Why the pressure limit matters far more than the volume, the seal pressures that decide whether you can ventilate, and the nitrous oxide problem.
The pressure rule
The manufacturer's maximum volume is routinely mistaken for a target. It is a ceiling, not a goal.
60 cmH₂O
absolute maximum, any device
40–44 cmH₂O
safer working ceiling
34 cmH₂O
pharyngeal mucosal perfusion pressure
50–70%
of max volume usually seals
Why 34 cmH₂O is the number that matters
Pharyngeal mucosal perfusion pressure is roughly 34 cmH₂O. Sustained intracuff pressure above that causes mucosal ischaemia — which is the whole mechanism behind the complications below.
Hence the universal ceiling of 60 cmH₂O, and the argument from many authors for keeping to 40–44 cmH₂O in practice.
Inflating to the stated maximum often makes the seal WORSE, by tenting the cuff away from the periglottic tissues. Most devices seal adequately at 50–70% of the maximum volume.
Maximum cuff volumes by size
These are maxima, not targets. Inflate to a manometer-verified pressure, not to a number of millilitres.
Size
Patient weight
Classic / Unique / Flexible
ProSeal
Supreme
1
< 5 kg
4 mL
4 mL
5 mL
1.5
5–10 kg
7 mL
7 mL
8 mL
2
10–20 kg
10 mL
10 mL
12 mL
2.5
20–30 kg
14 mL
14 mL
20 mL
3
30–50 kg
20 mL
20 mL
30 mL
4
50–70 kg
30 mL
30 mL
45 mL
5
70–100 kg
40 mL
40 mL
45 mL
6
> 100 kg
50 mL
—
—
The devices with nothing to inflate
The i-gel has no inflatable cuff at all — a thermoplastic elastomer bowl that softens and moulds at body temperature. Nothing to inflate, nothing to monitor, and no nitrous oxide problem.
The same applies to the SLIPA and similar non-inflatable designs.
Oropharyngeal leak (seal) pressures
This — not cuff volume — is what determines whether positive pressure ventilation is feasible.
Device
Typical seal pressure
Notes
Classic / Unique
~20 cmH₂O
Lowest of the group
Supreme
~24–28 cmH₂O
Integral drain tube, single use
i-gel
~24–30 cmH₂O
Non-inflatable, with gastric channel
ProSeal
~30–32 cmH₂O
Highest seal; drain tube for gastric access
Second-generation devices
ProSeal, Supreme and i-gel all have a gastric drain channel — the defining feature of a second-generation device. This reduces, but does not eliminate, aspiration risk.
Their higher seal pressures are what make controlled ventilation practical, whereas the Classic at ~20 cmH₂O leaks readily.
Practical points
Use a manometer. Estimating by pilot-balloon palpation is unreliable and consistently overshoots — studies repeatedly find intracuff pressures well above 100 cmH₂O when clinicians inflate by feel.
Nitrous oxide diffuses INTO the cuff. With N₂O in the fresh gas flow, intracuff pressure rises steadily through the case and can double within 30 minutes. Either recheck and vent periodically, or inflate the cuff with the anaesthetic gas mixture rather than air.
Consequences of overinflation: sore throat, dysphagia and hoarseness commonly; and less often but more seriously, lingual, hypoglossal or recurrent laryngeal nerve injury.
Sizing errors compound the problem. A device that is too small gets over-inflated to compensate for a poor seal — the worst combination. If you need the maximum volume to achieve a seal, go up a size instead.
Same principle, different tube
The identical logic applies to tracheal tube cuffs: keep below tracheal mucosal perfusion pressure, conventionally 20–30 cmH₂O, and remember that nitrous oxide diffuses into those cuffs too.