Introduction
The lungs are the vital organs that participate in gas exchange. Airway resistance is essential for lung function. It is created due to the frictional forces of the airways against the airflow. It is essential at physiological levels to maintain lung function. Airway resistance can get elevated in some pulmonary diseases resulting in respiratory failure.
What Is Airway Resistance?
Various factors are associated with breathing; one such factor involved in breathing is the resistance to the airflow in the lung. Airway resistance is the change in the transpulmonary pressure essential to produce a unit flow of gas through the airways. It is the pressure difference between the mouth and the alveoli, divided by the airflow. The air that moves into and out of the lungs should overcome friction. Air also contains a frictional force that stops two things from sliding past each other.
Airway resistance is the measure of the resistance to the airflow caused by friction. The resistance to the airflow determines the ease of breathing. Airway resistance in the trachea causes turbulent airflow, while the airway resistance in the bronchi and bronchioles is responsible for a laminar airflow that allows a smooth flow of air to the distal segments of the lungs. Various factors like airflow velocity, the diameter of the airway, and lung volume can influence airway resistance.
How Do Lungs Develop?
Lung development happens in the embryonic, fetal, and postnatal stages of life. The major airways are formed at four to seven weeks during the embryonic period of development. The rest of the airways and the bronchial tree are formed between five and seventeen weeks during the fetal period. The distal airways develop between 16 to 26 weeks.
The lung looks like a tubular gland during the formation of the bronchial tree. Bronchial buds that are formed between four to seven weeks become the bronchi later. The bronchi then bifurcate to form bronchioles, and the branching continues. Fetal breathing movements are evident from the 10th week. This allows the movement of amniotic fluid in and out of the lungs and causes stretching of the lung tissue to increase the caliber of the airways.
What Is the Mechanism of Breathing?
Nerve impulses are sent to the respiratory muscles, and the muscles contract to allow expansion of the lungs in the intrathoracic space. This increases the lung volume, and there is a slight decrease in intrapulmonary pressure. This pressure drop allows air from the external environment to move into the lungs. This commences the inhalation of air.
The respiratory muscles then relax, and the intrathoracic space decreases slightly. This reduces the lung volume and causes the intrapulmonary pressure to increase. This creates a pressure difference between the external atmosphere and the intrapulmonary atmosphere and causes the air to move out of the lungs, thereby commencing exhalation.
What Is the Significance of Airway Resistance?
Airway resistance is essential because the heart and brain require a high amount of oxygen to function normally. It is also essential for patients on ventilators. If airway resistance is increased, it will prevent proper ventilation and result in air trapping, atelectasis, and pulmonary hypertension. Increased airway resistance increases the pressure required to deliver a fixed volume of gas and causes airway problems.
What Conditions Cause Changes in Airway Resistance?
Various factors influence airway resistance. It depends on the diameter of the airway and the airflow. Airway resistance is less in laminar flow and increased in a turbulent flow. Multiple pulmonary diseases can elevate airway resistance resulting in air trapping in the lungs. This limits the gas exchange and causes respiratory failure. The following conditions cause changes in airway resistance:
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Asthma - Asthma is a common disease caused by hyper-responsiveness to an irritant. Inhalation of the irritant causes airway inflammation, bronchospasms, and increased mucus production. These changes increase airway resistance during expiration.
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COPD (Chronic Obstructive Pulmonary Disease) - COPD refers to chronic bronchitis, bronchiectasis, emphysema, and refractory asthma. COPD increases airway resistance and results in air trapping in the lungs.
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Chronic Bronchitis - Chronic bronchitis is caused by repeated exposure to irritants which initiates an inflammatory response and increased mucus production with mucociliary dysfunction. The dysfunctional mucociliary response exacerbates inflammation and increases airway resistance resulting in air trapping in the lungs.
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Emphysema - Emphysema causes the loss of pulmonary connective tissue. The lungs cannot contract and push air out of the lungs without elastin. This increases airway resistance and results in air trapping.
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Neurological Injuries - Neurological injuries impact sympathetic output and result in uninhibited parasympathetic input. This causes constriction of bronchial smooth muscles, narrowing of the airways, and an increase in airway resistance.
How Is Airway Resistance Measured?
The airway resistance is measured using the following tests:
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Spirometry - This test measures the patient’s inhalation and exhalation over time. It measures maximal inspiration, forceful exhalation, and continued exhalation. This test provides forced vital capacity and forced expiratory volume in one second. This ratio determines the type of lung pathology present.
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Body Plethysmography - It is the most common method that measures airway resistance. A plethysmograph is an air-tight chamber in which the patient sits inside. The patient is asked to put the tube in the chamber into the mouth. It has a flowmeter to measure the flow rate. The patient is asked to breathe normally and against the closed tube. It records the values for mouth pressure, alveoli pressure, and flow rate. Airway resistance is calculated using these values.
How Is Increased Airway Resistance Managed?
Increased airway resistance is managed with medications like
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Inhaled Corticosteroids - It decreases capillary permeability, the inflammatory response, and mucus production and reduces airway resistance.
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Beta-2-Agonists - This inhibits the contraction of the smooth muscles of the airway and allows dilation of the airway, and increases airflow. This decreases airway resistance. Short-acting beta-agonists are used during attacks and exacerbations, while long-acting beta-agonists are used for long-term therapy.
Conclusion
Airway resistance at physiological levels is significant for the normal function of the lungs, heart, and brain. Changes in airflow resistance can cause complications like a respiratory failure. Airway resistance is dependent on various physiological factors. Increased airway resistance is a treatable condition. Various medications are available for the management of increased airway resistance.
