Modes of mechanical ventilation refer to the various mechanical ventilator strategies employed to deliver a breath in patients that require mechanical ventilation. The mode refers to the method of respiratory 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 continuous mandatory ventilation (CMV).
Terminology
There has been extensive discussion regarding the nomenclature of mechanical ventilation, particularly in relation to the classification and terminology of ventilation modes. Despite these efforts, considerable confusion persists in the field. Ongoing initiatives aim to standardize and clarify this terminology, and, more recently, the International Organization for Standardization has revised its relevant standards to incorporate more precise definitions for modes of ventilation.
Taxonomy for mechanical ventilation The taxonomy is a logical classification system based on 10 maxims of ventilator design:
10 maxims
How modes are classified A structured method is commonly used to classify modes of mechanical ventilation based on three key attributes: the control variable, the breath sequence, and the targeting scheme. The control variable is the parameter directly regulated during inspiration. It is classified as pressure when inspiration is initiated with a preset inspiratory pressure or when pressure varies in proportion to inspiratory effort. It is classified as volume when both tidal volume and inspiratory flow are preset. If neither condition applies, the control variable is categorized as time.
The breath sequence is determined by analyzing whether breath initiation (triggering) and termination (cycling) are controlled by the patient or the ventilator. Based on these characteristics, breaths may be classified as mandatory or spontaneous and organized into sequences such as continuous mandatory ventilation (CMV) or intermittent mandatory ventilation (IMV).
The targeting scheme describes how the ventilator achieves the desired breath parameters. This may involve fixed operator-defined settings (set-point targeting) or dynamic adjustments made by the ventilator to achieve a specified goal (adaptive targeting), among other approaches.
Examples of mode classification Assist/Control Volume Control (e.g., Covidien PB 840)
In this mode, inspiratory volume and flow are preset, establishing volume as the control variable. Each breath is volume-cycled, meaning that inspiration is terminated by the ventilator; such breaths are therefore classified as mandatory. Because all breaths are mandatory, the breath sequence is continuous mandatory ventilation (CMV). The operator directly sets the waveform parameters, indicating a set-point targeting scheme. Accordingly, the mode may be classified as volume-controlled continuous mandatory ventilation with set-point targeting (VC-CMV).
SIMV Volume Control Plus (e.g., Covidien PB 840)
In this mode, the operator sets the tidal volume but not the inspiratory flow. Since specifying volume alone is insufficient to define volume control, the control variable is classified as pressure. The presence of spontaneous breaths between mandatory breaths defines the breath sequence as intermittent mandatory ventilation (IMV). The ventilator adjusts inspiratory pressure between breaths to achieve a target average tidal volume, reflecting an adaptive targeting scheme. This mode may therefore be classified as pressure-controlled intermittent mandatory ventilation with adaptive targeting (PC-IMV, adaptive).
Descriptions of common modes
Mechanical ventilation machines are available with both invasive modes (such as intubation) and non-invasive modes (such as BPAP). Invasive has to do with the insertion of medical devices or tubes internal to the patient, while non-invasive is completely external to the patient, as for example in using a tightly fitting mask or other device that covers the patient's nose and mouth.
Assist mode, control mode, and assist-control mode A basic distinction in mechanical ventilation is whether each breath is initiated by the patient (assist mode) or by the machine (control mode). Dynamic hybrids of the two (assist-control modes) are also possible, and control mode without assist is now mostly obsolete.
Airway pressure release ventilation
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.
Airway pressure release ventilation needs to be well understood to use it effectively and to prevent ventilator-induced lung injury. 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.
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. The term APRV has also been used in American journals where, from the ventilation characteristics, BIPAP would have been perfectly good terminology. But BiPAP 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.
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 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.
Pressure-regulated volume control Pressure-regulated volume control is an Assist Controlled Ventilation (ACV) 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 pressure-control mode with adaptive targeting.
Continuous positive airway pressure Continuous positive airway pressure (CPAP) is a non-invasive positive pressure mode of respiratory support. CPAP is a continuous pressure applied 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.
Automatic positive airway pressure Automatic positive airway pressure (APAP) is a form of CPAP that automatically tunes the amount of pressure delivered to the patient to the minimum required to maintain an unobstructed airway on a breath-by-breath basis by measuring the resistance in the patient's breathing.
Bilevel positive airway pressure Bilevel positive airway pressure (BPAP) is a mode used during non-invasive ventilation (NIV). First used in 1988 by Professor Benzer in Austria,Benzer H (1988) Ventilatory support by intermittent changes in PEEP levels. 4th European Congress on Intensive Care Medicine. Baveno-Stresa 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-cycle change of the applied CPAP level.
CPAP/APAP, BPAP, and other non-invasive ventilation modes have been shown to be effective management tools for chronic obstructive pulmonary disease, acute respiratory failure, sleep apnea, etc.
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.
Medical uses BPAP has been shown to be useful in reducing mortality and reducing the need for endotracheal intubation when used in people with chronic obstructive pulmonary disease (COPD).
High-frequency ventilation (Active) The term active refers to the ventilator's 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 bellows and piston force positive pressure in and apply negative pressure to force an expiration.
High-frequency ventilation (Passive) The term passive refers to the ventilator's non-forced expiratory system. In a HFV-P scenario, the ventilator uses pressure to apply an inspiratory breath and then returns to atmospheric pressure to allow for a passive expiration.
This is seen in High-Frequency Jet Ventilation, sometimes abbreviated HFJV. Also categorized under High Frequency Ventilation is High Frequency Percussive Ventilation, sometimes abbreviated HFPV. With HFPV it utilizes an open circuit to deliver its subtidal volumes by way of the patient interface known as the Phasitron.
Volume guarantee 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
Positive end-expiratory pressure Positive end expiratory pressure (PEEP) 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 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, and an increase in PEEP increases the PaO2.
Pressure support