Which of the following statement(s) is/are true concerning CO2 transfer in the lung?

a. Carbon dioxide excretion is a direct function of alveolar ventilation

b. Normally end tidal CO2 should be identical to PaCO2

c. The gradient between end tidal and arterial CO2 can be an indirect measure of nonperfused alveoli

 The amount of carbon dioxide excretion is directly related to alveolar ventilation. While oxygenation is a function of matching blood flow to alveoli, carbon dioxide excretion is a direct function of ventilation or hyperventilation of alveoli with some blood flow. Normally the end tidal CO2 represents mixed alveolar gas which is at equilibrium with pulmonary capillary blood, hence with arterial blood. Therefore, the end tidal CO2 and the PaCO2 should be identical. End tidal CO2 measurement is a very useful continuous measurement of PaCO2 which can be used as a monitor when the lung is normal, as in ventilator weaning. Furthermore, the gradient between end tidal and arterial CO2, when it is large, acts as an indirect measure of nonperfused alveoli and/or compression volume. In patients who are ventilated with positive pressure ventilation, a significant end tidal PaCO2 gradient occurs only when peak airway pressures are very high (over 30 cm H2O) and the compression volume is a significant component of each exhaled breath.