Question

Explain static and dynamic lung compliance related to mechanical ventilation

Answer 1

The compliance of a system is defined as the change in volume that occurs per unit change in the pressure of the system. In layman terms, compliance is the ease with which an elastic structure can be stretched. Compliance is, therefore, basically a measurement of the elastic resistance of a system. Pulmonary compliance (C) is the total compliance of both lungs, measuring the extent to which the lungs will expand (change in volume of lungs) for each unit increase in the trans-pulmonary pressure (when enough time is allowed for the system to reach equilibrium) [1]. It is one of the most important concepts underpinning mechanical ventilation used to manage patient respiration in the operating room (OR) or intensive care unit (ICU) environment. To better understand pulmonary compliance, certain terminologies should be briefly reviewed. The following formula is used to calculate compliance:

Lung Compliance (C) = Change in Lung Volume (V) / Change in Transpulmonary Pressure {Alveolar Pressure (Palv) – Pleural Pressure (Ppl)}.

Transpulmonary pressure is the pressure gradient between the inside alveolar pressure and outside pleural pressure. It mainly measures the force of lung elasticity at each point of respiration (recoil pressure). Alveolar pressure is the air pressure inside the alveoli. Pleural pressure is the pressure of the fluid present inside the space between visceral pleura (layer adhered to lungs) and parietal pleura (chest wall lining layer). Normally the total compliance of both lungs in an adult is about 200 ml/ cm H2O. Physicians rely on this concept to understand some pulmonary pathologies and help guide therapy and adjust ventilator pressure and volume settings.

Issues of Concern

Types of Compliance:

For practical purpose, lung compliance is measured by different methods. Based on the method of measurement, lung compliance can be described as static or dynamic [2].

Static Compliance: It is the representation of pulmonary compliance at a given fixed volume when there is no airflow and muscles are relaxed. This situation takes place when transpulmonary pressure equals the elastic recoil pressure of the lungs. It only measures the elastic resistance. It is measured with a simple water manometer, but electrical transducers are now more commonly used. In the conscious individual, it is difficult to achieve complete certainty of respiratory muscle relaxation. But the compliance measurement is considered valid since the static pressure difference is unaffected by any muscle activity. In case of a paralyzed individual as in the operating theatre, it is straightforward to measure static compliance using recordings captured through electrical transducers. Therapeutically, it is used to select the ideal level of positive end-expiratory pressure, which is calculated based on the following formula:

Cstat = V / (Pplat – PEEP)

Where,

Pplat = Plateau pressure, PEEP = Positive End Expiratory Pressure

Dynamic Compliance: It is the continuous measurement of pulmonary compliance calculated at each points representing schematic changes during rhythmic breathing [2]. It monitors both elastic and airway resistance. Airway resistance depends on the air viscosity, density, and length and radius of airways. Except for the radius of the airway, all other variables are relatively constant. Thus, airway resistance can be physiologically altered by changes in the radius of the airway bronchus.

Compliance

When different readings of the lung volume are taken at specific measured pressure points and then plotted on a diagram, a pressure-volume curve representing both elastic and airway resistance properties of the lung is obtained [3] The two meeting points are end-inspiratory, and end-expiratory points and the line connecting them provides the measurement of dynamic compliance of the lungs. The area falling between this line and both the curves represents the excess work required to overcome the airway resistance during inspiration and expiration. This curve is also called the Hysteresis curve [4]. You can see that the lung is not a perfect elastic structure. The pressure required to inflate the lungs is higher than the pressure required to deflate them.

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