Ventilators Health Article

Definition

A ventilator is a device used to provide assisted respiration and positive-pressure breathing.

Purpose

Ventilators are used to provide mechanical ventilation for patients with respiratory failure who cannot breathe effectively on their own. They are also used to decrease myocardial gas consumption or intracranial pressure, provide stability of the chest wall after trauma or surgery, and when a patient is sedated or pharmacologically paralyzed.

Description

Different types of ventilators can be programmed to provide several modes of mechanical ventilation. A brief overview of each type and mode follows.

Negative-pressure ventilators

The original ventilators used negative pressure to remove and replace gas from the ventilator chamber. Examples of these include the iron lung, the Drinker respirator, and the chest shell. Rather than connecting to an artificial airway, these ventilators enclosed the body from the outside. As gas was pulled out of the ventilator chamber, the resulting negative pressure caused the chest wall to expand, which pulled air into the lungs. The cessation of the negative pressure caused the chest wall to fall and exhalation to occur. While an advantage of these ventilators was that they did not require insertion of an artificial airway, they were noisy, made nursing care difficult, and the patient was not able to ambulate.

Positive-pressure ventilators

Postive-pressure ventilators require an artificial airway (endotracheal or tracheostomy tube) and use positive pressure to force gas into a patient's lungs. Inspiration can be triggered either by the patient or the machine. There are four types of positive-pressure ventilators: volume-cycled, pressure-cycled, flow-cycled, and time-cycled.

VOLUME-CYCLED VENTILATORS. This type delivers a preset tidal volume then allows passive expiration. This is ideal for patients with acute respiratory distress syndrome (ARDS) or bronchospasm, since the same tidal volume is delivered regardless of the amount of airway resistance. This type of ventilator is the most commonly used in critical care environments.

PRESSURE-CYCLED VENTILATORS. These ventilators deliver gases at a preset pressure, then allow passive expiration. The benefit of this type is a decreased risk of lung damage from high inspiratory pressures, which is particularly beneficial for neonates who have a small lung capacity. The disadvantage is that the tidal volume delivered can decrease if the patient has poor lung compliance and increased airway resistance. This type of ventilation is usually used for short-term therapy (less than 24 hours). Some ventilators have the capability to provide both volume-cycled and pressure-cycled ventilation. These combination ventilators are also commonly used in critical care environments.

FLOW-CYCLED VENTILATORS. Flow-cycled ventilators deliver oxygenation until a preset flow rate is achieved during inspiration.

TIME-CYCLED VENTILATORS. Time-cycled ventilators deliver oxygenation over a preset time period. These types of ventilators are not used as frequently as the volume-cycled and pressure-cycled ventilators.