Mechanical Ventilation Explained

Modes and Terms of Mechanical Ventilation

Most modern mechanical ventilators are positive pressure ventilation.  An iron lung is an example of negative pressure ventilation.   A ventilator is a device used to support, assist or control respiration (inclusive of the weaning period) through the application of positive pressure to the airway when delivered via an artificial airway, specifically oral/nasal endotracheal or tracheostomy tube. 

Note: Ventilation and lung expansion devices that deliver positive pressure to the airway (for example: CPAP, Bipap, bi-level, IPPB and PEEP) via non-invasive means (for example: nasal prongs, nasal mask, full face mask, total mask, etc.) are not considered ventilators unless positive pressure is delivered via an artificial airway (oral/nasal endotracheal or tracheostomy tube).

Pressure-cycled ventilators: Gas is allowed to flow into the lungs until a present airway pressure limit is reached, at which time a valve opens allowing exhalation to ensue. The volume delivered by the ventilator varies with changes in airway resistance, lung compliance, and integrity of the ventilatory circuit.

Volume-cycled ventilators: Gas flows to the patient until a preset volume is delivered to the ventilator circuit, even if this entails a very high airway pressure.

It is important for clinical staff working with individuals on mechanical ventilation to understand the different modes.  Individuals require mechanical ventilation for different reasons.  It is used for individuals with respiratory failure who are unable to breathe on their own.  Different modes in mechanical ventilation help to determine how far along a patient is in the weaning process and the patient’s current respiratory status.

Controlled Mechanical Ventilation (CMV)

In controlled mechanical ventilation, the ventilator provides a mechanical breath on a preset timing.  Patient respiratory efforts are ignored.  This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient.  It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing. 

Assist Control

In assist control, the ventilator delivers preset volume or pressure when triggered by negative inspiratory effort above the triggering threshold. The patient may trigger additional machine assisted breaths above the set rate.  If patient does not trigger the ventilator with sufficient inspiratory effort, the ventilator automatically delivers a preset volume, ensuring minimum minute ventilation.  The clinician sets the tidal volume, back-up rate, sensitivity and flow rate. 

An example of ventilator settings is assist control of 12 (set rate) and volume control of 600. When the patient takes a deep enough breath to trigger the ventilator, the patient is provided with the preset volume of 600. If the patient does not take enough breaths on their own (spontaneous breaths), then the ventilator will provide the breath.  In this case, if the patient does not take any spontaneous breaths, the ventilator would provide 12 breaths.

Synchronized Intermittent Mechanical Ventilation (SIMV)

In synchronized intermittent mechanical ventilation (SIMV) the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 seconds – thus a respiratory rate of 12 results in a 5 second cycle time). Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor. When the ventilator senses the first patient breathing attempt within the cycle, it delivers the preset ventilator breath. If the patient fails to initiate a breath, the ventilator delivers a mechanical breath at the end of the breath cycle. Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath. However, SIMV may be combined with pressure support (see below). SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate, which requires the patient to take additional breaths beyond the SIMV triggered breath.

Pressure Support Ventilation (PSV)

In pressure support ventilation, a fixed amount of pressure (set by the clinician) augments each breath.  Pressure support ventilation was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that “supports” ventilation during inspiration.  Thus, for example, SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported.  However, while the SIMV mandated breaths have a preset volume or peak pressure, the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (e.g. 10-25%).  The peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath. PSV can be also be used as an independent mode.  

In pressure support ventilation, the patient has control over the rate, inspiratory and inspiratory flow rate.  The tidal volume is determined by the level of PSV, patient effort and pulmonary mechanics.

Continuous Positive Airway Pressure (CPAP)

A continuous level of elevated pressure is provided through the patient circuit when in continuous positive airway pressure mode. No cycling of ventilator pressures occurs and the patient must initiate all breaths. In addition, no additional pressure above the CPAP pressure is provided during those breaths. CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs.

Terms of Mechanical Ventilation

Positive End Expiratory Pressure

Positive End Expiratory Pressure (PEEP) is a technique in which airway pressure greater than atmospheric pressure is achieved at the end of exhalation by the introduction of a mechanical impedance to exhalation. In patients on mechanical ventilation, PEEP is one of the key parameters that can be adjusted depending on the patient’s oxygenation needs, and is typically in the range of 0 to 15 cmH2O. 

 Positive end expiratory pressure is functionally the same as CPAP, but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle.  After delivery of the set amount of breath by the ventilator, the patient then exhales passively. The volume of gas remaining in the lung after a normal passive expiration is termed the functional residual capacity (FRC). The FRC is primarily determined by the elastic qualities of the lung and the chest wall.  In many lung diseases, the FRC is reduced due to collapse of the unstable alveoli, leading to a decreased surface area for gas exchange and intrapulmonary shunting, with wasted oxygen inspired.  Adding PEEP can reduce the work of breathing (at low levels) and help preserve and increase FRC, by preventing alveolar collapse. 

Fraction of inspired air (Fi02)

The fraction of oxygen in inspired gas. For example, the FiO2 of ambient air is 0.21; the oxygen concentration of ambient air is 21%. In patients on mechanical ventilation, the FiO2 is one of the key parameters that can be adjusted depending on the patient’s oxygenation needs, and is typically in the range of 0.30 (oxygen concentration of 30%) to 1.0 (oxygen concentration of 100%)

Tidal Volume (TV)

Tidal volume is the lung volume representing the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. In a healthy, young human adult,tidal volume is approximately 500 mL per inspiration or 7 mL/kg of body mass.  Tidal volume can be set on a ventilator to deliver a set amount of volume.  

I:E ratio

 Inspiratory time to expiratory time ratio.  Normal is 1:2 to 1:4 or 5.

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