Respiratory dead space is that portion of the respiratory system not involved in gas exchange. The percent of tidal volume involved in ventilation of dead space is a measure of respiratory efficiency.
Bohr equation of dead space
(dead space volume) / (tidal volume) =
= ((PACO2) - (PCO2 of expiratory gas sample)) / (PACO2)
Since PaCO2 approximates PACO2, this becomes:
Physiologic dead space
(dead space volume) / (tidal volume) =
= ((PaCO2) - (PCO2 of expiratory gas sample)) / (PaCO2)
Interpretation:
• The normal ((dead space volume) / (tidal volume)) ratio is 0.2 - 0.3.
• Increased dead space ventilation can be seen in patients with ventilation of distal air spaces with nonfunctioning alveoli or in patients on ventilators with high mean airway pressures
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