Assume that the chest wall static compliance is 0.31 liter/cm H_2O and that the
ID: 179021 • Letter: A
Question
Assume that the chest wall static compliance is 0.31 liter/cm H_2O and that the pulmonary static compliance is 0.19 liter/cm H_2O when a ventilator system is at its resting volume (FRC). Evaluate the static compliance for the total respiratory system. See Figure 9.2 of the 4th edition of Webster. Based on a lung circuit model, how would you model the effect of chronic obstructive pulmonary' disease (COPD)? What respiratory' measurements would you perform and what results would you expect? Based on a lung circuit model, how would you model the effect of cystic fibrosis? What respiratory measurements would you perform and what results would you expect? What is the difference between static and dynamic lung compliance?Explanation / Answer
D) Compliance is the ability of lungs to expand is expressed using a measure known as LUNG COMPLIANCE. It also refers to Distensibility of lung.
Static Compliance is the relationship between volume change of lung and transpulmonary pressure change i.e., measured under known static conditions (Zero Airflow). It reflects elastic resistance of the lung and chest wall.
It represents pulmonary compliance during periods without gas flow such as during inspiratory pause. It is calculated by using the formula
Cstat = VT / Pplat - PEEP
Where as
VT = Tidal volume
Pplat = Plateau pressure
PEEP = Post End Expiratory Pressure
Factors affecting Static Compliance are Lung olume, Pulmonary blood volume, Age, Disease, Restriction of chest expansion and Recent ventilatory history.
Dynamic Compliance - Airflow is zero at the point of flow reversal during normal respiratory cycle. Measurements of lung compliance made using these points reflect Dynamic compliance. It reflects airway resistance as well as elastic properties of lung and chestwall.
It represents pulmonary compliance during periods of gas flow such as during active inspiration. It is always less than or equal to static compliance. It is calculated by using the formula
Cdyn = VT / PIP - PEEP
where as,
VT = Tidal volume
PIP = Peak inspiratory pressure
PEEP = Positive End Expiratory Pressure
Factors affecting Dynamic Compliance are Decreased dynamic compliance is seen especially with increased airways resistance. Examples are Asthma, Emphysema etc., Emphysema increases specific lung compliance but decreases dynamic compliance.
Compliance also changes in various disease states. For example, in Fibrosis lungs become stiff making a large pressure necessary to maintain a moderate volume. Such lungs would be considerd Poorly compliant.
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