The best way to determine the depth of a patient’s respiration is to observe the degree of chest-wall movement during inspiration and expiration.
When you take a breath in, the chest expands as your lungs fill with air. The deeper the breath, the more air enters the lungs and the greater the expansion of the chest.
Therefore, a larger breath has more depth and is more visible. If the respiration is shallow, then it may not be as visible. You can also feel the depth of respiration by placing your hand on the patient’s chest.
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What is Respiration?
Respiration is the physiological process of inhaling oxygen and exhaling carbon dioxide. This process, known as gas exchange, is essential for maintaining cellular metabolism.
The air that enters the lungs during an inspiratory breath contains oxygen molecules that reach the alveoli. Then oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses from the blood into the alveoli.
The oxygen-rich blood is then carried to the heart, where it is pumped to the rest of the body. The carbon dioxide-rich blood is returned to the lungs, where it is removed from the body by exhaling.
The Process of Gas Exchange
For gas exchange to occur, there must be a difference in partial pressures of oxygen and carbon dioxide between the alveoli and the blood. The partial pressure of a gas is defined as the pressure that would be exerted by that gas if it were present alone in a container.
During inspiration, the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and decreasing the pressure within it. This decrease in pressure causes air to flow into the lungs until the pressure within the lungs is equal to that of the atmosphere.
The air that flows into the lungs contains more oxygen than the air that is exhaled. This difference in partial pressure of oxygen creates a pressure gradient that drives the diffusion of oxygen from the alveoli into the blood.
At the same time, carbon dioxide diffuses from the blood into the alveoli. The partial pressure of carbon dioxide in the blood is higher than that in the alveoli, so the diffusion gradient is reversed.
What Controls Breathing?
The act of breathing is controlled by the respiratory center, which is located in the medulla oblongata of the brainstem. The respiratory center is stimulated by changes in the partial pressure of carbon dioxide in the blood.
When the partial pressure of carbon dioxide increases, the respiratory center is stimulated to increase the rate and depth of breathing. This increase in ventilation removes the excess carbon dioxide and decreases the partial pressure of carbon dioxide in the blood.
Conversely, when the partial pressure of carbon dioxide decreases, the respiratory center is stimulated to decrease the rate and depth of breathing. This decrease in ventilation allows carbon dioxide to accumulate in the blood, which increases the partial pressure of carbon dioxide.
How to Determine the Depth of a Patient’s Respiration
Again, you can determine the depth of a patient’s respiration by evaluating how much chest-wall movement you can observe during a breathing cycle. This is a subjective measurement.
The movement and depth of respiration are generated by the following:
- Intercostal muscles
The diaphragm is the primary muscle of respiration. It’s a dome-shaped muscle that separates the thoracic cavity from the abdominal cavity.
When the diaphragm contracts, it moves downward and flattens out. This increases the volume of the thoracic cavity and decreases the pressure within it. This decrease in pressure causes air to flow into the lungs until the pressure within the lungs is equal to that of the atmosphere.
The intercostal muscles are a group of muscles that run between the ribs. They are accessory muscles of respiration and help the ribcage expand so that air can flow into the lungs.
They help expand and shrink the size of the thoracic cavity by working together with the diaphragm. There are external and internal layers of intercostal muscles.
The external intercostal muscles are responsible for helping elevate the ribs during inspiration. The internal intercostal muscles, on the other hand, are responsible for helping depress the ribs during expiration.
What is the Chest Wall?
The chest wall is the structure that encloses the thoracic cavity. It consists of the ribs, sternum, and vertebral column.
The ribs are attached to the vertebral column in the back, while the sternum is located in the front. They form a cage that protects the lungs and organs of the thoracic cavity.
The sternum is a long, flat bone that is located in the center of the thoracic cavity. It articulates with the ribs and forms the sternocostal joints.
Depth of Respiration
The depth (i.e., volume) of breathing is known as the tidal volume. It is the amount of air that is inhaled or exhaled in a single breath. The average tidal volume is approximately 4–8 mL/kg of a patient’s ideal body weight (IBW).
The depth of respiration can be affected by the patient’s physiology and pulmonary mechanics. This includes the following:
- Lung compliance
- Airway resistance
- Muscle strength
- Size of the lungs
Lung compliance is a measurement of how easily the lungs expand. It is determined by the elasticity of the lungs and chest wall.
The compliance of the lungs naturally decreases with age and is also decreased in patients with respiratory disorders, pulmonary fibrosis, and interstitial lung disease.
Airway resistance is the measurement of the opposition to airflow that occurs as air moves through the lungs. It can be calculated with the following formula:
Raw = (PIP – Plateau pressure) / Flow
Therefore, airway resistance is the difference between the patient’s alveolar pressure and mouth pressure, divided by the flow at the mouth.
The strength of the respiratory muscles also contributes to the depth of respiration. The primary muscles of respiration (i.e., diaphragm and intercostal muscles) can weaken due to disuse, age, or neuromuscular disorders.
When the respiratory muscles are weak, they are unable to generate enough force to expand the thoracic cavity and facilitate airflow into the lungs. This leads to shallow breathing and a decrease in tidal volume.
Size of the Lungs
The size of the lungs also affects the depth of respiration. Smaller lungs have a smaller capacity, which limits the amount of air that can be inhaled. This can lead to shallow breathing and a decrease in tidal volume.
As previously mentioned, the most effective way to determine the depth of a patient’s respiration is to observe the degree of chest-wall movement during a breathing cycle.
The depth of respiration is an important indicator of a patient’s respiratory status. It can be affected by a variety of physiological and pulmonary factors.
Lung compliance, airway resistance, muscle strength, and size all play a role in determining a patient’s tidal volume. These factors should be considered when assessing the condition of a patient’s respiratory system. Thanks for reading, and, as always, breathe easy, my friend.
John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.
- Brinkman, Joshua E., et al. “Physiology, Respiratory Drive.” National Library of Medicine, StatPearls Publishing, Jan. 2022, www.ncbi.nlm.nih.gov/books/NBK482414.
- Clark, F. J., and C. Von Euler. “On the Regulation of Depth and Rate of Breathing.” National Library of Medicine, J Physiol, Apr. 1973, www.ncbi.nlm.nih.gov/pmc/articles/PMC1331381.
- Nicolò, Andrea, et al. “Control of the Depth and Rate of Breathing: Metabolic Vs. Non‐metabolic Inputs.” National Library of Medicine, J Physiol, 1 Oct. 2017, www.ncbi.nlm.nih.gov/pmc/articles/PMC5621491.