Jet Ventilation in ENT

Describe in brief the principle of Jet Ventilation. What  are  its applications in the management of ENT and Head & Neck diseases? Discuss in brief the advantages, limitations and complications of Jet ventilation . (2+2+2+2+2) Dec 2015

Jet ventilation is used as a lifesaving maneuver in CICV “cannot intubate – cannot ventilate” situation for oxygenation to avoid a severe desaturation of the patient.

Jet ventilation is of two types, i,e high or low frequency. HFJV is a versatile, safe and effective technique with growing indications for elective and emergency use.

The general principles of both methods are the same: jet-streams originating from high-pressure sources are cut by pneumatic or electronically controlled flow interruption devices.

Definition of Jet Ventilation^​1​

Douglas Sanders described a technique that allowed uninterrupted patient
ventilation concurrent with unhindered surgical access through an open, rigid bronchoscope.^​2​

HFJV is characterised by delivery of small tidal volumes (1-3mls/kg) from a high pressure jet at supraphysiological frequencies (1-10Hz) followed by passive expiration. ^​1​

Indications of Jet Ventilation

HFJV is indicated when it offers advantages over conventional ventilation. These indications fall into two main categories; to facilitate surgical access and to optimise pulmonary function.

Uses of Jet Ventilation in ENT

Diagnostic laryngoscopy requires a technique to provide an unimpeded view of the larynx, immobility of the vocal cords and complete control of the airway and ventilation. The delivery of small tidal volumes produces minimal vocal cord movement whilst high-pressure ventilation permits the use of narrow bore jet ventilation catheters. These impede surgical access less than conventional endotracheal tubes. Jet ventilation catheters are constructed from non-flammable, fluoroplastic material providing
advantages for use in laser surgery by reducing the chance of airway fire.

Thoracic Surgery

As in ENT surgery, the passage of a narrow catheter through the surgical field causes much less interference with surgery than the passage of a standard or double-lumen endotracheal tube. If a major conducting airway has been divided, the narrow catheter provides a relatively unobstructed, accessible circumference of trachea and bronchus so that the ends of a divided airway can be aligned for construction of an airtight anastomosis.
Use of HFJV during one-lung ventilation can be advantageous.

The non-dependent lung is held slightly distended, so minimising shunt by increasing MAWP, aiding perfusion and allowing carbon dioxide removal without necessity for large volume excursions. Selective HFJV of the non-dependent lung, while the dependent lung is ventilated with conventional intermittent positive-pressure, increases PaO2 compared with simple collapse of the non-dependent lung and conventional ventilation of the
dependent lung.

Critical Care

HFJV is particularly well suited for lung protective ventilation strategies. Low tidal volumes reduce the risk of over-distension. Reduced pressure swings during the ventilatory cycle and increased MAWP optimise end-expiratory lung volume, preventing collapse and cyclic atelectatrauma.

A Cochrane systematic review of elective HFJV versus conventional ventilation for respiratory distress syndrome in preterm infants showed no difference in mortality but showed benefit in pulmonary outcomes.

LOW FREQUENCY JET VENTILATION

LFJV is usually applied via hand-triggered devices such as the Sanders injector or Manu jet III.
Its application is usually limited to short investigative procedures such as
laryngoscopy or bronchoscopy.

Technique

Many techniques exist based on patient characteristics, surgical requirements and anaesthetic experience.
If a LaserJet catheter is used, the length of insertion should be determined by approximation, laying the catheter next to the airway. The red cuff is then positioned as a guide to depth of insertion. Most often, the stylet is removed as the catheter is rigid enough alone.

Anaesthesia is induced using an IV agent, following which low-dose neuromuscular blockade is administered and direct laryngoscopy performed. Unless using an additional ventilator to deliver anaesthetic vapours via superimposed conventional ventilation, it is necessary to adopt a Total Intravenous Anaesthesia (TIVA) technique.

The catheter is introduced and secured with tape on the left side of the patient’s mouth. This is to facilitate surgical access from the right. Jet ventilation is not commenced at this point, and the patient is ventilated with a bag and mask until positioned on the operating table
with the airway open. Alternatively, ventilation can be achieved via a laryngeal mask until the airway is open and HFJV can commence.

Once the airway is open, HFJV is started. The jet ventilator is attached to the central lumen of the catheter and the capnography and pressure tubing is connected to the monitoring lumen, both via luerloc to prevent disconnection under pressure. Initial ventilator parameters are set; It is usual to commence at a frequency of 100-150 per minute, with a DP of 1.0-1.5 bar, PP of 20 mbar and FiO2 of 1.0.

Two ventilators for jet ventilation are used during anaesthesia. These ventilators include “Monsoon” (Acutronic Medical Systems, Ag, 8816 Hirzel, Schweiz) and “Twinstream” (Carl Reiner GmbH,
Vienna, Austria). Both ventilators deliver ventilator breaths across a wide range of frequencies (4–1,600 breaths/ minute). They are also capable of delivering high- and low frequency ventilation simultaneously, usually named superimposed HFJV.

MECHANISMS OF GAS EXCHANGE^​2,3​

With conventional ventilation where tidal volumes exceed dead space, gas exchange is largely related to bulk flow of gas to the alveoli. With high frequency ventilation, the tidal volumes used are smaller than anatomical and equipment dead space and therefore alternative mechanisms of gas exchange occur.
• Pendelluft describes the movement of gas between lung units with different time constants – a property related to the product of compliance and resistance. Following inspiration, there is redistribution of inspired gas from full, fast-filling units to slower-filling units, augmenting gas exchange.

• Bulk flow may contribute partially to gas exchange as the leading edge of the gas front may actually reach a number of proximal alveoli.

Jet ventilation can be applied via supraglottic, transtracheal or subglottic approaches. The advantages and disadvantages of each technique are discussed below.

Supraglottic Approach

The supraglottic approach is advantageous as it allows a completely tubeless surgical field. However supraglottic techniques require the airway to be maintained during the procedure by the surgeon, and the quality of ventilation can be impaired by malalignment of the jet with the airway during attempts to access the operative site. There is also greater vocal cord movement when compared to the other techniques and a risk of blowing debris into the airway. It is not possible to monitor PAWP or end-tidal
CO2 (ETCO2) with the supraglottic approach.

With all approaches, peak and mean airway pressures increase linearly with increasing frequencies as the expiratory time is shortened. The rate at which these pressures increase is influenced by the approach used. The greatest rate of increase in airway pressure is seen with supraglottic techniques and the slowest rate of increase is seen with subglottic techniques.

Theoretically, it may seem that subglottic techniques increase airway pressures most rapidly because the jet is applied distally and expiratory flow is limited by the glottis/stenosis, whilst with supraglottic techniques the jet is applied proximally, and the glottis/stenosis impedes inspiratory flow as well as expiratory flow, thus reducing the rate of increase in airway pressure. However, the converse is true.
There are 4 reasons for this;

1. Venturi Effect – gas flow at a constriction speeds up causing a pressure drop and entrainment. Supraglottic jet ventilation is a true Venturi type of ventilation and the tidal volume is the sum of injected and entrained air.
2. Flow characteristics in conducting airways – turbulent flow exists in the conducting airways, with primarily laminar flow in distal airways and alveoli. Turbulent flow generated in the upper airway is conducted for a variable distance downstream and therefore a supraglottic point of injection creates a greater column of turbulent flow causing a greater resistance to expiratory flow and gas trapping.
3. Double jetting – when a supraglottic jet is applied some gas is reflected from the glottis/stenosis thereby increasing supraglottic pressure. This acts as a second high pressure jet source directing gas beyond the stenosis as well as creating an unfavourable pressure gradient for exhaust gases, thus impeding expiratory flow and increasing gas trapping.
4. Expiratory impedance – exhaust gas must be expired through the glottis/stenosis, which is only open during the expiratory phase of the cycle with supraglottic techniques. Expiratory impedance therefore
   depends on the inspiratory : expiratory (I:E) ratio and the area of the stenosis aperture. Supraglottic techniques should be avoided in small diameter stenoses due to rapid increases in airway pressures. With larger diameter stenoses, supraglottic techniques provide more efficient gas delivery with greater distending pressures (Venturi and double jet phenomenon), which may be required in patients with low respiratory compliance. Caution must be excised to avoid excessive gas trapping.

Transtracheal Approach

Transtracheal techniques provide the surgeon with operating conditions unhindered by anaesthetic equipment, and the anaesthetist controls the airway and ventilation. Entrainment is minimal allowing a consistent FiO2.
Transtracheal HFJV may be hazardous with small diameter stenoses (D<2d where D = stenosis diameter, d = jet catheter diameter), because the stenosis aperture is partially obstructed by the jet catheter, increasing exhaust impedance. With stenosis diameter <4.5 mm, airway pressure during
transtracheal jetting increases above that during subglottic jetting at equivalent stenosis size.

Subglottic Approach

The rate of increase in airway pressure is reduced with subglottic techniques. Supraglottic/stenotic pressures remain atmospheric and the pathway for exhaust gases is open throughout the ventilatory
cycle, reducing expiratory impedance and gas trapping. Subglottic techniques also produce consistent delivered oxygen concentrations since entrainment is minimal. PAWP and ETCO2 monitoring is also
possible with the subglottic technique. This is not a completely tubeless technique and may impede surgical access to posterior glottis particularly.

COMPLICATIONS

Barotrauma
Many complications associated with HFJV are due to use of a high pressure gas source. The high pressure jet may cause a rapid increase in airway pressure due to gas trapping if there is an inadequate expiratory pathway.
Increasing ventilation frequency and I:E ratio makes gas trapping more likely, and small diameter stenosis of the airway also predispose to this. Gas trapping impairs cardiac output; There is an inverse relationship between trapped gas volumes and cardiac index.

Excessive gas trapping is not an inevitable consequence of jet ventilation. Small amounts of gas trapping and minimal changes in cardiac index can be observed with subglottic ventilation at frequencies of 150 bpm.

Exposure to dry gas HFJV without humidification limits the duration of use. Prolonged exposure to dry gases under pressure causes traumatic airway injury leading to necrotising tracheobronchitis, atelectasis, loss of
ciliated epithelium, mucosal inflammation, and excessive mucous and airway plugging. High frequency jet ventilators (Monsoon, Acutronic Medical Systems) which deliver humidified gas can be used for longer periods.

Hypercapnia – CO2 elimination is dependent on the frequency, and tidal volume raised to the second power. Tidal volume is modulated by canging the Driving Pressure (DP), and the most effective way to increase CO2
elimination is to increase the driving pressure while leaving the frequency unchanged. Large increases in DP and frequency reduce CO2 elimination as expiratory time for exhaust of gases is reduced.

Transtracheal Jet Ventilation and Retrograde Intubation

Refrences

Conlon CE. High Frequency Jet Ventilation Anaesthesia tutorial of the week 271. ATOTW weekly. 2012 Oct 8.

HIGH FREQUENCY JET VENTILATION
ANAESTHESIA TUTORIAL OF THE WEEK 271