Figure 1.

Flow diagram of literature search and selection processes.

COPD, chronic obstructive pulmonary disease; RCT, randomized controlled trial.

^aTwo articles enrolled the same groups of patients but with different follow-up periods,^18,19 which were treated as 1 RCT,¹⁹ while one article provided three different comparisons,¹³ resulting in 3 separate RCTs, 15 RCTs in total were therefore included.