SEPT 10 Built from the handwritten pages you uploaded on 10 September 2026.
Cannulae & flow physics
Laminar and turbulent flow, Reynolds number, pressure unit conversions — and the cannula flow rates they explain.
IV cannula gauges and flows
Gauge / colour
Ext. diameter · length
Flow (ml/min)
14 G · Orange
2.0 mm · 45 mm
300
16 G · Grey
1.6 mm · 45 mm
150–200
18 G · Green
1.2 mm · 45 mm
75–100
20 G · Pink
1.0 mm · 33 mm
55
22 G · Blue
0.8 mm · 25 mm
25–36
24 G · Yellow
0.7 mm · 19 mm
15
Memorable sequence: 300 · 150 · 75 · 55 · 25 · 15, and the colours Orange, Grey, Green, Pink, Blue, Yellow from largest to smallest. Note the gauge number runs opposite to the diameter.
Why a short wide cannula wins
Flow follows the Hagen–Poiseuille relationship: Q = πΔPr⁴ / 8ηl.
Radius to the fourth power — doubling the radius increases flow sixteen-fold.
Length in the denominator — halving the length doubles the flow. This is exactly why a long central line is a worse resuscitation route than a short peripheral 14 G, despite its larger-sounding calibre.
Raising ΔP with a pressure bag, and reducing viscosity by warming the fluid, both increase flow further.
Laminar versus turbulent flow
Laminar
Turbulent
Occurs at
Low flow, smooth straight tube
High flow, orifices, branches, irregularity
Governed by
Hagen–Poiseuille
Graham's law
Depends on
Viscosity (η)
Density (ρ)
Flow ∝
ΔP (linear)
√ΔP — pressure must quadruple to double flow
Velocity profile
Parabolic — fastest at the centre, twice the mean
Flat, with eddies
Reynolds number
Re = ρvd / η — density × velocity × diameter, over viscosity.
Dimensionless. Below about 2000 flow is laminar; above about 2000 it becomes turbulent.
Clinical consequence: where flow is turbulent — as in a narrowed upper airway — density is what matters, so replacing nitrogen with helium (heliox) reduces resistance and the work of breathing. Heliox has no benefit in small airway disease, where flow is laminar and it is viscosity, not density, that counts.
Pressure unit conversions
1 atmosphere equals
760
mmHg
760
torr
14.7
psi
101.325
kPa
1.013
bar
1033
cmH₂O
Working conversions: 1 kPa ≈ 7.5 mmHg · 1 cmH₂O ≈ 0.74 mmHg · 1 bar ≈ 100 kPa.
Flowmeters and altitude
A rotameter is a variable orifice, constant pressure device. At low flows the annular gap behaves as a tube, so flow is laminar and governed by viscosity. At high flows it behaves as an orifice, so flow is turbulent and governed by density.
That is why rotameters are calibrated for one specific gas and are not interchangeable.
At altitude, atmospheric pressure falls, so gas density falls. A rotameter then under-reads — the actual flow delivered is greater than indicated, because the less dense gas supports the bobbin less well.
Oxygen flush: bypasses the vaporiser and delivers 35–75 L/min of pure oxygen — it dilutes the anaesthetic gases and can cause awareness, as well as barotrauma if used with a closed system.