Synopsis
Apply ventilation, perfusion, diffusion, compliance and gas-transport principles to calculate physiological values and distinguish mechanisms of hypoxaemia and ventilatory failure.
- Minute ventilation equals tidal volume times respiratory rate, but alveolar ventilation subtracts dead-space volume before multiplying; rapid shallow breathing can therefore waste most ventilation.
- Alveolar carbon dioxide varies inversely with alveolar ventilation when carbon dioxide production is stable, making hypercapnia principally a problem of inadequate effective ventilation.
- Ventilation-perfusion matching is regional: low V/Q lowers oxygenation, high V/Q wastes ventilation, and true shunt responds less to increased inspired oxygen than ordinary V/Q mismatch.
Reasoning priorities
Quantify effective fresh-gas delivery after accounting for physiological dead space.
Use respiratory rate multiplied by tidal volume minus dead-space volume; do not substitute minute ventilation when breath depth changes.
Worked reasoning
A model adult breathes 12 times per minute with tidal volume 500 mL and anatomical dead space 150 mL; carbon dioxide production is stable. Assume alveolar dead space is negligible, so physiological dead space equals the stated anatomical dead space.
- Calculate fresh gas per breath by subtracting dead space from tidal volume: 500 minus 150 equals 350 mL.
- Multiply 350 mL by 12 breaths per minute to obtain 4200 mL/min.
- The final alveolar ventilation is 4.2 L/min, whereas minute ventilation is 6.0 L/min.
- Predict that switching to 30 breaths/min at 200 mL with the same dead space would yield only 1.5 L/min alveolar ventilation despite unchanged 6.0 L/min minute ventilation.
- Verify each unit and use the contrast to confirm why rapid shallow breathing can cause carbon dioxide retention.