TABLE 2

Respiratory parameters during the hypercapnic protocol on continuous positive airway pressure (CPAP)

	Placebo			Morphine
	Baseline^#	+5 mmHg^¶	+7.5 mmHg⁺	Baseline^#	+5 mmHg^¶	+7.5 mmHg⁺
CO₂ change from baseline mmHg		4.87±0.53	8.03±0.80		5.33±0.29	7.95±0.51
PIF L·s⁻¹	0.57±0.04	0.68±0.04	0.76±0.05	0.62±0.03	0.68±0.03	0.70±0.04
V_T L	0.50±0.02	0.55±0.02	0.56±0.03	0.50±0.01	0.53±0.02	0.51±0.03
f_R breaths/min	13.0±0.6	14.1±0.4	15.2±0.6	11.1±0.5^§	12.3±0.5^§	13.0±0.7^§
t_I s	2.08±0.10	1.94±0.07	1.84±0.09	1.94±0.08^§	1.85±0.06^§	1.77±0.08^§
t_E s	2.69 ±0.14	2.39±0.06	2.20±0.10	3.57±0.18^§	3.26±0.15^§	3.09±0.17^§

Data are presented as mean±sem. CO₂: carbon dioxide; PIF: peak inspiratory flow; V_T: tidal volume; f_R: respiratory frequency; t_I: inspiratory time; t_E: expiratory time. ^#: therapeutic CPAP during non-rapid eye movement sleep; ^¶: 5 mmHg increase in end-tidal CO₂ above baseline condition; ⁺: 7.5 mmHg increase in end-tidal CO₂ above baseline condition; ^§: significant difference compared to the equivalent condition during placebo. Refer to the text for overall changes in respiratory parameters with increasing CO₂ levels.