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

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

Answer: (D)

Minute volume is how much air moves in and out in one minute, and it depends on two things: how much air you take in with each breath (tidal volume) and how many breaths you take per minute (respiratory rate). If tidal volume is reduced, you’re delivering less air per breath, so even with a normal or even slightly reduced rate, the total air moved each minute drops. That’s why shallow breathing causes a decreased minute volume—the tidal volume is small, so the product of tidal volume and respiratory rate falls.

Increasing tidal volume would boost minute volume, not decrease it. A respiratory rate of 20 breaths per minute is normal for many adults and doesn’t by itself reduce minute volume. A slight decrease in rate could lower minute volume, but the most direct and impactful way to reduce it among the options is the shallow breathing, which minimizes the air per breath.