Jet ventilation

Jet ventilation features in both exams - the principles of jet ventilators in the Primary, and its clinical use in shared airway surgery in the Final.

Curriculum

Primary FRCA: "Ventilators: principles including pressure and flow generators, cycling, jet and oscillator ventilators"
Final FRCA: "Commonly used methods of local and general anaesthesia for patients having surgery on or below the vocal cords including techniques of ventilation"

Past exam questions

February 2023 CRQ - Airway stents (56.1% pass rate)
- Considered a difficult question at standard setting; complications were answered well, but "the finer points of jet ventilation were not well answered"
March 2019 CRQ - Laser surgery (67.0% pass rate)
- The final part was poorly answered; examiners felt many candidates lacked experience of laryngeal surgery and were unsure how patients are ventilated and oxygenated during it

Resources

Definition

Intermittent delivery of gas at high pressure into an open airway
Each jet is terminated by a flow interrupter (pneumatic or electronic)
Expiration is passive - it relies on lung and chest wall recoil, and therefore on an unobstructed route for gas to escape

Variables

Frequency - jets per minute
Driving pressure (DP) - the injection pressure of the jet
- The lungs are not exposed to this pressure, as the system is open
Inspiratory time - as a % of the cycle or an I:E ratio
FiO₂ of the jet
- Alveolar FiO₂ is lower than that set, because room air is entrained alongside the jet (Venturi effect)
Tidal volume is not set; it depends on DP, inspiratory time, entrainment and the patient's respiratory system

1 Hz (60 min^-1) is the arbitrary threshold between low- and high-frequency jet ventilation

	Low-frequency (LFJV)	High-frequency (HFJV)
Frequency	Physiological e.g. 8 - 12 min^-1	Supraphysiological e.g. 100 - 150 min^-1
Tidal volume	Greater than dead space	Less than dead space (1 - 3 ml/kg)
Gas exchange	Bulk flow	Mainly non-bulk flow mechanisms
Delivery	Hand-held injector (e.g. Manujet) or automated ventilator	Automated ventilator only (e.g. Monsoon, TwinStream)
Typical uses	Short procedures e.g. rigid bronchoscopy CICO via cricothyroid cannula	Laryngo-tracheal surgery Procedures needing a motionless field
Surgical field	More movement	Less movement of larynx, trachea and lungs

Manual low-frequency jet ventilation

Pipeline O₂ at 4 bar is delivered via a hand-held injector, attached with a Luer-lock connection
Start at a low pressure (e.g. 1 bar) and increase until the chest rises; use the lowest effective pressure
Rate similar to conventional ventilation e.g. 12 min^-1
Allow complete chest recoil before the next jet to prevent gas trapping and barotrauma

Superimposed HFJV

LFJV and HFJV delivered simultaneously
High-frequency component maintains lung inflation → improves oxygenation
Low-frequency component provides larger tidal breaths → improves CO₂ elimination

In LFJV, as with conventional ventilation, gas exchange is mostly by bulk flow

In HFJV tidal volume is less than dead space, so other mechanisms must contribute (and are incompletely understood)

Mechanisms in HFJV

Bulk flow: a minor contribution, reaching alveoli close to the conducting airways
Laminar flow: in small airways gas moves fastest centrally; high-frequency jets accentuate this, so fresh gas travels down the centre of the airway while gas at the margins moves outwards
Taylor dispersion: fast-moving central gas mixes outwards to the slower-moving margins
Pendelluft: gas redistributes between lung units with different time constants, from fast-filling to slow-filling units
Cardiogenic mixing: cardiac contraction agitates adjacent lung tissue
Molecular diffusion: at the alveolar-capillary interface

Supraglottic

Jet delivered via the surgical laryngoscope or rigid bronchoscope, above the glottis

Advantages	Disadvantages
Completely tubeless field	Ventilation depends on the surgeon keeping the scope aligned with the airway
	Most rapid rise in airway pressure
	Vocal cord movement
	Blood and debris blown down into the airway
	No airway pressure or EtCO₂ monitoring; unpredictable FiO₂

Subglottic

Jet delivered via a narrow catheter passed through the vocal cords, tip in the upper-to-mid trachea

Advantages	Disadvantages
Slowest rise in airway pressure - gas can escape through the glottis throughout the cycle	Catheter in the surgical field; may impede access to the posterior larynx
Minimal vocal cord movement	Barotrauma risk if the upper airway becomes obstructed
Minimal entrainment → predictable FiO₂	Laser-resistant catheters are still a potential fuel source
Airway pressure and EtCO₂ monitoring via a second lumen
Blood and debris blown out of the airway

Transtracheal

Percutaneous cannula, usually through the cricothyroid membrane
Can be sited awake under local anaesthesia before induction in patients with upper airway pathology, as a backup route for oxygenation

Advantages	Disadvantages
Tubeless field	Invasive - bleeding, infection, malposition
Predictable FiO₂	Kinking or blockage of the cannula
Can be placed awake before induction	Highest complication rate of the three routes
	Still needs a patent upper airway for expiration

Why airway pressure rises fastest with supraglottic jetting

Venturi effect - entrained room air adds to the delivered volume
Turbulent flow in the conducting airways increases resistance to expiration
Double jetting - gas reflected back off the glottis acts as a second jet
Gas can only leave through the glottis during expiration, rather than throughout the cycle

Supraglottic techniques should therefore be avoided with small-diameter stenoses

Automated jet ventilators

E.g. Monsoon (Acutronic), TwinStream (Carl Reiner)
Pipeline O₂ and air at 4 bar pass through a pressure regulator to solenoid valves, which open intermittently to release pulses of gas
Can deliver low-, high- and superimposed-frequency ventilation
Safety features
- Pause pressure - airway pressure measured at the catheter tip between jets
- Jetting stops automatically if pause pressure exceeds a preset limit (usually 20 - 30 mbar), e.g. due to outflow obstruction
- Laser mode - reduces FiO₂ while the laser is in use
- Humidification of gases

Subglottic catheters

Double lumen - one for jetting, one for airway pressure and EtCO₂ monitoring
Made of laser-resistant material
Hunsaker Mon-Jet - distal basket centres the catheter in the trachea, keeping the jet off the mucosa
LaserJet - no basket, so easier to insert but less stable; a marker at the lips is used to monitor depth

Transtracheal catheters

E.g. Ravussin cannula - kink-resistant
Ventrain - uses the Venturi effect to actively suction gas back out through the cannula, allowing expiration even with significant upper airway obstruction

Typical starting settings for HFJV

Driving pressure 1 - 1.5 bar
Frequency 100 - 150 min^-1
Inspiratory time 40 - 50%
FiO₂ 1.0 (reduced during laser use)
Pause pressure limit ~20 mbar
Adjust according to chest wall movement, SpO₂ and CO₂

Optimising gas exchange

To improve oxygenation	To improve CO₂ elimination
Optimise jet alignment	Optimise jet alignment
↑ FiO₂	-
↑ Driving pressure	↑ Driving pressure
↑ Frequency	↓ Frequency
↑ Inspiratory time	↓ Inspiratory time

Tidal volume has the greatest influence on CO₂ elimination, so increasing driving pressure is the most effective way to reduce CO₂
Increasing frequency reduces tidal volume (and expiratory time), so can cause hypercapnia
Increasing inspiratory time improves oxygenation but shortens expiration, which may cause CO₂ retention and gas trapping
Change one variable at a time and observe the effect

Measuring CO₂

EtCO₂ is unreliable with an open system and small tidal volumes
Options include:

Briefly pausing HJFV to deliver a tidal volume breath for sampling
Transcutaneous CO₂ monitoring
Arterial blood gas analysis

If oxygenation deteriorates

Check jet alignment with the airway
Consider a pneumothorax
Consider converting to conventional airway management: facemask, supraglottic airway, tracheal intubation or surgical airway

Jet ventilation is indicated when it offers an advantage over conventional ventilation, usually because the site of surgery makes conventional ventilation impractical

Airway surgery

Surgery to the larynx, trachea and bronchi without a tracheal tube, e.g. via suspension laryngoscopy or rigid bronchoscopy
Unobstructed view, minimal vocal cord movement and reduced risk of airway fire during laser surgery
Airway resection - a narrow catheter can be passed across the surgical site while the anastomosis is formed
See also laser airway surgery, bronchoscopy and airway stenting

Thoracic surgery

HFJV to the non-dependent lung during one-lung ventilation improves oxygenation
Bronchopleural fistula - lower airway pressures and tidal volumes reduce the leak

Difficult airway

Pre-emptive transtracheal cannula where difficulty is anticipated
Rescue in a CICO situation via a cricothyroid cannula although high risk of barotrauma; current DAS guidance favours scalpel-bougie-tube front of neck access

Motionless field

HFJV reduces diaphragmatic and thoracoabdominal movement
E.g. radiofrequency ablation of liver tumours, AF ablation, extracorporeal shockwave lithotripsy

Relative contraindications

Airway obstruction: prevents expiration, leading to barotrauma
Airway trauma: jetting may worsen the injury and force gas into the tissues
Foreign body: may be blown further down the airway
Staff unfamiliar with the technique, particularly in an emergency

Respiratory

Barotrauma - pneumothorax, pneumomediastinum, subcutaneous emphysema
Hypoxia and hypercapnia
Airway soiling with blood, debris or gastric contents
Mucosal drying and injury, including necrotising tracheobronchitis, with prolonged use of unhumidified gas
Airway fire during laser surgery

Cardiovascular

Gas trapping → ↑ intrathoracic pressure → ↓ venous return and ↑ right ventricular afterload → hypotension and potentially RV failure
Arrhythmias secondary to hypercapnia and acidaemia
Pneumopericardium

Gastrointestinal

Gastric distension and, rarely, rupture; particularly with a misaligned supraglottic jet

Risk factors for barotrauma

The main cause is unrecognised obstruction to gas outflow
Obstructing lesions and tight stenoses
Laryngospasm
No neuromuscular blockade
Transtracheal route
Manual jet ventilation (lacks pressure monitoring and automatic cut-off)
High frequency or long inspiratory time (less time to exhale)

Preventing barotrauma

Ensure the upper airway remains patent for expiration
Manual jetting: allow full chest deflation between jets
Automated jetting: pause pressure monitoring with automatic cut-off
Use the lowest effective driving pressure

Perioperative management of the patient undergoing jet ventilation

Assessment

Nasendoscopy and CT to establish the level, degree and type of any airway narrowing
Identify and mark the cricothyroid membrane (with ultrasound if needed)

Planning

Team brief with the surgeon to agree primary and backup oxygenation plans
Plan for failure of jet ventilation in every case, with equipment prepared
Consider awake tracheal intubation if difficult airway management is anticipated

Monitoring

AAGBI standards
Depth of anaesthesia monitoring
Observation of chest wall movement
CO₂ monitoring for longer procedures (see above)

Anaesthetic technique

TIVA (e.g. propofol and remifentanil): volatile agents cannot be reliably delivered via an open system
Neuromuscular blockade optimises surgical conditions and avoids patient-ventilator dyssynchrony; rocuronium and sugammadex allow deep block with rapid reversal
Topical local anaesthesia of the airway, particularly if neuromuscular blockade is not used

Airway

Oxygenate via facemask, supraglottic airway or high-flow nasal oxygen until the jetting device is in place and the airway is open
Secure jet catheters with Luer-lock connections
Laser precautions - reduced FiO₂, laser-resistant catheters and an airway fire plan

Emergence

Oxygenate via facemask or supraglottic airway until the patient is awake
High-flow nasal oxygen is less suitable on emergence as it does not recruit collapsed lung, and CO₂ continues to rise in the apnoeic patient

Disposition

Extubation may be high-risk given the underlying pathology and surgery; choose an appropriate post-operative location with a clear airway rescue plan
Observe for signs of barotrauma (e.g. subcutaneous emphysema, pneumothorax) and airway oedema or bleeding