Resp Care
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Application of positive end-expiratory pressure (PEEP) in acute lung injury patients under mechanical ventilation improves oxygenation and increases lung volume. The effect of PEEP is to recruit lung tissue in patients with diffuse lung edema. This effect is particularly important in patients ventilated with low tidal volumes. ⋯ In patients with acute respiratory distress syndrome in whom the lungs have been near-optimally recruited by PEEP and tidal volume, the use of recruitment maneuvers as adjuncts to mechanical ventilation remains controversial. The application of PEEP in patients with unilateral lung disease may be detrimental if PEEP hyperinflates normal lung regions, thus directing blood flow to diseased lung regions. In patients with air flow limitation and lung hyperinflation, the application of additional external PEEP to compensate for intrinsic PEEP and flow limitation frequently decreases the inspiratory effort to initiate an assisted breath, thus decreasing breathing work load.
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Alveolar (and thus arterial) P(O2) and P(CO2) clearly depend on minute ventilation. However, we need to balance gas exchange goals against the risk of overstretching, especially of the healthier regions of the lung. The plateau pressure is probably the best easily-obtained marker of the risk of stretch in the lung, and a commonly quoted threshold is 30--35 cm H(2)O, the normal maximum transalveolar pressure at total lung capacity. ⋯ In addition, the small V(T) of HFV prevents excessive end-inspiratory distention. Although considerable clinical data support the use of HFV in pediatric patients at risk for ventilator-induced lung injury, there are few data from adults. Whether HFV will prove valuable in well-designed open lung strategies in the adult population still has to be determined.
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A number of adjuncts to mechanical ventilation have been the focus of recent research. Automatic tube compensation (the regulation of airway pressure by estimation of tracheal pressure) appears to be an ideal approach to unloading the resistive effort imposed by the endotracheal tube. Randomized controlled trials have recently been performed with high frequency oscillation (HFO), partial liquid ventilation (PLV), and prone positioning. ⋯ With both HFO and prone positioning we will have to wait for additional randomized clinical trials before the status of those techniques can be determined. No randomized trials of tracheal gas insufflation have been performed. Of major concern with tracheal gas insufflation is the lack of a commercial product.
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Increased understanding of the mechanisms and effects of acute respiratory failure has not been accompanied by more precise criteria by which the clinician can determine when intubation should be carried out and invasive positive-pressure ventilation (IPPV) instituted in a given patient. The indications traditionally offered in reviews and textbooks have tended to be either so broad as not to be very helpful in an individual case, or of questionable clinical relevance and too cumbersome for practical use. This review updates the indications for IPPV in adult patients with acute respiratory failure by examining available evidence from clinical trials and by considering new management alternatives that have become available in the last 20 years. ⋯ However, in other settings of acute hypoxemic respiratory failure, such as acute lung injury and acute respiratory distress syndrome, this has not been shown. The use of IPPV may improve outcomes in patients with severe cardiogenic shock. However, IPPV has not proven to be beneficial in traumatic brain injury and flail chest, in the absence of other indications.
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The tremendous progress in microprocessor-driven ventilator technology over the last years has facilitated the introduction of a broad variety of different ventilatory modes into the clinical practice of mechanical ventilation. Many of these newer modalities are designed for partial ventilatory support, which might reflect the complexity of the issue of patient ventilator interactions when spontaneous breathing activity is present compared to controlled mechanical ventilation. There are reasons to believe that allowing some degree of spontaneous breathing activity during mechanical ventilation is useful not only to gradually withdraw ventilatory assistance in the process of weaning but also to avoid some of the adverse effects of mechanical ventilation in the early phase of acute respiratory failure when classically controlled modes of ventilation are used. It is the aim of this article to review the effects of preserved spontaneous breathing activity during mechanical ventilation with different ventilatory modalities in acute respiratory failure patients.