What is energy cost of breathing?

Like all activities even breathing would be possible only by spending energy. Normally oxygen cost of breathing in a healthy person at rest is about 2% of total oxygen consumption.  When the lungs are diseased, the oxygen cost of breathing increases as the rate of breathing increases and accessory muscles are used for respiration.  More oxygen requirement for the respiratory muscles may decrease oxygen supply to other organs.  Oxygen deprivation in the tissue level leads to a cyclical increase in rate of breathing. Again respiratory muscles are put into more use, needing more energy or increased energy cost of breathing. The respiratory muscles tire over time heralding the onset of respiratory failure.

Hence significant increase in energy cost of breathing will lead to respiratory failure and tissue hypoxia.