NAVA: Neurally Adjusted Ventilatory Assist

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Picture of Danelle Howard
Danelle Howard
Registered Respiratory Therapist, cross-trained in the Pulmonary Lab, caring for critically ill patients one breath at a time. Professional interests: mechanical ventilation, capnography, and waveforms.

The Pre-brief

You’re doing your vent rounds when the nurse yells, “Respiratory, fix the vent”. You notice patient-ventilator dyssynchrony.  Twenty minutes later, multiple ventilator mode changes, and multiple adjustments made to the rise time, the cycle time, and the flow rate yet your patient is still dyssynchronous.  It is at this time that you may think about NAVA.  

NAVA, Neurally Adjusted Ventilatory Assist, is a proportional ventilatory mode that uses the diaphragm’s vagal nerve stimulation to provide ventilatory assistance in synchrony with the patient’s respiratory efforts. NAVA reduces patient-ventilator dyssynchrony by utilizing the diaphragm’s electrical activity to trigger and cycle inspiration, optimizing ventilator assistance continuously in proportion to patient’s demands. NAVA monitors diaphragm activity via an OG or NG catheter with an electrode that sits near the diaphragm. NAVA allows for guided weaning, and allows for better sedation management. Spontaneously breathing patients more than 500 grams and with a working diaphragm can benefit from NAVA, as long as the clinician can place the required catheter. NAVA can be used invasively, non-invasively, and after extubation to monitor respiratory effort and muscular exhaustion on a personalized level.

Two ways to use NAVA 

It is important to note that NAVA functions as both a mode and a measurement.  As a measurement, you can drop in a catheter, ventilate the patient as you would traditionally, and measure and monitor Edi.  Edi can even be monitored in standby mode. NAVA requires a special Edi Catheter which is a diagnostic tool that continuously monitors the diaphragm’s electrical activity providing information regarding the respiratory drive in real-time. This diagnostic tool provides information to the clinician via a waveform. This special Edi Catheter will replace the NG or OG catheter and has a dual purpose. This catheter still functions as an OG and NG tube, allowing tube feeds and medications to be given through. The clinician can also attach it to low wall intermittent suction. Unlike the normal NG and OG catheter, this special Edi catheter is recommended to be changed every five days. The Special Edi catheter has ten electrodes. Nine of those electrodes read the diaphragm’s electrical activity and searches for any changes in electrical activity 62.5 times per second.

Switching to NAVA mode enables the ventilator to trigger and ventilate based on the measured data. Let’s focus on ventilation NAVA mode.

How it works 

Let’s think about how we breathe. The body’s sensors respond to chemical stimuli, which are received and interpreted by the brain’s respiratory center. The central nervous system sends electrical signals from the brain to the diaphragm through the phrenic nerves. The diaphragm then contracts, the chest wall and the lungs expand, causing pressure gradient changes. When the signal stops, the diaphragm relaxes, and inhalation stops. 

In conventional mechanical ventilation, the clinician sets an inspiratory time, respiratory rate, and tidal volume. This can cause patient dyssynchrony due to the mismatch of ventilator inspiratory time versus patient neural inspiratory time, the ventilator simply cannot meet the patient’s demands, and/or the ventilator being insensitive to hyperinflation and intrinsic PEEP. 

What does Edi mean 

Edi is the patient’s respiratory drive. The diaphragm’s electrical activity is delivered to the ventilator, displays a reading on the ventilator, and by using the same signal, the ventilator instantaneously meets the patient’s efforts creating patient-ventilator synchrony. By instantaneously meeting the patient’s demands, there will be no flow and/or cycle dyssynchrony.

There are a few values the clinician should look at. The Edi peak and the Edi min are two of these values. Edi represents the electrical activity of the diaphragm between inspiratory efforts. The Edi peak is the highest Edi value during one breath cycle while the Edi min is the lowest Edi value during one breath cycle. The changing amplitude of the Edi signal often suggests modifying the ventilator settings. If the Edi level reading goes up, this number represents under assistance (increased work of breathing by the patient). Getinge recommends increasing the NAVA level if the Edi level is greater than 15. Note, however, Edi level for COPD patients may be higher than 10-20. On the other spectrum, a decreasing Edi can signify over assistance of the ventilator, oversedation of the patient, and/or that weaning and extubation can be started. Another value to watch is the NAVA level setting. The NAVA level set on the ventilator reflects the amount of work of breathing that the ventilator will take over from the patient. PEEP and FiO2 are set normally. Clinicians tend to focus on the PIP reading displayed on the ventilator monitor, which doesn’t always need to be our focus.  However, for reference, what represents PIP in Nava Mode.

PIP= NAVA Level x Edi (Peak – Min) + PEEP

How to initially set the ventilator 

  • Set NAVA level at 1.0 with intentions to target Edi peak 10 +/- 5
  • Set trigger Edi.5 initially. Can be adjusted from 0 to 2
  • Increase NAVA level in small increments if adjustments are needed
  • Assess patient comfort

Improves Synchrony

There are a lot of benefits for the patient population that can be ventilated with NAVA. A significant benefit is patient-ventilator synchrony. With NAVA, the respiratory muscles and the ventilator are driven by the same signal. Support is initiated by the diaphragmatic neural drive and matched to the patient’s neural demands, thus allowing the patient to determine their own respiratory pattern. The ventilator cycles on instantaneously as the patient’s neural inspiration starts and cycles off instantaneously with the onset of the patient’s neural expiration, thus decreasing ineffective effort. This also reduces the over-assistance or under assistance of the ventilator, preventing diaphragm atrophy and/or volutrauma. 

With improved patient-ventilator synchrony comes improved lung protection. Note, in NAVA, the respiratory rate may be higher than in traditional ventilator modes since the patient will not have ineffective effort.

Weaning

NAVA level modification should be done once a day with the plan to decrease the level. Weaning can be the same as when weaning on pressure support with a daily SBT. When weaning, reduce NAVA levels regularly by 2. Decreasing the NAVA level while weaning should only reflect a minimal decrease in tidal volume with a small increase in Edi.  When extubated, the Edi can be left in place to continue monitoring. If Edi remains stable, it can then be removed.

Contraindications

Not every patient is a candidate for NAVA. Patients with a neuromuscular blockade, esophageal bleeding, severe hiatal hernia, the inability to place OG/NG catheter, and/or an actively used cardiac pacemaker are not candidates for NAVA.

The Debrief

  • Synchrony between patient and ventilator helps minimize patient discomfort and VILI
  • May reduce ventilator days, improve hemodynamic stability, and reduce use of sedatives
  • Edi measurement allows bedside monitoring of diagram activity

References

  1. Navalesi,P., Colombo, D., & Della Corte, F. (2010). NAVA Ventilation.  Miverva Anestesiologica, 76(5), 346-352
  2. Skorko, A., Hadfield, D. (2013). Advances in Ventilation- Neurally Adjusted Ventilatory Assist (NAVA). Journals.sagepub.com/doi/10.1177/174114371301400409

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