Compliance is a measure of stretchability or distensibility. It occurs due to the tissues' tendency to return to their original position after removing an applied force. Compliance can be defined as the change in the volume of the lungs over the change in pressure. Compliance is the extent to which the lungs can expand for each unit increase in transpulmonary pressure. Transpulmonary pressure is the difference between intrapleural pressure and alveolar pressure. The alveolar pressure is the pressure within the alveolar sac. In contrast, intrapleural pressure is the pressure of the fluid present between the two layers of the lung, known as the visceral and the parietal pleura, respectively. If the intrapleural pressure is more negative, the lungs increase in volume to expand, whereas if the intrapleural pressure becomes positive, the lungs will collapse, which decreases the lung volume.
During exhalation, the lung volume is much more for a given intrapleural pressure; therefore, compliance is more in exhalation than in inhalation. It is denoted by liter per cm of H2O. The total compliance of both lungs in an adult is 200 mL per cm of H2O. So, with a 1 cm increase in transpulmonary pressure, the lung will increase by 200 mL. Compliance of the lungs alone is approximately two times that of the lungs and thorax. The stiffer the lungs, the lesser the lung compliance. Lung compliance is inversely proportional to the elastance, elastic resistance, or elastic recoil. The elasticity is the resistance to the stretch. Elasticity is defined as the change in pressure over the volume change. It is the force at which the lung recoils for given distension.
What Are the Factors That Can Affect the Compliance of the Lungs?
Lung compliance can be affected by the following:
The Elasticity of Lungs:
Pulmonary Fibrosis: Compliance decreases, whereas the elasticity increases.
Emphysema: In emphysema, compliance is increased, whereas the elasticity is decreased. They have a low forced expiratory volume.
The Elasticity of the Chest Wall:
The compliance is decreased in the following factors:
The most important factor affecting lung compliance is the elastic property of the lung contributed to by the surface tension of the alveoli. A thin, aqueous liquid layer with associated surface tension lines the alveolar epithelium. The surface tension parallels the lung tissue elasticity to tend to collapse the alveoli. The lipoprotein complex lessens the impact of the surface tension.
The surface tension-reducing substance secreted by the type 2 alveolar epithelial cells or pneumocytes. It is the mixture of lipids like dipalmitoyl phosphatidylcholine or lecithin and proteins. The function of the surfactant is to decrease the surface tension and thus prevent the alveoli from collapsing, allowing them alveoli to expand.
So, compliance increases, and the breathing workload decreases. It also prevents pulmonary edema. Hormones that increase surfactant production are glucocorticoid and thyroxine. If there is a surfactant deficiency, the baby can develop hyaline membrane disease or infant respiratory distress syndrome (IRDS). It is more commonly seen in premature delivery and diabetic mothers.
What Are the Types of Lung Compliance?
Compliance can be of two types:
Static Compliance: Compliance is at a fixed volume when there is no airflow and muscles are relaxed. This can only happen when the transpulmonary pressure and the elastic recoil pressure of the lungs are equal.
Dynamic Compliance: It is the continuous measurement of the compliance at each point representing schematic changes during rhythmic breathing. It monitors both compliance and airway resistance. The factors depending on airway resistance are air viscosity, density, length, and radius of the airways. Except for the airway radius, the rest all remain constant.
What Is the Clinical Significance of Lung Compliance?
Pulmonary diseases are associated with changes in lung compliance. Examples of such a disease that can affect lung compliance are
Emphysema is a condition in which there is destruction and enlargement of the lung parenchyma distal to the terminal bronchiole. The most significant risk factor for emphysema is cigarette smoking. Smoking stimulates inflammatory cells, such as macrophages and polymorphonuclear cells, which release elastase and cause elastolysis and emphysema. The elastic recoil property of the lungs is decreased due to a deficiency of alpha -1 antitrypsin or an external factor such as smoking. Due to the decreased elastic recoil, such patients will have increased lung compliance and decreased elastic properties. The alveolar sacs have a high residual volume, which causes difficulty in exhaling the excess air out of the lungs and resulting in difficulty in breathing. They have a low forced expiratory volume. A characteristic feature of emphysema is the barrel chest which is a rounded and enlarged chest appearance due to hyperinflation. Typically, the anteroposterior to transverse ratio is 5:7 or 1:2, but in the case of emphysema, it becomes 1:1 due to hyperinflation.
It is a disease in which the lung tissue becomes fibrosed with time, affecting the lung's normal functioning. A few harmful agents that can cause fibrosis of the lungs are infections, environmental agents, and chemicals. Fibrosis occurs due to the overproduction of collagen fibers. It is an immune response of the body in which simultaneous inflammation, remodeling, and repair take place. In lung fibrosis, elastin is replaced by collagenous tissue, which compresses the lung tissue and, thereby, decreases the compliance of the lungs. Such patients require higher work of breathing to inflate more rigid alveoli.
Infant Respiratory Distress Syndrome:
It is a life-threatening situation that requires immediate medical attention to maintain continuous positive pressure breathing. Premature newborns have little or no surfactant produced by the alveoli, so the alveoli tend to collapse. Surfactant decreases the surface tension and, thereby, increases the compliance of the lungs.
Atelectasis or Acute Respiratory Distress Syndrome:
It is the collapse of the alveoli and occurs in the dependent parts of the lungs. It is one of the most common complications after operative anesthesia. Due to the decrease in lung volume, lung compliance decreases, requiring a higher pressure to reinflate the alveoli.
Pulmonary compliance measures the stretchability of the lungs and chest wall. It is defined as the change in volume over the change in pressure. A few pathologies of the lung that can affect compliance are pulmonary fibrosis, emphysema, infant respiratory distress syndrome, and atelectasis.