Which of the following factors will cause a reduction in minute volume in an adult?

• A. Increased tidal volume
• B. Shallow breathing
• C. Respirations of 20 breaths/min
• D. Slight increase in respiratory rate

Answer: (B)

Minute volume equals tidal volume times respiratory rate. A drop in minute volume happens when either the amount of air moved per breath (tidal volume) decreases or the number of breaths per minute decreases. Shallow breathing lowers tidal volume, so less air is moved each breath and the total air moved per minute falls. The other patterns—increasing tidal volume, breathing at a normal-to-high rate, or even a slight rate increase—tend to maintain or raise minute volume if tidal volume stays about the same. So shallow breathing best explains the reduction in minute volume.