Jet Ventilation

Definition and Overview

• Miller's Anesthesia, 10e
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery, Vol 1

Classification

1. Frequency

2. Anatomical position of the jet cannula

Mechanism of Gas Exchange

1. Convective (bulk) flow - direct mass movement of gas into the airways
2. Laminar flow in small airways - streamlined gas movement at lower velocities
3. Pendelluft - asynchronous filling between lung units, redistributing gas
4. Cardiogenic mixing - cardiac pulsations augment gas mixing
5. Molecular diffusion - important at the alveolar level, especially at high frequencies
6. Taylor-type dispersion - interaction of convection and diffusion in asymmetric bifurcations

• Scott-Brown's, Vol 1

Note: The Ventrain device is an exception - it uses the Bernoulli principle to provide both inspiratory and expiratory assistance, making it usable even with complete upper airway obstruction.

Driving Pressure and Parameters

Clinical Indications

1. Laryngeal and airway surgery - provides a tubeless surgical field; eliminates the ETT as an obstacle and a laser fire source
2. Rigid bronchoscopy - maintains ventilation through the side port of the bronchoscope
3. Panendoscopy - supraglottic HFJV commonly used
4. Microlaryngoscopy / suspension laryngoscopy - subglottic or supraglottic approach
5. Emergency airway (CICV scenario) - transtracheal jet ventilation (TTJV) listed in ASA Difficult Airway Algorithm as an emergent invasive technique when mask ventilation and intubation have both failed
6. Thoracic radiology procedures - jet ventilation reduces respiratory motion
7. Facilitation of intubation - TTJV can open the glottis and produce bubbles that guide ETT placement under poor visualization

Anesthetic Considerations

• TIVA is mandatory when jet ventilation is used because volatile agents cannot be reliably delivered through an open system.
• Neuromuscular blockade is typically used (prevents coughing, gagging, movement).
• Oxygenation monitoring: pulse oximetry is adequate.
• Ventilation (CO₂) monitoring: PETCO₂ is unreliable during HFJV because tidal volume is smaller than dead space. Options:
  • Intermittent arterial blood gas (PaCO₂) - accurate but invasive
  • Temporarily suspend HFJV or reduce frequency to ≤ 10 breaths/min to obtain valid PETCO₂
  • Transcutaneous CO₂ (PtCO₂) - noninvasive and continuous but less precise and slower to respond
  • Respiratory inductance plethysmography (RIP) - detects disconnect or absent thoracic excursion
• Miller's Anesthesia, 10e

Contraindications

• Complete upper airway obstruction - absolute contraindication because there is no egress path for gas, leading to auto-PEEP and barotrauma. For TTJV specifically, a patent upper airway is mandatory for expiration.
• Subglottic obstruction (for TTJV)
• Damage to the cricoid cartilage, larynx, or trachea (prefer surgical tracheostomy)
• Coagulopathy (relative, for percutaneous TTJV)
• Severe obstructive pulmonary disease (relative)
• Distorted neck anatomy making cricothyroid puncture unsafe
• Pfenninger & Fowler's Procedures for Primary Care; Miller's Anesthesia, 10e

Complications

Equipment

• 12- to 16-gauge kink-resistant catheter (e.g., 6 Fr Cook coil-reinforced Teflon catheter) through the cricothyroid membrane (CTM), bevel caudally
• Confirm intra-tracheal placement by aspirating air before jetting
• Connect to high-pressure oxygen source (30-60 psi)
• Manual hand-operated valve or dedicated jet ventilator

• Dedicated automatic HFJV device (e.g., Monsoon, Acutronic)
• Pause pressure cut-off alarm to prevent breath stacking
• Hunsaker Mon-Jet tube (subglottic) - shown effective for microlaryngeal surgery

• Manujet (VBM) device - simpler but lacks the safeguards of automatic HFJV (no pressure alarm), hence higher complication risk

The Ventrain Device

• Uses the Bernoulli principle (not jet friction)
• Generates negative pressure during expiration (Expiratory Ventilation Assistance, EVA), facilitating active gas egress
• Can be used even with complete upper airway obstruction (a major advantage over conventional TTJV)
• Driven from a standard wall oxygen flowmeter
• Clinical trial data remain limited as of publication
• Scott-Brown's, Vol 1; Miller's Anesthesia, 10e

Recent Evidence (2024-2025)

Invasive Arterial Blood Pressure (IBP) Monitoring: Principles and Procedure

I. DEFINITION

II. INDICATIONS

1. Anticipated or current hemodynamic instability - high-risk vascular, cardiac, thoracic, neurosurgical, or major trauma surgery with risk of sudden blood loss or deliberate hypotension/hypertension
2. End-organ vulnerability to pressure changes - coronary artery disease, heart failure, cerebrovascular disease, aortic stenosis; where any delay in detecting hypotension risks ischemia
3. Frequent arterial blood sampling - ABG analysis in single-lung ventilation, ARDS, pulmonary hypertension, mechanical ventilation, respiratory failure
4. Metabolic monitoring - major fluid shifts, sepsis, liver transplant (electrolyte and acid-base sampling)
5. Vasopressor/inotrope titration - patients in ICU requiring continuous feedback for drug dosing
6. Anticipated respiratory compromise - ETCO₂ unreliable (V/Q mismatch, single-lung ventilation)
7. Noninvasive BP unreliable - obesity, peripheral vascular disease, arrhythmias (AF), frequent cycling needed

• Barash Clinical Anesthesia, 9e; Morgan & Mikhail, 7e

III. CONTRAINDICATIONS

• Morgan & Mikhail, 7e

IV. PRINCIPLES OF OPERATION

A. The Fluid-Coupled Transducer System

1. The arterial pressure wave travels as a hydraulic pulse through the fluid column in the tubing.
2. It displaces the transducer diaphragm, converting mechanical energy into an electrical signal.
3. Most modern transducers use a strain gauge principle: a silicon crystal or wire is deformed by the diaphragm; deformation changes electrical resistance.
4. The sensing elements are arranged as a Wheatstone bridge circuit - unequal resistances create a proportionate voltage differential that is detected and converted to mmHg.
5. The signal is amplified, filtered, and displayed as:
   • A continuous pressure waveform
   • Digital systolic/diastolic/mean readouts

• Morgan & Mikhail, 7e; Barash, 9e

B. Fourier Analysis and Frequency Response

• Any complex periodic waveform (like the arterial pulse) can be decomposed into a series of simple harmonic (sine) waves - Fourier's theorem.
• The arterial pressure waveform contains significant harmonic content from 1 to 30 Hz.
• For faithful reproduction, the monitoring system's natural frequency (fn) must exceed the highest frequency component of the arterial waveform - typically requires fn > 16-24 Hz.
• Disposable transducers alone have fn > 200 Hz, but adding tubing, stopcocks, and air bubbles rapidly reduces the fn of the assembled system.

C. Natural Frequency (fn) and Damping Coefficient (ζ)

Underdamping (ζ < 0.6)

• Causes: air bubbles (paradoxically a small air bubble can increase natural frequency temporarily then degrade it further with larger bubbles), long compliant tubing, excessive stopcocks
• Effect: systolic BP overestimated, diastolic BP underestimated; exaggerated oscillations on the waveform; deep dicrotic notch visible
• Mean arterial pressure (MAP) remains accurate even in underdamped systems (provided calibration is correct)

Overdamping (ζ > 0.7)

• Causes: air bubbles (large), kinked catheter, clot in catheter, excessive tubing length, very compliant tubing
• Effect: systolic BP underestimated, diastolic BP overestimated; flat, sluggish waveform with loss of fine features
• MAP again remains the most accurate measurement

D. The Fast-Flush (Square-Wave) Test

1. Briefly open the flush valve to pressurize the system to ~300 mmHg, then release abruptly - this creates a square-wave artifact.
2. Observe the oscillations after the square wave:

V. ZEROING AND LEVELING

Zeroing

• Opens the transducer stopcock to atmosphere so the crystal senses only atmospheric pressure
• Sets the zero reference as local atmospheric pressure
• The "Zero Sensor" button on the monitor is activated
• Should be verified periodically - transducer baselines can drift over time

Leveling

• Positions the transducer at the appropriate anatomical reference point
• Standard: midaxillary line (approximates the right atrium / phlebostatic axis)
• Neurosurgery (sitting/beach-chair position): zero at the level of the external auditory meatus (approximates the Circle of Willis) - gives a reading representing cerebral perfusion pressure
• If the operating table is raised or lowered without moving the transducer, apparent BP changes - always re-level when table position changes

• Miller's Anesthesia, 10e; Barash, 9e; Morgan & Mikhail, 7e

VI. ARTERIAL SITE SELECTION

Allen Test

• Tests collateral ulnar supply to the hand before radial cannulation
• Technique: exsanguinate hand → compress both radial and ulnar arteries → release ulnar only → thumb flushes pink within 5 seconds = adequate collateral
• 5-10 seconds = equivocal; > 10 seconds = inadequate collateral
• Prognostic limitation: test has poor predictive value; does not rule out catheter thrombus with subsequent distal emboli. Many practitioners omit it. Doppler/pulse oximetry are preferred alternatives.

VII. PROCEDURE: RADIAL ARTERY CANNULATION

Setup (before skin puncture)

1. Flush the pressure tubing-transducer system with normal saline and confirm all connections are secure and bubble-free
2. Zero and level the transducer to the midaxillary line
3. Position the wrist in supination with dorsiflexion (~60°); a roll under the wrist helps; tape fingers back to a board if needed

Technique (three methods)

Method 1: Direct Puncture (catheter-over-needle)

1. Palpate the radial pulse with index and middle fingers of the non-dominant hand; ultrasound guidance is preferred when any difficulty is anticipated
2. Clean skin with chlorhexidine (aseptic technique); infiltrate 1% lidocaine subcutaneously in awake patients
3. Advance a 20- or 22-gauge catheter-over-needle at 45° to the skin, directed toward maximal impulse
4. On flashback of arterial blood, lower angle to 30° and advance a further 1-2 mm to seat the catheter tip fully in the lumen
5. Advance the catheter off the needle into the arterial lumen, withdraw the needle
6. Apply proximal digital pressure to prevent blood loss during connection

Method 2: Seldinger (guidewire-assisted)

1. Puncture artery as above; on flashback, advance a flexible J-wire through the needle
2. Remove needle; advance catheter over the guidewire into the artery
3. Remove guidewire; connect to tubing

• Preferred for difficult access or tortuous vessels - reduces trauma

Method 3: Transfixion-Withdrawal

1. Advance catheter-needle through both walls of the artery ("transfix")
2. Remove needle; slowly withdraw the catheter until pulsatile flow is seen
3. Advance catheter into lumen and connect

Securing and Dressing

• Secure with waterproof tape or sutures
• Apply sterile dressing over insertion site
• Confirm waveform on monitor after connection

VIII. THE ARTERIAL WAVEFORM: INTERPRETATION

Additional waveform information

• Respiratory variation (swing): exaggerated phasic variation in pulse pressure during PPV suggests hypovolemia or excessive tidal volumes (SVV/PPV >10-13% predicts fluid responsiveness)
• Pulsus alternans: alternating larger/smaller beats - LV dysfunction
• Pulsus paradoxus: exaggerated respiratory variation in spontaneous breathing - tamponade, severe asthma
• Rate of upstroke: steep = high contractility; slow = poor LV function or AS
• Broad peak: increased SVR; narrow peak: low SVR
• Morgan & Mikhail, 7e

IX. CONTINUOUS FLUSH SYSTEM

• A pressurized bag of heparinized or plain saline (at 300 mmHg) is connected inline via a continuous flush device
• Delivers 1-3 mL/hour of flush fluid to keep the catheter patent
• Manual flushing (fast flush) should use < 5 mL to avoid retrograde embolization to the cerebral circulation
• Air must never be present when flushing - risk of cerebral air embolism (especially with axillary catheters)

X. COMPLICATIONS

• Morgan & Mikhail, 7e; Schwartz's Surgery, 11e; Barash, 9e

XI. OPTIMIZING SYSTEM FIDELITY: PRACTICAL STEPS

Anaesthetic Gas Scavenging System (AGSS)

I. DEFINITION AND PURPOSE

• Nitrous oxide (N₂O)
• Volatile halogenated agents (isoflurane, sevoflurane, desflurane, halothane)
• Exhaled mixtures of the above from patients

II. WHY SCAVENGING IS NECESSARY: HEALTH EFFECTS OF CHRONIC EXPOSURE

• Miller's Anesthesia, 10e

III. REGULATORY THRESHOLDS

NIOSH Recommended Exposure Limits (Time-Weighted Average, TWA)

• Published by NIOSH in 1977; not legally enforceable but widely adopted
• OSHA additionally recommends: no worker exposed to halogenated agents > 2 ppm for > 1 hour, and 8-hour TWA for N₂O < 25 ppm
• Note: NIOSH limits were established before desflurane and sevoflurane were introduced but are considered similarly applicable to newer agents
• Barash Clinical Anesthesia, 9e

IV. SOURCES OF OPERATING ROOM CONTAMINATION

A. Anaesthetic Technique Factors

1. Failure to turn off gas flow control valves at the end of a case
2. Poorly fitting face masks (leakage around seal)
3. Flushing the breathing circuit to atmosphere
4. Filling vaporizers (spillage of liquid agent)
5. Use of uncuffed tracheal tubes
6. Use of circuits difficult to scavenge (e.g., Jackson-Rees / Mapleson F/E circuits)

B. Equipment Failure Factors

• Leaks in high-pressure hoses or nitrous oxide cylinder mountings
• Leaks in the low-pressure circuit of the anaesthesia machine
• Leaks at the CO₂ absorber assembly in the circle system
• Bellows ventilator failures discharging driving gas into the circuit
• Sidestream capnographs/multigas analysers withdrawing sampled gas (50-250 mL/min) - this gas must be returned to the circuit or directed to the scavenging system
• Barash, 9e

V. THE FIVE COMPONENTS OF A SCAVENGING SYSTEM

[1] Gas-Collecting Assembly
         ↓
[2] Transfer Tubing (Transfer Means)
         ↓
[3] Scavenging Interface
         ↓
[4] Gas Disposal Tubing
         ↓
[5] Gas Disposal Assembly (Active or Passive)

Component 1: Gas-Collecting Assembly

• Captures excess anaesthetic gas at its source - the APL (adjustable pressure-limiting) valve during spontaneous/manual ventilation, and the ventilator pressure-relief (pop-off) valve during mechanical ventilation
• Gas passing through either valve accumulates here and is directed to the transfer tubing
• On some ventilators (e.g., GE 7100/7900), the ventilator drive gas (O₂) is also exhausted into the scavenging system - this can overwhelm the evacuation system at high fresh gas flows + high minute ventilation, causing spillage into the room
• Most other pneumatic ventilators exhaust their drive gas separately (directly to room or through a vent at the back of the machine)

Component 2: Transfer Tubing (Transfer Means)

• Conducts waste gas from the gas-collecting assembly to the scavenging interface
• Diameter must be 19 mm or 30 mm (ASTM F1343-91 standard) - a deliberately non-standard size to prevent misconnection to the 22-mm breathing circuit tubing
• Many manufacturers colour-code with yellow bands for additional identification
• Must be rigid enough to resist kinking - occlusion is dangerous because this component is upstream from the pressure-relief valves of the interface; an occlusion here will cause dangerous positive pressure buildup in the breathing circuit
• Should be as short as possible; some machines have separate tubes from the APL valve and the ventilator relief valve that merge before entering the interface

Component 3: Scavenging Interface

A. OPEN INTERFACE

• Open to the atmosphere - no valves; provides positive- and negative-pressure relief by simply venting to room air through relief ports
• Used only with active (vacuum) disposal systems
• Requires a reservoir canister: waste gas enters the canister intermittently in surges; vacuum continuously draws gas from the bottom; room air is entrained through the open relief ports at the top to balance the system
• Vacuum flow rate must be adjusted (via a flowmeter/vacuum control valve) so that:
  • Vacuum flow rate ≥ volume of waste gas entering the system (otherwise gas spills out through relief ports into the room)
  • Some room air is always being entrained (confirming adequate vacuum)
• Does not require separate positive- or negative-pressure relief valves because the open canister inherently provides both functions
• Efficacy depends on: vacuum flow rate, reservoir volume, and the flow characteristics within the canister (turbulence can cause early spillage even before reservoir is full)

B. CLOSED INTERFACE

• Isolated from atmosphere except through specific relief valves
• Two subtypes:

• A single positive-pressure relief valve (opens at ~5 cmH₂O)
• Used with passive disposal systems only
• Waste gas flows by its own positive pressure (weight of heavier-than-air gases) through to the disposal system
• No reservoir bag required
• No negative-pressure relief valve (passive systems cannot generate negative pressure)

• A positive-pressure relief valve (opens at ~5 cmH₂O) - vents to room if system pressure exceeds this
• At least one negative-pressure relief valve (opens at ~-0.5 cmH₂O) - entrains room air if excessive suction threatens to pull gas from the patient's breathing system; a backup valve opens at -1.8 cmH₂O
• A reservoir bag (typically 5 L): stores waste gas intermittently between evacuations
• The operator must adjust the vacuum control valve to keep the reservoir bag appropriately inflated - neither overdistended (B label on Dräger bags) nor completely deflated (C label); correct inflation = label A

• Gas leaks to room only when the reservoir bag is fully inflated and pressure rises enough to open the positive-pressure relief valve - clinically this often triggers a "high PEEP" or sustained airway pressure alarm

Comparison: Open vs. Closed Interface

Component 4: Gas Disposal Tubing

• Conducts waste gas from the interface to the gas disposal assembly
• Must be collapse-proof (reinforced)
• Should run overhead where possible to minimise the chance of accidental occlusion or kinking
• Connection to the vacuum system uses a DISS-type connector (Diameter Index Safety System) to prevent misconnection

Component 5: Gas Disposal Assembly (Active vs. Passive)

Active Disposal

• Uses a central vacuum pump (hospital's central suction or a dedicated vacuum pump)
• Mechanically draws waste gases out and vents them to the outside of the building via ducting
• Requires the interface to have a negative-pressure relief valve (because the system generates subatmospheric pressure)
• Most common in contemporary operating theatres

Passive Disposal

• No mechanical suction - relies on the positive pressure of heavier-than-air gases
• Vents through: wall/ceiling/floor outlets, or into a non-recirculating HVAC exhaust (it is mandatory that it be non-recirculating - gas must not re-enter the OR atmosphere)
• Requires only positive-pressure relief at the interface
• Less common in modern theatres; acceptable in lower-resource settings

VI. CLASSIFICATION SUMMARY

AGSS
├── Active System (vacuum-driven)
│     ├── Open Interface + Active Disposal
│     └── Closed Interface + Active Disposal (most common overall)
└── Passive System (pressure-driven)
      └── Closed Interface + Passive Disposal

VII. HAZARDS AND PROBLEMS

VIII. ENSURING EFFECTIVE SCAVENGING: PRACTICAL MEASURES

1. Pre-use check: Verify scavenging system is connected, operational, and vacuum is appropriately set (part of the daily machine checkout - must be performed manually)
2. Tight face mask fit during induction and recovery
3. Use cuffed tracheal tubes whenever possible
4. Turn off all gas flows at the end of a case before disconnecting the circuit
5. Fill vaporizers carefully to avoid liquid spillage
6. Return sidestream analyser gas to the breathing circuit or route it to the scavenging system
7. Adjust vacuum to keep the reservoir bag appropriately inflated (closed active system)
8. Low-flow anaesthesia technique reduces the volume of WAG produced
9. Theatre ventilation: operating theatres should have non-recirculating ventilation with ≥ 15-20 air changes per hour to dilute any residual trace gases

IX. LOW-FLOW SCAVENGING SYSTEMS (DGSS)

• A gas-tight metal container with a 3-L reservoir bag
• An internal pressure transducer senses when pressure from the APL/ventilator relief valve reaches +0.5 cmH₂O
• A solenoid valve opens and draws gas until internal pressure reaches -0.5 cmH₂O, then closes
• Vacuum is used only when needed, in short pulses, rather than continuously
• Produces a more concentrated WAG stream - facilitating agent reclamation technologies
• Relevant because inhaled anaesthetics are greenhouse gases with global warming potential; agent reclamation/recycling is becoming increasingly important
• Barash, 9e

X. ENVIRONMENTAL CONSIDERATIONS

• Desflurane has the highest GWP (~2,540 over 100 years)
• Sevoflurane: GWP ~130
• Isoflurane: GWP ~510
• N₂O: long atmospheric lifetime; significant ozone-depleting and greenhouse gas