MEDizzy - A healthy 65-year-old man with a tidal volume (VT) of 0.45 L

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Respiratory Physiology

A healthy 65-year-old man with a tidal volume (VT) of 0.45 L has a breathing frequency of 16 breaths/min. His arterial PCO2 is 41 mm Hg, and the PCO2 of his expired air is 35 mm Hg. What is his alveolar ventilation?

ExplanationAlveolar ventilation is the difference between tidal volume (VT) and dead space multiplied by breathing frequency. VT and breathing frequency are given, but dead space must be calculated. Dead space is VT multiplied by the difference between arterial PCO2 and expired PCO2 divided by arterial PCO2. Thus, dead space : 0.45 × (41 − 35/41) : 0.066 L. Alveolar ventilation is then calculated as (0.45 L − 0.066 L) × 16 breaths/min : 6.14 L/min.