Table 4. Correlation analysis of cut-off pressure with nitrous oxide and air at different time intervals.

Cutt off pressure	Nitrous Oxide		Air
	r	p-value	r	p-value
After 30 minutes	0.2	0.04	-0.06	0.5
After 60 minutes	1	<0.001	0.91	<0.001
After 90 minutes	0.1	0.37	0.02	0.87