Ventilators are essential medical equipment used to aid patients with respiratory difficulties. Their primary function is to assist or replace spontaneous breathing by providing mechanical ventilation. There are two general classes of mechanical ventilators: negative-pressure and positive-pressure ventilators.

Negative-Pressure Ventilators

Negative-pressure ventilators create a vacuum around the chest or body to draw air into the lungs, simulating breathing. This method does not require an artificial invasive airway; as a result, it is categorized as a form of non-invasive ventilation (NIV).

The "iron lung," developed in the 1950s during the polio epidemic, was the first form of negative pressure ventilation. It intermittently reduces intrathoracic pressure, causing air to rush into the upper airway, followed by passive expiration when the machine cycles off. Portable negative-pressure ventilators are available for home use, primarily for patients with chronic neuromuscular diseases and central nervous system problems.

Positive-Pressure Ventilators

Positive-pressure ventilators differ from negative-pressure ventilators in their mechanism of lung inflation. Instead of exerting negative pressure to draw air into the lungs, positive-pressure ventilators apply positive pressure to the airway, expanding the alveoli and facilitating inspiration. Expiration occurs passively. These ventilators require ET intubation or tracheostomy and are widely used in hospitals and homes.

Volume-Cycled Ventilators

Volume-cycled ventilators deliver a preset volume of air with each inspiration. The ventilator cycles off after this volume is delivered, allowing passive exhalation. Volume-cycled ventilators ensure consistent breaths but may cause barotrauma due to the potentially excessive pressures needed to deliver breaths.

Pressure-Cycled Ventilators

Pressure-cycled ventilators deliver airflow until a preset pressure is reached, and then they cycle off for passive expiration. The air volume can vary with changes in the patient's airway resistance or compliance, possibly leading to inconsistent tidal volumes and compromised ventilation.

High-Frequency Oscillatory Support Ventilators

These ventilators deliver high respiratory rates, low tidal volumes, and high airway pressures. They are used to open alveoli in situations characterized by closed small airways, such as atelectasis and ARDS, and are believed to protect the lungs from pressure injury.