11 - 15 Physics SBAs for the Primary FRCA
Question 11
You offer Entonox to a patient in the Emergency Department while you get ready to perform an erector spinae block for their rib fracture pain.
Which of the following is false regarding Entonox?
- It is a 50:50 mixture of oxygen and nitrous oxide
- It is produced by bubbling oxygen through liquid nitrous oxide
- The Poynting effect ensures both components remain in their gaseous form
- It is supplied in cylinders with a blue collar
- The component gases risk separating out below -5.5°C
Answer
- It is supplied in cylinders with a blue collar
Entonox is a 50:50 oxygen nitrous mix, stored at 13 700 kPa in its gaseous form.
It is produced by bubbling oxygen through liquid nitrogen, employing the 'Poynting effect' wherein the two gases essentially dissolve in one another, ensuring they remain in their gaseous form.
Below the pseudocritical temperature of -5.5°C, you run the risk of the two gases separating out via a process called lamination. If this happens then the patient will receive just oxygen and no pain relief to begin with, followed by a hypoxic mixture of nitrous oxide as the cylinder empties. Bad news.
This is why entonox cylinders should be stored on their side and above 5°C.