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.
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