Fig. 4. Illustration of the oral mask, nasal mask, and application of both masks and their interface with the two noninvasive cardiac output monitors. One noninvasive cardiac output monitor was placed between the nasal mask and the breathing circuit (flowmeter 1), and the other was placed between the oral mask and the breathing circuit (flowmeter 2).  A and  B are the photos of the oral and nasal masks, respectively.  C shows the application of the nasal and oral masks held by two separate straps. Reprinted with permission from Liang Y  et al. , Anesthesiology 2008; 108:998–1003.