Relationships among various ventilator-controlled (shaded circles) and pulmonary mechanics (unshaded circles) that determine minute ventilation during pressure-limited, time-cycled ventilation. Relationships among various ventilator-controlled (shaded circles) and pulmonary mechanics (unshaded circles) that determine minute ventilation during pressure-limited, time-cycled ventilation. The relationships between the circles joined by solid lines are described by simple mathematical equations. The dashed lines represent relationships that cannot be calculated precisely without considering other variables, such as pulmonary mechanics. Thus, simple mathematical equations determine the time constant of the lungs, the pressure gradient, and the inspiratory time. These, in turn, determine the delivered tidal volume, which when multiplied by the respiratory frequency, gives the minute ventilation. Alveolar ventilation can be calculated from the product of tidal volume and frequency when dead space is subtracted from the former. From Carlo WA, Greenough A, Chatburn RL. Advances in conventional mechanical ventilation. In: Boynton BR, Carlo WA, Jobe AH, eds. New Therapies for Neonatal Respiratory Failure. Boston, Mass: Cambridge University Press; 1994. Waldemar A. Carlo, and Namasivayam Ambalavanan Pediatrics in Review 1999;20:e117-e126 ©1999 by American Academy of Pediatrics