Brazilian recommendations of mechanical ventilation 2013. Part I

Abstract

Perspectives on invasive and noninvasive ventilatory support for critically ill patients are evolving, as much evidence indicates that ventilation may have positive effects on patient survival and the quality of the care provided in intensive care units in Brazil. For those reasons, the Brazilian Association of Intensive Care Medicine (Associação de Medicina Intensiva Brasileira - AMIB) and the Brazilian Thoracic Society (Sociedade Brasileira de Pneumonia e Tisiologia - SBPT), represented by the Mechanical Ventilation Committee and the Commission of Intensive Therapy, respectively, decided to review the literature and draft recommendations for mechanical ventilation with the goal of creating a document for bedside guidance as to the best practices on mechanical ventilation available to their members. The document was based on the available evidence regarding 29 subtopics selected as the most relevant for the subject of interest. The project was developed in several stages, during which the selected topics were distributed among experts recommended by both societies with recent publications on the subject of interest and/or significant teaching and research activity in the field of mechanical ventilation in Brazil. The experts were divided into pairs that were charged with performing a thorough review of the international literature on each topic. All the experts met at the Forum on Mechanical Ventilation, which was held at the headquarters of AMIB in São Paulo on August 3 and 4, 2013, to collaboratively draft the final text corresponding to each sub-topic, which was presented to, appraised, discussed and approved in a plenary session that included all 58 participants and aimed to create the final document.

Figure 1

Trigger asynchronies identified in volume-, flow- and pressure-time curves, indicated by arrows. Negative deflections in the pressure-time curves represent the patient’s inspiratory effort (muscle pressure), which are only visible when the esophageal pressure is monitored. Panel A) Lost efforts. The first arrow indicates a weak stimulus, which is unable to trigger the ventilator, thus resulting in a small positive flow wave and minimal tidal volume. The second arrow points to effort during expiration, which failed to trigger the ventilator and merely sufficed for the flow to return to baseline and become slightly positive. Panel B) Double-triggering. Example in volume-controlled ventilation. The patient’s inspiratory efforts persist at the time of cycling-off, thus triggering another cycle. The corresponding volumes are added together (stacking), and the airway pressure increases, causing the high-pressure alarm to go off. Panel C) Auto-triggering. In the support pressure mode, some cycles are triggered without a patient inspiratory effort, which can be facilitated by leaks; this is observed in the volume-time curve, which does not return to baseline (the inspired volume is greater than the expired volume). Figures obtained at Xlung.net, a virtual mechanical ventilation simulator. Available at: http//:www.xlung.net.

Figure 2

Flow asynchrony. In volume-controlled mode, the flow rate was adjusted below the patient’s demand; the patient thus maintained muscle effort throughout inspiration, and the curve consequently became concave and upward. The asynchrony exhibits increasing intensity from the first to the third cycle, as represented in the figure. The negative deflections in the pressure-time curve represent the patient’s inspiratory effort (muscle pressure) and are only visible when esophageal pressure is monitored. Figures obtained at Xlung.net, a virtual mechanical ventilation simulator. Available at: http//:www.xlung.net.

Figure 3

Cycling asynchronies during pressure support ventilation. In the first cycle, the cutoff point of 25% of the peak inspiratory flow (percentage of the cycling criterion) was reached rapidly; the ventilator’s inspiratory time was therefore shorter than the time desired by the patient. This is shown in the expiratory segment of the flow curve, which tends to return to the baseline as a result of the patient’s inspiratory effort, which is still present. The last cycle represents the opposite situation, i.e., delayed cycling. The flow reduction occurs very slowly, which is typical of airway obstruction; the cycling threshold is therefore reached with some delay. Sometimes, the cycle is interrupted by a contraction of the respiratory muscles, which causes an increase above the support pressure adjusted at the end of inspiration (not shown in this figure). Figures obtained at Xlung.net, a virtual mechanical ventilation simulator. Available at: http//:www.xlung.net

Figure 4

Inspiratory pause maneuver and estimation of the airway resistance and pause (or plateau) pressure. VCV - volume-controlled ventilation. Paw - airway pressure; PEEP - positive end-expiratory pressure; Vt - tidal volume; Pel - elastic pressure; Palv - alveolar pressure.