Table 2

Theoretical flow ratios and pressures compared with 18G cannula.

	Internal diameter (mm)	Length (mm)	Calculated flow ratio compared with 18G cannula	Actual flow ratio compared with 18G cannula				Actual flow ratio compared with 18G cannula
	Internal diameter (mm)	Length (mm)	Calculated flow ratio compared with 18G cannula	50 mm Hg	100 mm Hg	150 mm Hg	200 mm Hg	20 mL/min	30 mL/min	40 mL/min	50 mL/min
18G cannula	0.9	45	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0	1.0
20G cannula	0.8	32	0.88	0.91	0.78	0.78	0.77	0.84	0.78	0.79	0.81
5Fr umbilical line	1.372	400	0.61	0.52	0.37	0.37	0.36	0.17	0.15	N/A	N/A
4Fr umbilical line	1.067	400	0.22	0.35	0.25	0.31	0.25	N/A	N/A	N/A	N/A