Mechanical ventilation modes of ventilation

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Vishnu Vardhan Serla M.B.B.S. [2]

Modes of Ventilation

The modes of ventilation are as follows:

Conventional Ventilation

• The modes of air flow can be a concept of as classifications based totally on the way to manage the ventilator breath.
• Historically ventilators have been labeled based on how they decided whilst to stop giving a breath.
• As microprocessor is integrated into ventilator, the difference amongst those sorts has grown to be less clear as ventilators may use mixtures of all of those modes in addition to float-sensing, which controls the ventilator breath based on the flow rate of gas as opposed to a specific extent volume, pressure and time.

Breath Termination

• In a volume-cycled ventilator the ventilator delivers a preset volume of gas with each breath, the positive pressure is terminated once the specified volume of breath is delivered.
• In a pressure-cycled ventilator, the breath is terminated once a preset pressure is reached within the ventilator.
• In a time-cycled ventilator, the termination of the breath occurs after a certain specified time period.

• Pressure and volume modes of ventilation have their own limitations.
• These modes are flow-variable, volume centered, pressure regulated, time-limited modes, which means that in place of providing a precise tidal volume every breath, a goal volume is about and the ventilator will vary the inspiratory flow at every breath to obtain the goal extent at the lowest feasible top peak pressure.
• The inspiratory time deadlines the length of the inspiratory cycle and consequently the I: E ratio.
• Pressure regulated modes together with PRVC or Draeger can maximum effortlessly be a notion of as turning an extent mode right into a pressure mode with the introduced advantage of retaining more control over tidal volume than with strictly strain-manage.

Breath Initiation

• The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath.
• Similar to the termination classification noted above, microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications.
• Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above.

• Assist Control (AC)
  • In this mode, the ventilator provides a mechanical breath with either a preset tidal volume or peak pressure every time the patient initiates a breath.
  • Traditional assist-control used only a preset tidal volume when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV.
  • However the initiation timing is the same--both provide a ventilator breath with every patient effort. In most ventilators, a backup minimum breath rate can be set in the event that the patient becomes apneic.
  • Although a maximum rate is not usually set, an alarm can be set if the ventilator cycles too frequently.
  • This can alert that the patient is tachypneic or that the ventilator may be auto-cycling a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt.
• Synchronized Intermittent Mandatory Ventilation (SIMV)
  • In this mode the ventilator provides a preset mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - 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.
• Controlled Mechanical Ventilation (CMV)
  • In this mode 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.
• Pressure guide ventilation (PSV)
  • This become developed as a technique to decrease the work of breathing in-among ventilator mandated breaths. consequently, for example, SIMV might be blended with PSV so that extra breaths beyond the SIMV programmed breaths are supported.
  • However, even as the SIMV mandated breaths have a preset volume or top pressure, the PSV breaths are designed to reduce quickly when the inspiratory flow reaches a percent of the peak inspiratory glide (e.g. 10-25%). also, the peak strain set for the PSV breaths is often a decreased strain than that set for the SIMV breath.
  • PSV may be also be used as an independent mode. However, considering the fact that there is normally no returned-up rate in PSV, suitable apnea alarms have to be set on the ventilator.
• Continuous Positive Airway Pressure (CPAP)
  • A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation, decrease the work of breathing, and decrease the work of the heart (such as in left-sided heart failure - CHF).
  • Note that 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.
• Positive End Expiratory Pressure (PEEP)
  • Its 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 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 FRC.

• High Frequency Ventilation (HFV)

High Frequency ventilator

• High-Frequency Ventilation refers to ventilation that occurs at rates significantly above that found in natural breathing (as high as 300-900 "breaths" per minute).
• Within the category of high-frequency ventilation, the two principal types are flow interruption and high-frequency oscillatory ventilation (HFOV).
• The former operates similarly to a conventional ventilator, providing increased circuit pressure during the inspiratory phase and dropping back to PEEP during the expiratory phase.
• In HFOV the pressure wave is driven by an electromagnetically controlled diaphragm similar to a loudspeaker. Because this can rapidly change the volume in the circuit, HFOV can produce a pressure that is lower than ambient pressure during the expiratory phase.
• This is sometimes called "active" expiration. In both types of high-frequency ventilation the pressure wave that is generated at the ventilator is markedly attenuated by passage down the endotracheal tube and the major conducting airways.
• This helps protect the alveoli from volutrauma that occurs with traditional positive pressure ventilation.
• Although the alveoli are kept at a relatively constant volume, similar to CPAP, other mechanisms of gas exchange allow ventilation (the removal of CO2) to occur without tidal volume exchange.
• Ventilation in HFV is a function of frequency, amplitude, and I:E ratio and is best described graphically as the area under the curve of an oscillatory cycle.
• Amplitude is analogous to tidal volume in conventional ventilation; larger amplitudes remove more CO2.
• Paradoxically, lower frequencies remove more CO2 in HFOV whereas in conventional ventilation the opposite is true.
• As frequency increases, the total time for a single cycle decreases (the oscillatory curve is shortened thereby decreasing the area under the curve and thus ventilation). I-time is set as a percentage of total time (usually 33%).
• Amplitude is a function of power and is subject to variability due to changes in compliance or resistance. Therefore, power requirements may vary significantly during treatment and from patient to patient.
• Patient characteristics and ventilator settings determine whether PaCO2 changes may be more sensitive to amplitude or frequency manipulation.
• In HFOV, mean airway pressure (MAP) is delivered via a continuous flow through the patient circuit which passes through a variable restriction valve (mushroom valve) on the expiratory limb. Increasing the flow through the circuit and/or increasing the pressure in the mushroom valve increases MAP.
• The MAP in HFOV functions similarly to PEEP in conventional ventilation in that it provides the pressure for alveolar recruitment.