Jet Ventilation

Jet ventilation is the use of high-pressure (1-3 bar), high frequency ventilation to facilitate oxygenation and ventilation. Jet ventilation:

• Is used in the operating room for:
  • Diagnostic laryngoscopy and glottic ENT surgery
    • Allows unimpeded view of larynx
      Narrow bore catheters.
    • Immobility of vocal cords
      Small tidal volumes cause less movement.
  • Thoracics
    • Selective jet ventilation to non-dependent lung ↑ PaO₂
  • Radiofrequency ablation
    • Jet ventilation reduces breathing-related motion of abdominal and thoracic organs
• Achieves gas exchange via alternate means compared with conventional ventilation
  When tidal volumes are smaller than dead space volumes, bulk flow is relatively less effective.
  • Pendelluft
    Movement of gas between lung units with different time constants. Fast-filling alveoli will redistribute inspired gas to slow-filling alveoli.
  • Convective streaming
    Higher velocity central jets will diffuse to lateral zones at end-expiration.
  • Cardiogenic mixing
    Cardiac pulsations agitate lung tissue and improve molecular diffusion.
• Requires:
  • Close communication with surgeon
    Jet fired between laser pulses to prevent cord movement.
  • Paralysis
  • TIVA
  • Careful control of FiO₂
    
    • No laser: 100% O₂
    • Laser: 30% O₂
      Consider ↑ FiO₂ between pulses or whilst waiting for biopsy results, etc.

Mechanisms

May be either:

• Manual
  Low frequency.
• Automated
  Low (8 bpm) to high (up to 300bpm) frequency.
  
  • Reduced barotrauma compared to manual jet ventilation

Manual Jet Ventilation

Initial setup:

• Driving pressure: 3.5 bar (0-3.5 bar)
• Respiratory rate: ~8
• Inspiratory time: ~1s, aiming 4-5s expiration
  Keep hand or eyes on chest throughout to ensure full expiration occurs - high risk for barotrauma.

Automated Jet Ventilators

• Deliver heated, humidified gas jets in a square wave at 1-10Hz
• Clinician can control driving pressure, frequency, inspiratory time, and composition of gas
• Have alarms and emergency shutdown devices in presence of high airway pressures

Advantages:

• Minimises vocal cord, bronchial, and mediastinal excursion
• Can be passed through surgical field to jet into trachea
• Less haemodynamic impact compared with IPPV
• CO can be augmented using ECG synchronisation
• Avoids potential for ETT fire

Initial Setup:

• Driving pressure: 2 bar
  1-3 bar acceptable range.
• Frequency: 200/min
  150-300cpm. ↑ frequency reduces tidal volume.
• FiO2: 1.0
• I-time: 20%
  Adjust 20-70%. ↑ inspiratory time ↑ tidal volume.
• Pause pressure: 2mbar
  10-40mbar. This is the pressure measured at the tip of the ventilation catheter ~1ms before the next jet.

Adjustment:

• ↑ oxygenation and ventilation requires ↑ pressure, not rate

Intraoperative Management:

• Check for movements of the chest and abdomen
• Check CO₂ if sampling port connected
  Not an accurate assessment of arterial CO₂, and is easily lost if the port is blocked or kinked.
• Turn off HFJV prior to removing catheter to prevent contamination

Techniques

Supraglottic ventilation usually required for posterior cord lesions.

May be:

• Supraglottic
• Infraglottic
• Transtracheal

Supraglottic{#supra}

Jet kept in glottis and gas jetted through the glottis. * Pros: * Cheap * No tube in surgical field * Cons: * Greater rate of ↑ in airway pressure compared to subglottic ventilation
Counter-intuitive, but occurs due to: * Venturi effect
Greater entrained air with supraglottic approach ↑ tidal volume and reduces FiO₂. * Double jetting
Supraglottic jet is reflected from glottis, ↑ supraglottic pressure and impeding expiratory flow. * Expiratory impedance
Expiration can only occur during the expiratory phase of the cycle, which is a function of the I:E ratio of the jet ventilator. * No CO₂ monitoring * Cannot monitor peak airway pressure * Inadequate ventilation
If aim is incorrect. Monitor: * Sound * Chest movement * Movement of surgical field ↑ compared to subglottic approaches Can’t ventilate whilst lasering.

Infraglottic

Jet catheter placed in trachea via glottis:

• Pros
  • Slow rate of pressure ↑
  • Consistent FiO₂
    Minimal entrainment.
  • Expiration can occur through a subglottic stenosis throughout the respiratory cycle, reducing expiratory impedance
• Cons
  • Requires an intra-tracheal catheter
    Obstructs surgical field.
    • Hunsaker Monjet
      
      • Basket assists central direction of jet
      • Laser-resistant
      • Should be placed >7.5cm post the cords so the monitoring port is at an adequate depth
        
      • Monitoring port easily blocked or kinked
    • LaserJet
      • Similar to Hunsaker but without the basket
      • Monitoring lumen for airway pressure and CO₂

Transtracheal

Jet catheter placed in trachea via front of neck:

• Pros
  • Anaesthetic advantages of infraglottic jet ventilation
  • Surgical advantages of supraglottic jet ventilation
• Cons
  • Requires front of neck access
  • Risk of gas trapping with small diameter stenoses
    Air pressure exceeds that of infraglottic jetting when stenosis diameter is <4.5mm.

Complications

• Barotrauma
  May lead to subcutaneous emphysema, pneumothorax, or pneumomediastinum. Occurs due to:

  • Excessive inspiratory pressures
  • Prolonged inspiratory time
  • Outflow obstruction

• Gas trapping
  Outflow obstruction leading to rise in airway pressures and occlusion of venous return.

• Dry gas trauma
  Occurs if jetting without humidified gas. Leads to epithelial necrosis, mucous buildup, and airway pluggin.

• Hyperoxia
  Due to fall in FiO₂ or ↓ airway pressure and atelectasis.

• Hypercapnoea
  Can be avoided by ↑ driving pressure (to ↑ tidal volume) and ↓ frequency (to ↑ E time).

  • Malignancy
    Theoretical risk of spreading tumour cells.
  • Risk of airborne contamination if papilloma; wear masks

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