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Therapeutic aerosols are often used in mechanically ventilated patients, most commonly to administer beta-agonist bronchodilators. However, beta-agonists should be avoided in ARDS, in which they have been shown to increase mortality. Other aerosols that might be administered during mechanical ventilation included anticholinergics, steroids, antibiotics, and prostacyclins. Therapeutic aerosols can be delivered using a nebulizer or MDI. A variety of factors affect aerosol delivery during mechanical ventilation (Table 35-1).
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With the traditional jet nebulizer, about 5% of the dose placed into the device is deposited in the lower respiratory tract. There are a number of disadvantages associated with jet nebulizer use during mechanical ventilation. Contamination of the lower respiratory tract can occur if the nebulizer is the source of bacterial aerosols. The continuous flow from the nebulizer may increase tidal volume during volume ventilation or pressure during pressure ventilation. Continuous flow from the nebulizer makes triggering more difficult and increases resistance of expiratory filters and pneumotachometers. Some of these disadvantages can be offset by using the nebulizer control of the ventilator, which powers the nebulizer only during inspiration and may compensate for the additional flow added by the nebulizer.
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The vibrating mesh nebulizer uses a mesh or plate with multiple apertures to produce an aerosol. They require electric power for operation of the control unit. These devices have a high drug output, and their residual volume is negligible. The mesh nebulizer overcomes some of the issues associated with the jet nebulizer because it adds no gas flow into the circuit and the device can remain in the circuit between treatments. The mesh nebulizer is placed between the ventilator and the humidifier (Figure 35-1). Aerosol delivery is more efficient with the mesh nebulizer compared to the jet nebulizer.
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Continuous Aerosol Delivery
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Continuous aerosols can be delivered into the ventilator circuit using a mesh nebulizer and syringe pump. Continuous aerosol bronchodilators are used for severe acute asthma. Continuous aerosol vasodilators (eg, prostacyclin) are used to improve oxygenation with refractory hypoxemia and to decrease pulmonary artery pressure with pulmonary hypertension.
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Many of the complications of jet nebulizer during mechanical ventilation are avoided by use of a MDI. Pulmonary deposition from a MDI is similar to the jet nebulizer (5%). Either MDI or nebulizer can be used effectively in mechanically ventilated patients. The MDI can be introduced into the ventilator circuit using an elbow adapter, inline adapter, or chamber adapter. However, for the same number of actuations, the greatest pulmonary deposition occurs with the chamber adapter. To maximize delivery, the MDI should be actuated at the beginning of inhalation. As with the nebulizer, the endotracheal tube is a formidable barrier to aerosol penetration. An issue with the newest generation of MDIs is their cost, which is high compared to that of a nebulizer.
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The usual dose from a nebulizer is about 10 times the dose with a MDI. Because the usual dose from the MDI is only a fraction of that with a nebulizer and the deposition from each is similar, more drug may be deposited in the lungs with the nebulizer—particularly with a mesh nebulizer. Thus, the mesh nebulizer may be more effective and convenient than MDI if high doses are required (eg, status asthmaticus). The dose of inhaled medications (nebulizer or MDI) may need to be increased in intubated patients due to the decreased pulmonary deposition secondary to the endotracheal tube.
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Evaluation of Response
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Response to an inhaled bronchodilator includes decreased peak airway pressure, plateau pressure, auto-PEEP, and resistive pressure (peak minus plateau pressure). More sophisticated measurements such as airway resistance and flow-volume loops may be useful in selected patients to evaluate bronchodilator response.
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Aerosol Delivery During Noninvasive Ventilation
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Aerosol therapy during noninvasive ventilation (NIV) can be delivered by MDI with a chamber spacer or nebulizer. A number of factors affect aerosol delivery during NIV. These include the type of ventilator, mode of ventilation, circuit conditions, type of interface, type of aerosol generator, drug-related factors, breathing parameter, and patient-related factors. Despite the effects of continuous flow, high inspiratory flow, leaks, humidity, and asynchrony, significant therapeutic effects have been reported with inhaled bronchodilator administration during NIV. Careful attention to the technique is required to optimize therapeutic effects of inhaled therapies during NIV.