⚡ Chapter 31: Newer Modes of Ventilation

APRV · ASV · PAV · NAVA · Liquid ventilation · Mechanisms, advantages, disadvantages, clinical applications

🔍 Core Concepts: Newer Modes of Ventilation

📌 APRV (Airway Pressure Release Ventilation)
CPAP at two levels (Phigh, Plow) with brief releases. Long Thigh (80-95% cycle), short Tlow (0.4-0.8s). Allows spontaneous breathing throughout. Higher MAP, lower peak pressures. Used in ARDS.
📊 ASV (Adaptive Support Ventilation)
Assist-control, pressure-targeted, time-cycled. Automatically sets frequency-tidal volume pattern to minimise work of breathing based on respiratory mechanics. Uses expiratory time constant (RCe).
🎛️ PAV (Proportional Assist Ventilation)
Ventilator pressure proportional to patient effort (gain on flow and volume). All breaths spontaneous. Improves synchrony, adapts to changing mechanics. Contraindicated in respiratory depression or large air leaks.
🧠 NAVA (Neurally Adjusted Ventilatory Assist)
Uses diaphragmatic electrical activity (Edi) via oesophageal catheter to trigger and cycle ventilator. Proportional support, near-physiological synchrony. Requires Edi catheter, not for MRI.
💧 Liquid Ventilation
Perfluorocarbon (PFC) liquid. Total liquid ventilation (TLV) or partial (PLV). Low surface tension, high O2 solubility. Experimental: ARDS, CDH, RDS.
⚠️ Key Advantages
APRV: lower peak pressures, spontaneous breathing. NAVA: best synchrony, reduces sedation. PAV: adapts to patient effort. ASV: automatically optimises I:E ratio.

⚙️ Stepwise Approach: Selecting Newer Ventilator Modes

1
Identify patient need & physiology
Severe ARDS with refractory hypoxaemia → consider APRV or HFOV. Severe asynchrony on conventional modes → NAVA or PAV. Weaning difficulty → ASV or NAVA.
2
APRV initiation
Set Phigh (20-30 cmH2O), Thigh (4-6 sec), Plow (0 cmH2O), Tlow (0.4-0.8 sec). Adjust Tlow to achieve desired PaCO2 (shorter Tlow increases CO2 removal). Allow spontaneous breathing.
3
NAVA setup
Place Edi catheter (oesophageal). Verify Edi signal (P waves, QRS seen). Set NAVA level (0.5-3 μV/cmH2O). Provide backup ventilation. Monitor Edi max and patient-ventilator synchrony.
4
PAV considerations
Contraindicated if respiratory depression (bradypnoea) or large air leak. Set gain (resistive and elastic unloading). Runaway phenomenon if high drive. Set pressure/volume safety limits.
5
ASV for weaning
Set desired minute ventilation and % support. Machine measures RCe (time constant) and sets I:E ratio automatically. Use in patients with changing mechanics.
6
Monitor & escalate
If no improvement on newer modes within 24-48h, reassess. May need to switch back to conventional mode or consider ECMO. Ensure sedation and analgesia tailored to mode (NAVA may reduce sedation needs).