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What Are the Benefits of Different Ventilatory Techniques?
INTRODUCTION
A broad variety of techniques and modes of mechanical ventilation are now available to physicians, thanks to improvements in technology. For the most part, the design of these techniques is based on sound physiologic principles. Nevertheless, there is limited evidence that ventilatory techniques and modes affect hard outcomes. Additionally, the existing randomized controlled trials (RCTs) do not indicate the superiority of any specific mode; they only support certain general strategies for mechanical ventilation, such as tidal volume (TV) limitation and the use of ventilator liberation protocols. It can be argued that clinicians should choose only those modes and techniques that are time honored and have been used in the few existing positive RCTs. Although this approach will benefit a broad population, it is common experience that many patients require a more articulated strategy. In these cases, knowledge of the benefits of the different ventilatory techniques helps the clinician to individualize respiratory care, using the available modes within a general strategy that is supported by solid evidence.
OPTIONS: DESCRIPTIONS OF VENTILATORY MODES
Assist Control Ventilation
During assist control ventilation (ACV), the ventilator delivers a mandatory breath every time the patient initiates an inspiration. A backup respiratory rate is set to guarantee that the patient always receives a minimal number of breaths, even in the absence of spontaneous inspiratory activity. Mandatory breaths can be delivered with either volume or pressure control. During ACV, the inspiratory time is preset and invariable. In ACV, the mandatory breaths can be set to volume control breaths, where a TV is guaranteed, or pressure control breaths, where the TV will be determined by the patient’s compliance. ACV also includes dual-controlled ventilation modes such as volume control with autoflow, pressure control with volume guarantee, or pressure-regulated volume control. These dual-controlled modes deliver a decelerating inspiratory flow, adjusting the inspiratory pressure breath by breath to maintain the TV close to a target value.
Pressure Support Ventilation
Pressure support ventilation (PSV) assists each inspiratory attempt by the patient with a pressure-limited breath, thus partitioning the work of breathing between the patient and ventilator. , The patient maintains partial control of TV and respiratory rate; the operator allows the patient to perform more or less work by modifying the level of inspiratory pressure. PSV differs from ACV in the lack of a backup rate and in the fact that, during PSV, inspirations have variable durations and are terminated when inspiratory flow decreases below a predetermined threshold value.
Synchronized Intermittent Mandatory Ventilation
Synchronized intermittent mandatory ventilation (SIMV) assists with a mandatory breath during only an adjustable fraction of patient’s inspiratory attempts. Unlike ACV, additional inspirations are either unassisted or partially assisted with PSV. During SIMV, higher mandatory rates are used for patients who require higher levels of ventilatory assistance and are progressively decreased during the weaning process, which allows the patient to accomplish more unsupported breaths.
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