What is Mechanical Ventilation?
Mechanical ventilation involves the use of a machine to help a patient who is unable to breathe spontaneously. Therefore, it is indicated for patients who are unable to maintain adequate ventilation.
Ventilation is the process of taking in oxygen during inhalation while removing carbon dioxide during exhalation.
When a patient is unable to do this on their own, a ventilator can be used to assist with or completely take over the ventilatory process.
Some of the most common reasons why a patient may require mechanical ventilation include:
- Insufficient oxygenation: When a patient is not receiving enough oxygen (i.e., hypoxemia), it can impact the functionality of tissues and vital organs of the body. Mechanical ventilation can help deliver oxygen to the lungs, which is then distributed throughout the body.
- Insufficient ventilation: When a patient is not removing enough carbon dioxide from their body, it results in increased acidity of the blood (i.e., respiratory acidosis). Mechanical ventilation helps the patient remove carbon dioxide during exhalation.
- Acute lung injury (ALI): This is an injury to the lungs that occurs from an acute event such as sepsis, pneumonia, aspiration, or trauma.
- Severe asthma: During an asthma exacerbation, the airways constrict and make it difficult to move air in and out of the lungs. This can lead to respiratory failure, which often requires ventilatory support.
- Severe hypotension: Conditions that cause extremely low blood pressure, such as shock, sepsis, and congestive heart failure (CHF), often require mechanical ventilation.
- Inability to protect the airway: When a patient is at risk of aspirating secretions into the lung, they may require intubation and mechanical ventilation to protect their airway.
- Upper airway obstruction: Conditions that cause upper airway obstructions, such as epiglottitis and laryngeal edema, can prevent patients from being able to move air into the lungs. Therefore, mechanical ventilation can help bypass the obstruction.
In general, mechanical ventilation is indicated whenever a patient’s spontaneous breathing is not adequate to sustain life.
In such a case, a ventilator would be used to provide breathing support until the patient’s underlying condition is reversed.
A patient cannot survive without adequate ventilation and oxygenation. Therefore, there are no true contraindications for mechanical ventilation.
However, there may be some circumstances where a patient chooses not to receive mechanical ventilation, such as when they have a DNR (Do Not Resuscitate) order in place.
This means that the patient legally wishes not to receive life-saving interventions. In these cases, the patient’s goals of care must be respected.
Principles of Mechanical Ventilation
A practitioner must learn and understand the principles of mechanical ventilation in order to administer support to patients in need. This includes:
- Ventilation: The process of moving air into and out of the lungs.
- Oxygenation: The process of absorbing oxygen into the bloodstream.
- Lung compliance: The lung’s ability to expand and contract.
- Airway resistance: The impedance of airflow through the respiratory tract.
- Deadspace ventilation: The volume of ventilated air that does not participate in gas exchange.
- Respiratory failure: The inability of the lungs to oxygenate the blood or remove carbon dioxide from the body.
Each principle is important in determining the amount of ventilatory support that is delivered to the patient by the machine.
What is a Mechanical Ventilator?
A mechanical ventilator is a breathing machine that uses positive pressure to deliver ventilatory breaths to patients who are in need of assistance. The machine consists of several parts that work together to generate positive pressure that helps force air into the lungs.
Mechanical ventilation is an intervention that can provide short or long-term support while the patient’s underlying condition is treated.
It is often indicated for patients with cardiopulmonary disorders but is also common in postoperative patients who are recovering from anesthesia.
How Does a Ventilator Work?
Ventilators work by using positive pressure to deliver breaths to the patients. However, an artificial airway must be inserted into the patient’s trachea before being connected to the machine.
This process is known as intubation, which involves the insertion of an endotracheal tube through the mouth and into the trachea.
Once the tube is in place, it establishes a link between the patient and the ventilator so that positive-pressure breaths can be delivered.
Ventilators are not used to heal and treat a patient of their underlying disease. Rather, they are used to provide breathing support until the patient is stable and treated with medications and other modalities.
Mechanical Ventilation Benefits
There are many benefits for patients who are receiving mechanical ventilation, including the following:
- Decreases work of breathing: The ventilator assists with the patient’s breathing, which can help to decrease the amount of energy and work required for each breath.
- Maintains adequate oxygenation: The ventilator can deliver an FiO2 of up to 100% to help with oxygenation. It also can deliver positive end-expiratory pressure (PEEP), which is helpful in patients with refractory hypoxemia.
- Helps remove carbon dioxide: The ventilator can help the patient remove carbon dioxide from their body with an increased respiratory rate or tidal volume.
- Provides stability: The ventilator helps keep the patient stable, allowing medications and other modalities to reverse their underlying condition.
The benefits of mechanical ventilation often far outweigh the risks, which is why it is such a common intervention in the field of respiratory care. However, there are some complications that can occur.
Mechanical Ventilation Complications
Mechanical ventilation is necessary for patients who are critically ill; however, it does come with some risks and complications, including the following:
- Barotrauma: An injury to lung tissue that results in alveolar overdistention caused by increased levels of pressure.
- Ventilator-associated pneumonia (VAP): A type of pneumonia that develops 48 hours or more after a patient has been intubated and placed on the ventilator.
- Auto-PEEP: A complication of mechanical ventilation that occurs when a positive pressure remains in the alveoli at the end-exhalation phase of the breathing cycle.
- Oxygen toxicity: A type of cell damage that can occur when a patient is exposed to high levels of oxygen for an extended period of time.
- Ventilator-induced lung injury (VILI): An acute lung injury that occurs while a patient is receiving mechanical ventilatory support.
However, the risks and complications of mechanical ventilation can be minimized with proper care and monitoring by medical professionals.
Types of Mechanical Ventilation
There are four primary types of mechanical ventilation, each with its own indications, settings, contraindications, and risks. The different types include:
- Positive-pressure ventilation
- Negative-pressure ventilation
- Invasive mechanical ventilation
- Noninvasive ventilation
Positive-pressure ventilation is the most common type of mechanical ventilation. It’s known as “conventional mechanical ventilation” and is generally what people are talking about when they say that “someone is on the ventilator.”
This type works by using positive pressure that is greater than the atmospheric pressure to push air into the lungs. The air then fills the alveoli, where the exchange of oxygen and carbon dioxide takes place.
Negative-pressure ventilation is not as common as positive-pressure ventilation, but it may still be used in certain situations. This type works by generating negative pressure outside of the thoracic cavity that is less than atmospheric pressure.
As a result, air moves from an area of higher pressure (outside the body) to an area of lower pressure (inside the lungs). Some examples of negative-pressure ventilation include:
- Iron lung: A negative-pressure ventilator that was invented in the 1920s that was primarily used to treat patients with polio.
- Cuirass ventilation: A type of negative-pressure ventilation that is delivered through a tight-fitting garment that covers the chest and abdomen.
Invasive Mechanical Ventilation
Invasive mechanical ventilation is a type that involves the insertion of an artificial airway into the trachea. This establishes a direct connection between the ventilator and the patient’s lungs.
There are two primary types of artificial airways that can be used:
- Endotracheal tube
- Tracheostomy tube
An endotracheal tube is a long, thin tube that is inserted through the nose or mouth and then passed down the throat into the trachea.
A tracheostomy tube, on the other hand, is a shorter tube that is inserted through a small incision in the neck and then directly into the trachea.
Noninvasive ventilation (NIV) is a type of ventilatory support that doesn’t require the insertion of an artificial airway. It requires the use of a face mask that creates a tight seal over the patient’s nose or mouth.
This allows the machine to force oxygen-rich air into the patient’s lungs using positive pressure. The two primary types of NIV include:
Noninvasive ventilation is commonly indicated to improve oxygenation and ventilation and to provide relief for respiratory distress prior to intubation and conventional mechanical ventilation.
A ventilator mode is a setting that determines how the machine will deliver breaths to the patient. The characteristics of each mode determine how the ventilator functions.
Ventilator Control Variables
There are two primary control variables in mechanical ventilation:
- Volume control (VC): A type of ventilation where the delivered volume can be set (i.e., controlled) by the operator. Since the delivered volume is fixed, the patient’s peak inspiratory pressure (PIP) will vary depending on their lung compliance and airway resistance.
- Pressure control (PC): A type of ventilation where the delivered level of pressure can be set (i.e., controlled) by the operator. Since the delivered pressure is fixed, the patient’s tidal volume will vary depending on their lung compliance and airway resistance.
The primary advantage of volume-controlled ventilation is that a set volume allows the operator to regulate the patient’s minute ventilation.
The primary advantage of pressure-controlled ventilation is that it protects the lungs from overinflation due to too much pressure, which prevents barotrauma and ventilator-induced lung injuries.
Types of Ventilator Modes
There are several types of ventilator modes, including the following:
- Assist/Control (A/C)
- Synchronous Intermittent Mandatory Ventilation (SIMV)
- Pressure Support Ventilation (PSV)
- Continuous Positive Airway Pressure (CPAP)
- Volume Support (VS)
- Control Mode Ventilation (CMV)
- Airway Pressure Release Ventilation (APRV)
- Mandatory Minute Ventilation (MMV)
- Inverse Ratio Ventilation (IRV)
- High-Frequency Oscillatory Ventilation (HFOV)
Each ventilator mode is different and has its own characteristics. This includes unique settings and how the machine will deliver breaths to the patient.
We’ve covered each mode in more detail in our comprehensive guide to the modes of mechanical ventilation; however, here’s a brief overview of the two primary modes:
- Assist/Control (A/C)
- Synchronous Intermittent Mandatory Ventilation (SIMV)
The assist/control (A/C) mode is used to deliver a minimum number of preset mandatory breaths by the ventilator, but the patient can also trigger assisted breaths.
Therefore, the patient can make an effort to breathe, and the machine will use positive pressure to assist in delivering the breath.
This mode provides full ventilatory support; therefore, it is commonly used when mechanical ventilation is first initiated. It helps keep the patient’s work of breathing requirement very low.
Synchronous Intermittent Mandatory Ventilation (SIMV)
The synchronous intermittent mandatory ventilation (SIMV) mode delivers a preset minimum number of mandatory breaths, but it also allows the patient to initiate spontaneous breaths in between the preset breaths.
Since the patient is able to initiate spontaneous breaths, it means they are contributing to some of their minute ventilation. Therefore, SIMV is indicated when a patient only needs partial ventilatory support.