Dead space ventilation is defined as the volume of ventilated air that does not participate in gas exchange. It’s often referred to as “wasted ventilation,” as the inhaled air is not involved in the exchange of oxygen and carbon dioxide.
In other words, dead space occurs when plenty of air reaches the alveoli, but there is a lack of perfusion available for gas exchange to occur.
In this article, we will discuss what dead space ventilation is, its causes, and its clinical significance.
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Types of Dead Space
There are three primary types of dead space:
Anatomical Dead Space
Anatomical dead space is the volume of air in the conducting airways that does not participate in gas exchange. The conducting airways are made up of the nose, trachea, and bronchi.
When you take a breath in, air first enters through the nose and then travels down the trachea into the bronchi. The air that remains in this area that does not participate in gas exchange is the anatomical dead space.
Note: Anatomical dead space is estimated to be approximately 1 mL/lb of ideal body weight.
For example, if a patient has an ideal body weight of 162 lbs, their anatomical dead space would be approximately 162 mL.
Alveolar Dead Space
Alveolar dead space is the volume of air that reaches the alveoli but does not participate in gas exchange due to a lack of perfusion. This can occur due to:
- Decreased cardiac output
- Heart failure
- Blood loss
- Pulmonary embolism
Alveoli are tiny air sacs in the lungs where gas exchange occurs. Air that reaches the alveoli but doesn’t participate in gas exchange is considered alveolar dead space.
Again, this occurs when there is a lack of blood flow (i.e., perfusion) to this area.
Physiologic Dead Space
Physiologic dead space is the total volume of air that does not participate in gas exchange. Therefore, it is the sum of the anatomical and alveolar deadspace.
Physiologic Dead Space = Anatomical Dead Space + Alveolar Dead Space
This type occurs throughout the entire respiratory zone, which includes the following:
- Alveolar duct
- Alveolar sac
In healthy adults, the majority of the physiologic dead space is made up of anatomical dead space. Therefore, alveolar dead space only accounts for a small percentage of the total physiologic dead space.
Note: Physiologic dead spaces typically increases in lung diseases that cause a ventilation/perfusion (V/Q) mismatch.
What is Ventilation?
Ventilation is defined as the process of moving air into and out of the lungs. This is accomplished by the contraction and relaxation of the diaphragm and intercostal muscles.
The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. When it contracts, it flattens and moves downward. This action increases the volume of the thoracic cavity and decreases its internal pressure.
As a result, air flows into the lungs during inhalation. The opposite occurs during exhalation when the diaphragm relaxes and moves upward. This action decreases the volume of the thoracic cavity and increases the internal pressure.
In other words, the diaphragm is responsible for making sure air flows into and out of the lungs. Again, this process is known as ventilation.
What is Perfusion?
Perfusion is defined as the process of moving blood through pulmonary circulation. This is accomplished by the contraction of the right ventricle of the heart.
When the right ventricle contracts, it pumps blood from the right atrium into the pulmonary artery. The pulmonary artery then branches off into smaller arteries that lead to the lungs.
These arteries eventually branch off into even smaller arteries (i.e., capillaries) that surround the alveoli. The blood in these capillaries picks up oxygen from the alveoli and delivers it to the tissues.
Simultaneously, the capillaries surrounding the alveoli pick up carbon dioxide from the tissues and deliver it to the alveoli. The carbon dioxide is then removed from the body during exhalation.
What Causes Increased Dead Space Ventilation?
There are several factors that can contribute to an increase in dead space ventilation. Some examples include:
- Dysfunctional alveoli
- Decreased perfusion
- Decreased cardiac output
- Pulmonary embolism
- Acute respiratory distress syndrome (ARDS)
- Increased alveolar-capillary permeability
- Endotracheal intubation
This results in a lack of gas exchange at the alveolar level, which causes the PaCO2 to increase and the PaO2 to decrease. That is because less carbon dioxide is being removed from the body and less oxygen is being delivered to the tissues.
Over time, this can lead to respiratory acidosis (i.e., a buildup of carbon dioxide in the blood) and tissue hypoxemia (i.e., a lack of oxygen in the tissues).
In severe cases, respiratory failure can occur, which is a life-threatening condition that requires immediate medical attention.
How to Calculate Dead Space
Physiological dead space can be calculated using the Bohr equation, which is given as a ratio of dead space to tidal volume:
(VD/VT) = (PaCO2 – PeCO2) / PaCO2
For example, let’s say an adult patient has a PaCO2 of 42 mmHg and a PeCO2 of 33 mmHg. What is the deadspace to tidal volume ratio?
Now, all you have to do is plug the numbers into the formula:
(VD/VT) = (42 – 33) / 42
(VD/VT) = 9 / 42
(VD/VT) = 0.21
Dead space is an important concept to understand when it comes to respiratory physiology. It’s a measurement of inhaled air that does not participate in gas exchange.
This occurs when air remains in the conducting airways, or when it reaches alveoli that are not perfused with blood. When this occurs, the inhaled air is “wasted,” meaning that it’s not available for the exchange of oxygen and carbon dioxide.
An increase in dead space ventilation can be caused by a variety of factors, including dysfunctional alveoli, decreased perfusion, and pulmonary embolism. In severe cases, respiratory failure can occur, which is a life-threatening condition that requires immediate intervention.
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.
- NCBI. (n.d.). National Library of Medicine. Retrieved October 14, 2022, from https://www.ncbi.nlm.nih.gov/books/NBK482501/.
- Egan’s Fundamentals of Respiratory Care. 12th ed., Mosby, 2020.
- Cardiopulmonary Anatomy & Physiology: Essentials of Respiratory Care. 7th ed., Cengage Learning, 2019.