A reduction in minute volume in an adult is caused by factors that reduce either tidal volume or respiratory rate. Specifically, shallow breathing (which decreases tidal volume) will reduce minute volume. Minute volume is the product of tidal volume and respiratory rate, so a decrease in either of these factors leads to a decreased minute volume. Other factors that can reduce tidal volume include increased airway resistance, decreased lung compliance, and obstructions in the airway. Additionally, during certain conditions such as obesity, mechanical pressure on the diaphragm can reduce tidal volume and thus minute volume. Overall, shallow breathing is a primary factor that reduces minute volume in adults.