3.2 Advanced Ventilation (APRV, NAVA)

Airway Pressure Release Ventilation (APRV)

Airway Pressure Release Ventilation (APRV) is an inverse-ratio, pressure-controlled mode of mechanical ventilation that allows unrestricted spontaneous breathing throughout the entire respiratory cycle. It is primarily indicated as a rescue oxygenation strategy for patients with severe, refractory Acute Respiratory Distress Syndrome (ARDS) who have failed conventional lung-protective ventilation (VC-CMV or PC-CMV).

Physiological Principles and Mechanism of Action

APRV operates by maintaining a high continuous positive airway pressure ($P_{high}$) for a prolonged duration ($T_{high}$) to optimize alveolar recruitment, maximize mean airway pressure ($P_{aw}$), and improve ventilation-perfusion ($V/Q$) matching. This prolonged high-pressure phase is periodically interrupted by a brief "release" phase to a lower pressure ($P_{low}$) for a short time ($T_{low}$) to facilitate passive exhalation and carbon dioxide elimination. The patient can breathe spontaneously at both pressure levels, although the majority of spontaneous breaths occur at the $P_{high}$ level.

Initial Settings and Dosing Protocol

When transitioning a patient from conventional ventilation to APRV, the clinician must establish parameters based on the patient's existing pulmonary mechanics:

• $P_{high}$ (High Pressure): Set to match the measured plateau pressure ($P_{plat}$) from conventional volume-control ventilation or the peak pressure from pressure-control ventilation. This is typically initialized between 28 and 30 cm H2O. To prevent volutrauma and barotrauma, $P_{high}$ should generally be capped at a maximum of 35 cm H2O.
• $T_{high}$ (Time at High Pressure): Represents the recruitment phase. It is set between 4.0 and 6.0 seconds, ensuring that the lungs remain inflated for the vast majority of the respiratory cycle.
• $P_{low}$ (Low Pressure): Set at 0 cm H2O. A setting of 0 cm H2O maximizes the pressure gradient between $P_{high}$ and $P_{low}$, facilitating rapid expiratory gas flow and optimal CO2 clearance during the brief release phase.
• $T_{low}$ (Time at Low Pressure / Release Time): Set between 0.5 and 0.8 seconds. This is the most critical setting in APRV and must be meticulously titrated.

Titration of $T_{low}$ and the Prevention of Derecruitment

The duration of the release phase ($T_{low}$) must be short enough to prevent the alveoli from collapsing (derecruitment). In ARDS, the lungs have a short expiratory time constant due to decreased compliance. If $T_{low}$ is too long, the lungs will empty completely, leading to atelectrauma. To set $T_{low}$ correctly, the clinician must analyze the expiratory flow-time waveform:

1. Identify the Peak Expiratory Flow Rate (PEFR).
2. Titrate $T_{low}$ so that the expiratory flow is truncated (cut off) when it reaches 50% to 75% of the PEFR (often targeted at 60%).
3. Truncating the flow at this point leaves a significant volume of gas trapped in the lungs, creating "intrinsic PEEP" or auto-PEEP. This keeps the unstable alveoli splinted open throughout the release phase.

Weaning APRV: The "Drop and Stretch" Method

Weaning is accomplished by gradually decreasing $P_{high}$ and extending $T_{high}$. This transitions the patient toward a continuous CPAP profile with spontaneous breathing.

• Drop: Decrease $P_{high}$ in increments of 1 to 2 cm H2O (down to a baseline of 10-12 cm H2O).
• Stretch: Increase $T_{high}$ in increments of 0.5 to 1.0 seconds (stretching up to 10-15 seconds).
• Once the patient is at a $P_{high}$ of 10-12 cm H2O and $T_{high}$ is stretched to 12-15 seconds with minimal releases, they are effectively breathing on CPAP and can be evaluated for extubation.

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Neurally Adjusted Ventilatory Assist (NAVA)

Neurally Adjusted Ventilatory Assist (NAVA) is an advanced, proportional mode of mechanical ventilation that delivers pressure support in direct proportion to the electrical activity of the diaphragm (EAdi).

Electrode Catheter Placement and Signal Verification

NAVA requires the insertion of a specialized nasogastric or orogastric tube containing an array of miniaturized electrodes positioned at the distal end.

1. Placement: The catheter is advanced into the esophagus until the electrode array is positioned at the level of the diaphragm.
2. Verification: The clinician verifies catheter placement using the ventilator's catheter positioning tool, which displays a multi-channel retrocardiac electrocardiogram (ECG). Correct placement is confirmed when:
   • P-waves are visible on the upper leads.
   • QRS complexes decrease in amplitude from the top to bottom leads.
   • The EAdi signal waveform shows a distinct, rhythmic upward deflection matching the patient's inspiratory effort.
3. Signal Metrics: The ventilator monitors the peak EAdi ($EAdi_{peak}$, representing respiratory drive strength) and minimum EAdi ($EAdi_{min}$, representing tonic diaphragmatic activity). Normal $EAdi_{peak}$ values range from 5 to 15 microvolts ($\mu V$).

Dosing and Proportional Support

The clinician sets the NAVA Level (expressed in cm H2O/$\mu V$), which serves as the amplifier of the neural signal: $\\text{Delivered Pressure} = \\text{NAVA Level} \\times (\\text{EAdi} - EAdi_{min}) + \\text{PEEP}$

• Initial NAVA Level: Typically set between 1.0 and 2.0 cm H2O/$\mu V$. It is adjusted to achieve a target tidal volume (6-8 mL/kg IBW) and control the respiratory rate while keeping the patient's $EAdi_{peak}$ within a healthy range of 5-15 $\mu V$.
• Triggering and Cycling: A breath is triggered when the EAdi signal rises by 0.5 $\mu V$ above $EAdi_{min}$. The breath cycles off to exhalation when the EAdi signal drops to 70% of its peak value. Because the trigger and cycle are neural, pneumatic delays are completely eliminated.

Clinical Advantages and Safety Backups

• Overcoming Auto-PEEP: In patients with severe COPD, air trapping creates a threshold load that makes triggering conventional ventilators difficult or impossible (missed triggers). NAVA bypasses this completely because the EAdi signal triggers the breath before any airway pressure or flow changes occur.
• Auto-Protection against Over-Assistance: If the NAVA level is set too high, the patient's respiratory center detects the excess volume and naturally reduces diaphragmatic effort (lowering the EAdi signal), which automatically decreases the ventilator's pressure output.
• Safety Backups: If the EAdi signal is lost (e.g., catheter displacement or sedation-induced respiratory depression), NAVA automatically switches to a pressure-control backup mode.