Box plots showing changes of tidal volume during one-hand mask ventilation (MV) and tidal volume during two-hand MV in 20 non–sleep-disordered breathing (SDB) patients with lower apnea hypopnea index (gray boxes) and 20 SDB patients with higher apnea hypopnea index (white boxes). Median values are indicated by horizontal bar within each box; bars above and below each box represent 25th and 75th percentiles, and the end of vertical lines denote minimum and maximum. Note the progressive increase of the tidal volume in both groups, clear differences between the groups, and greater tidal volume by two-hand MV. IBW = ideal body weight.