Modes of mechanical ventilation

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Modes of mechanical ventilation are one of the most important aspects of the usage of mechanical ventilation. The mode refers to the method of inspiratory support. In general, mode selection is 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 intermittent mandatory ventilation (IMV) and controlled mechanical ventilation (CMV).[1] There have been substantial changes in the nomenclature of mechanical ventilation over the years, but more recently it has become standardized by many respirology/pulmonology groups.[2][3] Writing a mode is most proper in all capital letters with a dash between the cycle and the strategy (i.e. PC-IMV, or VC-MMV etc.)

Cycle[edit]

Cycling is the method for how a ventilator knows to give a breath and stop a breath. Cycling is the governing system for how a breath will ultimately be applied. Parameters vary but rate (f), I:E and other similar parameters are almost always set by the clinician alongside the cycle.

Volume controlled[edit]

Flow-volume loop

Volume controlled systems of ventilation are based on a measured volume variable that is set by the clinician. When the ventilator detects the set volume having been applied, the ventilator cycles to exhalation. This is measured various ways by each brand and model. Some ventilators measure using a flow sensor at the circuit wye while some measure where the expiratory circuit plugs into the expiratory port on the ventilator body, which means that the machine will give all support to take breath to the patient.

Pressure-controlled[edit]

Pressure-controlled cycling is based on an applied positive pressure that is set by the clinician. In pressure-controlled modes, the tidal volume is variable, as the ventilator is using only the pressure as a measurement for cycling. Most ventilators calculate pressure at the expiratory circuit though some measure near the circuit with a proximal pressure line.

Spontaneously controlled[edit]

Spontaneously controlled cycling is a flow-sensed mode dependent on a spontaneously breathing patient to cycle. Spontaneously controlled ventilation is typically only in reference to continuous spontaneous ventilation, also called continuous positive airway pressure (CPAP).Dependent

Negative pressure-controlled[edit]

Negative pressure ventilation cycles by producing a 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.[4] When the negative pressure stops being applied, the chest returns to atmospheric pressure and the inspired air then is exhaled.[4]

Strategy[edit]

Airway pressure release ventilation[edit]

Airway pressure release ventilation graph

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.[5]

Airway pressure release ventilation is usually utilized as a type of inverse ratio ventilation. The exhalation time (Tlow) is shortened to usually less than one second to maintain alveoli inflation. In the basic sense, this is a continuous pressure with a brief release. APRV currently the most efficient conventional mode for lung protective ventilation.[6]

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.[7] The term APRV has also been used in American journals where, from the ventilation characteristics, BIPAP would have been perfectly good terminology.[8] But BiPAP(tm) is a trademark for a noninvasive ventilation mode in a specific ventilator (Respironics Inc.).

Other manufacturers have followed with their own brand names (BILEVEL, DUOPAP, BIVENT). 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.

Continuous mandatory ventilation[edit]

Continuous mandatory ventilation (formerly known as Assist Control or AC) is a mode of ventilation where breaths are delivered based on set variables. The patient may initiate breaths by attempting to breathe. Once a breath is initiated, either by the patient or by the ventilator the set tidal volume is delivered. Continuous mandatory ventilation used to also be called Volume Control or Assist Control Volume Control (AC/VC), though this is no longer recommended. Since nomenclature of mechanical ventilation is only recently standardized[9] there are many different names that historically were used to reference CMV but now reference Assist Control.[9] Names such as: volume control ventilation, and volume cycled ventilation in modern usage refer to the Assist Control mode.

Controlled mechanical ventilation in its original form had no patient sensitivity. A breath set was a breath delivered. Continuous mandatory ventilation was created out of the need for patient-initiation in breaths. Fundamentally, Continuous mandatory ventilation is controlled mechanical ventilation (CMV) with a sensitivity for patient breathing. The use of controlled mechanical ventilation requires the patient be completely unconscious, either pharmacokinetically or otherwise in a coma.

Continuous mandatory ventilation (formerly Assist Control or AC) is associated with profound diaphragm muscle dysfunction and atrophy.[10] Continuous mandatory ventilation is no longer the preferred mode of mechanical ventilation.[11]

Intermittent mandatory ventilation[edit]

Intermittent mandatory ventilation is similar to continuous mandatory ventilation in two ways: The minute ventilation (VE) is determined (by setting the respiratory rate and tidal volume); and the patient is able to increase the minute ventilation. However, IMV differs from continuous mandatory ventilation in the way that the minute ventilation is increased. To be specific, patients increase the minute ventilation by spontaneous breathing rather than patient-initiated ventilator breaths. The ventilator breaths are synchronized with patient inspiratory effort.[12][13] IMV with pressure support is the most efficient and effective mode of mechanical ventilation.[14]

Intermittent mandatory ventilation has not always had the synchronized feature, so the division of modes were understood to be SIMV (synchronized) vs IMV (not-synchronized). Since the American Association for Respiratory Care established a nomenclature of mechanical ventilation the "synchronized" part of the title has been dropped and now there is only IMV.

Mandatory minute ventilation[edit]

Mandatory minute ventilation (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, in general, 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 an optimal mode for weaning in neonatal and pediatric populations and has been shown to reduce long-term complications related to mechanical ventilation.[15]

Pressure-regulated volume control[edit]

Pressure-regulated volume control is an IMV based mode. Pressure-regulated volume control utilizes pressure-limited, volume-targeted, time-cycled breaths that can be either ventilator- or patient-initiated.

The peak inspiratory pressure delivered by the ventilator is varied on a breath-to-breath basis to achieve a target tidal volume that is set by the clinician.

For example, if a target tidal volume of 500 mL is set but the ventilator delivers 600 mL, the next breath will be delivered with a lower inspiratory pressure to achieve a lower tidal volume. Though PRVC is regarded as a hybrid mode because of its tidal-volume (VC) settings and pressure-limiting (PC) settings fundamentally PRVC is a volume-control mode.

Continuous positive airway pressure[edit]

continuous positive airway pressure (CPAP) is a non-invasive positive pressure mode of ventilation (NPPV). CPAP is simply a pressure applied at the end of exhalation to keep the alveoli open and not fully deflate. This mechanism for maintaining inflated alveoli helps increase partial pressure of oxygen in arterial blood, an appropriate increase in CPAP increases the PaO2.

Bilevel positive airway pressure[edit]

Bilevel positive airway pressure (BPAP) is a mode used during noninvasive positive pressure ventilation (NPPV). First used in 1988 by Professor Benzer in Austria,[16] 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.[17] 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.[18]

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.

High-frequency ventilation (Active)[edit]

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 (sometimes abbreviated HFOV) the oscillation bellow and piston force positive pressure in and apply negative pressure to force an expiration.[19]

High-frequency ventilation (Passive)[edit]

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. This is seen in High-Frequency Jet Ventilation, sometimes abbreviated HFJV.

Volume guarantee[edit]

Volume guarantee an additional parameter available in many types of ventilators that allows the ventilator to change its inspiratory pressure setting to achieve a minimum tidal volume. This is utilized most often in neonatal patients who need a pressure controlled mode with a consideration for volume control to minimize volutrauma.

Spontaneous breathing and support settings[edit]

Positive-end expiratory pressure[edit]

Positive end expiratory pressure is pressure applied upon expiration. PEEP is applied using either 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, in effect 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.[20]

Pressure support[edit]

Pressure support is a spontaneous mode of ventilation also named Pressure Support Ventilation (PSV). The patient initiates every breath and the ventilator delivers support with the preset pressure value. With support from the ventilator, the patient also regulates their own respiratory rate and their 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.[21][22]

Pressure support improves oxygenation,[23] ventilation and decreases work of breathing.

Also see adaptive support ventilation.

Other ventilation modes and strategies[edit]

Closed loop systems[edit]

Adaptive Support Ventilation[edit]

Adaptive Support Ventilation is the only commercially available closed-loop mode of mechanical ventilation to date that uses "optimal targeting". This targeting scheme was first described by Tehrani in 1991,[24][25] and was designed to minimize the work rate of breathing, mimic natural breathing, stimulate spontaneous breathing, and reduce weaning time.[26]

Automatic Tube Compensation[edit]

Automatic Tube Compensation (ATC) is the simplest example of a computer-controlled targeting system on a ventilator.

The goal of ATC is to support the resistive work of breathing through the artificial airway

Neurally Adjusted Ventilatory Assist[edit]

Neurally Adjusted Ventilatory Assist (NAVA) is adjusted by a computer (servo) and is similar to ATC but with more complex requirements for implementation.

In terms of patient-ventilator synchrony, NAVA supports both resistive and elastic work of breathing in proportion to the patient’s inspiratory effort

Proportional Assist Ventilation[edit]

Proportional assist ventilation (PAV) is a mode in which the ventilator guarantees the percentage of work regardless of changes in pulmonary compliance and resistance.[27]

The ventilator varies the tidal volume and pressure based on the patient's work of breathing. The amount it delivers is proportional to the percentage of assistance it is set to give.

PAV, like NAVA, supports both resistive and elastic work of breathing in proportion to the patient’s inspiratory effort.

Liquid ventilation[edit]

Liquid ventilation 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.[28]

Total liquid ventilation[edit]

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.[29][30][31]

Partial liquid ventilation[edit]

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.[32]

See also[edit]

References[edit]

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