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Clinical Case

First, we are going to need a functioning ventilator. Let’s review the basic components of a ventilator circuit, then learn how to set one up!

Assembling the ventilator

At RNSH, we use the Drager Evita V500. Like a number of modern ICU ventilators, instead of depending on valves to regulate an external compressed (and thus pressurised) gas source, this ventilator uses a turbine to deliver gas flow to the patient.

At its most basic, the ventilator requires a (1) power source, (2) a source of fresh gas (oxygen and medical compressed air) and (3) tubing to connect to the patient.

Starting at the wall oxygen, try to identify each component of your ventilator circuit and describe its role.

Hover over each component for more details (there are 6 to find).

Now let’s review how to set the ventilator up before our patient arrives from the ED.

Ventilator and Circuit Testing

Having connected each of the component parts, the ventilator must be tested before starting ventilation and every time the ventilator circuit is changed.

A) Resistance and compliance of the ventilator tubing
B) O2 and Air Supply
C) Humidification of the circuit
D) Leaks within the circuit
E) Calibration of O2 sensors

All except (C) Humidification of the circuit. 

Routine testing of modern ventilators usually involves a device check (checking gas supply, O2, flow and pressure calibration, alarms and safety valves), and a breathing circuit check (checking circuit leaks, compliance and resistance).

 

Different breathing circuits have different volumes and are made of different materials. With a change in circuit there may be small changes in dead space, resistance and compliance of the circuit which, as we will discuss in the next section, determine the volume of gas delivered to the patient, and the flow and pressures with which the breath is given.

During the ventilator check, the type of circuit is directly entered into the ventilator, then the ventilator checks that the circuit performs as expected.

Let’s review some these principles below: these are fundemental concepts that apply to both the ventilator circuit and the patient.

Ventilator circuit compliance, resistance and dead space

‘Compliance’ refers to a change in volume for a given change in pressure.

It is a measure of the ease with which something is distended or stretched. It is measured as ml/ cm H2O.

Usually, when considering compliance of the respiratory system, we are referring to pulmonary compliance – the ‘stretchability’ of lung tissue. We will discuss this further later on. When testing the ventilator, we also need to consider the circuit compliance.

Circuit ‘compliance’ refers to the change in circuit volume for a given change in pressure.

Highly compliant circuits increase their volume as pressure increases (more stretchable), where less compliant circuits do not change their volume.

Differing circuit compliance can influence the volume of gas delivered by the ventilator at a set pressure. Thus, it is important that adjustments be made for every new circuit attached to the ventilator so that accurate volumes are of gas are delivered to the patient.

‘Resistance’ refers to the change in pressure for a given change in flow. It is a measure of the opposition to gas flow through the circuit and airway.

Again, this can refer to both resistance within the patient (airways and tissue frictional resistance), or circuit resistance.

Circuit ‘resistance’ refers to the change in pressure for a given change in flow within the circuit tubing. Circuits with high resistance generate high pressures for a given rate of gas flow. Each of the following components of the circuit can increase the resistance to gas flow within the circuit.

  • Valves
  • Filters
  • Heat and Moisture Exchange (HME) devices
  • The endotracheal tube (ETT)

We will discuss it further later on, but many modern ventilators have an “Automatic Tube Compensation (ATC)” mode to compensate for circuit resistance. This setting applies additional pressure during inspiration via closed-loop control of continuously calculated tracheal pressure to compensate for the higher resistance and increased work of breathing associated with the ETT.

As narrower tubes have a higher resistance, the ETT size needs to be programmed into the ventilator.

Lets review this again later on…

A) Valves
B) Filters
C) HME
D) The ETT

The inner diameter of the circuit is the most important determinant of resistance to flow, and to a lesser degree length of the circuit (recall the Hagen-Poiseuille equation). Inner diameter is smallest at the ETT.

You will therefore be relieved to know that the patient in this case was intubated with an ETT size 8.0 rather than a smaller one.

‘Dead space’ refers to the volume of gas that is inspired but does not participate in gas exchange.

In a ventilated patient, this volume comprises three volumes:

  1. Anatomical Dead Space: The volume of gas in the conducting zones of the lung- the trachea, and bronchi.
  2. Physiological Dead Space: This includes both anatomical dead space, as well as alveolar dead space- ventilated alveoli that are not participating in gas exchange, usually due to a pathological process.
  3. Circuit Dead Space: Volume of the ventilator circuit

Clearly, a circuit with a large volume will contribute to an increased dead space, compared to a smaller volume. It is key to note, however, that in a duel limb ventilator circuit, it is the dead space beyond the “Y” connector (between inspiratory and expiratory limbs) that contributes to dead space.

In certain circumstances, circuit dead space may be adjusted in order to manipulate alveolar ventilation and pCO2.

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