Effectiveness and safety of Neurally Adjusted Ventilatory Assist (NAVA) mechanical ventilation compared to standard conventional mechanical ventilation in optimizing patient-ventilator synchrony in critically ill patients: a systematic review protocol

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Abstract

Review question/objective

The objective of this systematic review is to systemically identify, appraise and synthesize the best available evidence regarding the safety and effectiveness of invasive mechanical ventilation (IMV), in optimizing patient-ventilator interaction by using the Neurally Adjusted Ventilatory Assist (NAVA) compared with conventional IMV modalities in critically ill pediatric and adult patients in intensive care units (ICUs).

Review question/objective

Does NAVA have a better capability in reducing patient-ventilator asynchrony compared to the conventional IMV among critically ill pediatric and adult patients on ventilator support in an intensive care environment?

Review question/objective

Is NAVA safer than the conventional IMV to be used in critically ill pediatric and adult patients in an intensive care environment?

Background

Invasive mechanical ventilation is a common intervention in ICUs. It is used to sustain respiratory function in acute respiratory failure when a patient's ventilatory capabilities are unable to adequately meet the metabolic demands of the body.1–3 The goals of mechanical ventilation are to reduce excessive respiratory effort, ensure adequate oxygenation,4 avoid ventilator induced lung injury, and optimize patient-ventilator synchrony.5,6,7 Patient-ventilator asynchrony (PVA) occurs frequently in ICUs. Numerous investigators have reported a number of asynchrony incidences. It ranges from 10-88% of breathing observations during assisted mechanical ventilation.8,9,10–15 Patient-ventilator asynchrony may occur as a result of inadequate or excessive sedation16 and suboptimal ventilator settings,17–21 and it is associated with adverse clinical consequences including hypoxia, cardiovascular compromise, anxiety and fear,22 patient discomfort,9,18,23–25 sleep fragmentation,26,27 prolonged mechanical ventilation,9 and possible diaphragmatic injury.28–30 It is also associated with a longer duration of mechanical ventilation, longer ICU stay, and increased morbidity11 and mortality.7 Therefore, identifying the best modality of IMV to optimize patient-ventilator interaction is a necessary empirical goal in minimizing adverse outcomes and providing optimal care to ventilated critically ill patients.

Background

Internationally, epidemiology of IMV in adult ICUs from an analysis of the Simplified Acute Physiological Score III (SAPS III) Project database demonstrates that 53% of intensive care patients are mechanically ventilated.31 In addition, IMV is required in 17%-64% of children in pediatric intensive care units (PICUs).32–34 The rates of IMV have increased more than 10% over a seven-year period,35 contributing to a substantial increase (approximately 44%) in the annual critical care cost and accounting for 13% of total hospital costs.36 Additionally, intensive care costs for mechanically ventilated patients is more than US$2000 per day.37

Background

Mechanical ventilation can be used to support a patient's ventilatory system by delivering breaths, which can be either controlled or assisted.4,38,39 It can be set to control airway pressure, flow, volume and respiratory timing or a combination of them and to synchronize with the patient's neural breathing efforts (neural trigger) and inspiratory effort.4,40 In synchronizing with the patient's inspiratory effort It works also either by the reversal of the expiratory flow (flow sensor) or by a drop in the pressure (pressure sensor). These mechanisms are known as a pneumatic trigger.41

Background

Patient-ventilator asynchrony can be defined as a mismatch between the patient and ventilator inspiratory and expiratory times.30,42,43 It is also referred to as the difficulty of harmonizing the respiratory cycle generated by the complex respiratory control system with the mechanical cycle of the ventilator,44 and the uncoupling of the mechanical delivered breath (ventilator) and the neural respiratory effort (patient). Patient-ventilator asynchrony can be detected by measurement of electrical activity of respiratory muscles (diaphragm or transverse abdominus) or esophageal pressure43,45 or ventilator graphic waveforms.9,19 Patient-ventilator asynchrony is associated with the conventional assist modes, which are influenced by multi-factors related to both ventilator and patient.46,22,47 Patient-related factors of asynchrony include respiratory mechanics, minute ventilation, respiratory muscle capacity, and respiratory drive. Ventilator factors include the method of respiratory triggering, i.e. pneumatic trigger and neural trigger. Furthermore, the interface of the ventilator circuitry and humidification system can contribute to PVA.18,48

Background

Triggering asynchrony is found to be only one type of problem associated with suboptimal patient-ventilator interaction.19 Asynchrony events are more frequent with pneumatic triggered compared to neural triggered IMV.49,50,51–53 Several clinical studies and a meta-analysis comparing conventional controlled modes to patient triggered ventilation modes in neonates demonstrate a shorter duration of ventilation in the latter modes.54–58 Unsuccessful weaning in prolonged weaning patients is associated with a high incidence of ineffective triggering.10 Muscle fiber injury and diaphragm injury and atrophy are caused by excessive assistance and prolonged support from mechanical ventilation.59–61 Conventional ventilation can induce loss of inspiratory muscle force, as much as 75%.62,63 An asynchrony index at least 10% contributes to a longer duration of mechanical ventilation as well as a higher rate of tracheostomy in medical patients.9 However, it is not associated with prolonged IMV in trauma patients.13 Ventilator asynchronized patients tend to receive excessive levels of ventilator support,9 and sedation.11 Additionally, adjusting the pressure support level64 and the sedation level can alter PVA.12 Furthermore, there has been a report that 42% of all increases of sedation account for PVA.65 Conversely, greater sedation is associated with increased risk of ineffective effort.12 Therefore, reduced duration of mechanical ventilation, promoted spontaneous breathing,66–71 and reduced sedation are factors that contribute to positive outcomes in mechanical ventilated patients72–77 that may be caused by optimizing patient-ventilator interaction.

Background

An ideal approach in optimizing patient-ventilator interaction would be to connect the patient respiratory centers to the ventilator, as similarly and naturally as the brain stem is connected to the respiratory muscles via the phrenic nerves.4 The technique of transforming neural drive into ventilatory support output is by measuring of the neural excitation of the diaphragm, which is a diaphragmatic electrical activity (EAdi). The diaphragmatic electrical activity signal is then used to control NAVA. The diaphragmatic electrical activity is generated by the neural respiratory output signal from the brain stem, and is modulated by input from multiple respiratory reflexes feedback to the respiratory centers.40

Background

Recently, advances in computer technology have made it possible to obtain reliably diaphragmatic electrical activity, free of artifacts and noise and in real time.78–80 A new modality of neurally trigger ventilator was introduced to a clinical practice to improve patient-ventilator synchrony.39 A neurally trigger mechanical ventilation is called Neurally Adjusted Ventilatory Assist(NAVA),40 the latest development of mechanical ventilation that became available to clinicians in a clinical setting. Neurally Adjusted Ventilatory Assist may be considered to be an assist mode where the level of ventilatory assist is proportional to diaphragm muscle electrical activity. The timing and intensity of the EAdi signal both determine the timing and intensity of the ventilatory assist, resulting in a high level of synchrony.41 The diaphragmatic electrical activity signal reliably monitors and controls the ventilatory assist.81

Background

Neurally Adjusted Ventilatory Assist uses EAdi to trigger and cycle off the ventilatory assist and to control the inspiratory ventilation.82 The diaphragmatic electrical activity is obtained from the crural portion of the diaphragm via a nasogastric feeding tube with an array of eight bipolar electrodes mounted at its distal end. The signals are amplified, band-pass filtered and digitized.4,83 With NAVA, the ventilators apply pressure to the airway opening throughout inspiration in proportion to the EAdi signal times. A preset gain constant is referred to as the NAVA level. Therefore, during inspiration, peak airway pressure (Paw) is instantaneously coupled to EAdi. The support delivery is under the patient's control. This corresponds to patient demands, irrespective of variations in muscle length or contractility.84

Background

Several studies have evaluated the impact of increasing pressure support (PSV) levels versus NAVA levels, using similar methods of setting the ventilator. All studies show that NAVA averts the risk of over assistance when the assist level increased gradually. In addition, NAVA does not depend on measurement of airway pressure or flow, and is synchronous with inspiratory (neural) efforts, which is independent of the presence of leaks or intrinsic positive end expiratory pressure (iPEEP), therefore, brings about improved patient-ventilator synchrony.14,85–90 In contrast, there has been a report that a very high level of NAVA results in unstable periodic breathing patterns with delivery of high tidal volume, followed by periods of apnea and signs of discomfort.91

Background

Based on original physiological concepts, NAVA adds a new modality to patient-ventilator interaction during spontaneous breathing by using EAdi. There is compelling evidence that NAVA improves patient-ventilator interactions and increases respiratory variability in comparison with conventional pneumatic triggering ventilators, which have a number of limitations in correcting the inappropriate timing and delivering of pressure. Many investigators have conducted numerous clinical trials to evaluate the safety and efficacy of NAVA since it was first introduced into clinical practice. There is clear evidence that NAVA is safe and effective in optimizing patient-ventilator synchrony compared to the conventional mechanical ventilation modalities.14,52,85,86,89,92,93 Several clinical trials (ongoing studies) have been registered to evaluate a newly advanced neural trigger ventilation modality.94 However, to date there is no systematic review available to inform and guide clinicians in the clinical setting regarding safety and effectiveness of NAVA. Therefore, a systematic review to analyze and synthesize the best available scientific evidence is proposed to measure outcomes across included studies regarding the safety and effectiveness of NAVA as a solution to inefficient patient-ventilator interactions.

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