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Mechanical ventilators are life-saving devices that support or replace spontaneous breathing. They deliver breaths to patients through varying methods known as ventilator modes. Understanding these modes is critical for healthcare providers managing patients with respiratory failure.
There are three ventilatory modes: full support, partial support, and spontaneous. These are described below.
Full Support Modes
Full support modes include controlled mechanical ventilation, continuous mandatory ventilation, and pressure control ventilation.
Controlled Mechanical Ventilation
Controlled mechanical ventilation delivers a preset tidal volume and respiratory rate, providing full ventilator support. It's primarily indicated for apneic patients.
Continuous Mandatory Ventilation (Assist-Control)
Continuous Mandatory Ventilation is also called Assist-control (A/C) ventilation. In A/C ventilation, the ventilator delivers a preset tidal volume or pressure at a preset respiration rate. If the patient initiates a breath between the machine's breaths, the ventilator delivers at the preset volume or pressure (assisted breath). As a result, every breath is the preset volume or pressure, regardless of whether the patient or the ventilator initiates it.
Pressure Control Ventilation
Pressure-controlled ventilation is another full support mode with a set inspiratory pressure limit. When the patient takes a breath, the ventilator delivers a controlled amount of gas up to the maximum inspiratory pressure allowed.
Partial Support Modes
Partial support modes, where the patient and the ventilator share responsibility, include intermittent mandatory ventilation (IMV), synchronized intermittent mandatory ventilation (SIMV), and proportional assist ventilation (PAV).
Intermittent Mandatory Ventilation (IMV)
IMV provides a combination of mechanically assisted breaths and spontaneous breaths. Mechanical breaths are delivered at predetermined intervals and a selected tidal volume, regardless of the patient's exertion. Although the patient can increase the respiratory rate by initiating inspiration between ventilator-delivered breaths, these spontaneous breaths are limited to the tidal volume generated by the patient.
Synchronized Intermittent Mandatory Ventilation (SIMV)
SIMV delivers a preset tidal volume and a specific number of breaths per minute. During the intervals between ventilator-delivered breaths, patients have the freedom to breathe spontaneously, without any assistance from the ventilator, on those additional breaths. As the patient's ability to breathe spontaneously increases, the preset number of ventilator breaths decreases, and the patient does more breathing work.
Proportional Assist Ventilation (PAV)
Proportional Assist Ventilation (PAV) is a ventilation method where the ventilator generates pressure in response to the patient's inspiratory efforts. The ventilator synchronizes with the patient's own ventilatory efforts. As the patient generates higher inspiratory pressure, the ventilator increases the pressure, amplifying the patient's breathing effort. It occurs without a specific target pressure or volume, ensuring optimal support and synchronization.
Spontaneous Breathing Modes
Spontaneous modes of mechanical ventilation, where the patient assumes responsibility for almost all breathing, include pressure support ventilation and continuous positive airway pressure.
Pressure Support Ventilation (PSV)
PSV applies a pressure plateau to the airway throughout the patient-triggered inspiration to decrease resistance within the tracheal tube and ventilator tubing. Pressure support is reduced gradually as the patient's strength increases.
Continuous Positive Airway Pressure (CPAP)
CPAP refers to a mode of spontaneous ventilation where the ventilated patient controls almost all aspects of the breath. All that is set is the FIO2 and PEEP. There is one level of pressure on both inspiration and expiration. No set respiratory rate, tidal volume, or peak inspiratory pressure (PIP) exists. The patient determines their respiratory rate and tidal volume.
Special Modes
Special modes of ventilation have been developed for specific situations. These include neurally adjusted ventilatory assist (NAVA), independent lung ventilation (ILV), and airway pressure release ventilation (APRV).
Neurally Adjusted Ventilatory Assist (NAVA)
NAVA requires an esophageal catheter to monitor the diaphragm's electrical activity. This mode delivers assisted breaths in synchrony with the patient's breath.
Independent Lung Ventilation (ILV)
ILV is used in patients with unilateral lung disease. Each lung is ventilated separately via a double-lumen endotracheal tube.
Airway Pressure Release Ventilation (APRV)
APRV is a time-triggered, pressure-limited, time-cycled mode of mechanical ventilation that allows unrestricted, spontaneous breathing throughout the ventilatory cycle. During the inflation period, breaths can be spontaneously initiated by both the ventilator and the individual.
