Mechanical Ventilator Settings and Basic Modes

Understanding mechanical ventilator settings and the common modes of mechanical ventilation is the key to successful mechanical ventilation. This article covers the common mechanical ventilator settings and common modes of ventilation.

The initial ventilator settings are as follows:

• The initial mode is usually assist-control mode
• Tidal volume setting is dependent of the lung status. Normal tidal volume is 12 mL/kg ideal body weight; in patients with COPD, the tidal volume is 10 mL/kg ideal body weight and in patients with ARDS it is set to 6-8 mL/kg ideal body weight
• Respiratory Rate is set to 10-12 breaths per minute
• Fraction of inspired oxygen (FIO2) is set at 100%
• A Sigh is not necessary
• PEEP setting is dependent of the first arterial blood gas, for example ie, shunt greater than 25% and an inability to oxygenate with an FIO2 less than 60%

Usually the initial mode of ventilation is the assist-control mode. In this mode, the tidal volume and rate are preset and guaranteed to be delivered to the patient. The frequency and timing of the breaths is then affected by the patient.

If the patient does not initiate inspiration, the ventilator automatically delivers the preset rate and tidal volume. This is to ensure minimum minute ventilation is achieved.

Some ventilator settings are common between conventional modes of ventilation. These common ventilator settings are;

• Fraction of inspired oxygen
• Positive end expiratory pressure
• Trigger sensitivity

A fraction of inspired oxygen (FIO2) is the amount of oxygen being delivered to the patient. The minimum setting for FiO2 can only be set as low as 21% which is equal to the normal fraction of oxygen in the atmosphere.

The aim of mechanical ventilation is to provide effective oxygenation to the patient. The FIO2 should always be set at 100% until adequate arterial oxygenation is achieved and for the patient's safety after intubation.

Setting an FIO2 of 100% usually protects the patient from hypoxemia incase any difficulty or problem occur as a result of the intubation procedure.

An FIO2 of 100% together with PaO2 helps to easily calculate the next desired FIO2 and estimate the shunt fraction.

The FiO2 should then be quickly reduced to the minimal level required to maintain adequate oxygenation. This is to avoid barotrauma.

Positive End Expiratory Pressure (PEEP) is defined as the pressure maintained in the respiratory system at the end of exhalation. This pressure serves a purpose of trying and maintaining the lungs open by preventing atelectasis. PEEP is generally in at a minimum level of 5 centimeters of water.

PEEP therapy is effective when it is used in patients with a diffuse lung disease resulting in an acute reduction in functional residual capacity (FRC). A functional residual capacity is the volume of gas that remains in the lung at the end of a expiration.  A reduction of FRC is because of the collapse of the unstable alveoli.

Applying PEEP increases alveolar pressure and alveolar volume.

Trigger sensitivity is a criteria used by the mechanical ventilator to determine if the patient is making an effort. There are two triggers; the flow trigger and pressure trigger.

Flow trigger is defined as a change in baseline flow that is required for the ventilator to determine if a breath has been requested by the ventilated patient. This flow trigger is measured in liters per minute.

Pressure trigger is the change in baseline pressure measured in centimeters of water that is required for the ventilator to determine if a breath has been requested by the ventilated patient.

Tidal volume and rate

The rule of 12-12 is used in determining the tidal volume and rate in patients without a preexisting lung disease. This means that a tidal volume of 12 mL/kg of ideal body weight is delivered 12 times per minute in the assist-control mode.

In patients with COPD, the tidal volume and rate are set using the 10-10 rule in order to prevent overinflation of the lungs, hyperventilation, and auto-PEEP. This means that a tidal volume of 10 mL/kg ideal body weight is delivered 10 times a minute in the assist-control mode.

Patients having an acute respiratory distress syndrome receive a lower tidal volume of 6-8ml/kg ideal body weight for their lungs to function well and avoid volutrauma. This strategy is known as lung-protective ventilation

An ideal body weight is the patient's predicted weight that is based on height and birth gender but not their actual body weight.

When using assist control modes, the respiratory rate should be set at least high enough so as to achieve a minute ventilation that is predicted for the patient. The respiratory rate can be set even higher if the patient has a known acid base imbalance during the time of intubation.

When using a low tidal volume, sighs are set at 1.5-2 times the tidal volume and are delivered 6-8 times an hour if the peak preasure and plateau pressures are within the normal limit. This is to prevent development of microatelectasis.

How do you determine predicted minute ventilation?

Predicted minute ventilation is determined by multiplying ideal body weight of the patient by 100 milliliters per minute.

Take an example, you have a patient with ideal body weight of 60 kgs, the minute ventilation for this patient is 60*100= 6000 milliliters per minute (6L/minute).

What are the common modes of ventilation?

There are three most commonly used modes in mechanical ventilation

1. Pressure assist control
2. Volume assist control and
3. Pressure support

Pressure assist control and volume assist control modes are usually used in acute phase of mechanical ventilation or when the patient has minimal or no drive to breath.

These modes are classified as control modes because they do 100% of the work for the patients while on the ventilator. Therefore they are the most efficient for the patient in complete respiratory failure.

On the other hand, pressure support is used in patients who have an intact respiratory drive.

After getting an overview, let’s have a look at these modes and their settings.

Volume Assist Control (AC-VC)

This mode requires a frequency of respirations per minute to be set. Patients who are attached to the ventilator then can trigger additional breaths that are greater than the set respirations per minute. If these patients cannot meet the trigger criteria, then the machine takes over and triggers all of the breaths.

Assist control modes share the common setting of requiring a frequency, which is a respiration per minute. And patients can trigger some of the breaths or more breaths than what is set by the ventilator if they would like.

If the trigger sensitivity criteria is met, the ventilator will deliver a breath to the patient, and if there is no trigger criteria being met, the ventilator will deliver a breath at the set frequency.

In volume assist control, the trigger sensitivity is flow, and it is set at 2 liters per minute.

When patients begin to actively interact with the ventilator, a trial of a spontaneous mode, such as pressure support, should be considered.

One of the setting that needs to be set is tidal volume. The normal tidal volume is 6-8ml/kg of an idea l body weight.

How do you calculate an ideal body weight?

The following formula is used in calculating ideal body weight for women and men.

For men

Ideal body weight =50+2.3 x(height in inches-60)

For women

Ideal body weight= 45+2.3 x (height ininches-60).

The reason behind using an ideal body weight instead of an actual body weight is because. An actual body weight will overestimate the tidal volume required.

Patients are required to be ventilated with a lower tidal volume of 6-8ml/kg.

Another setting is the flow.

Inspiratory flow

Flow is the speed to deliver the required tidal volume. Flow is given in litres per minute. And once the target volume is reached, the breath delivery has ended.

A normal inspiratory flow is set between 50-60 Litres per minute is set to minimize discomfort when a patient starts to make an effort.

The frequency is set to 14 breaths per minute. And this gives us a minute ventilation, which commonly has the label VE,

A positive expiratory pressure (PEEP) is set at 5cmH2O so as to reduce atelectasis or collapse of the lung alveoli.

Inspiratory: Expiratory ratio

I:E Ratio is also affected by respiratory rate.

The normal set I:E ratio is 1:1.5 and 1:2.

A normal person has an inspiratory time between 0.75 second to 1 second. But in some respiratory diseases such as asthma, the airway is usually obstructed. In these patients it is usually beneficial to allow more time for exhalation to occur by increasing the inspiratory flow or decreasing the I:E ratio to about 1:3 or 1:4.

The inspiratory-to-expiratory ratio is actually the simplest parameter to de