Noninvasive Mechanical Ventilation
Kerry B. Broderick
Peter M.C. DeBlieux
Patients with severe respiratory complaints frequently present to 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. Recently, 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; and a decreased length of stay in the ICU.
TECHNOLOGY OF NONINVASIVE MECHANICAL VENTILATION
Noninvasive mechanical ventilators have several characteristics that are distinct from standard critical care ventilators. NPPV offers a more portable technology because of the reduced size of the air compressor. Because of this reduction in size, these noninvasive ventilators do not develop pressures as high as their critical care ventilator counterparts do. Noninvasive ventilators have a single-limb tubing circuit that delivers oxygen to the patient and allows for exhalation. To prevent an accumulation of carbon dioxide, this tubing is continuously flushed with supplemental oxygen delivered 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 the helmet. Ideally, interfaces should be comfortable, offer a good seal, minimize leak, and limit 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 pressure thresholds to initiate airflow (see Chapter 7). It provides positive airway pressure throughout the respiratory cycle. This static, positive pressure is maintained constantly during inhalation and exhalation. This mode should never be used in patients who may have apneic episodes because of the lack of a backup 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, indicated by a decrease in inspiratory flow rate, or a maximum inspiratory time is reached, typically 3 seconds. Tidal volume varies breath to breath and is determined by 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.
Spontaneous mode relies on patient effort to trigger inhalation. In this mode, a patient breathing at a low rate can develop a respiratory acidosis. The spontaneous/timed (ST) mode prevents this clinical consequence. The trigger in the ST mode can be the patient’s effort, or an elapsed time interval that is predetermined by a set respiratory backup rate. If the patient does not initiate a breath in the prescribed interval, then IPAP is triggered. For machine-generated breaths, the ventilator cycles back to EPAP based on a set inspiratory time. For patient-initiated breaths, the ventilator cycles as it would in the spontaneous mode.
Conceptually, one can consider BiPAP as CPAP with pressure support (PS). The pressure during the inspiratory phase is termed IPAP and is analogous to PS, a pressure boost during inspiratory efforts. The pressure during the expiratory phase is termed EPAP and is analogous to CPAP, or PEEP, positive pressure maintained during the entire respiratory cycle. The IPAP is necessarily set higher than EPAP by a minimum of 5 cm H2O, and the difference between the two settings is equivalent to the amount of PS provided.
The keys to successfully using NPPV on an emergency basis are patient selection and appropriate aggressiveness of therapy—that is, before resorting to endotracheal intubation and mechanical ventilation.
INDICATIONS AND CONTRAINDICATIONS
The indications for NPPV in the emergency setting are straightforward. The eligible patient must have a patent, nonthreatened airway; be conscious and cooperative; and have an existing, ventilatory drive. Patients who may benefit from NPPV could be hypercarbic, hypoxemic, or both. Patients with an acute exacerbation of chronic obstructive pulmonary disease (COPD), congestive heart failure exacerbation, severe pneumonia, status asthmaticus, or mild postextubation stridor might all be considered for NPPV. NPPV is contraindicated if the patient has a threat to the airway, is unable to cooperate, or is apneic. If the patient is in extremis, with very poor oxygen saturations and severe or worsening ventilatory inadequacy, immediate intubation is usually indicated. In such cases, it is not appropriate to delay intubation for a trial of NPPV. This is a relative contraindication, though, and clinical judgment is required. In some cases, the situation permitting, NPPV can be used to enhance preoxygenation of a patient for whom intubation is planned.
The objectives of NPPV are the same as those for invasive mechanical ventilation: to improve pulmonary gas exchange, to alleviate respiratory distress, to alter adverse pressure/volume relationships in the lungs, to permit lung healing, and to avoid complications. Patients on NPPV must be monitored closely, using familiar parameters such as vital signs, oximetry, capnography, chest radiograph, bedside spirometry, and arterial blood gases (ABGs).
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