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Jet Ventilation

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Jet Ventilation
Photo by Nick Fewings / Unsplash

Take home messages

  • The main risks are barotrauma and failure to oxygenate
  • Jet ventilation can be high or low frequency, or a bit of both
  • It can also be supraglottic, infraglottic, or trans-tracheal

A case

As if rendering a stranger unconscious and apnoeic wasn't already stressful (and awesome) enough, occasionally you might decide to up the excitement factor a little further by involving the concept of jetting.

Here's a case from our list last week, for some context.

A 55 year old man with leucoplakia of the left vocal cord requiring laser surgery. The surgeon wanted a tubeless field, so we opted for supraglottic low frequency jet ventilation.

Why bother?

From a surgical perspective, the ideal operating conditions involve a perfectly stationary patient, unobstructed access to bloodless vocal cords, and all the time in the world to operate.

The difference between ENT and autopsy is that ENT surgeons are willing to compromise on these to at least some degree.

Prior to 1967, trying to share an airway between surgeon and anaesthetist was a clumsy affair of intermittent facemask ventilation followed by brief bursts of operating while the anaesthetist held their breath.

In 1958, Safar conceptualised a ventilating bronchoscope with a sidearm through which continuous ventilation could be applied throughout surgery. This still however required the proximal end to be occluded to deliver an adequate tidal volume.

So it wasn't until 1967 when Sanders decided to try and use the Venturi principle by firing a jet of high pressure oxygen through a needle on the end of the bronchoscope, that the whole concept really took off.

Even better, it was referred to as a 'bronchoflator', which we at Anaestheasier will be calling it henceforth.

The whole point of jet ventilation is that rather than relying on a cumbersome plastic tube rammed between diseased vocal cords, you can blow-dart your oxygen therapy into the trachea from a distance, leaving much more space for the surgeons to play.

Could our surgeon last week have managed with a microlaryngoscopy tube and intermittent positive pressure ventilation?

For this case, almost certainly.

Did supraglottic jetting make their life easier?

Probably, a bit.

Was this a perfect excuse to use some cool kit?

Absolutely.

How it works

If you fire high pressure fluid through a nozzle, not only does it exit that nozzle with enthusiasm, it also reliably drags some of the surrounding fluid along for the ride as well.

There are three forces at play:

  • Viscous shear
  • Turbulent entrainment
  • Venturi principle

The former two are beyond the scope for the FRCA exams - you just need to be able to appreciate that more gas is delivered than just that leaving the nozzle.

Therefore with intermittent bursts of this high pressure, you can fire a small tidal volume down the open trachea without a tube or supraglottic airway to form a seal.

Then you interrupt the flow briefly (either manually or electronically) to allow the chest to recoil, and some alveolar gas and CO2 to escape, and you do it again.

Supra or infra?

You can choose one of three launch sites for your jet:

  • Supraglottic
  • Infraglottic
  • Transtracheal

And each has their benefits, drawbacks and enthusiasts.

Supraglottic

  • Usually a dedicated side port on the surgeon's rigid bronchoscope
  • Delivered jet entrains pharyngeal gas and delivers a higher airway pressure than other routes
  • Totally tubeless surgery with maximal access for surgeon
  • Jet can vibrate larynx
  • May need to pause at certain points
  • Very tricky to monitor pressures and EtCO2 without a measurement catheter
  • Blood and secretions risk being blown into the airway

You're basically running entirely off chest movement and saturations to guide your settings.

Infraglottic

  • A fine bore catheter is passed through the cords like an ET tube
  • The catheter often has holes at the side that allow the catheter to automatically centralise itself in the trachea
  • The catheter has a separate lumen for measuring airway pressures and EtCO2
  • Because the jet is delivered below the cords, there is less entrainment of ambient gas
  • There is minimal vocal cord movement (although there is a catheter in the way)
  • Ventilation is less affected by the surgeon's instrument positioning
  • Exhaust gas can escape throughout the respiratory cycle
  • Blood and secretions will be blown away from the airway
  • Much higher risk of barotrauma if the upper airway becomes obstructed as the jet will continue to be delivered below the obstruction

Transtracheal

  • Similar principle to infraglottic (it is in fact a form of infraglottic ventilation)
  • The transtracheal route adds an element of risk of potentially generating and aggressively ventilating a false passage
  • The tube is more likely to kink or get blocked
  • It does, however, completely avoid the surgical field, providing all the tubeless benefits of supraglottic jetting
  • All the other risks and benefits are the same as for the infraglottic route

Some versions have a bi-directional set up, where the jet alternates with a suction mechanism, meaning gas is actively removed from the respiratory tree before the next jet. This makes it somewhat safer in the event of upper airway obstruction, but adds a substantial degree of complexity.

What we did

This is one of those rare situations that deviates from the standard 'volatile or TIVA is acceptable', because you can only really do TIVA when you don't have an airway.

đź’ˇ
Muscle relaxation is very helpful for avoiding ventilator dyssynchrony.

BIS (or equivalent) is advised, and very helpful, however be warned that surgical faffage at the face end can cause interference, so don't be thrown by numbers that jump around a little once they get started.

  • Remifentanil 50 mcg/ml Minto set to give 1 mcg/kg bolus over two minutes, then 0.15 mcg/kg/min maintenance
  • Propofol 1% Marsh set to plasma target of 7.0 mcg/ml for induction then dropped to 4.0 mcg/ml for maintenance
  • Rocuronium 50 mg
  • Intubation with hyperangulated videolaryngoscope and MLT 5.0 mm

The tube was to allow conventional ventilation while the surgeons faffed around setting up the laser and their fiddly ventilating bronchoscope.

Once they were satisfied with their set up and their view, and the jet ventilator was fully set up and attached to the ventilating side port of the scope (supraglottic), we deflated the cuff of the MLT, gave 25 mg more rocuronium for the resulting laryngospasm, and pulled the tube out.

Jetting time

Now the most important question - what fiddly dials and numbers can you mess around with?

There are four principal variables that you can control with a jet ventilator:

Driving pressure

  • This is the pressure applied by the ventilator
  • It is not the pressure felt by the lung

Remember Bernoulli's principle states that high pressure gas fired through a narrow orifice will dramatically increase in velocity, but the pressure will drop.

This doesn't mean you can't still do damage, mind.

Inspiratory time

  • This is the proportion of the jetting cycle spent applying a breath vs allowing the patient to exhale
  • More inspiratory time = better oxygenation
  • Less inspiratory time = better CO2 clearance

It's the same idea as conventional ventilation.

Frequency

  • How many jets per minute
  • This can be high or low frequency

60 breaths per minute is usually used as the entirely arbitrary cutoff for determining high and low.

Low frequency

You're aiming do deliver a jet for long enough that you generate a tidal volume that is greater than the patient's dead space.

  • Start with a low driving pressure (1 bar ish)
  • Increase this slowly until you can see the chest rise
  • The timing should be adjusted so the next breath isn't delivered until the chest has fully relaxed to the exhaled position (to prevent breath stacking)

High frequency

  • You're firing much shorter bursts of lower volume (less than the dead space)
  • A flow interrupter controlled by solenoid valves delivers rapid fire jets at up to 1600 per minute
  • There is less time to exhale so more PEEP is generated

Because the smaller volume delivered and the much shorter timings, the peak pressure is generally lower with high frequency than normal IPPV.

Gas composition

  • By nature of the underlying physics, jetting will always entrain some room air and therefore dilute the FiO2 to at least some degree
  • You can however alter how much oxygen comes out the nozzle

This is of crucial importance when using lasers, as a high FiO2 of dry jetted air is a significant fire hazard.

đź’ˇ
Once the laser is set up and ready to go, bring the FiO2 down below 0.3

How does it actually ventilate them?

The short answer is we don't really know.

The longer answer is that it's a combination of several proposed mechanisms.

Bulk flow

  • Gas moves as a bulk package into the lungs, much like during normal IPPV

This plays a much smaller role in high frequency ventilation, as there simply isn't sufficient time for that much gas to move all the way down to the alveoli in bulk, meaning other mechanisms must be at play.

Laminar flow

  • Smooth, non-turbulent flow in layers in the small airways
  • Gas moves much more quickly in the middle of the airway
  • It is almost stationary at the edges

Taylor-type dispersion

  • Also called convective streaming
  • High speed gas in the middle gets dragged to the slower streams at the edges
  • This causes more mixing than diffusion would by itself

Cardiogenic mixing

  • The beating heart squidges up against the lung parenchyma and causes regular changes in intrathoracic pressure
  • This causes movement of gas throughout the respiratory tree

Molecular diffusion

  • This is just simple diffusion from an area of high concentration to a lower one
  • It's a minor effect but still, not to be sniffed at, especially in the really small airways

Pendelluft effect

  • Also called collateral ventilation
  • Gas will 'swing' between alveoli of different time constants
  • Those that fill faster will then dispense gas into slower filling areas

This process occurs during normal ventilation as well, but it is a more pronounced effect during high frequency ventilation.

What should I do if it isn't working?

If the chest isn't moving and the sats are dropping, you need to boot the surgeon (politely) out of the pharynx and rapidly evaluate the following:

  • Is the jet adequately aligned with the trachea?
  • Has your catheter become blocked or kinked?
  • Have you blown open a false passage or pneumothorax?
  • Are the benefits of jet ventilation now outweighed by actually having a proper airway in place?

You don't have to commit to jetting - you can switch to facemask, SGA or intubation at any time if required - and can switch back again if appropriate, safe and feasible.

What can go wrong?

As with any complex airway management technique, the risk of losing the airway, aspiration and failure to ventilate with all the disastrous sequelae that follow are omnipresent.

  • You can't easily humidify the inspired gas, because such a significant proportion is entrained room air
  • After a while this will lead to drying and inflammation of the tracheal mucosa
  • You can fill the pharynx with warm humid gas to be entrained, but this generally isn't done for short procedures
  • There is a risk of barotrauma from the jet itself, either because it's misaligned and firing at the mucosa, or the airway becomes obstructed and the gas can't escape

Specific complications of jet ventilation

  • Pneumothorax
  • Subcutaneous emphysema
  • Pneumomediastinum
  • Airway soiling
  • Haemodynamic instability due to CO2 retention
  • Airway fire
  • Gastric rupture
  • Necrotising tracheobronchitis (from excessive dry gases)

Emergence

After the surgeons had finished, we simply popped in an i-gel, got the patient breathing, and took them round to recovery.

Easy.

Useful Resources

References and Further Reading

Primary FRCA Toolkit

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