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COMPLIANCE OF THE RESPIRATORY SYSTEM
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Compliance is a mechanical property used to describe the elastic behavior of the lung, chest wall, or respiratory system as a whole. It is defined as a change in volume (ΔV) divided by the change in pressure (ΔP) needed to cause the change in volume, and can be expressed mathematically as:
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If a patient who is breathing spontaneously inhales 500 mL of air and has an intrapleural pressure of −5 cm H2O prior to inhalation and an intrapleural pressure of −10 cm H2O following inhalation, the compliance of the respiratory system (both lung and chest wall) can be calculated as follows:
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It should be noted that transpulmonary pressure (intrapleural pressure minus alveolar pressure) is generally used to calculate total pulmonary compliance during spontaneous ventilation. Intrapleural pressure was used in this example because alveolar pressure remains constant during spontaneous ventilation without obstruction of the airway.
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Lung compliance in a healthy adult is normally 150-200 ml × cm H2O−1, chest wall compliance in a healthy adult is normally 200 ml × cm H2O−1, and total compliance of the chest wall and lungs together is normally 100 ml × cm H2O−1. The relationship between separate lung and chest wall compliances and total respiratory compliance can be expressed as:
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Compliance is the inverse of elastance. If a lung has high elastance, it will by definition have a low compliance. It can also be helpful to think of compliance as the inverse of “stiffness.” A “stiff” tissue will have a low compliance.
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STATIC AND DYNAMIC COMPLIANCE
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In anesthesiology practice, the concept of compliance is most often encountered during positive pressure ventilation of a patient where information provided by the ventilator can be used to calculate compliances. Static compliance is defined as pulmonary compliance without the presence of gas flow. The plateau pressure during an inspiratory hold maneuver minus the peak end-expiratory pressure (PEEP) can be used as the ΔP to calculate static compliance. Dynamic compliance is defined as pulmonary compliance during gas flow. The peak inspiratory pressure (PIP) minus PEEP can be used as the ΔP to calculate dynamic compliance.
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Compliance is affected by the factors listed in Table 136-1.
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