Discuss the definition, patient selection process, application, assessment, and complications of pressure targeted ventilation modes in critically ill patients.
Describe bilevel positive airway pressure, pressure-controlled/inverse ratio, volume-guaranteed pressure modes, airway pressure release ventilation (APRV), biphasic ventilation, adaptive support ventilation (ASV), proportional assist ventilation (PAV), and high-frequency ventilation used to support critically ill patients.
ADVANCED MODES OF MECHANICAL VENTILATION
New Concepts: Mechanical Ventilation
For years, volume ventilation was the dominant form of ventilation. But in the 1990s, numerous pressure modes emerged, and are now commonly used in critical care units to ventilate patients from the acute to weaning stages of illness. Although selected characteristics of these pressure modes are attractive, some of the modes are not well understood and superior outcomes associated with their use have not been described. Results of studies of acute respiratory distress syndrome (ARDS) suggest that traditional ventilator modes are injurious to the lung. Thus, clinical applications of ventilation and the use of specific modes during the acute stage of illness focus on “protecting the lung” and improving patient outcomes. These concepts are described here.
Mechanical Ventilation of Acute Respiratory Distress Syndrome
ARDS was previously described as the most severe presentation of acute lung injury (ALI), but the term ALI has been eliminated in favor of the labels “mild,” “moderate,” and “severe” ARDS. The Berlin Definition of ARDS consists of categories that identify the timing of the condition, chest imaging criteria, origin of lung edema, and oxygenation status. The severity stratification of mild, moderate, and severe are based on the PaO2/FiO2 score and positive end-expiratory pressure (PEEP) level. ARDS results from an acute insult to the body that may be direct (ie, specific lung condition such as pneumonia) or indirect (ie, condition outside the lung such as sepsis). The release of mediators and a host of other toxic substances affect the alveolar–capillary membrane adversely and result in a noncardiac pulmonary edema. Pathology includes decreased lung compliance, shunting, and refractory hypoxemia. Mortality rates are as high as 50%. To date, there is no definitive treatment for ARDS. Therapy focuses on managing the underlying condition, optimizing mechanical ventilation, and providing supportive care until the lungs heal.
Studies of patients with ARDS show that large tidal volume delivery results in greater lung damage and higher mortality rates than low tidal volume ventilation. Although low tidal volume ventilation leads to hypoventilation and hypercapnia in the ARDS patient, mortality rates are lower with this approach. In addition, studies show that the use of PEEP decreases mortality in ARDS by opening collapsed lung units and preventing collapse during exhalation. This effect is called lung recruitment. In general, the levels of PEEP required to recruit the lung are relatively high (eg, 14-20 cm H2O).
While results of recent studies have changed the management of patients ...