Alveolar ventilation accounts for dead space when relating tidal volume to respiratory rate. Rapid, shallow breathing can reduce gas exchange even when minute ventilation appears unchanged.
VA = (VT - VD) × RR
Hypoventilation can raise PaCO₂ and cause respiratory acidosis.
Gas exchange and the oxygen dissociation curve
O₂ diffuses from alveoli to blood, while CO₂ moves from blood to alveoli. Lower pH, higher CO₂, and higher temperature shift the oxygen dissociation curve to the right.
Right shift: oxygen is released to tissues more easily.
Left shift: hemoglobin holds oxygen more tightly.
Causes of hypoxemia
Compare V/Q mismatch, diffusion defect, shunt, and hypoventilation using the A–a gradient and response to supplemental oxygen.
V/Q mismatch: increased A–a gradient and usually improves with oxygen.
Shunt: PaO₂ improves only slightly despite oxygen.
Hypoventilation: PaCO₂ rises while the A–a gradient may stay normal.
Obstructive ventilatory defect
Asthma and COPD characteristically reduce FEV₁/FVC. Expiration becomes prolonged, air trapping develops, and residual volume can increase.
Chronic bronchitis: productive cough for at least 3 months in 2 consecutive years.