Application of neurally adjusted ventilatory assist in neonates☆
Introduction
Diaphragmatic electromyography (EMG) was first used by Petit in 1959 to evaluate respiratory muscle function [1]. Since then, diaphragmatic EMG has been utilized by a variety of investigators, between 1983 and 1994, to study diaphragmatic activity, sleep state and response to CO2 in infants [2], [3], [4], [5]. In 1987 Daubenspeck et al. described a new technique to evaluate the diaphragmatic EMG using an array of seven sequential electrode pairs on an esophageal catheter [6]. In the 1990s Sinderby and Beck expanded this concept with the introduction of embedded electrodes in a nasogastric tube that detected a reliable diaphragmatic EMG signal. This signal reflects the patient's neural respiratory drive in real time, and minimizes artifacts and noise [8], [9], [10]. This new, minimally invasive, bedside technology has been integrated into a commercially available mechanical ventilator (Servo-I; Maquet, Solna, Sweden) that converts this electrical activity into a proportionally assisted and synchronized breath known as neurally adjusted ventilatory assist (NAVA) [10], [11], [12].
Section snippets
Electrical activity of the diaphragm (Edi)
The diaphragmatic EMG is also referred to as the electrical activity of the diaphragm (Edi). The magnitude of this diaphragmatic activation (and hence the Edi signal) is controlled by adjusting the nerve fiber recruitment (the number of nerves that are sending the stimulus) and the rate coding (stimulation frequency). A specialized nasogastric tube, containing an array of eight bipolar electrodes (sensors are placed above the feeding holes), is positioned in the lower esophagus at the level of
Control of neuroventilatory coupling
During spontaneous breathing electrical excitation of the diaphragm occurs when the respiratory signal originating in the brainstem travels via the phrenic nerve to the diaphragm. The diaphragm then contracts, resulting in expansion of the chest muscles causing negative pressure in the chest so that air is drawn into the lung. The lung subsequently expands with changes in pulmonary pressures, flow, and volume (Fig. 3) [14], [22]. A variety of biological sensors provides neural feedback and
Patient–ventilator synchrony
Mechanical ventilation provides appropriate unloading of the respiratory muscles and maintains adequate gas exchange until the respiratory disease that is responsible for the patient's respiratory insufficiency has improved [10], [25]. Diaphragmatic dysfunction and atrophy have been associated with short-term ventilation of ≤7 days [26], [27], [28], long-term ventilation of >12 days, and failure of normal pulmonary growth and maturation [28]. Ventilator management with partial support
How NAVA works: Edi-controlled patient–ventilator interaction
The respiratory center in the brainstem sends a message to the diaphragm via the phrenic nerves to activate the diaphragm during spontaneous breathing. The electrodes embedded in the Edi catheter detect the electrical activity of the diaphragm and transmit the signal via the wires in the nasogastric tube to the ventilator (Fig. 3: neural trigger). The ventilator assists the spontaneous breath of the patient by delivering pressure directly and linearly proportional to the Edi. The peak
Safety features of NAVA ventilation
NAVA, like other modes of ventilation, has safety features available to protect the patient. The PIP limit can be set to prevent excessively large breaths. The peak inspiratory pressure delivered pops off at 5 cmH2O below the set limit. When setting this limit there needs to be consideration to allow the patient the ability to generate sufficient inspiratory pressure to enable intermittent lung recruitment (Fig. 5). If the level is set too low, the patient will be unable to take sigh or
NAVA terminology
The development of NAVA has introduced new terminology.
‘Edi’ is the electrical activity of the diaphragm and can be thought of as a respiratory vital sign. The Servo-I displays Edi not only as a waveform but also numerically on a breath-by-breath basis. For each breath, the highest Edi value of the waveform, the Edi peak, represents neural inspiratory effort and is responsible for the size and duration of the breath. The lowest Edi (Edi min) represents the spontaneous tonic activity of the
Using NAVA to unload respiratory muscles
When the respiratory muscles are unable to maintain adequate ventilation and oxygenation in certain disease states, partially or totally transferring workload from the respiratory muscles to the ventilator may be advantageous. The inspiratory muscle workload is lessened and the respiratory drive is reduced as ventilatory assist increases. This is reflected by a lower Edi. If the NAVA level is increased, the workload is shifted from the patient to the ventilator. Even at the highest NAVA levels,
Determining the appropriate NAVA level
The Edi, in conjunction with the NAVA level, controls the NAVA ventilator support. However, unlike pressure or volume-targeted modes, the delivered pressure during NAVA is continually adjusted based on the neural feedback from the respiratory centers. As the NAVA level is increased, the patient either maintains the Edi signal at the current level, resulting in increased delivered inspiratory pressure, or the patient decreases the Edi signal to maintain the same inspiratory pressure. Adult and
Use of Edi to determine appropriate ventilator support
With conventional ventilation, clinical comfort and blood gases are used to determine the level of support. Edi has the potential to be used to determine optimal ventilatory support. Edi peak can be influenced by the amount of ventilatory support provided. Overventilation will suppress spontaneous respiratory drive and decrease the Edi signal. Conversely, underventilation results in increased respiratory drive and higher Edi signals [36]. The availability of normative data in both term and
Studies in animals
NAVA effectively unloaded the work of respiratory muscles without increasing delivered tidal volume [31], [45]. Vagally mediated reflexes produced tonic Edi which functioned to prevent lung derecruitment and decreased with the addition of PEEP [45]. NAVA was as effective as low tidal volume strategies in reducing the incidence and severity of VILI, non-pulmonary organ dysfunction [46], and VIDD [47]. Non-invasive ventilation with NAVA (NIV NAVA) also provided synchronous ventilation via a
Conclusion
NAVA ventilation utilizes the Edi signal, representing the patient's neural respiratory drive, to synchronize ventilatory support on a breath-by-breath basis. This utilizes all facets of physiologic respiratory control to provide optimal ventilation based on the patient's ongoing needs. NAVA provides patients, even premature neonates, the ability to use physiologic feedback to control ventilation and enhance comfort for each breath. The Edi signal provides the caregiver access to previously
Conflict of interest statement
Dr Stein and Ms Firestone are speakers for MAQUET.
Funding sources
None declared.
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Sections of this chapter have been adapted with permission from a chapter previously published in: Stein H, Firestone KS, Rimensberger P. Synchronized mechanical ventilation using electrical activity of the diaphragm in neonates. Clinics in Perinatology 2012, vol. 39, p. 525–42.