Basics of Ventilators: Part 2

Introduction

This article builds on the information provided in Ventilators Part 1 and will describe the different ventilator modes. This article will discuss:

• Pressure-controlled ventilation
• Controlled, mandatory ventilation
• Controlled, intermittent ventilation
• Pressure-support ventilation

Understanding the disease process helps the provider select the appropriate mode of ventilation.

Related Video – Ventilator Basics – Part 1

Pressure-controlled Ventilation

Pressure and volume are the primary adjustable variables. In pressure-controlled ventilation, the provider predetermines and presets the pressure, giving them total control over the amount of pressure delivered to the patient. 

Pressure-controlled ventilation is most useful in disease processes that cause the lungs to become less compliant, often due to acute respiratory distress syndrome (ARDS). 

While the pathophysiology of ARDS is complex, patients ultimately end up with stiff lungs that are not compliant and do not readily accept pressure and volume. If these patients are given too much pressure, they may experience barotrauma and volutrauma. When these complications occur, the ventilator can damage healthy lung tissue.

Pressure-controlled modes are useful when the lungs are not compliant. However, when the volume-controlled mode is used for a noncompliant lung, the ventilator will give the patient as much air as it takes to reach the predetermined volume. If the pressure is too high, barotrauma and volutrauma can result. Therefore, the preferred ventilator mode for noncompliant lungs is the pressure-controlled mode.

A chest X-ray displays acute respiratory distress syndrome (ARDS), a disease process that causes the lungs to become stiff and noncompliant.

Read: Basics of Ventilators: Part 1

In pressure-controlled ventilation, the provider sets:

• Peak inspiratory pressure (PIP)*
• Respiratory rate (F)
• Inspiratory time (T)*
• Positive end-expiratory pressure (PEEP)
• Fraction of inspired oxygen (FiO₂)

*PIP and T are the two settings that make pressure-controlled ventilation unique. 

With disease processes similar to ARDS, a high peak inspiratory pressure can lead to barotrauma and volutrauma. When there is resistance within the airway, such as bronchospasm and mucus plugs, the peak inspiratory pressure increases.

Another factor to consider with pressure-controlled ventilation is the plateau pressure. Peak inspiratory pressure is the highest amount of pressure applied at the beginning of inspiration, while plateau pressure is the amount of pressure that is sustained throughout the inspiratory cycle. 

The lung’s compliance determines plateau pressure. With noncompliant lungs, the plateau pressure can increase. The ability to preset the peak inspiratory pressure and the respiratory time allows the provider to have more control over the plateau pressure.

In pressure-controlled ventilation, the peak inspiratory pressure can be titrated to meet a determined tidal volume. The amount of pressure delivered can be adjusted to meet the optimal tidal volume.

The ability to titrate to a determined tidal volume makes pressure-controlled ventilation useful in treating patients with neuromuscular diseases. These patients may be able to initiate spontaneous breaths but require extra pressure to pull in the tidal volumes needed for adequate ventilation and oxygenation.

Advantages of Pressure-controlled Ventilation

Pressure-controlled ventilation is excellent for disease processes where lung compliance is low. There is much less risk for barotrauma and volutrauma because the pressure is predetermined.

With ARDS and other disease processes of low lung compliance, oxygenation can be difficult. Pressure-controlled ventilation is useful because it allows for improved oxygenation.

There is also a better chance for patient and ventilator synchrony when using pressure-controlled ventilation. This mode is more comfortable for patients because the ventilator responds to the patient’s inspiratory effort.

Consider pressure-controlled ventilation for diseases where lung compliance is low, such as ARDS.

Disadvantages of Pressure-controlled Ventilation

The downside of pressure-controlled ventilation is the tidal volume and the minute ventilation will vary based on airway resistance and lung compliance. As airway resistance or lung compliance changes, the tidal volume and minute ventilation will also change.

There is also a risk for hypoventilation, especially in patients who have disease processes where they can’t take full, spontaneous breaths.

Controlled Versus Intermittent Mandatory Ventilation

The two modes that are used most often in volume-controlled or pressure-controlled ventilation are:

• Controlled, mandatory ventilation (CMV). his mode provides maximum ventilatory assistance. The goal for the patient is to decrease the work of breathing. In this mode, the amount of ventilation the patients do for themselves should decrease.

Patients on CMV are in acute respiratory failure. Patients will receive a predetermined amount of pressure, whether the ventilator or patient triggers initiates the breath.

• Intermittent, mandatory ventilation (IMV). In this mode, the patient initiates and triggers breaths on their own. When the patient initiates a spontaneous breath, they determine the pressure and inspiratory time.

IMV mode is reserved for patients who are not as sick or may be weaning off the ventilator. There is less ventilatory assistance, so the patient’s tidal volumes, minute ventilation, and arterial blood gases need to be monitored to ensure the patient is ventilating and oxygenating appropriately.

Related Video – What is Adaptive Support Ventilation?

Pressure-support Ventilation

Another form of ventilation is pressure-support ventilation. Pressure-support ventilation is used for patients who are fully, spontaneously breathing and is used as a weaning mode. This mode is for patients who are ready to fully come off the ventilator.

The main difference of pressure-support ventilation compared to other ventilation modes is that the patient triggers each breath. The ventilator is simply there to provide additional pressure to the patient’s breath to help them achieve a tidal volume.

Pressure-support ventilation is important because this is the mode used to wean the patient off the ventilator. 

To determine if a patient is ready to be extubated, the provider completes a spontaneous breathing trial (SBT). This process involves using the lowest settings on the ventilator and assessing the patient’s ability to maintain ventilation and oxygenation. Patients are typically extubated when they achieve pressure support of 5–10 mm Hg and a PEEP of 5 on 40% FiO₂.

Understanding ventilators and their modes is essential to providing critical care. It is essential the correct mode is chosen to meet the emergency medical situation.