31: Compliance and Resistance Measurement

Published on 06/03/2015 by admin

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Last modified 06/03/2015

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PROCEDURE 31

Compliance and Resistance Measurement

PURPOSE:

Clinical measurements of compliance and resistance are performed to assess trends in respiratory status, determine the effectiveness of therapy, and titrate therapy.

PREREQUISITE NURSING KNOWLEDGE

• Compliance is a measure of lung (and chest wall) distensibility. Conditions that decrease compliance include acute lung injury (ALI), acute respiratory distress syndrome (ARDS), pulmonary edema, atelectasis, pneumonia, obesity, pulmonary fibrosis, and kyphoscoliosis. Compliance increases with emphysema.2,5

• Resistance is a measure of how easily gases move down the airways. Conditions that adversely affect resistance include bronchospasm, secretions, and endotracheal tube size.2,5

• Compliance and resistance are reflected in the patient on mechanical ventilation by changes in peak inspiratory and plateau pressures (i.e., volume modes) and by changes in volume (i.e., pressure modes). With monitoring of changes in volume per unit change in pressure (mL/cm H2O), trends can be measured and therapies adjusted.2.5

• Although spirometry or plethysmography are required for the exact measurement of airway flow resistance and lung compliance, two clinical measurements are frequently used to estimate the contributions of each in a patient on mechanical ventilation: dynamic compliance (Cdyn), which is more accurately called dynamic characteristic, and static compliance (Cstat).

image The measurements of Cdyn and Cstat are obtained while the patient is on a volume mode of ventilation. Cdyn requires that the delivered volume be divided by the peak inspiratory pressure (PIP) minus positive end-expiratory pressure (PEEP). PIP reflects both the contribution of airway resistance (how easily gases flow down the airways) and lung compliance (dispensability of the lung). Thus the measurement of Cdyn, which does not separate resistance and compliance is a measure of the overall state of the lung (inclusive of compliance and resistance). For this reason, dynamic characteristic is a more accurate term for this measurement than is dynamic compliance.2,3,5

image Cstat is measured during a breath-hold maneuver (i.e., end inspiration). By stopping gas flow, the pressure in the system equilibrates, and the resultant pressure reflects the pressure required to distend the lungs separate from the pressure needed to move gases down the airways. The pressure measured during the breath hold is called static pressure (also called plateau, alveolar, or distending pressure). By also subtracting PEEP, this number becomes the denominator for the calculation of Cstat (i.e., tidal volume ÷ [plateau pressure − PEEP]). The normal gradient between PIP and plateau pressure is 10 to 15 cm H2

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