NAVA monitoring

There are several reasons to use NAVA monitoring (visible as Edi or electrical activity of the diaphragm), as provided by the specialist nasogastric tube:

1. To establish whether the patient is breathing or not. There are several potential explanations for a 'flat' neural drive (no Edi waveform):

The specialised NAVA catheter is in the wrong location
The patient is receiving sedation and/or muscle relaxants that is suppressing neural drive - it is a clinical decision as to whether this is appropriate.
The patient is receiving significant control-mode ventilatory support, that this support is suppressing the neural drive - initiating a spontaneous mode (Pressure Support/NAVA mode) may or may not result in the restoration of neural drive.
The patient is being 'over-oxygenated' and hypoxic drive has been lost
The patient has a brain or phrenic nerve injury that prevents neural drive being delivered to the diaphragm

2. To establish the trend in neural drive and the impact of other supportive care interventions. For example, the patient maybe on a lower level of support (eg standard flow-triggered pressure support) and the neural drive is stable (ie not rising). If the patient looks clinically stable, then the additional information from the NAVA monitoring may allow a more prolonged 'sprint' at lower support and thereby accelerate the overall weaning.

3. To properly describe asynchrony (eg missed breaths).

It is surprisingly difficult to establish whether neural drive is present without NAVA Monitoring. Once the neural drive is being monitored it becomes quickly obvious that many aspects of supportive care in ventilated patients, heavily impacts on their neural drive. This includes sedation boluses; changes in inspired oxygen concentration; changes in cerebral perfusion pressure; changes in ventilatory support mode, and changes in ventilatory support level. One of the objectives of UK NAVA is to establish whether this additional knowledge helps improve patient outcome. In other words, does the fact that the neural drive is visible, change patient management in a clinically advantageous direction.

1. Presence/magnitude of Edi

The NAVA catheter (specialised Nasogastric Tube) allows the absence of neural drive (commonly caused by over sedation/over ventilation or both) to be overtly visible.

2. Trend of EDi

The magnitude of the neural drive can be tracked over time and the impact of different interventions (eg mobilising the patient; airway changes eg downsize) on the drive, can be assessed.

3. Review of asynchrony

When the NAVA catheter (specialist nasogastric tube) is in place, the neural drive visualisation can be superimposed on whatever manadatory or spontaneous support mode is being given. This means that during nursing handover, or clinician assessment (eg before ward round), the presence and format of any asynchrony can be described.

This patient was a gentleman in multiple organ failure with a tracheostomy and global sarcopenia related to prolonged critical illness. The NAVA catheter was placed uneventfully. At 9 of pressure support the neural drive is relatively flat patient likely a little over sedated and over oxygenated as well. When the pressure support is reduced, the neural drive to diaphragm is restored. This disappears again as the pressure support is increased again. It proved relatively striaghtforward to decrease pressure support and transition the patient to NAVA with consequent acceleration of weaning. Essentially the neural drive monitoring was prompting good standard ICU practice of sedation holds and spontaneous breathing trials.

Monitoring neural drive in patient with significant agitation on waking/delirium. Very high Edipeak (eg 34 uV). Good neural drive despite Propofol and Dexmedetomidine infusions on significant amounts. Neural drive and pressure support seem well syncronised.

Monitoring neural drive in patient with significant agitation on waking/delirium. Edipeak rapidly fell (within one breath) when partner reassured her (to <20uV). Actually patient was not requiring much ventilatory pressure support at all.

Monitoring - over sedation/ over ventilation. The neural drive of the patient is completely absent. Without NAVA monitoring this is unknown. Whether this leads to better ventilatory care is unknown. For example, it may prompt a sedation hold or a reduction in respiratory support (if clinically appropriate). In other words the presence of the NAVA monitoring is accelerating standard good ABC practice in relation to liberation of patients from ventilation.

Monitoring - Patient post cardiac arrest, receiving neuroprotective sedation and ventilation with suppressed neural drive. In this case, the suppression of neural drive is clinically justified. However, it is still potentially useful to the clinical team, and reminds them that the diaphragm is off-loaded and that there is potential for adverse affects from the necessary supportive care.

Monitoring neural drive in patient with severe bronchospasm ahead of transition to NAVA. Significant asynchrony between the control breaths and the neural drive is visible.

Monitoring neural drive in patient breathing spontaneously in pressure support.

Monitoring a patient with severe global muscle weakness in pressure support to look at neural drive asynchrony - delayed triggering is visible on some breaths.

Monitoring a patient with global weakness - nurse noted autotriggering at end of this video. There is also premature triggering of pressure support breaths.

This screen shot shows the trend in neural drive decreasing from quite high levels (50uV) to low level in a patient with a high spinal injury as the pressure support was increased. The video files show some natural variation in neural drive as patient sleeps and then wakes (but still low level 4-12uV). The level of synchrony between the patient's neural drive and the pressure support breaths is visible-on some breaths a delay in the flow trigger is visible, suggesting that NAVA mode may convey some benefit to the patient. Although spinal injury will be an exclusion in the UK NAVA trial, this clinical case illustrates the use of NAVA monitoring and the importance of trend view and neural drive overlay on the pressure-time curve.

Monitoring a patient with severe global muscle weakness in pressure support to look at neural drive asynchrony - missed breaths are visible. When the patient was placed in NAVA, the respiratory rate appeared to increase but this was due to resolution of the missed breaths- in NAVA mode all breaths were supported and each pressure support breath was mapped to the neural drive.

Monitoring a patient post cardiac surgery had residual sedation but also some 'over ventilation'. When the pressure support was decreased, the neural drive of the patient became visible. When the pressure support was increased again, the neural drive again was completely suppressed. The increased awareness of 'over ventilation' as well as 'oversedation' may accelerate weaning from ventilatory support. NAVA monitoring may promote good 'ABC' practice in relation to liberating patients from invasive mechanical ventilation.

These video capture windows show a patient with severe Guillain-Barre Syndrome Syndrome on 14 pressure support versus 0 of pressure support. PEEP 5 in both settings. The neural drive is largely absent at 14 of pressure support. It then does appear when the pressure support is removed. Hoever, the neural drive remains abnormally low, indicating potential involvement of the phrenic nerve from the underlying disease. The NAVA monitoring was useful here, in revealing the point at which neural drive was completely suppressed by ventilatory support and by allowing some assessment of asynchrony. It also allows the impact of treatments like IVIG/plasmapheresis to be assessed.

Monitoring can continue to occur if the patient is extubated or ventilated with a home ventilator on ICU.

Leave the NAVA catheter connected to the ventilator

and the standby screen will have the option for 'Edi Monitoring'. This allows the clinical team to use the ventilator purely as a monitor. We recommend NAVA monitoring for at least 48 hours post extubation. It allows the effects on neural drive of 'high flow' or NIV or trachy cuff-down to be monitored.

The trend will display automatically.