8.5 NAVA, Edi & Proportional Support

8.5 NAVA, Edi & Proportional Support

Patient-ventilator dyssynchrony can increase work of breathing, discomfort, sedation exposure, and ineffective ventilation in neonatal and pediatric intensive care. Conventional pneumatic modes rely on pressure or flow sensors located in the ventilator or proximal circuit. In infants with compliant chest walls, high airway resistance, rapid respiratory rates, and uncuffed endotracheal tubes, pneumatic triggering is notoriously vulnerable to auto-triggering (from circuit leaks or cardiogenic oscillation) or missed triggers (inability of weak inspiratory efforts to overcome intrinsic PEEP).

Neurally Adjusted Ventilatory Assist (NAVA) and advanced proportional modes bypass pneumatic limitations by coupling the ventilator directly to the patient's neurological respiratory drive.

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Electrophysiological Foundations of NAVA

NAVA uses the patient's own central neural drive to dictate the onset, size, and termination of each assisted breath. The physiological sequence operates as follows:

+-----------------------------------------------------------------------------+
|                        THE NAVA NEURO-VENTILATORY AXIS                      |
+-----------------------------------------------------------------------------+
|                                                                             |
|   1. Respiratory Center (Medulla / Pons)                                    |
|          |                                                                  |
|          v                                                                  |
|   2. Phrenic Nerve Conduction                                               |
|          |                                                                  |
|          v                                                                  |
|   3. Diaphragmatic Depolarization (Action Potentials at Crural Motor Units) |
|          |                                                                  |
|          v                                                                  |
|   4. Edi Catheter Detection (Miniature Bipolar Electrodes)                  |
|          |                                                                  |
|          v                                                                  |
|   5. Microprocessor Amplification (Proportional Pressure Delivery)          |
|                                                                             |
+-----------------------------------------------------------------------------+

Because diaphragmatic electrical activity precedes the pneumatic pressure or flow change, NAVA can reduce trigger delay and improve synchrony. Actual delay and proportionality depend on catheter position, signal processing, ventilator, apnea settings, leak, and the patient's neural drive; they are not guaranteed to be below one universal millisecond value.

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The Edi Catheter: Sizing, Insertion & Anatomical Verification

The Edi catheter is a medical-grade feeding tube (available in sizes $6\text{ to }16\text{ French}$) equipped with an array of 9 miniature ring electrodes that form 8 sequential bipolar electrode pairs near its distal tip.

+-----------------------------------------------------------------------------+
|                     EDI CATHETER POSITIONING SCREEN CRITERIA                |
+------------------------------------+----------------------------------------+
| Anatomical Channel                 | Electrophysiological Signal Profile    |
+------------------------------------+----------------------------------------+
| Positioning display                 | Interpret the catheter’s ECG lead pattern and the device’s Edi-position highlight together. |
+------------------------------------+----------------------------------------+
| Expected transition                | P-wave visibility and QRS amplitude change as electrodes cross the diaphragm; the exact highlighted leads depend on catheter position and system display. |
+------------------------------------+----------------------------------------+
| Confirmation                       | Follow the current ventilator and catheter manual, then verify a stable Edi signal and feeding-tube position before use. |
+------------------------------------+----------------------------------------+

Insertion & Verification Technique

1. Depth Estimation: Catheter insertion depth is estimated using the modified Nose-Ear-Xiphoid (NEX) distance formula or manufacturer nomograms based on crown-to-rump length and weight.
2. The 4-Channel Verification Window: On the ventilator interface (e.g., Getinge Servo-u / Servo-n), the system displays four real-time bipolar electromyographic tracings derived from the electrode pairs.
3. Confirm position: Use the manufacturer’s Edi catheter positioning screen: evaluate which leads are highlighted, the direction of P-wave and QRS changes across the electrode array, signal stability, and the system’s on-screen guidance.
4. Correct malposition: Advance or withdraw only according to the device display and catheter instructions, then reconfirm both Edi signal and feeding-tube position. Do not infer direction from a memorized color rule that may differ by display or software version.

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Clinical Metrics of the Edi Signal

The $Edi$ signal is quantified in microvolts ($\mu\text{V}$) and continuously monitored as two distinct clinical values:

Microvolts (uV)
      ^
      |             /\                     /\
Peak  |----        /  \                   /  \        <-- Edi_peak (Effort / Drive)
      |           /    \                 /    \
      |          /      \               /      \
Min   |----     /________\             /________\     <-- Edi_min (Tonic Activity)
      +--------------------------------------------------> Time

1. $Edi_{\text{peak}}$ (Peak Diaphragmatic Activity)

• Definition: The maximum electrical activity recorded during the inspiratory phase of neural breathing.
• Clinical Significance: Reflects the total neural respiratory drive generated by the brainstem. It represents the patient's physical effort to breathe.
• Interpretation: There is no universal normal Edi_peak target. Trend the patient’s own signal with respiratory effort, comfort, tidal volume, gas exchange, sedation, neural disease, catheter position, and changes in support. A low value may reflect over-assistance, low drive, medication, weakness, or signal loss; a high value may reflect load, distress, insufficient assist, agitation, or artifact.

2. $Edi_{\text{min}}$ (Tonic Diaphragmatic Activity)

• Definition: The baseline electrical activity recorded during the expiratory pause.
• Clinical Significance: Represents basal diaphragmatic tone. In neonates, active tonic contraction of the crural diaphragm prevents end-expiratory alveolar collapse and preserves functional residual capacity (FRC).
• Interpretation: Edi_min varies with age, lung volume, sleep, support, and signal quality; use trends rather than a universal normal range.

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Machine Controls & The NAVA Proportional Assist Equation

In NAVA, the clinician does not set an inspiratory pressure or tidal volume. Instead, the clinician sets the NAVA Level, which acts as a proportional multiplier:

$\text{Delivered Airway Pressure} = \text{PEEP} + (\text{NAVA Level} \times [Edi_{\text{peak}} - Edi_{\text{min}}])$

Where:

• Airway Pressure: Instantaneous pressure delivered at the airway opening ($\text{cmH}_2\text{O}$).
• PEEP: Positive end-expiratory pressure set by the clinician ($\text{cmH}_2\text{O}$).
• NAVA Level: Proportional assist factor set in units of $\text{cmH}_2\text{O} / \mu\text{V}$ (selected and titrated using the manufacturer’s method, the patient’s response, and the local protocol).
• $Edi_{\text{peak}} - Edi_{\text{min}}$: Net neural drive for that individual breath ($\mu\text{V}$).

+-----------------------------------------------------------------------------+
|                       NAVA PROPORTIONAL ASSIST DYNAMICS                     |
+-----------------------------------------------------------------------------+
|                                                                             |
|   Quiet, Resting Breath (Edi = 6 uV)    --> Low Pressure Assist Delivered   |
|   Deep, Stressed Breath (Edi = 18 uV)   --> High Pressure Assist Delivered  |
|   Cough / Sigh (Edi = 25 uV)            --> Maximum Assist (Up to P_max)    |
|                                                                             |
+-----------------------------------------------------------------------------+

Cycling Mechanics: Neural Cycling-Off

Conventional pressure support often cycles when inspiratory flow falls to a set fraction of peak flow, which can be disrupted by leak. NAVA cycles from the fall in Edi; some Servo implementations use a default around 70% of peak Edi. Verify the ventilator software and settings rather than treating that percentage as universal or equating it exactly with cessation of neural drive.

Safety Settings & Back-Up Ventilation

• Upper Pressure Limit ($P_{\text{max}}$): An essential safety parameter set to prevent excessive pressure delivery during high $Edi$ spikes (e.g., crying, vigorous coughing). Set the upper pressure limit and alarms according to the current ventilator instructions; verify how that model limits NAVA pressure rather than assuming one offset.
• Apnea Back-Up Ventilation: If the $Edi$ signal is lost due to catheter displacement, central apnea, or administration of neuromuscular blocking agents, the ventilator can transition after the configured apnea interval to preset backup ventilation. Verify apnea time, backup settings, alarms, and the model-specific transition.

Noninvasive NAVA (NIV-NAVA)

NAVA can be delivered noninvasively via nasal prongs, nasal masks, or a RAM cannula. Because neural triggering and cycling do not depend on a pneumatic leak signal, NIV-NAVA can improve synchrony despite substantial interface leak. Leak still affects pressure transmission, oxygen delivery, monitoring, comfort, and backup ventilation, so it must be assessed and managed.

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