Ultimately, effective ventilation
is measured by the partial pressure
of CO2 in arterial blood (PaCO2).
All expired CO2 comes from alveolar
gas. During normal breathing,
the breathing rate or depth adjusts
to maintain a steady PaCO2 between
35 and 45 mm Hg. Hyperventilation
manifests as a low
PaCO2; hypoventilation, as a hig
hPaCO2. During exercise or certain
disease states, increasing breathing
depth is far more effective than increasing
the RR in improving alveolar
ventilation.