Mechanical Ventilators

Quick Answer

Mechanical ventilators deliver positive pressure breaths to patients who cannot breathe adequately themselves. Classification is by power source (pneumatic, electric, or combined), drive mechanism (bellows, piston, turbine), and cycling variable (volume, pressure, time, or flow). Modern anaesthesia ventilators use ascending bellows (visible disconnect alarm) with electronic control for precise delivery of tidal volumes (typically 6-8 mL/kg IBW) at set respiratory rates (10-14/min) and I:E ratios (1:2 default). Triggering initiates inspiration via flow sensing (most sensitive, 1-3 L/min threshold), pressure sensing (-0.5 to -2 cmH₂O), or time (mandatory breaths). Cycling terminates inspiration when the set volume, pressure, time, or flow threshold (typically 25% of peak inspiratory flow for PSV) is reached. Ventilator modes include Volume-Controlled Ventilation (VCV - guaranteed tidal volume, variable pressure), Pressure-Controlled Ventilation (PCV - guaranteed pressure, variable volume), Synchronized Intermittent Mandatory Ventilation (SIMV), and Pressure Support Ventilation (PSV). Waveform analysis of pressure-time, flow-time, and pressure-volume loops enables detection of auto-PEEP, inadequate inspiratory flow, overdistension, and patient-ventilator dyssynchrony. Safety features include disconnect alarms, high/low pressure alarms, apnoea backup, and oxygen failure protection. Understanding ventilator physics and operation is essential for safe anaesthetic practice.

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Classification of Ventilators

By Power Source

Mechanical ventilators require energy to generate positive pressure breaths. The power source fundamentally determines the ventilator's operational characteristics. [1,2]

Pneumatic (Gas-Powered):

• Driven entirely by compressed gas (oxygen or air at 280-600 kPa)
• No electrical power required for basic function
• Examples: Ohmeda 7000, Penlon Nuffield 200
• Advantages: Simple, reliable, suitable for MRI environments
• Disadvantages: Higher gas consumption, limited modes

Electric:

• Electric motor drives piston, bellows, or turbine
• Requires continuous electrical supply
• Examples: Servo-i, Puritan Bennett 840
• Advantages: Efficient gas use, sophisticated modes
• Disadvantages: Power failure vulnerability, electrical hazards

Combined (Pneumatic-Electric):

• Pneumatic power for gas delivery
• Electronic control for timing and monitoring
• Most modern anaesthesia workstation ventilators
• Examples: GE Aisys, Drager Primus
• Advantages: Reliable delivery with precise control
• Disadvantages: Complexity, maintenance requirements [3]

By Drive Mechanism

Bellows (Bag-in-Bottle): The bellows system consists of a collapsible bellows within a rigid chamber. Driving gas compresses the chamber exterior, squeezing the bellows and delivering fresh gas mixture to the patient.

Ascending (Standing) Bellows:

• Bellows rises during exhalation as patient exhales
• Failure to rise indicates disconnection or major leak
• Visual disconnect alarm - crucial safety feature
• Standard in modern anaesthesia ventilators

Descending (Hanging) Bellows:

• Bellows falls during exhalation under gravity
• Will continue cycling even if disconnected (DANGEROUS)
• Rarely used in modern practice
• Still found in some developing world equipment [4,5]

Piston:

• Electric motor drives piston in cylinder
• Very precise volume delivery
• Linear displacement proportional to tidal volume
• Examples: Drager Apollo, some ICU ventilators
• Advantages: Accurate at all flows, minimal compression losses
• Disadvantages: Mechanical wear, noise

Turbine (Blower):

• High-speed rotary turbine generates flow
• Can operate without compressed gas (room air)
• Flow sensor provides feedback control
• Examples: Hamilton-C1, some transport ventilators
• Advantages: Portable, no gas supply needed
• Disadvantages: Limited FiO₂ without O₂ supply [6]

By Cycling Variable

The cycling variable determines what terminates inspiration. This classification is fundamental to understanding ventilator modes.

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Ventilator Modes

Volume-Controlled Ventilation (VCV)

VCV delivers a set tidal volume regardless of lung compliance or resistance. The ventilator generates whatever pressure is necessary to deliver the target volume.

Settings:

• Tidal volume (VT): 6-8 mL/kg ideal body weight
• Respiratory rate (RR): 10-14 breaths/min
• Inspiratory flow rate: 30-60 L/min
• Flow pattern: Square (constant) or decelerating

Characteristics:

• Guaranteed minute ventilation (VT × RR)
• Airway pressure varies with compliance/resistance
• Square flow pattern: shorter inspiratory time
• Decelerating flow: lower peak pressures, better distribution

Advantages:

• Predictable minute ventilation
• Easy to calculate expected parameters
• Standard for intraoperative ventilation

Disadvantages:

• High peak pressures with poor compliance
• Risk of barotrauma if compliance suddenly decreases
• Flow dyssynchrony possible [7,8]

Pressure-Controlled Ventilation (PCV)

PCV delivers breaths at a set inspiratory pressure for a set inspiratory time. Tidal volume varies with lung mechanics.

Settings:

• Inspiratory pressure (Pinsp): 10-25 cmH₂O above PEEP
• Inspiratory time (Ti): 0.8-1.5 seconds
• Respiratory rate: 10-14/min
• Rise time/slope: 0-0.4 seconds

Characteristics:

• Constant pressure throughout inspiration
• Decelerating flow pattern (intrinsic)
• Tidal volume dependent on compliance and resistance
• Lower peak pressures than VCV for same mean airway pressure

Advantages:

• Pressure-limited (may reduce barotrauma risk)
• Better gas distribution in heterogeneous lung disease
• Improved patient comfort
• Decelerating flow pattern

Disadvantages:

• Variable tidal volume with changing mechanics
• Requires vigilant monitoring
• Hypoventilation possible if compliance worsens [9,10]

Synchronized Intermittent Mandatory Ventilation (SIMV)

SIMV combines mandatory ventilator breaths with patient-triggered spontaneous breaths. Mandatory breaths are synchronized to patient effort within a timing window.

Types:

• SIMV-VC: Mandatory breaths are volume-controlled
• SIMV-PC: Mandatory breaths are pressure-controlled
• Often combined with PSV for spontaneous breaths

Settings:

• Mandatory rate: 4-12/min
• VT or Pinsp for mandatory breaths
• PSV level for spontaneous breaths (optional)

Advantages:

• Allows spontaneous breathing
• Weaning mode
• Maintains respiratory muscle function
• Smooth transition from controlled to spontaneous

Disadvantages:

• Variable minute ventilation
• Work of breathing through circuit
• Potentially slower weaning than PSV alone [11]

Pressure Support Ventilation (PSV)

PSV augments every spontaneous breath with a set pressure boost. The patient triggers, controls rate, and influences inspiratory time.

Settings:

• Pressure support level: 5-20 cmH₂O above PEEP
• Flow cycling threshold: typically 25% of peak inspiratory flow
• Rise time: adjustable on some ventilators

Characteristics:

• Patient-triggered (requires spontaneous effort)
• Pressure-limited
• Flow-cycled (inspiration ends when flow decreases)
• Tidal volume determined by patient effort and PS level

Cycling:

• Inspiration terminates when inspiratory flow decreases to threshold
• Default: 25% of peak inspiratory flow
• Adjustable 5-80% on modern ventilators
• Lower threshold = longer inspiration
• Higher threshold = shorter inspiration

Advantages:

• Improved patient comfort
• Reduces work of breathing
• Effective weaning mode
• Maintains respiratory muscle function

Disadvantages:

• Requires intact respiratory drive
• Apnoea risk (need backup ventilation)
• Ineffective with leaks (mask ventilation)
• Variable tidal volume [12,13]

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Triggering

Triggering is the mechanism by which the ventilator detects patient inspiratory effort and initiates a breath.

Flow Triggering

Flow triggering detects a change in base flow through the circuit when the patient inhales.

Mechanism:

1. Continuous bias flow circulates through circuit (typically 2-5 L/min)
2. Patient inspiration diverts some flow to lungs
3. Flow sensor detects decreased expiratory flow
4. When flow difference exceeds threshold, breath triggered

Settings:

• Flow trigger sensitivity: 1-5 L/min (typically 2 L/min)
• Lower value = more sensitive, easier to trigger
• Higher value = less sensitive, prevents auto-triggering

Advantages:

• Most sensitive trigger mechanism
• Lowest work of breathing for patient
• Reduced trigger delay
• Preferred for spontaneous modes

Disadvantages:

• Auto-triggering with circuit leaks
• Sensitive to circuit condensation
• May auto-trigger with cardiac oscillations [14]

Pressure Triggering

Pressure triggering detects a negative pressure deflection in the circuit when the patient attempts to inhale.

Mechanism:

1. Patient effort generates negative pressure
2. Pressure transducer detects pressure drop below threshold
3. Breath initiated when threshold reached

Settings:

• Pressure trigger sensitivity: -0.5 to -2 cmH₂O (relative to PEEP)
• More negative = less sensitive
• Less negative = more sensitive

Advantages:

• Less affected by leaks
• Simple mechanism
• Reliable with older ventilators

Disadvantages:

• Higher work of breathing than flow triggering
• Trigger delay (patient must generate negative pressure)
• May miss weak efforts (insensitive) [15]

Time Triggering

Time triggering delivers mandatory breaths at fixed intervals regardless of patient effort.

Mechanism:

• Breath delivered at end of set expiratory time
• No synchronization with patient
• Based on set respiratory rate

Clinical Use:

• Controlled mechanical ventilation (paralyzed patient)
• Backup for apnoea
• When synchronization not desired

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Cycling Variables

Volume Cycling

Inspiration terminates when the set tidal volume has been delivered.

Mechanism:

• Volume measured by flow sensor integration
• VT = ∫Flow × dt
• Breath ends when integral reaches target

Considerations:

• Compression volume in circuit affects delivered volume
• Modern ventilators compensate for circuit compliance
• Compliance compensation: VT delivered = VT set + (Circuit compliance × Peak pressure)

Time Cycling

Inspiration terminates after a preset inspiratory time.

Mechanism:

• Timer initiated at breath start
• Inspiration continues for set duration
• Independent of volume delivered

Pressure-Controlled Ventilation:

• Time-cycled by definition
• Ti typically 0.8-1.5 seconds
• I:E ratio determined by Ti and respiratory rate

Flow Cycling

Inspiration terminates when inspiratory flow decreases to a threshold.

Mechanism:

• Flow monitored throughout inspiration
• When flow drops to percentage of peak (e.g., 25%)
• Expiratory valve opens

Clinical Relevance:

• Used in PSV modes
• Adjustable threshold allows optimization:
  • COPD: higher threshold (shorter Ti) prevents auto-PEEP
  • ARDS: lower threshold (longer Ti) improves distribution

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Limiting Variables

A limiting variable is a parameter that cannot be exceeded during inspiration.

Pressure Limiting

• Maximum pressure that can be generated
• Safety feature to prevent barotrauma
• Set as Pmax or pressure alarm limit
• If limit reached, volume delivery may be incomplete

Volume Limiting

• Maximum volume delivered per breath
• Less commonly used as primary limit
• Some ventilators limit to prevent over-delivery

Flow Limiting

• Maximum inspiratory flow rate
• Determines minimum inspiratory time
• High flow = shorter Ti = more time for exhalation

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Anaesthesia Machine Ventilators

Bellows Ventilators

Modern anaesthesia workstation ventilators use ascending bellows in a bag-in-bottle configuration integrated with the circle breathing system.

Components:

1. Bellows assembly: Standing bellows in rigid housing
2. Driving gas circuit: Separate from patient gas
3. Spill valve: Controls pressure/volume delivery
4. Control electronics: Manages timing and parameters
5. Pressure transducers: Monitor airway and bellows pressure
6. Flow sensors: Measure delivered and expired volumes

Operation:

1. Fresh gas enters circle system
2. During inspiration, driving gas compresses bellows housing
3. Bellows compress, delivering gas mixture to patient
4. Spill valve modulates pressure during delivery
5. During exhalation, driving gas vents, bellows re-expand
6. Patient exhales into bellows

Fresh Gas Decoupling: Modern ventilators decouple fresh gas flow from delivered tidal volume. During inspiration, fresh gas is diverted to a reservoir rather than adding to the patient circuit. This prevents:

• Tidal volume augmentation by fresh gas flow
• Variation in VT with changes in fresh gas flow [16,17]

Piston Ventilators

Design:

• Electric motor drives piston in cylinder
• Cylinder volume equals maximum tidal volume
• Piston displacement controls delivered volume
• Direct drive = no compression losses

Advantages:

• Highly accurate volume delivery
• Low fresh gas consumption
• Effective at very low tidal volumes (neonatal)
• Minimal circuit compliance effects

Disadvantages:

• Cannot exceed cylinder volume
• Mechanical complexity
• Motor wear and noise [18]

Key Specifications (Typical Modern Anaesthesia Ventilator)

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Modern Ventilator Features

Autoflow (Pressure-Regulated Volume Control - PRVC)

Autoflow combines the benefits of volume-controlled and pressure-controlled ventilation.

Mechanism:

1. Target tidal volume set by clinician
2. Ventilator delivers test breath at low pressure
3. Compliance calculated from delivered volume
4. Subsequent breaths delivered with pressure control
5. Pressure automatically adjusted to achieve target VT
6. If compliance changes, pressure adapts

Advantages:

• Guaranteed volume with decelerating flow
• Lower peak pressures than VCV
• Automatic adaptation to changing mechanics
• Reduced work of breathing

Disadvantages:

• May increase pressure inappropriately (coughing, light anaesthesia)
• Complex algorithm may be unpredictable
• Not universal terminology across manufacturers [19]

Adaptive Support Ventilation (ASV)

ASV automatically selects optimal ventilation pattern based on patient mechanics.

Principles:

• Based on Otis equation for minimal work of breathing
• Calculates optimal rate-VT combination
• Patient weight and %MinVol input by clinician
• Automatically adjusts Pinsp and rate

Clinical Use:

• Weaning mode
• Automated adaptation
• Reduced clinician workload

Neurally Adjusted Ventilatory Assist (NAVA)

NAVA uses electrical activity of the diaphragm (Edi) to trigger and proportion support.

Mechanism:

• Specialized oesophageal catheter detects Edi
• Support proportional to Edi amplitude
• Natural neural coupling between effort and assist

Advantages:

• Eliminates trigger delays
• Proportional support
• May improve patient-ventilator synchrony
• Potentially protective against over-assistance

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Ventilator Settings

Tidal Volume (VT)

Principles:

• Target: 6-8 mL/kg ideal body weight (IBW)
• IBW calculation: Males: 50 + 0.91 × (height in cm - 152.4)
• Females: 45.5 + 0.91 × (height in cm - 152.4)
• Lung-protective ventilation: 6 mL/kg IBW
• Higher volumes (8-10 mL/kg) acceptable for healthy lungs during short procedures

Clinical Considerations:

• Obese patients: Use IBW, not actual body weight
• Excessive VT increases ventilator-induced lung injury risk
• Monitor plateau pressure (<30 cmH₂O target) [20,21]

Respiratory Rate (RR)

Typical Range: 10-14 breaths/min

Determinants:

• Target minute ventilation
• CO₂ production (metabolism, temperature)
• Dead space ventilation
• Oxygenation requirements

Special Situations:

• Raised ICP: Target normocapnia (12-14/min)
• COPD/asthma: Lower rate allows complete exhalation
• Laparoscopy: May need increased rate for CO₂ clearance

I:E Ratio

The ratio of inspiratory time to expiratory time.

Default: 1:2 (1 second inspiration : 2 seconds expiration)

Calculated:

• Total cycle time = 60/RR
• Ti = Total cycle time × (I ratio / (I + E ratios))
• Te = Total cycle time - Ti

Clinical Variations:

PEEP (Positive End-Expiratory Pressure)

PEEP maintains positive pressure in the circuit throughout the respiratory cycle.

Mechanism:

• Expiratory valve partially closed
• Pressure at end-exhalation remains above atmospheric
• Prevents alveolar collapse

Physiological Effects:

• Increases FRC
• Recruits collapsed alveoli
• Improves V/Q matching
• Reduces intrapulmonary shunt
• Decreases venous return (preload)
• May increase PVR

Typical Settings:

• Routine anaesthesia: 5-8 cmH₂O
• Obese patients: 8-12 cmH₂O
• ARDS: Titrated (typically 10-15 cmH₂O)

Complications:

• Hypotension (reduced preload)
• Increased ICP
• Over-distension of compliant alveoli
• Barotrauma
• Increased dead space [22]

FiO₂

Titration Principles:

• Target SpO₂ 94-98% (92-96% in COPD)
• Start higher during induction/emergence
• Reduce to minimum required during maintenance
• 100% for preoxygenation and airway management

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Waveform Analysis

Pressure-Time Waveform

The pressure-time curve displays airway pressure changes during the respiratory cycle.

VCV Waveform:

• Sharp pressure rise during inspiration
• Peak inspiratory pressure (PIP) reached
• Plateau pressure visible with inspiratory pause
• Pressure returns to PEEP during exhalation

PCV Waveform:

• Rapid rise to set pressure
• Constant pressure plateau during inspiration
• Return to PEEP

Diagnostic Value:

Flow-Time Waveform

Inspiration:

• Square wave flow in VCV (constant flow mode)
• Decelerating flow in PCV and PSV

Expiration:

• Passive, determined by lung mechanics
• Normally returns to zero before next breath
• Failure to return to zero indicates auto-PEEP

Diagnostic Value:

• Inspiratory flow not reaching set level: High resistance
• Expiratory flow not reaching zero: Auto-PEEP
• Sawtooth pattern: Secretions, water in circuit [23]

Pressure-Volume Loops

P-V loops provide information about respiratory system mechanics.

Normal Loop:

• Counterclockwise direction
• Inspiration: bottom right quadrant
• Expiration: top left quadrant
• Compliance = ΔV/ΔP (slope of loop)

Abnormal Patterns:

Flow-Volume Loops

Normal Pattern:

• Inspiration: Above x-axis (positive flow)
• Expiration: Below x-axis (negative flow)
• Expiratory flow should return to zero

Abnormal Patterns:

• Scooped expiratory limb: Airflow obstruction
• Expiratory flow not reaching zero: Auto-PEEP
• Irregular pattern: Patient-ventilator dyssynchrony [24]

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Alarms and Safety Features

Pressure Alarms

High Pressure Alarm:

• Triggers when airway pressure exceeds set limit (typically Pmax 35-40 cmH₂O)
• Inspiration terminated to prevent barotrauma
• Causes: Obstruction, decreased compliance, light anaesthesia/coughing

Low Pressure Alarm:

• Triggers when pressure fails to reach minimum during inspiration
• Indicates: Disconnection, major leak, inadequate VT

PEEP Alarm:

• Monitors that end-expiratory pressure maintained
• Low PEEP: Leak, disconnection
• High PEEP: Expiratory obstruction, auto-PEEP

Volume Alarms

Low Minute Ventilation:

• Expired minute volume below threshold
• Indicates: Apnoea, leak, shallow breathing

Low Tidal Volume:

• Individual breath volume below threshold
• Indicates: Disconnection, leak, weak patient effort

Apnoea Alarm and Backup

Detection:

• No detected breath within apnoea interval (typically 20-30 seconds)
• Patient on spontaneous/support mode

Response:

• Audible/visual alarm
• Automatic transition to backup ventilation
• Preset backup mode activates (controlled ventilation)

Oxygen Failure Protection

Oxygen Failure Warning Device (OFWD):

• Activates when O₂ supply pressure falls below 200 kPa
• Audible alarm (whistle or electronic)
• Powered by failing oxygen supply (no batteries needed for pneumatic)

Oxygen Ratio Controller:

• Prevents delivery of hypoxic mixture
• Minimum 25% O₂ with N₂O
• Linked flowmeters or electronic monitoring [25,26]

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Ventilation in Special Circumstances

Obesity

Challenges:

• Reduced FRC (up to 50% reduction)
• Increased oxygen consumption
• Decreased chest wall compliance
• Rapid desaturation during apnoea
• Atelectasis formation

Ventilator Settings:

• VT: 6-8 mL/kg IBW (NOT actual weight)
• PEEP: 8-12 cmH₂O (higher than normal)
• Recruitment maneuvers may be beneficial
• Higher RR may be needed
• 30° head-up positioning

Evidence:

• Higher PEEP improves oxygenation in obese patients
• Driving pressure should be minimized
• Consider CPAP during preoxygenation [27]

ARDS (Acute Respiratory Distress Syndrome)

Lung-Protective Ventilation:

• VT: 6 mL/kg IBW (4-6 mL/kg if needed)
• Plateau pressure: <30 cmH₂O
• Driving pressure (Pplat - PEEP): <15 cmH₂O
• PEEP: Titrated (moderate-high strategy)
• Permissive hypercapnia may be accepted

Recruitment:

• Stepwise PEEP titration
• Recruitment maneuvers (sustained inflation 30-40 cmH₂O for 30-40 seconds)
• Prone positioning in severe ARDS

ARDS Network Protocol:

• Demonstrated 22% relative mortality reduction with low VT
• Landmark evidence for lung-protective ventilation [28,29]

One-Lung Ventilation (OLV)

Indications:

• Thoracic surgery (lung resection, esophagectomy)
• Lung isolation for contamination/hemorrhage

Settings:

• VT: 5-6 mL/kg IBW (lower than two-lung)
• RR: Increased to maintain minute ventilation
• PEEP: 5-10 cmH₂O to non-dependent lung
• FiO₂: Typically 100% initially

Hypoxemia Management:

1. Confirm tube position (bronchoscopy)
2. Increase FiO₂
3. Apply CPAP to non-ventilated lung
4. Increase PEEP to ventilated lung
5. Consider intermittent re-inflation
6. Early surgical clamping of pulmonary artery [30]

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Troubleshooting Common Problems

High Airway Pressure

Systematic Approach:

1. Check patient: Coughing, light anaesthesia, bronchospasm
2. Check circuit: Kinking, obstruction, water accumulation
3. Check ETT: Kinked, secretions, endobronchial intubation
4. Check ventilator: Settings, malfunction

Differentiate Compliance vs Resistance:

• Inspiratory pause: If PIP high but Pplat normal → resistance problem
• If both PIP and Pplat elevated → compliance problem

Low Tidal Volume/Minute Ventilation

Causes:

• Disconnection (most common)
• Circuit leak
• ETT cuff leak
• Sample line leak (sidestream capnography)
• Inadequate driving pressure (PCV mode)

Approach:

1. Check connections
2. Listen for leak
3. Check cuff pressure
4. Bag patient manually to confirm circuit integrity

Auto-PEEP (Intrinsic PEEP)

Detection:

• Expiratory flow not returning to zero before next breath
• End-expiratory occlusion maneuver reveals positive pressure
• Difficulty triggering ventilator

Management:

1. Identify cause (obstruction, high minute ventilation)
2. Reduce respiratory rate (increase Te)
3. Increase I:E ratio (e.g., 1:3 or 1:4)
4. Reduce tidal volume if appropriate
5. Treat bronchospasm
6. Consider applying external PEEP (controversial) [31]

Patient-Ventilator Dyssynchrony

Types:

• Trigger dyssynchrony: Missed triggers, auto-triggering
• Flow dyssynchrony: Inadequate flow delivery
• Cycle dyssynchrony: Premature or delayed cycling

Management:

• Optimize trigger sensitivity
• Match flow to patient demand
• Adjust cycling criteria (PSV)
• Ensure adequate sedation if indicated
• Consider mode change

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Checking Procedures

Pre-Use Checklist (Based on AS/NZS 4187 and Manufacturer Guidelines)

Before First Case:

1. Power supply connected and functioning
2. Gas supplies adequate and connected
3. Bellows/piston assembly correctly installed
4. Circuit connected and leak-free
5. Self-test completed (automated)
6. Alarms set appropriately
7. Backup ventilation available (self-inflating bag)

Leak Test:

1. Set VT and RR
2. Occlude patient port
3. Observe pressure rise to set Pmax
4. Ventilator should alarm (high pressure)
5. Release occlusion, observe normal cycling

Alarm Test:

1. Disconnect circuit - low pressure alarm
2. Occlude circuit - high pressure alarm
3. Apnoea mode - apnoea alarm
4. Low oxygen - oxygen failure alarm (if testable)

Between Cases

1. Change circuit (or single-patient use)
2. Change breathing filter
3. Visual inspection of bellows/connections
4. Confirm fresh gas flow settings
5. Reset alarms for new patient

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Indigenous Health Considerations

Remote and Rural Ventilator Access

Aboriginal and Torres Strait Islander communities in remote Australia, and Māori communities in rural New Zealand, face significant challenges regarding mechanical ventilator availability and maintenance.

Equipment Challenges:

Ventilator availability in remote settings is often limited to basic transport ventilators. The Royal Flying Doctor Service (RFDS) provides retrieval services, but initial stabilization may rely on self-inflating bags or basic automatic resuscitators. Power supply reliability affects ventilator use - solar and diesel generators may be inconsistent, necessitating battery backup capability. Biomedical engineering support is often hundreds of kilometers away, requiring GP anaesthetists to be competent in basic troubleshooting.

Clinical Implications:

Higher rates of chronic respiratory disease, rheumatic heart disease, and renal disease in Indigenous populations increase ventilatory support requirements. Retrieval times exceeding 4-6 hours necessitate careful attention to battery life, oxygen supply, and monitoring during transport.

Cultural Considerations:

When introducing mechanical ventilation:

• Involve Aboriginal Health Workers (AHWs) and Indigenous Liaison Officers
• Allow time for family consultation (extended family involvement is culturally important)
• Use culturally appropriate communication with visual aids and interpreters
• Respect end-of-life preferences and cultural protocols

Telemedicine enables specialist support for ventilator management in remote areas, reducing unnecessary retrievals. [32]

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Assessment Content

SAQ Practice Question (20 marks)

Question: A 65-year-old woman (actual weight 95 kg, height 160 cm) with COPD is undergoing laparoscopic cholecystectomy under general anaesthesia. Pneumoperitoneum is established at 12 mmHg. The anaesthesia ventilator is set to VCV mode with VT 600 mL, RR 12/min, I:E 1:2, and PEEP 5 cmH₂O. You notice peak inspiratory pressure has increased from 22 to 38 cmH₂O and the capnograph shows failure of expiratory flow to return to baseline.

(a) Calculate the ideal body weight for this patient and the appropriate tidal volume for lung-protective ventilation. Show your working. (4 marks)

(b) Explain the pathophysiology causing the observed changes in airway pressure and capnograph waveform. (4 marks)

(c) List the ventilator adjustments you would make to optimize ventilation in this patient with COPD and pneumoperitoneum. Justify each change. (6 marks)

(d) Describe how you would use an inspiratory pause maneuver to differentiate between resistance and compliance problems. What values would indicate each? (6 marks)

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Model Answer:

(a) IBW Calculation and Appropriate VT (4 marks)

Ideal Body Weight (IBW) for females: IBW = 45.5 + 0.91 × (height in cm - 152.4) [1 mark] IBW = 45.5 + 0.91 × (160 - 152.4) IBW = 45.5 + 0.91 × 7.6 IBW = 45.5 + 6.9 IBW = 52.4 kg [1 mark]

Appropriate Tidal Volume: Lung-protective ventilation: 6-8 mL/kg IBW [1 mark] VT range = 6 × 52.4 to 8 × 52.4 VT range = 314 to 419 mL Recommended VT: 350-400 mL (current 600 mL is excessive) [1 mark]

(b) Pathophysiology of Observed Changes (4 marks)

Increased Peak Inspiratory Pressure:

• Pneumoperitoneum increases intra-abdominal pressure, restricting diaphragmatic excursion [1 mark]
• This decreases respiratory system compliance
• Additionally, the patient's obesity and COPD contribute to baseline abnormalities
• The set VT of 600 mL is excessive for this patient's IBW, requiring higher pressure to deliver [1 mark]

Capnograph Finding (Expiratory Flow Not Returning to Baseline):

• This indicates auto-PEEP (intrinsic PEEP/dynamic hyperinflation) [1 mark]
• COPD causes expiratory airflow limitation
• Current I:E ratio of 1:2 provides insufficient expiratory time
• Gas trapping occurs when the next breath begins before complete exhalation [1 mark]

(c) Ventilator Adjustments (6 marks)

(d) Inspiratory Pause Maneuver (6 marks)

Technique:

• Deliver a breath and then pause (hold) at end-inspiration for 0.5-2 seconds [1 mark]
• This allows equilibration of pressure throughout the respiratory system
• Read the plateau pressure (Pplat) when flow has ceased [1 mark]

Interpretation:

Differentiation: [2 marks]

• High PIP with normal Pplat (PIP - Pplat >5 cmH₂O):

  • Indicates increased airway RESISTANCE
  • Causes: Bronchospasm, secretions, kinked ETT, small ETT

• High PIP with elevated Pplat (both elevated, gradient <5 cmH₂O):

  • Indicates decreased COMPLIANCE
  • Causes: Pneumothorax, endobronchial intubation, pneumoperitoneum, pulmonary edema, ARDS

Clinical Values: [2 marks]

• Normal PIP - Pplat gradient: <5 cmH₂O
• Target Pplat: <30 cmH₂O (lung-protective)
• Driving pressure (Pplat - PEEP): Target <15 cmH₂O

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Viva Scenario (15 marks)

Examiner: "Please classify mechanical ventilators and describe the key principles of their operation."

Candidate: "Mechanical ventilators can be classified by power source, drive mechanism, and cycling variable.

By Power Source: Ventilators may be pneumatic, using only compressed gas; electric, using motor-driven mechanisms; or combined pneumatic-electric, which is most common in modern anaesthesia workstations where compressed gas powers delivery but electronics control timing and monitoring.

By Drive Mechanism: The three main types are bellows, piston, and turbine systems. Bellows ventilators use a bag-in-bottle configuration where driving gas compresses a bellows containing the patient gas mixture. Modern anaesthesia ventilators use ascending or standing bellows that rise during exhalation - if the bellows fails to rise, this provides an important visual disconnect alarm. Descending bellows fall during exhalation and will continue to cycle even if disconnected, making them dangerous and largely obsolete.

Piston ventilators use an electric motor to drive a piston, providing very precise volume delivery. Turbine ventilators use high-speed blowers and can operate without compressed gas.

By Cycling Variable: The cycling variable determines what terminates inspiration. Volume-cycled ventilators end inspiration when the set tidal volume is delivered. Time-cycled ventilators end inspiration after a preset duration, as in pressure-controlled ventilation. Flow-cycled ventilators end inspiration when flow decreases to a threshold, typically 25% of peak flow in pressure support ventilation."

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Examiner: "Good. Explain the different ventilator modes and when you would choose each."

Candidate: "The main modes are Volume-Controlled Ventilation, Pressure-Controlled Ventilation, SIMV, and Pressure Support Ventilation.

Volume-Controlled Ventilation (VCV) delivers a set tidal volume regardless of lung mechanics. The ventilator generates whatever pressure is necessary to achieve the target volume. This is the standard mode for routine intraoperative ventilation because it guarantees minute ventilation. The disadvantage is that if compliance decreases, pressures will rise, potentially causing barotrauma.

Pressure-Controlled Ventilation (PCV) delivers breaths at a set inspiratory pressure for a set time. Tidal volume varies with lung mechanics. PCV produces a decelerating flow pattern that may improve gas distribution. It is useful when we want to limit peak pressures, such as in ARDS or when compliance is variable. However, tidal volume must be monitored closely.

SIMV combines mandatory breaths with spontaneous patient breathing. It allows respiratory muscle activity to be maintained while ensuring minimum ventilation. Mandatory breaths are synchronized to patient effort when detected. SIMV can be used as a weaning mode, gradually reducing the mandatory rate.

Pressure Support Ventilation (PSV) augments every spontaneous breath with a set pressure. The patient controls rate and inspiratory time. PSV reduces work of breathing and is used for weaning and in patients with intact respiratory drive. It requires apnoea backup because no mandatory breaths are delivered.

Mode Selection:

• Routine anaesthesia in paralyzed patient: VCV
• Poor compliance with pressure concerns: PCV
• Weaning or maintaining respiratory muscle function: SIMV or PSV
• Spontaneous breathing with support: PSV"

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Examiner: "How would you recognize and manage auto-PEEP in a ventilated patient?"

Candidate: "Auto-PEEP, also called intrinsic PEEP or dynamic hyperinflation, occurs when air trapping leads to positive end-expiratory alveolar pressure that exceeds the set PEEP.

Recognition:

The key diagnostic sign is failure of expiratory flow to return to zero before the next breath begins. This is visible on the flow-time waveform as the expiratory limb not reaching the baseline.

Other signs include:

• Progressive increase in peak airway pressure
• Difficulty triggering the ventilator (patient effort does not reach trigger threshold)
• Cardiovascular compromise from reduced venous return
• Measured total PEEP higher than set PEEP during end-expiratory occlusion

The end-expiratory occlusion maneuver quantifies auto-PEEP: with the expiratory valve occluded at end-expiration, the airway pressure equilibrates with alveolar pressure, revealing total PEEP. Auto-PEEP equals total PEEP minus set PEEP.

Management:

The goal is to increase expiratory time to allow complete exhalation.

First, identify and treat the underlying cause: bronchospasm requires bronchodilators; secretions require suctioning.

Ventilator adjustments include:

1. Reducing respiratory rate - this directly increases expiratory time
2. Changing I:E ratio to 1:3 or 1:4 - further prolongs expiration
3. Reducing tidal volume - smaller breaths empty faster, and this maintains minute ventilation if rate is slightly increased
4. Reducing inspiratory flow rate in some cases - though this actually increases inspiratory time

Adding external PEEP is controversial but may be beneficial by reducing the work required to trigger the ventilator. External PEEP should be less than measured auto-PEEP.

In severe cases, brief disconnection allows complete exhalation, though this causes hypoxia and should only be used in emergencies."

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Examiner: "Describe the safety features of modern anaesthesia ventilators."

Candidate: "Modern anaesthesia ventilators incorporate multiple safety features at various levels.

Pressure Safety: High pressure alarms terminate inspiration if airway pressure exceeds the set maximum, typically 35-40 cmH₂O, preventing barotrauma. Low pressure alarms detect failure to generate adequate pressure, indicating disconnection or major leak.

Volume Safety: Low tidal volume and low minute ventilation alarms detect inadequate ventilation. These alarms integrate with the disconnect alarm function.

Disconnect Detection: Ascending bellows provide visual indication of disconnection - if the bellows fails to rise during exhalation, something is wrong. Electronic monitoring of expired volume and pressure provides additional disconnect detection.

Apnoea Protection: When a patient on spontaneous or support modes fails to breathe within the apnoea interval, typically 20-30 seconds, an alarm sounds and backup controlled ventilation activates automatically.

Oxygen Safety: The oxygen failure warning device provides an audible alarm when oxygen supply pressure falls below 200 kPa. The oxygen ratio controller or link-25 system prevents delivery of hypoxic mixtures by mechanically or electronically linking oxygen and nitrous oxide flows to maintain minimum 25% oxygen.

Fresh Gas Decoupling: Modern ventilators decouple fresh gas flow from tidal volume delivery. During inspiration, fresh gas is diverted to a reservoir rather than adding to the patient circuit, preventing unpredictable tidal volume augmentation.

Self-Test Functions: Automated pre-use checks verify leak integrity, sensor calibration, valve function, and alarm responses before the ventilator is used on a patient. These are typically mandatory before first use each day."

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