How to assimilate information accurately and why aortic dissections and PEs get missed- the pretest probability

Medical school and medical training teaches us that we do tests to confirm the presence or absence of disease. This is the wrong way to think about things. A better concept is to realise that we start with a certain pre-test probability of a disease, which is determined by the base rates of that disease in the population and the patient’s clinical history. Tests can only ever modify this pre-test probability into becoming more or less likely. At a certain point the disease may become so unlikely that testing for it causes more harm than good. This greater harm may come from radiation, reactions to things such as contrast dyes, harmful therapy that might be initiated as a result of a false positive result e.g. antibiotics for a blood culture result that is a contaminant, or simply the fact that time is wasted not pursuing the most likely diagnosis. Other times the disease remains so likely that you may have to pursue repeat testing (take for example the high false-negative rate of COVID swabs).

Consider this scenario. You are the on call house officer. You get paged to the ward to review a 35 year old patient who is having abdominal pain. He was admitted 6 hours ago with severe central chest pain that came on over a matter of seconds and lasted 2 hours. His troponins and ECG have been normal. He has now developed abdominal pain of the same severity and also reaching its peak over a matter of seconds. Concerned about the possibility of aortic dissection you look for mediastinal widening on the chest Xray, pulse defecits, or any neurological symptoms as you know these are the things to look for in a dissection. None of these things are present. Satisfied, you order further ECGs and troponins. The next day you find out he died overnight of an aortic dissection. The next day your consultant tells you “it just shows you how useless clinical exam findings are for aortic dissection- you can’t rely on them. Most dissections have a normal Xray!”

Is this correct? Are these clinical exam findings useless? Is the chest Xray normal in most dissections, as commonly quoted? Well, not quite. They are actually reasonably good tests, including the chest Xray (1,2). The problem is not taking into account the pre-test probability of an aortic dissection, which in this case is high based on the clinical history. Continue reading

The test is not the disease

When I was a trainee intern we had a patient on my general medical placement present with 2 days of right arm swelling and tenderness, with dilated superficial veins over her arm and upper chest. Her d-dimer was normal. She had an ultrasound of the upper limbs looking for a DVT. This was negative. With a negative d-dimer and USS we were all ready to discharge the patient home (who was otherwise well), however the consultant, a mentor of mine, insisted on a CT venogram. We all rolled our eyes. Eye rolling turned into eye widening as the scan showed the subclavian vein thrombosis we had all been missing. Continue reading

Why does hypoxia happen?

It is hard to avoid anything covid related in medicine at the moment. This pesky virus does raise an important point about pathophysiology however. There has been a collective surprise at the degree of hypoxia these patients can have despite a chest X-ray that doesn’t look that bad and at how comfortable these patients can appear despite their severe hypoxia.
Except there is nothing particularly new about this and it has long been a pitfall for the intern seeing hypoxic patients on the ward. Often one comes across a patient with post operative atelectasis (or viral pneumonia!) who has some opacities on chest X-ray (such as the one pictured above) but a degree of hypoxia seemingly unexplained by the chest X-ray. These patients are often sent for unnecessary CTPAs looking for PE.
Firstly, the sensitivity of chest X-ray for pneumonia and atelectasis is not as great as we think it is. Changes to the lung parenchyma are often more extensive than what is visualized on X-ray. Secondly the hypoxia is occurring on a microscopic level. Hypoxia occurs when there is mismatch between ventilation and perfusion i.e. blood flow to an alveolus is in excess of the lowered oxygen tension in that diseased alveolus. The larger volume of hypoxic blood that mixes with ‘good blood’, the worse it is. This is called shunting. Hypoxic pulmonary vasoconstriction is the reflex that protects against this mismatch but this reflex becomes less efficient with age, the presence of vasodilator drugs (pretty much antihypertensive or antianginal drug you care to mention), and a whole host of other physiological factors some of which may be specific to the disease itself.
Unwell septic patients with their high cardiac outputs will have a large volume of blood rushing through their pulmonary circulation which further decreases ventilation perfusion matching. The end result is that the degree of hypoxia is related much more to things occurring at microscopic level that we can’t see on an X-ray (on top of the fact that X-ray doesn’t tell us the true extent of parenchymal changes anyway). It is not uncommon to have post operative patients with a chest X-ray that looks like the one above but that are on 50-60% inspired oxygen.
These patients are tachypneic (because hypoxia contributes to the ventilatory drive) but they may not appear overly distressed because the work of breathing is more mediated by lung mechanics and the stiffness of the lung rather than the degree of hypoxia. If you’ve ever seen congenital cardiac babies with right to left cardiac shunts and resting saturations of 75% you’ll know what I mean. They can look blue but pretty happy.
So if you have a good reason for shunting (a high pre-test probability) such as being immediately postoperative or having a diagnosis of a viral pneumonia, that probability remains high despite what the chest X-ray might show you, and there is not necessarily a reason to go chasing a PE or invoking the presence of an unknown hemoglobinopathy (as many people are speculating with Covid). Of course, this doesn’t mean that looking for a PE is never indicated and clinical judgement in the individual situation is paramount. But you have to evaluate the patient in front of you, and not a computer screen, and if you understand the pathophysiology you’ll have a much better chance of doing this.
Till next time…

Why is hypoxia not part of the Wells Criteria?

Whether you have done medical or surgical runs you will have spent plenty of time trying to figure out whether a patient has a pulmonary embolism. Many clinicians will hang their hat on the presence or absence of hypoxia. You may then wonder why hypoxia is not actually part of the esteemed Wells criteria.

Well, it turns out the presence of hypoxia in PE is quite variable. In fact, not uncommonly patients with massive PE may have normal oxygen saturations, a phenomenon confirmed both by the literature (1) and my own observations. To understand why, we have to understand why hypoxia might occur in the first place.

It has actually taken a while for people to figure out why hypoxia occurs in PE, although it is still not 100% transparent. Many people assume that the problem is V (ventilation)/Q (perfusion) mismatching, where Q is decreased due to the obstruction. This is not quite correct. V > Q results in hypercarbia, but not hypoxia. The problem is that Q is redistributed to other lung units, without a matching rise in V. This results in regions of lung with low V/Q, away from the embolism (2,3). This seems to be the most likely cause of hypoxia.

This explains why massive pulmonary embolism may not cause hypoxia. Remember that massive pulmonary embolism is defined by the presence of RV strain and cardiogenic shock. If most of the pulmonary arterial tree is obstructed there is nowhere for the blood to be redistributed, minimizing the ability for areas of low V/Q to form. Additionally, if the patient has cardiogenic shock, low cardiac output reduces Q, reducing the inequality. Therefore, paradoxically, improving oxygen saturations may be a sign of worsening shock (4).

Hypoxia is therefore not correlated with the severity of pulmonary embolism. Patients with severe PE may not be hypoxic. If your patient appears shocked, or just looks terrible, you cannot use the absence of hypoxia to rule out PE.

On the other hand, small PEs may also not cause hypoxia, if they are too small for significant redistribution of pulmonary blood flow to occur.

All of this leads onto the next point, which is the utility of ABGs when you suspect PE. No doubt at some point you will have been asked to obtain an ABG in a patient where PE is suspected. The origins of this were some small, flawed studies suggesting that a normal A-a gradient on an ABG could rule out PE in combination with other features. This has been proven false in a more rigorous study (5) – a normal A-a gradient is equally likely in patients with or without PE initially suspected of having PE. This study concluded that ABGs had limited diagnostic value in the investigation of PE. Hopefully now you understand why.

Additionally most of these ABGs are requested on patients in whom it is clearly obvious from the end of the bed that there is an elevated A-a gradient. If you are on 5L of oxygen to maintain normal saturations and there is no clinical reason for hypoventilation, then you will have an elevated A-a gradient.

Till next time…


  1. Intensive Care Medicine. June 1977, Volume 3, Issue 2, pp 77–80| Cite as Massive pulmonary embolism without arterial hypoxaemia Pathophysiology in two cases. F. Jardin, J. Bardet, A. Sanchez, F. Blanchet, J. P. Bourdarias, A. Margairaz.
  2. Pulmonary Circulation. Gas Exchange and Pulmonary Hypertension following Acute Pulmonary Thromboembolism: Has the Emperor Got Some New Clothes Yet? John Y. C. Tsang, James C. Hogg First Published June 1, 2014 Review Article.
  3. Journal of Applied Physiology. Pulmonary embolization causes hypoxemia by redistributing regional blood flow without changing ventilation. William A. Altemeier, H. Thomas Robertson, Steve McKinney, and Robb W. Glenny. 01 DEC 1998
  4. Hemodynamic Factors Influencing Arterial Hypoxemia in Massive Pulmonary Embolism with Circulatory Failure FRAN(OIS JARDIN, M.D., FRANCIS GURDJIAN, M.D., PIERRE DESFONDS, M.D., JEAN-LUC FOUILLADIEU, M.D., AND ANDRI MARGAIRAZ, M.D. Circulation 59, No. 5, 1979.
  5. Diagnostic Value of Arterial Blood Gas Measurement in Suspected Pulmonary Embolism. MARC A. RODGER , MARC CARRIER , GWYNNE N. JONES , PASTEUR RASULI , FRANÇOIS RAYMOND , HELENE DJUNAEDI , and PHILIP S. WELLS.   PubMed: 1111212 Received: April 24, 2000

You’re taking the piss

Urinary tract infections are the scapegoat of the medical world. They make us lose our common sense, because once you find something that is easy to treat, you stop looking for anything else. This is termed satisfaction of search.

Let us look at two examples where a positive urine sample may lead the ward house officer astray, related to the domain of surgery.

Firstly, you have a patient who is POD4 after an anterior resection. You are called because they have become febrile. There is no obvious source on examination. You take cultures, and the mid stream urine comes back with a high number of white cells. You start the patient on cefuroxime for a presumed UTI. This is a frequent occurrence.

Unfortunately this decision neglects the basic rule of general surgery. This rule states that the main differentials in a febrile patient after abdominal surgery are as follows; anastomotic leak, anastomotic leak,  anastomotic leak and also anastomotic leak.

“But the urine is positive!”

Unfortunately pyuria is common in intraabdominal sepsis, presumably due to the infection rubbing up against the wall of the bladder and causing inflammation. The rate of sterile pyuria in appendicitis and diverticulitis for example can be anywhere from 25% to 70-80%, depending what literature you read (1,2).

This phenomenon is not even confined to intra-abdominal infection. 30% of patients presenting with pneumonia, sepsis, intra-abdominal infection, or enteritis have pyuria (3). Of these urine samples, only 30% were culture positive. Note that culture positivity does not imply a UTI- there will be a significant proportion of asymptomatic bacteriuria.

The second situation will be when you are not on general surgery, but convincing the surgical registrar to review your patient who has abdominal pain and a clinical presentation concerning for something surgical.

“But the urine is positive, why don’t you just treat the UTI?”

Take home message? Pyuria is common in patients with other serious sources of infection and you should remind yourself and others of this.

Till next time….

  1. Ther Adv Urol. 2015 Oct; 7(5): 295–298. Sterile pyuria: a forgotten entity. Sanchia Goonewardene and Raj Persad
  2. 09 Sterile Pyuria an Indication of Acute Appendicitis in Children. S. Lewis1, C. St. Laurent1, A. Ruiz-Elizalde1 1University Of Oklahoma College Of Medicine,Oklahoma City, OK, USA
  3. Sterile Pyuria in Patients Admitted to the Hospital With Infections Outside of the Urinary Tract. Jared B. Hooker, MS2, James W. Mold, MD, MPH, and Satish Kumar. JABFM March 2013:97-103


Tricky gut ischemia, the uselessness of lactate, and the importance of clinical suspicion

You are called to see Ms A, an 80 year old woman on the surgical ward, due to worsening abdominal pain and tachycardia. She was admitted 8 hours ago with the same abdominal pain and diarrhea and had a CT abdomen, which the radiology registrar has provisionally reported as showing non-specific pericolonic fat stranding. She has been treated as an infectious colitis. She has a history of ischemic heart disease, atrial fibrillation and claudication. Examination of her abdomen shows diffuse tenderness but no peritonism.

You call the surgical registrar to express your concern this lady might have ischemic gut. He informs you he is reassured by the CT findings, the lack of peritonism and the normal lactate, which you had decided to check because you have recently heard about the association between gut ischemia and elevated lactate. When you arrive at work the next morning you discover that overnight she had become septic, spiked her lactate to 8 and been taken for a laparotomy, where extensively necrotic bowel was found. She was palliated.

Ischemic gut is one of those diagnoses that is always tricky to make, as there is no lab test to help you and the examination findings can be non-specific, although “pain out of proportion to the exam” is what you might find in the textbooks.  Age confers an exponentially increasing risk, and past the age of 75 it becomes more likely than appendicitis or ruptured AAA (1). This is a fact which I certainly hadn’t appreciated and I suspect many people don’t, given the frequency with which we query the latter two on CT requests and the infrequency with which we query ischemic gut.

The first point to make abundantly clear is that peritonism is a late sign of extensive bowel necrosis so is not reassuring. The same applies to the finding of portal venous gas on an abdominal Xray, pictured below (2). The whole point is to diagnose the condition early enough that you can do something about it (either open or endovascular revacularisation). By the time these signs develop, the proverbial train has left the station.

portal venous gas

Continue reading

Supine masking of hypotension


Today’s post regards a common pitfall regarding the assessment of the hypotensive patient. Unfortunately, it will be based on anecdote rather than literature, but hopefully you won’t find the underlying assumptions too controversial.

Often you are called to the bedside of a patient with a hypotensive episode. You arrive to find them flattened on the bed, sometimes in the Trendelenburg position. You take stock of the situation and find the airway to be intact, the patient fully responsive, with warm peripheries and a good strong peripheral pulse. They are breathing comfortably. A blood pressure is taken and this is 110/70. The other vitals are normal as is the rest of the examination.

It is easy to conclude that the blood pressure has normalised and that this was therefore a self- resolving period of hypotension, most likely postural or vasovagal. What is important to remember is that, if you leave at this point, you have left the patient in a highly unnatural position. People don’t live their lives supine, nor indeed in Trendelenburg, and often their previous blood pressure recording taken in hospital will be with them sitting up. It is easy to gloss over this fact as you only arrive after the fact and therefore tend to only ever see the patient supine.

Before being reassured that the blood pressure is normal you must make sure it is normal in the sitting position. If they are sat up and now the blood pressure is 85/40, you clearly have an ongoing problem. Even the most posturally fragile old patients should maintain a normal blood pressure while sitting.

I have certainly ignored my own rule multiple times and it is extremely easy to do so because the human mind tends to only see what is there in front of you. Also, given the multitude of calls for abnormal vital signs, we latch onto any indication that the patient is actually OK and doesn’t need intervention. A few times when I ignored this rule I was later called for sustained hypotension (of course after they had been sat up) which turned out to be proper pathology.

The first of many lessons that you always need to take into account what we have done to the patient when making your assessment!