Pilbeam's Mechanical Ventilation Practice Exam

The time constant in mechanical ventilation represents the time required for the lung volume to reach approximately 63.2% of its change during any given respiratory phase. In this context, when considering how much of the exhalation phase is completed, the time constant helps in understanding the exponential decay of lung volume as it returns to its baseline.

For a time constant of 3 seconds during exhalation, the percentage of lung volume that would be exhaled at the end of one time constant is indeed 63.2%. However, it's important to understand how this continues over subsequent time constants. By the end of the second time constant (6 seconds), approximately 86.5% of the lung volume will be expelled, and by the end of the third time constant (9 seconds), about 95.0% of the lung volume will be exhaled. This illustrates the diminishing returns of exhaled volume as time progresses.

The correct response, 95.0%, reflects the cumulative effect of the time constant over multiple cycles, signifying that after 3 time constants, nearly all of the volume has been exhaled, which closely relates to effective ventilation strategies in practice. In many clinical settings, understanding these percentages helps in