as minor PE but I suspect they were all quite impressive and may well be termed “submassive” in these days of right heart strain and trops…

Most were a week post symptoms and the ABG was taken just after the diagnostic V/Q scan.

They garnered a huge 11 pts. The V/Q all showed reduced perfusion in areas well ventilated (which seems to be the definition of PE on a V/Q scan) and lots of other distal areas that were overperfused comared to how well ventilated they were. The more severe the V/Q scan changes the more severe the ABG abnormalities.

One of the big issues here is that they assume the cardiac output was normal or raised i their calculations but they don;t actually measure it. The prior study did measure CO in their patients and that it was reduced in all patients.

This doesn’t really help much in working out what’s going on to be honest

Paper #3

D’Alonzo GE, Bower JS, DeHart P, Dantzker DR. The mechanisms of abnormal gas exchange in acute massive pulmonary embolism. The American review of respiratory disease. 128(1):170-2. 1983. [pubmed]

I had to work off the abstract here as I couldn’t get full access. They studied two patients here, both with “massive PE” (they don’t provide a definition in the abstract) using the inert gas method and found that shunt was the main issue, not V/Q mismatch . They state that 20% and 39% of blood flow was through unventilated lung. They conclude that shunt is the main issue.

Paper #4

D’Angelo E. Lung mechanics and gas exchange in pulmonary embolism. Haematologica. 82(3):371-4. 1997. [pubmed]

This a review article written by one of the authots above. It’s the best I’ve found and is open access too. Bottom line: shunt and V/Q mismatch are the main causes of hypoxia. (are there any other options???)

It does try and explain why V/Q mismatch causes low O2 – apparently due to the sigmoidal shape of the O2 curve increases in ventilation cannot keep pace with either increased or decreased perfusion. Note this does not apply to CO2 as its curve is linear. This did ring a bell from my ICU reading years ago…

It highlights that some reports have noted bronchoconstriction and airway narrowing is part of PE – though perhaps this is clinically silent for most patients as I can’t say I’ve heard a great deal of wheezing in there.

The atelectasis that occurs (with resulting shunt) could be down to pneumoconstriction from low CO2 or it could be down to humoral mediators from the platelets in the clot surrounding it.



Both shunt (perfusing a totally unventialted lung segment) and V/Q mismatch (poor matching of ventilation to perfusion) are important causes of hypoxia in PE. The shunt is probably the important take away point as we commonly see patients with pleuritic pain for a week with what looks like consolidation/atelectasis on a CXR and we don’t entertain the diagnosis of PE as most of us were brought up believing that we should think of PE in patients with SOB and a clear CXR.

In my search I did find some great videos on basic mechanisms of hypoxia in all conditions which I’ve embedded below.

Any questions, comments, corrections are always welcome.

UPDATE: Martin on twitter noted that shunt is really just V/Q mismatch in the extreme. In particular shunt refers to perfusion of a completely unventilated lung segment. If the segment is perfused and partially ventilated then it is just an area of high V/Q (numerator bigger than the denominator). I hope this doesn’t confuse matters.

Image source: https://commons.wikimedia.org/wiki/File:Saddle_thromboembolus.jpg

Location based decision making

This is something that I’ve noticed happening to me for as long as I’ve been doing emergency medicine (coming up on 10 years now) – that the physical part of the department that you see the patient in has a significant impact on my decision making process. 

A patient with chest pain arrives in resus, the staffing ratio is much higher here. The expected rate of pathology for all staff working in the area is much higher. Before the doctor sees the patient it is likely that the patient will be on a trolley, on a monitor, often an IV cannula is inserted and lab tests are flying through the chute to the lab before anyone has even further assessed the patient. Often the patient is changed to a gown and an ECG will be done.

A second patient with chest pain arrives and is brought to the minors area. There are no cubicles so they sit in a chair awaiting a doctor’s assessment. No further testing or assessment is done. As there are still no cubicles available to assess the patient the doctor apologetically walks the patient to the psychiatry interview room as it is the only free space in the ED with a door that can close to give the patient even the slightest bit of dignity.

I find when I am the doctor in both those situations I make rather different decisions, or at the very least, I feel inclined to make different decisions even if I ultimately don’t do so. The assumption of course is that if the patient is in the minors area then it’s not possible for the patient to have serious pathology and indeed vice versa – if they’re in resus then it’s not possible for them to have minor pathology.

I suspect everyone working in emergency medicine as noticed this to some degree. Hopefully those who are thorough enough will be able to make appropriate and safe decisions (sometimes involving waiting until a cubicle is free and properly exposing and monitoring the patient) no matter what the environment.

However it is a useful reminder, once again, on how overcrowding in the ED is a threat to safe and effective medical care. Ireland has had its own crisis this week – which of course only means that we got in the papers; the ongoing major incident that is the result of the decision to locate all crowding in the ED has been going on for much longer… While people often view ED attendances as simply punters seen by nurses and doctors, there is rarely reflection on the on the fact that working in an overcrowded environment with the compromises it requires exposes patients to the harm of cognitive biases and poor decision making.

[featured image via wikimedia commons]

UPDATE: the good people at Emergency Medicine News have written an article about this for which I did a short email interview along with a few other actual smart people.

Exit Block: What it is and why it’s dangerous.

This a great video from the college on the significance of exit block and the effects it’s having on our patients and staff. This, along with the recruitment crisis is, I think the biggest issue facing emergency medicine in both Ireland and the UK.

Well worth a watch.

[H/T Simon Carley for the orignial tweet where i saw this]

Full Capacity Protocols

[image via NetDance on Flickr. CC License]

Hospitals are busy places. We have no space, no beds, no staff and inevitably less money to make this all happen. This is the situation we have.

We’re fairly pragmatic folks so we find ways to manage the work more efficiently and try and do more as an out patient or involve things like ADPs (accelerated diagnostic protocols)

But when we get slammed and have more admitted patients than you have trolleys to put them on then the system grinds to a halt and you can’t assess treat and admit/discharge anyone new.

In Stony Brook in New York, the hospital (and that’s the important bit, not just the ED) decided that when the ED was choked that they could put some of the stable patients as extras in the hallways of the wards. You can imagine what the ward staff thought of that.

This is a brief paper reviewing their experience.

Viccellio, Asa, Carolyn Santora, Adam J Singer, Henry C Thode, and Mark C Henry. “The Association Between Transfer of Emergency Department Boarders to Inpatient Hallways and Mortality: a 4-Year Experience..” Annals of Emergency Medicine 54, no. 4 (October 2009): 487–491. doi:10.1016/j.annemergmed.2009.03.005. PMID 19345442

This is a review of patient flow effectively and not a trial in any prospective sense. All they wanted to show was that this was happening and what the effects were.

It is not the highest quality science and does not claim to be.


  • 25% of those assigned to a hallway bed actually got a proper bed immediately
  • another 25% got a proper bed within an hour
  • the rest got a proper bed within 8 hrs.


Your hospital probably has more beds than they say they do. Spreading the crowding from one place to the whole hospital spreads the moral and professional responsibility to a hospital wide problem. It’s remarkable how that motivates resources.

Importantly it must be realised that this is no panacea for a poorly run hospital. In fact every time a hospital implements something like this it’s a sign that something is deeply wrong. However it can alleviate a crisis.

The Irish Association has a nice statement on FCPs. And indeed a nice EMJ paper on the same too.

EM docs are more burnt out than most but none of us are great…

The night shift insomnia that leaves me with about 4 hrs sleep a day has given me the chance to catch up with a bit of reading so here’s a paper for you.

This got a very amount of Twitter attention when it came out as it was a bit of a headline grabber:

Shanafelt, Tait D, Sonja Boone, Litjen Tan, Lotte N Dyrbye, Wayne Sotile, Daniel Satele, Colin P West, Jeff Sloan, and Michael R Oreskovich. “Burnout and Satisfaction with Work-Life Balance Among US Physicians Relative to the General US Population..” Archives of Internal Medicine (August 19, 2012): 1–9. doi:10.1001/archinternmed.2012.3199. PMID 22911330

First a quick run through of the study and then some thoughts


  • this was a massive survey of the AMA register of doctors compared with the general population. It was done effectively by mass emailing
  • the survey used the “gold standard” of burnout: the Maslach Burnout Inventory
    • the only problem here is that it’s a bit of a cumbersome tool so they let the docs fill in the whole survey whereas Joe Bloggs only filled in what the authors state are the predictive bits of the survey. They say that doing this has been studied before and is kosher but there you go…


  • only a 26% (7000/27000) response rate in the docs. A response rate of somewhere closer to 70% is considered important as it’s giving a much more representative of the people you’re surveying. If you think about it could be only the pissed off, grumpy docs answering the survey. Or maybe even the opposite and only the calm and cool docs with lots of free time filled it out
  • bottom line was that a lot of docs feel overworked and burnt out. And this is higher than the general population
  • the people with the highest symptoms of burn out were the EM docs. By a clear country mile it seemed. We were much better than the surgeons in terms of work-life balance but despite this we were still burnt out.


I think this is vitally important stuff.

Emergency Medicine is like a puppy – it’s for life not just for Christmas but it seems increasingly both from my own anecdotal experience and now represented in study form in various settings that we’re going to have real difficulty keeping docs in the specialty.

In the US there are comparatively huge numbers of trained Emergency Physicians compared with the UK/Irish model. These guys work shift patterns often for their entire career. They are well paid and work reasonable hours (I was quoted that 30 hrs a week was an average for an EP in the US – can anyone corroborate this?) Despite their resonable work life balance these guys are really burnt out.

Now the UK/Irish model is a service delivered by trainees and non-board certified EPs, (the “sickest looked after by the thickest” as some have joked) these guys are paid less and work more hours than fully trained EPs, of whom we have vanishingly few. Just imagine how much more burn out might apply to those docs who deliver hands on emergency care day in, day out (or night in, night out)…

As I enter my ninth year since graduation from med school with no clear end in sight to my training (largely my own fault I’ll admit) the importance of work-life balance and the threat of burn out becomes more and more apparent. Workforce planning is one of the biggest problems (along with overcrowding) that EM has to face in this part of the world, but if we are to address it in any way we must address sustainability and burn out.


Graham Walker did a survey for EM News on burnout that’s worth a read