The term noninvasive ventilation (NIV) refers to delivering positive-pressure ventilation through an interface (i.e., face mask, nasal mask, or nasal plugs) rather than intubating the trachea.1,2 It may be used to prevent acute respiratory failure (ARF) as a prophylactic treatment, to treat ARF, or as a curative treatment to avoid reintubation.3,4 There are different types or modes of NIV. The most frequently used are continuous positive airway pressure (CPAP) and bilevel positive airway pressure (BIPAP). CPAP is a method to deliver constant positive airway pressure during the inspiratory and expiratory phase of breathing.5 BIPAP refers to the association of two positive airway pressures (i.e., pressure support ventilation [PSV] during inspiration and lesser pressure value during expiration).5 The pressure applied during expiration is commonly incorrectly referred to as positive end expiratory pressure (PEEP). The correct term is expiratory positive airway pressure (EPAP), or the pressure delivered throughout the expiratory phase.
Some Emergency Physicians consider high-flow nasal cannula (HFNC) as a method to deliver NIV. NIV should be well known to all Emergency Physicians caring for patients with acute respiratory failure.6 It is used in the Emergency Department for NIV and preoxygenation prior to intubation.7-12 It is used as NIV by some prehospital providers.13-17 This chapter discusses CPAP and BIPAP in detail with some reference to HFNC. Other methods to deliver oxygen therapy are discussed briefly at the end of this chapter.
ANATOMY AND PATHOPHYSIOLOGY
CPAP works mainly by increasing intrathoracic pressure. It prevents airway and alveolar collapse. CPAP leads to decreased atelectasis and maintaining functional residual capacity (FRC). CPAP reduces left ventricular afterload, which may lead to an increase in cardiac output. CPAP helps to decrease the work of breathing in chronic obstructive airway disease by decreasing the inspiratory threshold load caused by intrinsic PEEP.5 CPAP may improve gas exchange by improving the ventilation-perfusion ratio and by increasing intraalveolar oxygen partial pressure. The tidal volume is dependent on respiratory muscles without unloading of inspiratory muscles.6 The increase in intrathoracic pressure can result in decreased venous return to the heart, decreasing left-sided output, and decreased blood pressure.18
The PSV portion of BIPAP is triggered by the patient’s inspiratory effort. This trigger will induce positive-pressure support that helps to unload the inspiratory muscles and decrease work of breathing. Tidal volume depends on inspiratory muscle effort, respiratory compliance, and pressure support provided by the ventilator. PEEP works during expiration by the same mechanisms that CPAP works and achieves the same goals.19 BIPAP provides pump function in addition to improving gas exchange.
HFNC is used to provide oxygen therapy (Figure 11-1).20-24 It has humidification built into the circuit and oxygen flow of 10 to 60 L/min. HFNC looks like a regular nasal cannula with bigger prongs. It provides a small positive pressure. HFNC ...