The Ventilator Circuit

INTRODUCTION — 

The ventilator circuit refers to the tubing that connects the ventilator to the patient, as well as any devices that might be connected to the circuit.

COMPONENTS OF THE VENTILATOR CIRCUIT — 

The ventilator circuit refers to the tubing that connects the ventilator to a patient, as well as any device that is connected to the circuit tubing. The most common devices include heaters and humidifiers, filters, suction catheters, and therapeutic aerosol generators (nebulizers and inhalers)

Circuit tubing — 

The ventilator circuit tubing is generally corrugated plastic (22 mm inside diameter for adults), which has universal connectors (22 mm outside diameter, 15 mm inside diameter) that connect the ventilator to the endotracheal tube (ETT), tracheostomy tube, or noninvasive interface. There are two types of circuits commonly used:

• Single limb circuit
• Dual limb circuit

Characteristics of the ventilator circuit — 

Dead space and compression volume are important characteristics of the ventilator circuit.

Dead space –

Mechanical dead space is that part of the circuit through which the patient rebreathes, and is thus an extension of the anatomic dead space. Dead space is increased by adding volume between the Y-piece and the patient interface. Examples of increased mechanical dead space include excessive connecting tubing and passive humidifiers.

Compression volume –

When the circuit is pressurized, gas is compressed in the circuit and the volume of the circuit increases due to its compliance. As much as 3 to 5 mL/cm H2O can be compressed in the ventilator circuit.

Heaters and humidifiers — 

During mechanical ventilation, inspired gas must be warmed and humidified. This is necessary to prevent desiccated respiratory secretions, ETT (or tracheostomy tube) occlusion, lower respiratory tract airway occlusion, and consequently atelectasis, during mechanical ventilation.

Filters — 

Bacterial filters are sometimes placed at the inlet and the outlet of the ventilator circuit. The inspiratory filter is intended to minimize the delivery of microorganisms from the gas supply to the patient, whereas the expiratory filter is intended to minimize contamination of the environment from the exhaled breath of the patient. 

Suction equipment — 

Tracheal secretions are removed during mechanical ventilation by passing a suction catheter through the endotracheal tube. Closed suction systems are used in many hospitals. With a closed system, the suction catheter is part of the ventilator circuit and, therefore, the patient can be suctioned without being disconnected from the ventilator. With an open system, the patient is disconnected from the ventilator and then the suction catheter is passed through the ETT. 

Nebulizers and inhalers — 

Inhaled medications, typically bronchodilators, can be introduced into the ventilator circuit by a pressurized metered-dose inhalator, jet nebulizer, or mesh nebulizer. The choice of delivery device is usually determined by institutional and clinician preference.

Other inhaled gases —

 Gases other than air and oxygen can be delivered during mechanical ventilation. The most common are inhaled nitric oxide (INO) and heliox. Heliox is introduced to the gas inlet of the ventilator (ie, same place that oxygen or air is introduced); note that the ventilator must be compatible with heliox delivery. INO, however, is introduced directly into the ventilator circuit via a specific port/adaptor.

Monitoring devices — 

Monitoring devices might be introduced into the circuit, typically between the Y-piece and endotracheal or tracheostomy tube. They include carbon dioxide monitors (ie, capnography), pressure monitors, and flow sensors.

Bag-valve resuscitator — 

Bag-valve resuscitators are kept at the bedside of mechanically ventilated patients to allow emergency ventilation should there be a ventilator failure. Use of the bag-valve resuscitator should be minimized by using strategies such as closed suction and portable ventilators during patient transport. These devices are often contaminated.

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