A pneumatic circuit, or pathway, is a series of tubes that allow gas to flow inside the ventilator and between ventilator and the patient. The pressure gradient created by the ventilator with its power source generates the flow of gas. This flow passes through the pneumatic circuit to the patient. The gas first is directed from the generating source inside the ventilator through the internal pneumatic circuit to the ventilator's outside surface. Gas then flows through an external circuit, or patient circuit, to the patient . It is exhaled by the patient and passes through an expiratory limb and exhalation valve.

 

Internal Pneumatic Circuit: If the ventilator's internal circuit allows the gas to flow directly from its power source into the patient, the machine is called a single circuit ventilator . Most lCU ventilators manufactured today are single circuit ventilators.

 

Another type of internal pneumatic circuit ventilator is the double circuit ventilator . In these machines, the primary power source generates a gas flow that compresses a mechanism such as a bag or bellows. The gas in the bag then flows to the patient.

 

Most commercially available ICU ventilators are single circuit, microprocessor controlled, positive pressure ventilators with closed loop elements of logic in the control system.

 

External Pneumatic Circuit: The external pneumatic circuit, or patient circuit, connects the ventilator to the patient's artificial airway. This circuit must have several basic elements to provide a positive pressure breath .

 

Basic Elements of a Patient Circuit : 1. Main inspiratory line, which connects the ventilator output to the patient's airway adapter or connector. 2. Adapter, which fits the main inspiratory line to the patient's airway (also called a patient adapter or Y-connector because of its shape) 3. Expiratory line, which carries expired gas from the patient to the exhalation valve 4. Expiratory valve, which releases exhaled gas from the expiratory line into the room air

 

These essential parts are aided by other components, which are added to the circuit to optimize gas delivery and ventilator function.

 

Adjuncts Used with a Patient Circuit 1. A device to warm and humidify inspired air 2. A thermometer to measure the temperature of inspired air 3. An apnea or low-pressure alarm that indicates leaks or that the patient is not ventilating adequately 4. A nebulizer line to power a micronebulizer for delivery of aerosolized medications 5. A volume measuring device to determine the patient's exhaled volume' 6. Bacterial filters to filter gas administered to the patient and exhaled by the patient 7. A pressure gauge to measure pressures in the upper airway