We assume that the primary endpoint is the measurement of CO2 over a period of 24 hours and that this will be quantified in the analysis using area under the curve (AUC). If we compare the mean AUC between the diamorphine and fentanyl groups using a two sample t-test (with two-sided significance level 5%) then a study including 40 patients per group would have 80% power to detect a mean AUC difference between groups of 0.634 standard deviations and 90% power to detect a mean AUC difference of and 0.851 standard deviations.