VAQ 2 Answer

VAQ 2 Answer

This is our first “path result” VAQ, and it’s a beauty.  The scenario is made up (skip to the end to see the real case), but the pathology is the same.

Firstly some general concepts on answering path VAQ’s.
1) You must comment on every number.
Yes, every single number requires comment.
This can be tedious, but to leave out “normal” results, or not comment on numbers you don’t think are relevant is not answering the whole question, and is marks down the drain.
2) Quantify the severity of every deranged number
Writing “mild/moderate/severe/life-threatening” as a quantifier of the metabolic result shows that you have perspective and experience, and can help you guage the treatment required for management questions.
Just saying “sodium – low” is something a bus driver could do, as the normal ranges are provided for you. You want to appear smarter than a bus driver when commenting on results
3) Don’t just write “normal” for normal numbers.
You must still interpret what a normal result means in the context of other often very abnormal path results.  The classic is pseudo-hyponatraemia in hyperglycaemia, where the lab Na+ will be in the “normal range”, but when you correct it, it’s high. This applies to other “expected” or “corrected” numbers (eg K+ in acidosis, CO2/HCO3 in various acid/base conditions).

For example, In this question you could make a comment regarding the normal glucose level as follows:
   Glucose – normal – not cause for collapse.
Indicating that hypoglycaemia has been ruled out as a cause for the collapse.

Get the drift?  Doing lots of practice VAQ’s will allow you to build up a bank of “comments for normal results” that will definitely assist you on exam day.

5) Syhthesise at the start of at the end?
Different people have different opinions about this. Providing a synthesized summary statement of the main problem/diagnosis at the start shows that you’ve “picked” the diagnosis, and you may get some credit for this if you run out of time while you’re commenting on every number later.  The problem is often there is a differential diagnosis, or multiple diagnoses, that can’t be summarized into one clear statement.  If you took the approach of commenting on every number as you go, you will have “worked these out on paper”, so even if you run out of time and can’t write a summary statement, the examiners can see your train of thought and calculations, and as you are interpreting as you go, you’ll most likely have listed a differential as you went. This is where practice comes into it, and doing as many path VAQ’s as you can, so you get a feel for which ones are better written in which format.

It’s very easy to get sidetracked on path questions as there’s just so much to write about, and it’s very easy to run overtime.  But one of the GOLDEN VAQ RULES is not to rob Peter to pay Paul.  Regardless of where you are up to, at 7.5 minutes PUT YOUR PEN DOWN and turn to the next question.  You will have achieved the bulk of your marks by this point, and even 30sec-1min extra will leave you less time to do the next question, and potentially destroy your overall mark. Don’t do it.

Alright, enough of that, onto the question…
Apology: I should have deleted the ctCO2(P)c value, as this confuses things a bit. Sorry if it mislead you.

Important: Read the question
READ EVERY WORD OF THE QUESTION, and think laterally about what each term implies:
In this case the salient points are:
38yo = relatively young, but approaching middle age = fitness may be questionable
Male = prone to bravado and acts of relative stupidity
He’s a Doctor = “don’t backchat me, I know boats”

From Melbourne, doing heavy exercise in Darwin = Unacclimatised, from colder climate, in a tropical setting: at risk for heat illness, fluid balance issues, tropical infections.

Onto the results:
In the reading time you will have picked the severe hyponatraemia as the main problem (I hope you did, as it’s underlined for you!), and the most likely cause for the collapse.  You now have a choice: state this at the start, or leave it til you reach it. It’s up to you, but you’ll see that even though it’s 2/3 of the way down, you can score a lot of marks before you “get to the money” by being systematic and commenting on every number.

Use the terms “alkalaemia” or “acidaemia” when describing the pH (not alkalosis/acidosis). In this case there is a mild-moderate alakalemia.
You can “skip” the O2 for now to say that it’s a “respiratory” alkalosis”, due to moderate hypocapnoea pCO2 30.
Now is the “interpret” part. You need to suggest a reason that a hyponatraemic unacclimatised mountain biker would be hyper-ventilating his pCO2 down to 30.
Does he have a co-existing metabolic acidosis causing him to hyperventilate?
Unlikely with normal base excess -0.7
To be sure: Check Corrected Bicarb: In acute respiratory alkalosis, the HCO3 should drop 2 (from 24) for every drop of 10 in the pCO2 (from 40), which in this case it does, which implies adequate compensation and no co-existing metabolic acidosis/alkalosis.
You need to offer a few more options, such as:
CNS disturbance: (what bad things can happen to your brain during heavy exercise?) Eg: Subarachnoid haemorrhage, head injury…
Is he intubated and being hyperventilated?
Could he just be “post exertion” (ie he’s just puffed from over-exertion), could it be because he’s a doctor who’s feeling anxious ending up in an ED as a patient and is quite unwell? Whatever you think you need to make some comment on it.

The more
1) Relevant to the question and
2) Structured
you make these comments the better your answer reads.

Here’s a great link with a summary of the causes of Respiratory Alkalosis.

Low – consistent with venous sample.
What else could you say about this?
Something like: “Regardless, should have supplemental O2 applied”
Showing that you envisage this patient as quite unwell, as O2 is unlikely to do harm.

HCO3 & Base Excess:
You’ve already commented on these with the calculations above, I probably wouldn’t waste time repeating that they’re normal.

Yes it’s normal, so what?
2 approaches:
1) Think: what might you expect in this case?
Haemodilution leading to anaemia – hard to know without a previous Hb, but clearly this isn’t a significant problem.
2) Just make a statement: Anaemia not cause for collapse. Not polycythaemic.

NOW ONTO THE MONEY: Electrolytes
Again, choices:
Comment on each variable, or summarise, as all of the electrolytes & osmolality are low and obviously due to dilution/water excess. Given that the point of the question is a collapse in a young healthy man with a  Na+ of 119, I’d spend some time talking about just this.

Is this mild, moderate, severe or life threatening hyponatraemia?
This is poorly defined, and relates to risk of complications, which is also related to whether it’s acute or chronic.

Here is a link to a great podcast on sodium levels by some eminent EM gurus.  As you’ll hear, this is a contentious issue.

Direct link to download mp3:

Reagrdless, hyponatraemia has a huuuuge DDx, and you need to somehow condense this into your answer:
How can you do this succinctly?
1) Comment on the severity: I’d call this severe (2) Relate it to the question: ie it’s the likely cause of the collapse
3) Provide a differential diagnosis: This is the hard part:
This patient falls into the hypotonic category, which still has a big DDx depending on the fluid status.
What I’d do is mention that it’s likely acute, DDx depends on urine Na+/osmolality and clinical volume status
In this case it’s likely due to H20 Intoxication, compounded by excess sweat loss, so you could say “expect low urine osmolality, low urine Na+”
then mention some relevant negatives, Other causes eg: primary renal disease, large GIT/3rd space loss, Addison’s (K+ being low goes against this) or SIADH trigger = less likely
4) Highlight risks/complications:
High risk for:
Osmotic demyelination/Central Pontine Myelosis if over-corrected
5) Briefly describe treatment
Requires careful correction
Aim: no faster than 10-12 mmol/L/day or 0.5mmol/L/hr.
In acute/exercise induced low Na+: you can break the 0.5mml/L/hr rule, but don’t go more than 10-12mmol/L/day.
ie Only correct faster if the patient is seriously symptomatic at a rate of 1-2 mmol/L/hr (but only do this for 1-2 hours)

Indications for 3% saline:
Focal neurology
ie 100ml over 10 mins – will push Na+ up 2-3mmol/L – usually enough to stop seizure
Stop when Na+ >120
May need another 100ml over the next hour – only if still seizing
Then use:
N.Saline: if hypovolaemic
Fluid-restriction: if oedematous

Practical (non-exam) tip:
In real life, it’s very, very hard to raise the sodium in a nice slow linear fashion.  It tends to jump much quicker than you’d expect, or just not go up at all.  Unless the patient is fitting, go very slowly!

The other electrolytes:
Mild hypokalaemia/hypochloraemia: due to dilution/sweat loss
Likely to be asymptomatic. Check ECG.
Mild hypocalcaemia:
?Albumin – may correct to near normal
Anion gap
Normal – goes with lack of metabolic acidosis
Not cause for collapse
Mild  elevation – likely post exercise
Normal: Unlikley primary renal disease as cause for low Na+

If you didn’t do a summary at the start, you can try & bang out a summary highlighting the main issues, in POINT FORM:
Mild respiratory alkalosis & severe (likely acute) hyponatraemia
Likely due to H20 overload/sweat loss in unacclimatised exercising individual
Requires careful correction, admission (possible HDU), exclusion of other causes, (confirm with Urine Na+/osmolality & clinical evaluation).
Needs CK checked, and ECG.

So there you have it, the answer to VAQ question 2. Not hard to pick the abnormalities, but the essentials to answering this question well are:
1)    Mention all of the numbers and interpret them
2)    Give a succinct, point form answer on a condition with one of the longest differential diagnoses in medicine!
3)    Tie it together with the information in the stem.

Part b: (10%)
Of course the answer to part b) is “NO”. I was clearly off my rocker before going on this ride thinking I could keep up with Will Sargeant (acclimatized RDH FACEM &
endurance athlete extraordinaire) in the searing tropics on my shiny new bike…
The real case:
This is a real venous gas from a 29 year old, breastfeeding (losing fluid) mother of a 6 month old, who had flown to Darwin from Melbourne the previous day. She’d had such a busy day while travelling with her baby that she’d hardly eaten anything (reduced intake), drank a lot of water on the plane (dilution), then on her first morning in Darwin decided to take the baby out for a 2.5 hour walk in the pram, so she drank 2 litres of water before leaving (so she “wouldn’t get dehydrated”), and drank more water while walking, and she unsurprisingly collapsed. Luckliy for her she had no neurological events and was discharged after a brief admission for electrolyte correction, and education on tropical exercise! There but for the grace of God/Allah/Buddha/L.Ron Hubbard go I…

I didn’t specifically use these to prepare this answer, but you really should check them out…

Amazing Talk by Corey Slovis on Electrolyte Abnormalities
You CANNOT get exam answers wrong after you listen to this, this is a MUST LISTEN talk, and will definitely help you with your metabolic/electrolyte answers.

LITFL Article on Exercise Induced Hyponatraemia

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