Minute ventilation (Vm), the total amount of new air moved in and out of the airways and lungs each minute, is equal to the tidal volume (V T) multiplied by the respiratory rate (f). The normal Vt is about 7 mL/kg, or 500 mL in an adult, and the normal f is 12 breaths per minute. Therefore, the normal minute ventilatory volume required to maintain a partial pressure of carbon dioxide (P co2) of 40 mmHg averages about 6 L/min. However, the Vm required to maintain a normal Pco2 depends on the amount of CO2 being produced and the amount of dead space in the lung. People who are exercising or patients who are febrile or hypermetabolic have increased CO2 production and thus a Vm greater than 10 to 20 L/min, while patients who are severely hypothermic may have decreased CO 2 production and thus Vm equals 2 L/min. A Vm less than 2 L/min, even in a hypothermic patient, leads to respiratory acidosis. The f occasionally rises to as high as 40 to 50 breaths per minute, and the Vt can become almost as great as the forced vital capacity, which is about 4500 to 5000 mL, or 65 to 70 mL/kg in a young adult male. However, at rapid rates a person usually cannot sustain a Vt greater than 40 percent of the vital capacity for more than a few hours.
Approximately 30% of the air that a person breathes does not participate in the alveolar gas exchange process and is thus called dead space ventilation (V ds). Dead space ventilation is made up of anatomic dead space and alveolar dead space. Anatomic dead space is the volume of air that fills the conducting airways (trachea, bronchi, and bronchioles), while alveolar dead space is the volume of air in alveoli that are not perfused. The combination of alveolar and anatomic dead space is called the physiologic dead space, which in a young male adult with a V T of 500 mL is about 150 mL (about 2 mL/kg of body weight), or about 30% of the Vt.
Dead space can be increased by certain disease states, such as adult respiratory distress syndrome (ARDS), where the physiologic dead space can exceed 60% of the Vt. The increased dead space requires a tremendous increase in the Vm (usually by an increase in the respiratory rate) to prevent the development of respiratory acidosis.
The main function of the pulmonary ventilatory system is to continually renew the air in the alveoli, where it is brought in close proximity to the pulmonary capillary blood. The rate at which new air reaches these areas is called alveolar ventilation (V A), or Vm - Vds. Consider a patient with myasthenia gravis who is developing progressive respiratory failure. The patient's baseline V A is [Vm(500 * 12 = 6 L/min) - Vds(150 * 12 = 1.8 L/min) = Va(4.2 L/min)]. If the patient's Vt is decreased to 250 due to weakness, the patient cannot simply double f but must increase it almost 4 times, to 42 breaths per minute [(250 * 42 = 10.5 L/min) - (150 * 42 = 6.3 L/min) = 4.2 L/min], to maintain the same alveolar ventilation. An increased f increases the patient's work of breathing and may lead to respiratory failure.
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