Overview

Across most species, intake, circulation, and oxygen delivery are vital for living a long and healthy life. The respiratory and cardiovascular systems ensure the demand for oxygen is met, and are built to quickly adapt in situations when this demand changes. However, some individuals can be born with or develop problems within these organ systems that can disrupt normal function.

An example of this is acute respiratory distress syndrome (ARDS), a life-threatening inflammatory condition characterised by fluid buildup in the lungs.¹ This can trigger many symptoms, with low blood oxygen or ‘hypoxemia’ being a defining factor.²

This article aims to address the relationship between hypoxemia and acute respiratory distress syndrome, by overviewing the respiratory system and highlighting the key causes, symptoms and suitable treatment options for the disorders.

The respiratory system 

The respiratory system provides our body with the oxygen required to produce energy and function normally. The lungs are central to the respiratory system, working to pass the oxygen we inhale into the bloodstream and remove any unwanted carbon dioxide. That being said, there are several key parts of the respiratory system that work together to help you breath:³

• Mouth
• Nasal cavity
• Pharynx
• Larynx
• Trachea
• Bronchi
• Lungs
• Alveoli

The lungs are filled with tiny sacs called alveoli that are attached to branches known as bronchioles. Around the alveoli are a network of capillaries; small blood vessels that have a very thin outer wall.⁴ 

As you breathe in, air filled with oxygen travels down the trachea and expands the lungs. Oxygen passes from the alveoli through the capillary wall to enter the bloodstream. At the same time, carbon dioxide passes from the blood into the alveoli to be exhaled as a waste product.⁴ This is called gaseous exchange and is a vital process that provides us with the oxygen we need to generate energy.

Acute respiratory distress syndrome (ARDS)

Mechanisms of ARDS

ARDS causes a buildup of fluid in the alveoli, which leaks in from the capillaries.⁵ In healthy individuals, there are very fine gaps in the wall surrounding the capillaries to allow for the natural passing of fluid and inflammatory cells. However, in people with ARDS, these gaps can become enlarged due to infection and inflammation, leading to the disruption of the barrier.^1,5 This can reduce the amount of oxygen that enters the bloodstream.¹

In some individuals, an attempt is made to heal the inflamed alveoli by sending out a type of cell called fibroblasts.⁶ The cells aim to strengthen the lungs by depositing collagen, but this can have the opposite effect as scar tissue is generated.⁶ This is known as fibrotic ARDS, where the lungs become more rigid causing them to work harder than before to deliver oxygen to the bloodstream.

Causes

Whilst ARDS is a condition in itself, it is almost always triggered by the onset of another disease or through direct or indirect injury to the lungs. The most common causes are sepsis and pneumonia, two serious conditions that influence the 30-50% mortality rate seen in individuals with ARDS.⁵

Sepsis is a condition in which the body overreacts to invading microbes and germs, resulting in high levels of inflammation that can send the body into ‘septic shock’.⁷ In about 6-7% of sepsis cases, individuals develop ARDS as a complication.⁸

Pneumonia is the inflammation of either one or both of the lungs brought on by a viral, bacterial or fungal infection. This infection causes the alveoli in the lungs to become inflamed and filled with fluid in a very similar fashion to those with ARDS.⁹ 

Symptoms

As ARDS is a respiratory condition, many of the symptoms that an individual will experience are related to the lungs. Depending on the severity and the cause of its onset, however, the following symptoms may vary in type and intensity:¹⁰

• Rapid and shallow breathing 
• Shortness of breath 
• Chest pain
• Chesty cough i.e. produces phlegm 
• Bluish tinge to the skin and nails (cyanosis)
• Increased heart rate 
• Low blood pressure 

Hypoxemia 

Mechanisms of hypoxemia 

Whilst hypoxemia is often used interchangeably with hypoxia, these conditions are different. Hypoxemia is a condition in which an individual has low levels of oxygen in their blood, whilst hypoxia refers to reduced oxygen in the tissues of the body.² Although it is common to have both conditions, it is important to note that not all individuals will experience them simultaneously. 

For oxygen to pass from the lungs into the bloodstream, there needs to be much more oxygen in the alveoli compared to the blood. In people with hypoxemia, there is often a difference in the amount of air they breathe in vs the amount of oxygen their lungs can pass into their bloodstream, also known as a ventilation/perfusion mismatch.^4,2

Causes 

Similar to ARDS, hypoxemia normally occurs due to underlying health conditions. These health conditions normally originate in the respiratory or cardiovascular systems, due to their roles in oxygen intake and delivery. In some people, symptoms of hypoxemia are the first indications that there is something else going on. 

One of the most common causes of hypoxemia is a condition called chronic obstructive pulmonary disease. Toxins inhaled through smoking and air pollution have a devastating and damaging effect on the lungs, causing them to become inflamed and clogged with fluid and phlegm.¹¹ In a similar way that fluid affects gas exchange in ARDS, the damage caused by smoking in chronic obstructive pulmonary disease reduces the amount of oxygen that can enter the bloodstream, resulting in hypoxemia.²

Symptoms 

Hypoxemia is a serious condition that requires immediate medical attention. However, some of the symptoms can be quite non-specific, therefore it is important to seek the advice of a medical professional if you experience the symptoms listed below. Depending on the severity of hypoxemia, individuals will experience the following symptoms at different levels:²

• Headaches
• Bluish tinge to the skin and nails (cyanosis)
• Rapid heartbeat
• Confusion
• Difficulty breathing

Relationship between ARDS and hypoxemia

Whilst ARDS is a leading cause of hypoxemia, there are many other underlying causes aside from ARDS. However, the majority of people with ARDS will experience hypoxemia to some degree, making the relationship between the two conditions one-way to an extent. 

Fluid buildup in the lungs of individuals with ARDS reduces the surface area of the alveoli and therefore has a direct impact on the level of gas exchange that occurs between the lungs and bloodstream.

Ultimately less oxygen can pass into the blood, resulting in hypoxemia. Blood passing through the lungs without being oxygenated is a key feature of hypoxemia in ARDS and is known as intrapulmonary shunting.² Reduced oxygen flow around the body can have detrimental effects across all of the organ systems, which is why it is vital to seek medical attention if symptoms of hypoxemia or ARDS are experienced.

Treatments

The available treatments for hypoxemia and ARDS generally aim to improve oxygen delivery and lung efficiency. However, it is most important to treat the underlying condition that has triggered the onset of co-occurring ARDS and hypoxemia, to reduce the chance of long-term damage from the conditions.

Ventilation

Ventilation is a key treatment strategy to increase oxygen intake by removing some of the pressure on the lungs. The choice of ventilation technique is crucial, as too much mechanical ventilation can stretch the lungs and cause long-lasting damage.¹ For this reason, low-level mechanical ventilation has proven to be the most effective.¹²

Diuretics

Diuretics are often prescribed to help manage the fluid build-up in the lungs seen in ARDS. Diuretics are a type of medication that increase urination, to aid in the removal of excess fluid in the body. Diuretics are most beneficial when taken within 42 hours of meeting the criteria for an ARDS diagnosis, particularly in reducing the chance of death.¹³

Summary

Oxygen intake, circulation, and delivery are essential for a healthy life. The respiratory and cardiovascular systems work to meet oxygen demands and adapt quickly to changes. However, some individuals may have issues with these systems that affect normal function. Acute respiratory distress syndrome (ARDS) is marked by fluid accumulation in the lungs, with low blood oxygen, or hypoxemia, being a key symptom. 

The respiratory system is crucial for providing oxygen to the body, with the lungs playing a central role. In ARDS, fluid accumulates in the alveoli due to enlarged gaps in capillary walls caused by infection and inflammation, which reduces oxygen intake. Fibroblasts attempt to heal the lungs but may create scar tissue, resulting in fibrotic ARDS. 

ARDS is often triggered by conditions like sepsis and pneumonia, leading to severe inflammation and an increased risk of mortality. Hypoxemia, a related condition with low blood oxygen levels, shares common causes and symptoms with ARDS. The relationship between these commonly co-occurring conditions means the available treatments are closely aligned. Immediate medical attention is vital for both conditions. Treatments focus on improving oxygen delivery and addressing underlying causes, with options like ventilation and diuretics to manage symptoms and fluid buildup.