8.6 APRV in Pediatric Respiratory Failure

8.6 APRV in Pediatric Respiratory Failure

Airway Pressure Release Ventilation (APRV) in Pediatrics

Airway Pressure Release Ventilation (APRV) is an advanced open-lung mode designed for patients with severe, heterogeneous acute lung injury and Pediatric Acute Respiratory Distress Syndrome (PARDS).

Airway Pressure (cmH2O)
   ^
   |   +--------------------------+          +--------------------------+
P_high |---|                          |          |                          |
   |   |                          |          |                          |
   |   |   (Spontaneous Breaths   |          |   (Spontaneous Breaths   |
   |   |     at High CPAP)        |          |     at High CPAP)        |
   |   |                          |          |                          |
P_low  |--------------------------+--+-------+--------------------------+--+-----> Time
       |<------- T_high --------->|<-T_low->||<------- T_high --------->|<-T_low->|

Core Principles of APRV

1. Continuous CPAP with Intermittent Releases: APRV applies a continuous high airway pressure ($P_{\text{high}}$) for the vast majority of the respiratory cycle ($T_{\text{high}}$) to promote alveolar recruitment. It then intermittently releases pressure to a lower baseline ($P_{\text{low}}$) for a brief interval ($T_{\text{low}}$) to facilitate carbon dioxide elimination.
2. Unrestricted Spontaneous Breathing: The patient is free to take unassisted, spontaneous breaths at any point during the cycle, primarily at the $P_{\text{high}}$ plateau. Spontaneous breathing improves dependent lung ventilation, preserves diaphragmatic tone, enhances cardiac venous return, and reduces heavy sedation requirements.

Core APRV Settings & Titration

+-----------------------------------------------------------------------------+
|                            APRV PARAMETER TARGETS                           |
+-------------------+---------------------------------------------------------+
| Parameter         | Pediatric Clinical Setting & Titration Target           |
+-------------------+---------------------------------------------------------+
| P_high            | 20 to 28 cmH2O (matched to conventional plateau press)  |
+-------------------+---------------------------------------------------------+
| T_high            | 3.0 to 6.0 seconds (accounts for 80% to 90% of cycle)   |
+-------------------+---------------------------------------------------------+
| P_low             | 0 cmH2O (maximizes gas egress gradient to dump CO2)     |
+-------------------+---------------------------------------------------------+
| T_low             | 0.2 to 0.6 seconds (CRITICAL: set by expiratory flow)   |
+-------------------+---------------------------------------------------------+

The Expiratory Flow Waveform Rule: Setting $T_{\text{low}}$

The release time ($T_{\text{low}}$) is the most critical parameter in APRV. Setting $T_{\text{low}}$ too long causes complete alveolar derecruitment, whereas setting it too short causes severe carbon dioxide retention.

• The 75% Peak Expiratory Flow Rate (PEFR) Rule:
  • The therapist must observe the expiratory flow-time scalar.
  • When the ventilator releases from $P_{\text{high}}$ to $P_{\text{low}}$, gas accelerates rapidly to a negative peak (Peak Expiratory Flow Rate, PEFR).
  • Some APRV protocols adjust $T_{\text{low}}$ so expiration ends while substantial expiratory flow remains—often around 50%-75% of peak—but pediatric practice and ventilator algorithms differ. Use the protocol, flow curve, lung volume, CO2, and hemodynamic response.
  • Terminating the release while flow is still escaping traps gas inside the alveoli, creating intentional intrinsic PEEP (auto-PEEP) that keeps fragile alveoli stented open, preventing cyclic atelectotrauma.

Expiratory Flow (L/min)
   ^
 0 |------------------------------+              +------------------------------
   |                              |              |
   |                              |              |
   |             PEFR             |              |             PEFR
   |               |              |              |               |
   |               v              |              |               v
   |             +----+           |              |             +----+
   |             |    |           |              |             |    |
   |             |    |           |              |             |    |
   |             |    \           |              |             |    \
   |             |     \ <--- Terminate at 75% of PEFR         |     \ <--- Terminate at 75%
   v             +------\---------+              v             +------\---------+
                         |                                             |
                         |<-- T_low -->|                               |<-- T_low -->|

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Clinical Comparison: Conventional PCV vs. NAVA vs. APRV

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Worked Clinical Calculations

Calculation 1: Determining Delivered Pressure on NAVA

A 2-year-old child recovering from viral pneumonia is supported on invasive NAVA. Current settings:

• PEEP: $6\text{ cmH}_2\text{O}$
• NAVA Level: $1.5\text{ cmH}_2\text{O} / \mu\text{V}$
• $P_{\text{max}}$: $30\text{ cmH}_2\text{O}$

During a calm, resting breath, the monitor displays:

• $Edi_{\text{peak}}$: $12\ \mu\text{V}$
• $Edi_{\text{min}}$: $1.2\ \mu\text{V}$

$$\text{Delivered Airway Pressure} = \text{PEEP} + (\text{NAVA Level} \times [Edi_{\text{peak}} - Edi_{\text{min}}])$$ $$\text{Delivered Airway Pressure} = 6 + (1.5 \times [12 - 1.2]) = 6 + (1.5 \times 10.8) = 6 + 16.2 = 22.2\text{ cmH}_2\text{O}$$

If the child becomes agitated and pulls an $Edi_{\text{peak}}$ of $22\ \mu\text{V}$ with an $Edi_{\text{min}}$ of $2.0\ \mu\text{V}$:

$$\text{Delivered Airway Pressure} = 6 + (1.5 \times [22 - 2.0]) = 6 + (1.5 \times 20) = 6 + 30 = 36\text{ cmH}_2\text{O}$$

Because $36\text{ cmH}_2\text{O}$ exceeds $P_{\text{max}}$ ($30\text{ cmH}_2\text{O}$), the ventilator clamps delivered pressure at $P_{\text{max}} - 5 = 25\text{ cmH}_2\text{O}$ or the set $P_{\text{max}}$ limit to safeguard against barotrauma.

Calculation 2: Setting $T_{\text{low}}$ on APRV

A pediatric patient with severe ARDS exhibits a Peak Expiratory Flow Rate (PEFR) of $-40\text{ L/min}$ on the expiratory flow scalar when releasing from $P_{\text{high}}$.

• Target termination flow: $75\%$ of PEFR.

$$\text{Termination Flow} = -40\text{ L/min} \times 0.75 = -30\text{ L/min}$$

• The therapist adjusts $T_{\text{low}}$ on the ventilator until the valve closes and pressure returns to $P_{\text{high}}$ exactly when expiratory flow decelerates to $-30\text{ L/min}$. This prevents lung derecruitment.

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NPS Exam Traps

Exam Trap 1: Sedation, Paralysis, and NAVA

On the NPS exam, if a patient on NAVA is administered a neuromuscular blocking agent (e.g., vecuronium, rocuronium) or heavy narcotic boluses that abolish central respiratory drive, candidates must recognize that the Edi signal will drop to zero. NAVA cannot function without active diaphragmatic electromyographic signals. When $Edi$ ceases, the ventilator automatically drops into apnea back-up ventilation. You cannot use NAVA in a paralyzed patient.

Exam Trap 2: Troubleshooting Edi Catheter Malposition

If the ventilator monitor indicates an invalid $Edi$ signal and the positioning window shows the highest amplitude signals in the topmost channel with large P-waves, the catheter has slipped upward into the esophagus. Do not increase the NAVA level or replace the catheter; simply advance the catheter deeper until the middle channels highlight blue.

Exam Trap 3: Titrating T_low in Pediatric APRV

When an arterial blood gas shows hypercapnia in a child on APRV, candidates often instinctively lengthen $T_{\text{low}}$ to allow "more time for exhalation." In APRV, prolonging $T_{\text{low}}$ beyond the $75\%$ PEFR cutoff causes catastrophic alveolar collapse (derecruitment), worsening both hypoxemia and lung compliance. To blow off more $CO_2$ safely, the clinician should slightly reduce $T_{\text{high}}$ (increasing release frequency) or optimize spontaneous breathing.