Modes of mechanical ventilation


Modes of mechanical ventilation

Contents

Modes of mechanical ventilation are one of the most imporant aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. Mode selection is generally based on clinician familiarity and institutional preferences since there is a paucity of evidence indicating that the mode affects clinical outcome. The most frequently used forms of volume-limited mechanical ventilation are SIMV and AC.[1] Substantial changes in the nomenclature of mechanical ventilation over the years but more recently has become standardized by many respirology/pulmonology groups[2].

Positive and negative pressure ventilation

Negative pressure ventilation

Negative pressure ventilation works by producing an intermittent negative pressure around the chest and abdomen. Negative pressure moves across the chest and diaphragm and causes air to move into the lungs in the normal fashion[3]. When the negative pressure stops being applied, the chest returns to atmospheric pressure and the inspired air then is exhaled[3].

Disadvantages

Negative pressure ventilation has several disadvantages compared to positive-pressure ventilation, such as; The machines are less portable, more difficult to apply, and is infrequently used. Additionally in obstructive sleep apnea, negative pressure is actually contraindicated[3].

Types of negative pressure ventilators

  • Tank ventilators (see: Iron lung)
  • Body wrap ventilators (Wrap ventilators, also known as raincoat ventilators) This must be connected to an external negative pressure vacuum[3].
  • Cuirass (shell, turtleshell) ventilators consisting of a dome that fits on the chest. This must be connected to an external negative pressure vacuum[3].

Positive Pressure Ventilation

Positive pressure ventilation is achieved by applying positive pressure higher than atmospheric pressure at the airway opening. Increasing the pressure at the airway opening produces a pressure gradient that generates an inspiratory flow. This flow in turn results in the delivery of a breath.


Volume-controlled

Volume-controlled ventilation (formerly called volume-limited or volume-cycled ventilation) requires the clinician to set the peak flow rate, flow pattern, tidal volume, respiratory rate, positive end-expiratory pressure (applied PEEP), and fraction of inspired oxygen (FiO2). Inspiration ends after delivery of the set tidal volume.

Volume-controlled continuous mandatory ventilation (VC-CMV)

VC-CMV — Controlled Mechanical Ventilation (also called Controlled Volume Control) is the original name for Assist Control. When the addition of a patient initiated breath was added to the mode, Assist Control effectively replaced CMV entirely. Fundamentally, CMV is a volume-controlled mode where the tidal volume and frequency are set to deliver a minute volume with a complete disregard for patient effort.

The use of CMV requires the patient be completely unconscious, either pharmacokinetically or otherwise in a coma.

Since nomenclature of mechanical ventilation is only recently standardized[4] there are many different names that historically were used to reference CMV but now reference Assist Control.[4] Names such as: volume control ventilation, and volume cycled ventilation in modern usage refer to the Assist Control mode.

Assist control

AC — Assist Control (AC) is a mode of ventilation where breaths are delivered based on set variables. The patient may initate breaths by attempting to breathe. Once a breath is initated, either by the patient or by the ventilator the set tidal volume is delivered. Assist Control is also called Volume Control or Assist Control Volume Control (AC/VC).

This mode was created out of the need for patient-initiation in breaths. Fundamentally, AC is controlled mechanical ventilation (CMV) with a sensitivity for patient breathing.

The variables required in AC are: tidal volume (VT), respiratory rate (f), flow rate and trigger sensitivity (for sensing patient effort). Additional variables like peak-end expiratory pressure (PEEP) and pressure support (PS) may be added for additional support.

Expected outcomes and considerations

Assist Control is associated with profound diaphragm muscle dysfunction and atrophy.[5] AC is no longer the preferred mode of mechanical ventilation.[6]

Volume-controlled intermittent mechanical ventilation (VC-IMV)

VC-IMV — IMV is similar to AC in two ways: the minute ventilation is determined (by setting the respiratory rate and tidal volume); and the patient is able to increase the minute ventilation. However, IMV differs from AC in the way that the minute ventilation is increased. Specifically, patients increase the minute ventilation by spontaneous breathing, rather than patient-initiated ventilator breaths.

SIMV — SIMV is a the new form of IMV, in which the ventilator breaths are synchronized with patient inspiratory effort.[7][8] SIMV with pressure support is the most efficient and effective mode of mechanical ventilation.[9] In the same manner AC has become CMV, SIMV has become IMV and is now referred to as VC-IMV or PC-IMV.

Mandatory minute ventilation (MMV)

MMV — Allows spontaneous breathing with automatic adjustments of mandatory ventilation to the meet the patient’s preset minimum minute volume requirement. If the patient maintains the minute volume settings for VT x f, no mandatory breaths are delivered. If the patient's minute volume is insufficient, mandatory delivery of the preset tidal volume will occur until the minute volume is achieved. The method for monitoring whether or not the patient is meeting the required minute ventilation (VE) differs by ventilator brand and model, but generally there is a window of monitored time, and a smaller window checked against the larger window

(i.e., in the Dräger Evita® line of mechanical ventilators there is a moving 20-second window, and every 7 seconds the current tidal volume and rate are measured)

to decide whether a mechanical breath is needed to maintain the minute ventilation. MMV is the optimal mode for weaning in neonatal and pediatric populations and has been shown to reduce long term complications related to mechanical ventilation.[10]

  • Static factors set in this mode — VT, f, PEEP, PSupport, Ti

Pressure-controlled

Pressure-controlled continuous mandatory ventilation (PC-CMV)

PCV — Pressure Control Ventilation (PCV) also Pressure Control (PC) is a controlled mode of ventilation. The ventilator delivers a flow to maintain the preset pressure at a preset respiratory rate over a preset inspiratory time.[11]

The pressure is constant during the inspiratory time and the flow is decelerating. If for any reason pressure decreases during isnpiration, the flow from the ventilator will immediately increase to maintain the set inspiratory pressure.[12]

Pressure-controlled intermittent mandatory ventilation (PC-IMV)

Pressure regulated volume control (PRVC)

PRVCPressure regulated volume control is a pressure controlled mode (even though "volume control" is used in the name) with a VT set as a goal amount. Pressure varies with a peak pressure limit included to reduce lung trauma and use only the minimum pressure required to deliver the goal tidal volume (VT).

Airway pressure release ventilation (APRV)

APRV — Airway Pressure Release Ventilation is a time-cycled alternant between two levels of positive airway pressure, with the main time on the high level and a brief expiratory release to facilitate ventilation.[13]

This is a type of inverse ratio ventilation. The exhalation time (Tlow) is shortened to usually less than one second to maintain alveoli inflation. Fundamentally this is a continuous pressure with a brief release. APRV currently the most efficient conventional mode for lung protective ventilation.[14]

Different perceptions of this mode may exist around the globe. While 'APRV' is common to users in North America, a very similar mode, biphasic positive airway pressure (BIPAP), was introduced in Europe.[15] The term APRV has also been used in American journals where, from the ventilation characteristics, BIPAP would have been the appropriate terminology[16] . To further confusion, BiPAP© is a registered trade-mark for a noninvasive ventilation mode in a specific ventilator (Respironics Inc.). Other names (BILEVEL, DUOPAP, BIVENT) have been created for legal reasons. Although similar in modality, these terms describe how a mode is intended to inflate the lung, rather than defining the characteristics of synchronization or the way spontaneous breathing efforts are supported.


High Frequency Ventilation (HFV)

High frequency ventilation
Intervention
MeSH D006612

High frequency ventilation is a type of mechanical ventilation that employs very high respiratory rates (>150 (Vf) breaths per minute) and very small tidal volumes.[17][18] High frequency ventilation is thought to reduce ventilator-associated lung injury (VALI), especially in the context of ARDS and acute lung injury.[17] This is commonly referred to as lung protective ventilation.[19] There are different flavors of High frequency ventilation.[17] Each type has its own unique advantages and disadvantages. The types of HFV are characterized by the delivery system and the type of exhalation phase (active vs passive).

High Frequency Ventilation may be used alone, or in combination with conventional mechanical ventilation. In general, those devices that need conventional mechanical ventilation do not produce the same lung protective effects as those that can operate without tidal breathing. Specifications and capabilities will vary depending on the device manufacturer.

High-frequency ventilation (Active) (HFV-A)

The term active refers to the ventilators forced expiratory system. In a HFV-A scenario, the ventilator uses pressure to apply an inspiratory breath and then applies an opposite pressure to force an expiratory breath. In high-frequency oscillatory ventilation (a ventilator patented by sensormedics corp) the oscillation bellow and piston force positive pressure in and apply negative pressure to force an expiration.

High-frequency ventilation (Passive) (HFV-P)

The term passive refers to the ventilators non-forced expiratory system. In a HFV-P scenario, the ventilator uses pressure to apply an inspiratory breath and then simply returns to atmospheric pressure to allow for a passive expiration.

Continuous spontaneous ventilation

Continuous positive airway pressure (CPAP)

CPAP — Continuous positive airway pressure (CPAP) refers to the delivery of a continuous level of positive airway pressure. It is functionally similar to PEEP. The ventilator does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all breaths. Nasal CPAP is frequently used in neonates though its use is controversial. Studies have shown nasal CPAP to reduce ventilator time but an increased occurrence of pneumothorax was also prevalent.[20]

Bilevel positive airway pressure (BPAP)

BPAP — Bilevel positive airway pressure (BPAP) is a mode used during noninvasive positive pressure ventilation (NPPV). It delivers a preset inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP). BPAP can be described as a Continuous Positive Airway Pressure system with a time-cycled change of the applied CPAP level.[21] CPAP, BPAP and other non-invasive ventilation modes have been shown to be effective management tools for chronic obstructive pulmonary disease and acute respiratory failure.[22]

Often BPAP is incorrectly referred to as "BiPAP". BiPAP® is the name of a portable ventilator manufactured by Respironics Corporation; it is just one of many ventilators that can deliver BPAP.


Other modes and types of ventilation

Proportional Assist Ventilation (PAV)

PAV — Proportional assist ventilation is a mode in which the ventilator guarantees the percentage of work regardless of changes in pulmonary compliance and resistance.[23] The ventilator varies the tidal volume and pressure based on the patients work of breathing, the amount it delivers is proportional to the percentage of assistance it is set to give.

Liquid ventilation (LV)

Liquid ventilation (LV) — is a technique of mechanical ventilation in which the lungs are insufflated with an oxygenated perfluorochemical liquid rather than an oxygen-containing gas mixture. The use of perfluorochemicals, rather than nitrogen, as the inert carrier of oxygen and carbon dioxide offers a number of theoretical advantages for the treatment of acute lung injury, including:

  • Reducing surface tension by maintaining a fluid interface with alveoli
  • Opening of collapsed alveoli by hydraulic pressure with a lower risk of barotrauma
  • Providing a reservoir in which oxygen and carbon dioxide can be exchanged with pulmonary capillary blood
  • Functioning as a high efficiency heat exchanger

Despite its theoretical advantages, efficacy studies have been disappointing and the optimal clinical use of LV has yet to be defined.[24]

Total liquid ventilation (TLV)

Total liquid ventilation — In total liquid ventilation (TLV), the entire lung is filled with an oxygenated PFC liquid, and a liquid tidal volume of PFC is actively pumped into and out of the lungs. A specialized apparatus is required to deliver and remove the relatively dense, viscous PFC tidal volumes, and to extracorporeally oxygenate and remove carbon dioxide from the liquid.[25][26][27]

Partial liquid ventilation (PLV)

Partial liquid ventilation — In partial liquid ventilation (PLV), the lungs are slowly filled with a volume of PFC equivalent or close to the FRC during gas ventilation. The PFC within the lungs is oxygenated and carbon dioxide is removed by means of gas breaths cycling in the lungs by a conventional gas ventilator.[28]

Positive End Expiratory Pressure (PEEP)

PEEP — Positive end expiratory pressure is pressure applied upon expiration. PEEP is applied either using a valve that is connected to the expiratory port and set manually or a valve managed internally by a mechanical ventilator. PEEP is simply a pressure that an exhalation has to bypass, effectively causing alveoli to remain open and not fully deflate. This mechanism for maintaining inflated alveoli helps increase partial pressure of oxygen in arterial blood, an increase in PEEP increases the PaO2. [29]

Pressure Support (PS)

PS — Pressure support is a spontaneous mode of ventilation also named Pressure Support Ventilation (PSV). The patient initiates the breath and the ventilator delivers support with the preset pressure value. With support from the ventilator, the patient also regulates the respiratory rate and the tidal volume.

In Pressure Support, the set inspiratory pressure support level is kept constant and there is a decelerating flow. The patient triggers all breaths. If there is a change in the mechanical properties of the lung/thorax and patient effort, the delivered tidal volume will be affected. The user must then regulate the pressure support level to obtain desired ventilation.[30][31]

See also

References

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