Noninvasive Mechanical Ventilation

INTRODUCTION

Patients with severe respiratory symptoms are common in the emergency department (ED) and comprise >10% of all presentations. Over the past decade, ED presentations of asthma, pneumonia, and chest pain have increased. A thorough knowledge of mechanical ventilatory support, both invasive and noninvasive, is essential for practicing emergency medicine clinicians. This chapter discusses noninvasive positive-pressure ventilation (NPPV), while Chapter 7 focuses on mechanical ventilation after tracheal intubation. The use of NPPV has grown steadily as a result of evidence-based research, cost effectiveness, and consideration of patient comfort and complications.

The advantages of NPPV over mechanical ventilation include preservation of speech, swallowing, and physiologic airway defense mechanisms; reduced risk of airway injury; reduced risk of nosocomial infection; enhanced patient comfort; and a decreased length of stay in the ICU and hospital.

TECHNOLOGY OF NONINVASIVE MECHANICAL VENTILATION

Noninvasive ventilators have several characteristics that are distinct from standard invasive mechanical ventilators. NPPV offers a more portable technology because of the reduced air compressor size, but because of this, noninvasive ventilators are not able to generate pressures as high as standard critical care invasive ventilators. Noninvasive ventilators have a single-limb tubing circuit that delivers oxygen to the patient and allows for exhalation. To prevent accumulation of carbon dioxide, this circuit is continuously flushed with supplemental oxygen during the expiratory phase. Exhaled gases are released through a small exhalation port near the patient’s mask. During the respiratory cycle, the machine continuously monitors the degree of air leak and compensates for this loss of volume. NPPV is designed to tolerate air leak and compensates by maintaining airway pressures. This is in sharp contrast to the closed system found in invasive, critical care ventilators consisting of a dual, inspiratory and expiratory tubing system that does not tolerate air leak or compensate for lost volume. The device that makes physical contact between the patient and the ventilator is termed the interface. Interfaces for NPPV come in a variety of shapes and sizes designed to cover the individual nares, the nose only, the nose and mouth, the entire face, or fitted as a helmet. Ideally, interfaces should be comfortable and offer a good seal with minimal leak and limited dead space.

MODES OF NONINVASIVE MECHANICAL VENTILATION

In a manner analogous to invasive mechanical ventilation, understanding the modes of NPPV is based on knowledge of three essential variables: the trigger, the limit, and the cycle. The trigger is the event that initiates inspiration: either patient effort or machine-initiated positive pressure. The limit refers to the airflow parameter that is regulated during inspiration: either airflow rate or airway pressure. The cycle terminates inspiration: either a pressure is delivered over a set time period or the patient ceases inspiratory efforts.

Continuous Positive Airway Pressure

Continuous positive airway pressure (CPAP) is a mode for invasive and noninvasive mechanical ventilation. CPAP is not a stand-alone mode of assisted mechanical ventilation. It is equivalent to positive end-expiratory pressure (PEEP) and facilitates inhalation by reducing the pressure threshold to initiate airflow (see Chapter 7). Positive airway pressure is provided throughout the respiratory cycle with constant pressure maintained during both inhalation and exhalation. This mode should never be used in patients at risk of apnea, because of the lack of a backup respiratory rate.

Spontaneous and Spontaneous/Timed Modes

In spontaneous mode, the airway pressure cycles between an inspiratory positive airway pressure (IPAP) and an expiratory positive airway pressure (EPAP). This is commonly referred to as bilevel or biphasic positive airway pressure (BL-PAP or BiPAP). The patient’s inspiratory effort triggers the switch from EPAP to IPAP. The limit during inspiration is the set level of IPAP. The inspiratory phase cycles off, and the machine switches back to EPAP when it detects a cessation of patient effort. This is indicated by a decrease in inspiratory flow rate, or once a maximum inspiratory time is reached, which is typically set at 3 seconds. Tidal volume varies breath to breath and is determined by the degree of IPAP, patient effort, and lung compliance. Work of breathing (WOB) is primarily dictated by initiation and maintenance of inspiratory airflow, with additional WOB linked to active contraction of the expiratory muscles.

Only gold members can continue reading. Log In or Register to continue