It kinda irks me when someone describes a vital sign or lab value as “incompatible with life.”

Here’s a @tweetorial all about the extremes of physiology.

Case #1:
A 10 yo ____ presents with the following vital signs.
T 109F RR 30 HR 300 BP 142/116

Fill in the blank

Answer:

A chicken's "normal" Temp is 103-110F (w/ HR 220-360) & they live up to 11 yrs.
The Hummingbird would be quite bradycardia (“normal" HR 800-1200 when active)
The Desert ant (Cataglyphis bicolor) has a higher temp (up to 122F!) but doesn't live 10 yrs or have that BP

Case #2:
An *arterial* blood gas is obtained from a ___ showing
pH 7.37 / PCO2 50 / PaO2 20 / HCO3 26
(yup it really is arterial)

Fill in the blank

Answer: ABG is from a Weddel seal after holding its breath for >17 minutes!
Climbers on the summit of Mt Everest do get their PaO2 this low but would be hyperventilating therefore hypocarbic & alkalemic
Dogs (esp Spaniels) do get bronchiectasis, but would not be this hypoxemic

Case #3
The lab calls you with the following critical values.
Hematocrit 63.5%
Hemoglobin 23.8 g/dL
MCV 155 fL

You tell them not to worry because the sample came from a ___

Weddel seals have a Hct (60%+) w/ LARGE RBCs (MCV 150+), an adaptation to increase CaO2.

Bats also have Hct (63%) w/ SMALL RBCs (MCV 35), also optimizing for high CaO2.
Camels have "normal" Hct w/ small RBCs (MCV ~30); RBCs are elliptocytes, to lower viscosity in dehydration

Incidentally, there’s a tradeoff b/w RBC number & size (MCV). Mammals tend to have more smaller RBCs relative to birds. If you think an MCV of 155 is big, reptiles have even larger RBCs: The desert tortoise has big RBCs (MCV ~350) and pond turtles RBCs are even bigger (MCV 550).

Case #4
Now the lab is calling about a critical lactate result on an ABG:
pH 7.25 / PCO2 40 / HCO3 8 / Lactate 160 mmol/L
(yes you read that right!)

You reassure them. This is “normal” because the ABG was obtained from ___.

Answer:

C picta bellii (the painted turtle) can hibernate underwater for ~150 days; its lactate rises steadily reaching almost 200 mmol/L!

Olympic rowers can raise their lactate to >25 mmol/L (with pH 6.85)

No one is foolish enough to perform an ABG on a hibernating bear!

There isn’t anything intrinsically bad about lactate, especially when transiently elevated (such with exercise, epinephrine gtt, or seizure). Sustained lactic acidosis is a marker that should prompt you to look for other badness (hypo perfusion, organ ischemia, toxicity, etc).

Case #5
Now the lab is calling because of a critical change in a patient's BUN & creatinine since the last measurement 3 months ago.
3 mo ago BUN 22 ug/dL Cr 1.5 mg/dL
Today BUN 10 ug/dL Cr 3.1 mg/dL

You tell the lab it’s cool because the sample came from a __

Answer:
During hibernation, bears decrease urea & increase Cr

Birds have extremely low Cr; they excrete uric acid & don’t make much creatinine.

Vampire bats do have chronic azotemia (BUN 50+) due to high protein intake after ingesting blood but their Cr remains normal.

Remember that - like vampire bats - people with GI bleed can have markedly elevated BUN/Cr ratio due to absorption of protein from blood in the GI tract. BUN/Cr >36 can suggest an upper GI source in a patient with GIB. https://pubmed.ncbi.nlm.nih.gov/2229992/ 

I hope you’ve enjoyed this

I’d like to acknowledge the comparative vertebrate bio course I took @BrownUniversity, a great lecture by #ErikSwenson @uwpccm, & my 3yo for teaching me about Weddel seals after watching @Octonauts

Finally, I think the Weddel Seal deserves an emoji.