Respiratory rate (RR): number of breaths per minute
Tidal volume (VT): the volume of air that is inspired or expired in a single breath
Minute ventilation (VE)
Volume of air that a person breathes per minute
VE = VT x RR
Physiologic dead space (VD): volume of inspired air that does not participate in gas exchange
VD is the sum of the anatomic dead space and the alveolar dead space.
Anatomic dead space: the volume of air in the conducting zone, e.g., mouth, trachea (approx. ⅓ of the resting tidal volume)
Alveolar dead space: the sum of the volumes of alveoli that do not participate in gas exchange (mainly apex of the lungs); These alveoli are ventilated but not perfused.
Bohr equation determines the physiologic dead space: VD = VT x (PaCO2 - PeCO2)/(PaCO2)
In a healthy lung, VD equals the anatomic dead space (normal value: approx. 150 mL/breath).
Alveolar ventilation (VA)
Volume of gas that reaches the alveoli each minute
VA = (VT - VD) x RR
Positive end-expiratory pressure (PEEP)
Definition
Positive pressure in the lung maintained over the entire inhalation and expiration phases
Mechanism of action: keeps airway pressure above the atmospheric level at the end of exhalation → increases alveolar pressure and volume → reopens collapsed or unstable alveoli → improves ventilation/perfusion relation
Indications: PEEP is typically justified when a PaO2 of 60 mm Hg cannot be achieved with an FiO2 of 60%.
Almost all patients need a minimum amount of PEEP (3–5 cm H2O).
Higher PEEP is indicated in conditions involving a right-to-left pulmonary shunt pathology.
