Clearing the mist from our eyes: bronchodilators, mechanical ventilation, new devices, locations, and what you should know about bias flow.

Patients with obstructive lung disease often present with life-threatening respiratory failure, confounded by severe air-flow limitation, dynamic hyperinflation, intrinsic PEEP, reduced pulmonary compliance, hypercarbia, and hemodynamic instability. Inhaled bronchodilators play a vital role in the care of such patients and are one of the most widely prescribed medications in the critical care setting. Despite administration of high doses of bronchodilators and instituting noninvasive ventilation strategies, many patients are incapable of sustaining the high work of breathing that may be required to maintain effective alveolar ventilation. As such, emergency intubation and mechanical ventilation is considered a necessary and “life-saving” intervention. Mechanically ventilated patients with obstructive airway disease also present as some of the most technically challenging patients to stabilize and then to wean from the ventilator. Airway reactivity and resistance can be affected by infections, changes in lung volume, fluid balance, and other drugs.1 In addition to these intrinsic factors, there are a number of obscure resistive elements (eg, endotracheal tube [ETT]2,3 or exhalation valve4) within the ventilator system that can add to causal respiratory failure,5 increase the work of breathing,6-9 and potentially prolong ventilation. While little, other than extubation, can be done to avoid these extrinsic factors, bronchoconstriction and airflow limitation are usually reversible in mechanically ventilated patients. In fact, inhaled bronchodilators have been shown to reduce airway resistance10 and intrinsic PEEP,11,12 improve hemodynamics,13 and reduce the work of breathing.12 Therefore, immediate and effective bronchodilator therapy is pivotal to successful stabilization and weaning of mechanically ventilated patients. But it is not intuitively obvious to clinicians with ICU experience that many bronchodilator treatments given to intubated patients appear to have any clinical effect whatsoever. No one knows for sure, but this probably is often a result of very poor drug delivery to the lungs. Experimental data obtained from in vivo studies have demonstrated poor aerosol delivery to ventilated patients, with approximately 1%14 and 1–12%15-17 of the nominal dose being delivered to the peripheral airways of infants and adults, respectively. The paucity of human data makes it extremely difficult for clinicians to settle on one particular device or method for aerosol delivery. This is an important reason why the elusive practice of bronchodilator administration in ventilated patients has no standards. Thus, techniques and delivery devices differ wildly from one institution to the next. There are several practical issues complicating the efficacy of drug delivery during mechanical ventilation, including the patient’s lung mechanics, ventilator and ventilation mode, aerosol generator, heating and humidification of the inspired gas, position of the aerosol generator in the ventilator circuit, timing during the respiratory cycle, ETT size, tidal volume, and inspiratory flow rate.18 The majority of these factors have been described following well designed in vitro tests where each of these variables can be independently controlled. However, over the last decade there has been a proliferation of new aerosol delivery devices and a newer generation of microprocessor ventilators introduced into the clinical arenas. Further, humidification practices have changed to include selective use of both passive and active humidification systems during mechanical ventilation. No previous studies have objectively evaluated bronchodilator delivery using currently available devices with the variety of delivery options during mechanical ventilation.