7.1 Why Do we need mechanical ventilation – Respiratory Failure Type I

Yvonne Drasovean, RRT, M.Ed., FCSRT

7.1 Why Do we need mechanical ventilation – Respiratory Failure Type I

Respiratory failure occurs when the cardio-respiratory system is unable to provide adequate tissue oxygenation and/or CO₂ removal. This condition can become life threatening, and it is important for respiratory therapists to recognize impending respiratory failure and take appropriate action. Respiratory Failure can be classified as acute or chronic.

Key Takeaway

Going back to the analogy of lungs as balloons, it is a simple concept that too much volume or pressure applied to a balloon can over stretch or pop the balloon. Keep this concept in mind when learning the basics of mechanical ventilation. Lung protective strategies will be explored further in later chapters.

Acute Respiratory Failure (ARF) has a sudden onset and may present with hypoxemia alone, or hypoxemia may be accompanied by hypercapnia. Acute Respiratory Distress Syndrome (ARDS) is a severe form of Respiratory Failure, and will be discussed later. Chronic Respiratory Failure is often caused by chronic lung disease or neuromuscular disease.

Respiratory Failure

Acute Respiratory Failure is usually sudden in onset and is caused by temporary conditions such a pneumonia.

Chronic Respiratory Failure is caused by chronic disease such as Chronic obstructive Pulmonary Disease.

Chronic Respiratory Failure can also be caused by neuromuscular diseases or chest wall deformity.

Reasons for mechanical ventilation due to respiratory failure can fall into two main categories:

Hypoxemic respiratory failure (also called hypoxia or Type I respiratory failure) is the inability to oxygenate the body adequately. Issues with the lungs, circulating blood, or the heart can result in inadequate oxygen being delivered to the organs and tissue. If not corrected, this low oxygen can cause organ failure and irreversible damage.
Hypercapnic respiratory failure (also called pump failure or Type II respiratory failure) is characterized by high CO₂ levels and can occur because of compromised breathing or other illnesses in the body. If CO₂ increases to a level the body cannot clear by increasing breathing, mechanical ventilation is indicated. An ineffective drive to breathe can also occur if there is an injury to the brain or neurological control to breathing, leading to respiratory failure. It can occur with spinal or nerve injuries as well, where the triggers of ventilation do not result in an intact signal to breathe. These cases may require mechanical ventilation to take over the impaired drive to breathe.

Integrity of the complex system that is required for breathing and maintenance of normal O₂ and CO₂ levels compatible with metabolic demand is essential to all organ functions. Imagine this complex interaction between respiratory and cardiovascular systems as a chain with multiple links. Problems can arise at any level of this chain. Failure of one or more of links in the chain can lead to development of ARF.

The first three components of this chain represent the “ventilatory pump” and are responsible for alveolar ventilation. A problem with any of these components results in CO₂ retention which will eventually compromise oxygenation, limiting alveolar gas exchange. Problems with the next components, airway and lung tissue, result in inadequate oxygenation but normal CO₂ elimination, assuming the ventilatory “pump” function is maintained. The last link in the respiratory system chain, representing gas transport in circulation, can result in inadequate O₂ delivery to the tissues and CO₂ removal. Cases of shock severe anemia and/or hemoglobin dysfunction could also cause ARF. Often, multiple chain links are broken at the same time, resulting in ARF of multiple causes. For example, a patient who suffered a head injury, (involving the central nervous system) lung contusion (involving lung tissue) and hemorrhagic shock (involving circulation) may develop ARF from multiple causes without appropriate intervention.

Respiratory System Chain: Central Nervous System, Peripheral Nervous System, Respiratory Muscles and Thorax, Upper and Lower Airways, Lung Tissue, Cardiovascular System. — Figure 7.1.1: “Respiratory System Chain” by Freddy Vale, CC BY-NC-SA 4.0

Remember when we learned about those chemoreceptors in the brain which are sensing oxygen and CO₂ levels? Anything that interrupts the signal from the brain to the lungs, or if the signals or subsequent breaths are not adequate for what the body needs, will require ventilation. The body naturally wants to keep the CO₂ levels within a desired range and wants to take in enough oxygen to feed the body. When a patient gets sick or has any type of organ dysfunction, this can quickly cause a disruption in the balance of the body by increase the oxygen consumption or increasing the CO₂ production of the cells. When this happens, the body will try to compensate, but this compensation can only go so far. As patients get sicker, there may come a point where the oxygen levels are too low (hypoxic failure) or CO₂ levels are too high for the lungs to compensate (hypercapnia), and mechanical ventilation is necessary to help restore healthy levels of CO₂ or oxygen.

An elderly person's hand, with the finger tips turning blue due to lack of oxygen. — Figure 7.1.2: Photo by Doc James, CC BY-SA 3.0

Cyanosis will occur if hypoxic failure is left untreated. Mechanical ventilation will prevent oxygen levels from falling to this low level. The health care provider must always act quickly to treat hypoxic failure.

Hypoxemic Respiratory Failure or Type I Respiratory Failure

The main causes of Hypoxemic respiratory Failure (Type 1) are Ventilation/Perfusion (V/Q mismatch) or shunt, Alveolar hypoventilation, diffusion impairment, and decreased inspired oxygen.

Causes of Hypoxemia (Type 1 ARF): Low partial pressure inspired O2, alveolar hypoventilation, anatomical shunt, impaired diffusion, ventilation-perfusion dismatch. — Figure 7.1.3: “Causes of Type 1 ARF” by Freddy Vale, CC BY-NC-SA 4.0

Ventilation/Perfusion Mismatch

Key Takeaway

The greatest amount of perfusion and ventilation is in the dependent areas of the lungs.

At the apex of the lung = VA/Q ratio is increased

At the middle of the lung= VA/Q = 1

At the base of the lung = VA/Q ratio is decreased

Ventilation/Perfusion mismatch ([latex]\frac V{Q}[/latex] mismatch) is often a difficult concept to understand, so let’s review it as a cause of hypoxemic respiratory failure. The optimal [latex]\frac V{Q}[/latex] ratio is [latex]1[/latex], when alveolar ventilation matches blood flow available for gas exchange. However, ventilation and perfusion are not evenly distributed and matched in all areas of the lung. Under normal physiological conditions, in the upright individual, there is more ventilation than blood flow at the apex of the lung and more blood flow than ventilation at the base due to gravity [latex]\left(\frac{V}{Q}>1\right)[/latex]. In an individual in supine position, there will be more blood flow to the posterior areas of the lung (dependent areas, [latex]\frac{V}{Q}<1[/latex]).

Represented in Figure 7.1.4 blow:

Zone 1: At the top of the lung = VA/Q ratio is increased
Zone 2: At the middle of the lung= VA/Q = 1
Zone 3: At the bottom of the lung = VA/Q ratio is decreased

Figure 7.1.4: “Lung Zones” by Freddy Vale, CC BY-NC-SA 4.0

Disease process often disrupts this physiological balance, leading to hypoxemia. [latex]\frac{V}{Q}[/latex] varies based on lung zone and body position. This is often used to improve oxygenation in severe ARDS, where patients are placed in prone position, so that anterior areas of the lungs become dependent and available to increased blood flow for gas exchange.

A shunt is an extreme form of [latex]\frac{V}{Q}[/latex] mismatch, when there is no ventilation to match blood flow. [latex]2\%\text{ - }3\%[/latex] of the blood supply is normally shunted (anatomic shunt). Shunt increases, in certain abnormal conditions such as right to left shunting of blood flow though cardiac septal defects. Physiologic shunt leads to hypoxemia in abnormal conditions such as atelectasis or pulmonary edema. Shunt does not respond to supplemental oxygen, because gas exchange is not available [latex](\frac{V}{Q}=0)[/latex].

Object Lesson

Ventilation/perfusion mismatch usually responds to O₂ therapy, while shunt does not.

Watch this video for a quick review of [latex]\frac{V}{Q}[/latex] mismatch:

Video: “V-Q Mismatch” By macrophage [4:23] Transcript Available

Alveolar Hypoventilation

While alveolar hypoventilation is discussed mostly in the context of hypercapnic respiratory failure, it can also lead to hypoxemia. Alveolar hypoventilation refers to a decrease in the amount of fresh air reaching the alveoli, resulting in reduced O₂ intake and increased CO₂ retention.

When alveolar hypoventilation occurs, the exchange of oxygen and carbon dioxide in the alveoli is compromised. This can lead to a decrease in the O₂content within the alveoli, resulting in lower O₂ levels available for transfer into the bloodstream. As a result, the O₂saturation of arterial blood can decline, leading to hypoxemic respiratory failure.

Diffusion Limitations

Diffusion is defined as the movement of air and oxygen across the alveolar capillary membrane along a pressure gradient. Diffusion limitation is seen in conditions affecting gas exchange, such as interstitial lung disease, where destruction of interstitium leads to impaired gas exchange. Destruction of alveolar tissue can also impair diffusion. Abnormalities in pulmonary circulation may lead to impaired gas exchange due to reduced blood flow available.

Decreased Inspired Oxygen

Hypoxic respiratory failure caused by decreased inspired oxygen occurs when the body oxygen requirements are greater than available oxygen. Knowing that the concentration of oxygen is the same anywhere on the planet, [latex]21\%[/latex], this low inspired oxygen is seen at high altitude due to decreased atmospheric pressure, or in situations where the body’s increased oxygen requirements go unrecognized for prolonged periods of time without appropriate intervention (i.e. provide O₂ at higher FiO₂).