NAVA VERSUS TRADITIONAL MV MODALITIES IN PATIENT VENTILATOR SYNCHRONY AND SAFETY
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NAVA VERSUS TRADITIONAL MV MODALITIES IN PATIENT VENTILATOR SYNCHRONY AND SAFETY
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NAVA VERSUS TRADITIONAL MV MODALITIES IN PATIENT VENTILATOR SYNCHRONY AND SAFETY
Abbreviations
ABGs – Arterial blood gases
AECOPD – acute exacerbation of chronic obstructive pulmonary disease
ARDS – Acute respiratory distress syndrome
COPD – Chronic obstructive pulmonary disease
EAdi – Electrical activity of the diaphragm
Edi – Electrical activity of the diaphragm
ICU – Intensive care unit
NAVA – Neurally adjusted ventilatory assist
NIV – Noninvasive ventilation
PSV – Patient ventilator synchrony
PSP – Controlled pressure support
PVS – patient-ventilator synchrony
VT – Tidal volume
Introduction
Neurally adjusted ventilatory assist (NAVA) is a form of ventilation that employs the use of electrical activity of the diaphragm (EAdi) to provide ventilator aid to a patient in accordance with their effort. NAVA has contributed significantly in improving the patient-ventilator synchrony (PVS) and minimizes the health risk of over-assistance (Ferreira et al., 2017). NAVA guarantees sufficient inspiratory effort as well as gas exchange. The ventilator is ignited on and off and is centered on the value of EAdi, which reflects the neural respiratory command activity. The EAdi signal serves to monitor and control the ventilator assist. Usually, EAdi activates the assist when an individual tries to initiate inspiratory effort. NAVA is not dependent on the airway pressure measurements. Studies have shown that the respiratory demands of patient determine the level of assistance. In most cases, the EAdi signal as well as the level titration of NAVA are used to determine the extent of respiratory unloading (Baudin et al., 2015). Therefore, NAVA helps to avoid the over-assistance or under-assistance of patients. Further, it offers a protective mode of ventilation to the lungs particularly during spontaneous breathing. The EAdi signal is important in enhancing the detection of neural breathing efforts. As a result, NAVA assists to monitor sedation in a patient. NAVA minimizes the occurrence of double triggering and prevents the arousal of a patient during sleep. NAVA has been utilized successfully under various conditions including in individuals with obstructive and restrictive lung disease, in noninvasive ventilation through a helmet, and in noninvasive support in neonates. NAVA is an interesting mode of ventilation because it helps improve patient-ventilator synchrony resulting in improved clinical outcomes (Dries, 2016).
Contribution of NAVA in noninvasive ventilation support in newborns
The sustained use of mechanical ventilation such as endotracheal tubing has been reported to cause upper airway damage among the newborns and it predisposes them to various illnesses such as bronchopulmonary dysplasia and lung disease. Therefore, the lack of endotracheal intubation in respiratory supper is considered to be an important approach as postextubation treatment for breathing respiratory problems in newborns. Studies have identified NAVA as one of the new methods that can aid in the delivery of assisted ventilation and alleviate the challenges associated with mechanical ventilation. The ventilator in NAVA delivers synchronized breaths during the start and at the end, with each of the patient’s breath and is under the control of the patient (Longhini et al., 2015).
Studies conducted on newborns indicated that NAVA helps to improve the interaction of a patients with ventilator. In a study comparing the effectiveness of NAVA compared to conventional volume control within 12 hours. The findings from the study showed the presence of 4 differences between the patient’s respiratory rate and the ventilator. This was more than 5 times that of the patient, indicating backup ventilation or auto-triggering. Moreover, central apneas illustrated by flat EAdi waveforms are often decreased slightly with the use of NAVA (Schmidt et al., 2015).
Compared to mechanical ventilation and other conventional methods, NAVA has been shown to be successful in supporting preterm infants with low birth weight. A study to evaluate the interaction with NAVA demonstrated that the intervention could be implemented both invasively and noninvasively for a short period in infants with low body weight (640 g and children up to 3 years old). Further, invasive ventilation using NAVA highlights the relationship between the diaphragm’s electrical activity of the diaphragm (Edi) and ventilator pressure in addition to improving PVS.
Among the infants, NAVA helps to limit peak inspiratory pressures as well as tidal volume. Research has shown that in infants (up to 24 weeks old), low NAVA levels contribute in increasing the Edi. As the levels of NAVA increase, a certain level is attained in which the infants de-activate their diaphragms. This is referred to as down regulation of the Edi as illustrated in the figure below.
Figure: Illustration of the physiologic response of a pre-term infant to increasing NAVA levels (Firestone, Beck & Stein, 2016).
NAVA gives newborns and other patients the opportunity to utilize physiologic feedback in order to effectively manage their ventilation. The Edi signal enables the healthcare professionals to access patient information regarding central respiratory drive. Given that PVS is enhanced with NAVA, infants may require only small doses of sedation with this intervention (Terzi et al., 2012).
However, there are various limitations associated with NAVA in preterm newborns, with the key concern being the loss of EAdi signal. A NAVA catheter is essential and it needs to be placed correctly at the crural diaphragm level. If the catheter is disconnected or removed accidentally, the Edi signal will not be transmitted (Rahmani et al., 2012).
Application of NAVA in noninvasive ventilation via a helmet
Among individuals suffering from postextubation respiratory failure or COPD exacerbation, the use of NAVA helps to improve patient-ventilator interaction. It also improves comfort while having no effect on the arterial blood gases (ABGs) and EAdi. In addition to improving patient-ventilator interaction, NAVA reduces asynchronies in comparison to pneumatically triggered pressure support (PSP), which is the common technique of noninvasive ventilation (NIV) delivery. The helmet is a novel interface for noninvasive ventilation and it allows for administration of ventilation for longer periods with minimal interruptions and any NIV-associated side effects (Piastra et al., 2014).
Other studies have shown that provision of NAVA through a helmet is more effective the PSP with regards to triggering performance, comfort and patient-ventilator synchrony. Comfort is a major determinant of noninvasive ventilation outcomes. The helmet is considered to be comfortable and it helps decrease the intubation rates and both the in-hospital 90-days and 1-year mortality. The application of NAVA via a helmet has in many cases been characterized by enhanced pressurization and triggering performance. Moreover, the intervention facilitated its use in patients with more challenging situations such as COPD exacerbation (Cammarota et al., 2016).
In most cases, the key reason for instituting mechanical ventilation is to improve ABGs. Given that NAVA is known to exhibit high tidal volume (VT) variability, has the ability to enhance the flow of oxygen in arteries and ventilation distribution in regions of the lung. It is also expected to improve oxygenation. NAVA is reported to be superior to PSP in improving arterial oxygenation within a 24-hour period.
Over the past few decades, patient-ventilator asynchrony has been identified as a significant clinical problem. Individuals with rates of asynchronous breaths that exceed 10% of the total breath count experience worse outcomes such as prolonged durations of mechanical ventilation, lower chances of survival, decreased number of ventilator-free days, increased rates of tracheotomy and extended ICU stay. However, studies have outlined the benefits of NAVA in improving PVS in varied clinical conditions. The incremental ventilator assistance has a negative effect on the patient’s effort especially with the use of conventional interventions. NAVA on the other hand prevents EAdi reduction as well as the risk of significantly low efforts (Bello, De Pascale & Antonelli, 2013).
Another critical aspect of NAVA is its contribution to breathing pattern and lung volumes. Marked differences have been reported in breathing pattern between NAVA and PSP specifically in mechanically ventilated ICU patients. NAVA has the ability to maintain reduced VT and this has been demonstrated in patient with acute respiratory distress syndrome (ARDS) in the acute phase. Similar outcomes have also been reported in individuals with COPD. Compared with PSP, VT variability has been shown to be higher in NAVA, with the variability being similar in healthy individuals. Variability contributes in improves oxygenation while decreasing proinflammatory response. It is believed that the breathing pattern variability that is linked with the decreased risk of over-assistance in NAVA could explain the lack of central apneas during weaning among the non-sedated patients (Longhini et al., 2015).
Effects of NAVA on air distribution and dead space in individuals with acute exacerbation of COPD
A limited number of studies have been carried out to examine the effects of NAVA on air distribution as well as dead space in patients suffering from AECOPD. The available evidence indicates that NAVA aids in improving heterogeneous ventilation distribution by reducing diaphragm contraction.
Individuals suffering from AECOPD often experience increased number of cases of dead space and declining efficiency in ventilation. Over-assistance of these patients during pressure support ventilation may cause the problem to become worse. NAVA supplies adequate pressure assistance with regards to EAdi and minimizes the threat of over-assistance as a result of down regulation of the EAdi signal. This is considered to be the likely solution to increase dead space during mechanical ventilation in individuals with AECOPD (Fasano, Pisani & Nava, 2014).
In a study conducted by Sun et al. (2017), it was reported that ventilation distribution improved significantly among AECOPD patients compared to pressure support ventilation. Further, the diaphragm activities measured with the help of ultrasonography were found to be higher during NAVA as opposed to provision of the same support level during pressure support ventilation as indicated in the figure below.
Figure: Diaphragm activity in NAVA and pressure support ventilation (Sun et al., 2017).
Another significant finding with regards to administration of NAVA to individuals with AECOPD is that it helps in reducing the dead space fraction (Vd/Vt values).
During NAVA, EAdi is employed to stimulate and cycle the ventilator. Studies have suggested that NAVA may help to reduce the work of trigger compared to pressure support ventilation. With NAVA, the pressure delivered by the ventilator is followed by an increase in EAdi signal (Spadaro et al., 2015).
Therefore, it can be observed that NAVA is critical in increasing the distribution of ventilation specifically in the near-dorsal lung region. In addition, it reduces the Vd/Vt values in individual suffering from AECOPD. NAVA also plays a role in improving patient ventilator interaction in intubated individuals with AECOPD. Finally, NAVA helps to reduce the work of trigger (Meric et al., 2014).
Conclusion
NAVA is a novel technique of proportional assistance that offer numerous physiological advantages to patients with respiratory conditions compared to other traditional modes of partial support. The available evidence indicate that NAVA contributes greatly in improving patient-ventilator interaction, avoiding the risk of over-assistance especially in patients with COPD exacerbation and restrictive lung disease. NAVA also helps in preventing the occurrence of asynchronies. The respiratory pattern in NAVA has been reported to resemble spontaneous breathing pattern with regards to variability of VT (Navalesi & Longhini, 2015).
Despite the numerous advantages associated with NAVA, there exist some limitations. For instance, the need for insertion of a catheter limits its use in individuals with contraindications for a feeding tube. Moreover, alteration in the patient’s position may result in the displacement of the esophageal catheter from its appropriate position and subsequently may result in the loss Edi signal. Another limitation is that the neural drive which originates from the respiratory center may be affected sedation or disease. This may lead to respiratory patterns that have significantly low or significantly high variability (De Abreu & Belda, 2013).
Taking into account the available body of evidence, it can be concluded that NAVA is of great importance to patients faced with challenges in synchronizing with the ventilator.
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