What is the Difference Between Shunt and Dead Space

The main difference between the shunt and dead space is that shunt is the pathological condition in which the alveoli are perfused but not ventilated, whereas dead space is the physiological condition in which the alveoli are ventilated but not perfused. For example, in pneumonia, the area of the alveoli is well perfused although it is not well ventilated, leading to the shunt. Pulmonary embolism (PE) is an example of increased dead space resulted in decreasing perfusion relative to ventilation. 

Shunt and dead space are two conditions of lungs, resulting in impaired gas exchange. Moreover, they are examples of the ventilation-perfusion (V/Q) mismatch. 

Key Areas Covered 

1. What is the Shunt
     – Definition, Characteristics, Importance
2. What is Dead Space
     – Definition, Characteristics, Importance
3. What are the Similarities Between Shunt and Dead Space
     – Outline of Common features
4. What is the Difference Between Shunt and Dead Space
     – Comparison of Key Differences

Key Terms 

Alveolar Dead Space, Alveoli, Capillary Shunt, Dead Space, Gas Exchange, Perfusion, Shunt

What is the Shunt 

Shunt or pulmonary shunt is one of the two contributors to the ventilation-perfusion (V/Q) mismatch. Generally, pulmonary shunt can occur in two ways. They are the anatomical shunt and capillary shunt. Anatomical shunt occurs when the arterial blood returns to the pulmonary veins without passing through the pulmonary capillaries. On the other hand, a capillary shunt occurs when the blood passes through the capillaries of alveoli without ventilation.  

Capillary Shunt 

Typically, the capillary shunt is commonly identified as the shunt. In the capillary shunt, the alveoli have the proper perfusion, but not the ventilation. Therefore, it decreases the oxygenation of the deoxygenated blood, which passes to the lungs. Moreover, this poorly oxygenated blood subsequently returns to the heart and mixes up with the oxygenated blood, which comes from the other areas of the lungs with proper ventilation. However, this mixing lowers the total oxygen content of the arterial blood, which moves out from the heart. This leads to hypoxemia. Thus, the persistence of hypoxemia despite 100% oxygen inhalation refers to the shunt. 

Figure 1: Pneumonia

Furthermore, the capillaries which surround the alveoli without proper ventilation undergo constriction as they make the returning vessels hypoxic. When the whole lung becomes hypoxic, it also can undergo vasoconstriction. However, this can put enormous pressure on the right ventricle, leading to right ventricular dilation, which may even cause death. Moreover, lung disease always results in shunt. The common causes of a shunt include pneumonia and pulmonary edema, tissue trauma, atelectasis, mucus plugging, pulmonary arteriovenous fistulas, etc. 

What is Dead Space 

Dead space, total dead space or physiologic dead space is the second contributor to the ventilation-perfusion mismatch. Generally, physiological dead space has two components: anatomical dead space and alveolar dead space. Typically, anatomical dead space is the portion of the air which remains in the conducting airways where no gas exchange is possible. In comparison, alveolar dead space is the condition which results in when the alveoli of lungs have proper ventilation but, not proper perfusion.

Figure 2: Pulmonary Circulation

Furthermore, alveolar dead space alters with time according to the alterations in cardiac output as well as the pulmonary blood flow. However, the other reasons that impair the perfusion include cardiovascular shock, which reduces the blood flow to the lungs, emphysema, which is the presence of enlarged alveoli in contact with fewer capillaries, and pulmonary embolus, which is a clot, blocking the blood flow.  

Similarities Between Shunt and Dead Space 

• Shunt and dead space are two conditions of the lungs. 
• Moreover, they are examples of the ventilation-perfusion (V/Q) mismatch. 
• They result in impaired gas exchange, preventing whole air in breath to take part in gas exchange. 
• Furthermore, both conditions can occur in the same patient. 

Difference Between Shunt and Dead Space 

Definition 

Shunt refers to the pathological condition which results when the alveoli of the lungs are perfused with blood as normal, but the ventilation fails to supply the perfused region while dead space refers to the volume of air which does not take part in the gas exchange as it remains in the conducting airways or reaches alveoli, which are not perfused. 

Blood Supply to the Alveoli 

In shunt, there is regular blood supply to the lungs, while in dead space, alveoli have a poor blood supply. 

Ventilation of Alveoli 

In shunt, ventilation fails to supply an adequate amount of air to the alveoli while in dead space, ventilation supplies the regular amount of air to alveoli. 

Types 

Anatomical shunt and capillary shunt are the two types of shunt while anatomical dead space and alveolar dead space are the two types of dead space. 

Causes 

The common causes of a shunt include pneumonia and pulmonary edema, tissue trauma, atelectasis, mucus plugging, pulmonary arteriovenous fistulas, etc. while the main causes of dead space include cardiovascular shock, emphysema, pulmonary embolism (PE), etc. 

Conclusion 

The shunt is a pathological condition in which the alveoli with normal blood supply do not get proper ventilation. Therefore, hypoxemia persists even with the complete oxygen inhalation. Generally, this occurs in pneumonia. In comparison, dead space is the volume of the air which does not take part in gas exchange. Basically, this can occur due to the presence of the air in conducting airways. Also, it occurs due to the poor perfusion of alveoli even they get an adequate amount of air through ventilation. For example, it occurs in pulmonary embolism. Therefore, the main difference between the shunt and dead space is the amount of blood supply and amount of ventilation into the alveoli. 

References:

1. Whitten, Christine. “Ventilation Perfusion Mismatch – The Airway Jedi. Dead Space vs. Shunt.” The Airway Jedi, 2 July 2018, Available Here.

Image Courtesy:

1. “Lobar pneumonia illustrated” By Heart, Lung and Blood Institute (Public Domain) via Commons Wikimedia   
2. “2101 Blood Flow Through the Heart” By OpenStax College – Anatomy & Physiology, Connexions Web site. (CC BY 3.0) via Commons Wikimedia