Advanced Ventilation -- Back to Basics

Source: Advanced Ventilation 2025 Resp IST UPDATED — 35 slides Author: Sammy Reed, June 2025 (In-Service Training) Learning levels: Foundation = Band 5 | Intermediate = Band 6 | Advanced = Band 7+

1. Why Do We Breathe? — Gas Exchange Fundamentals

Learning level: Foundation

• Cells continuously use O2 for metabolic reactions, releasing energy from nutrients to produce ATP
• These reactions release CO2; excess CO2 causes acidity, which is toxic to cells and must be eliminated
• The respiratory system provides gas exchange; the CVS transports blood with gases — they work together
• The respiratory system also helps regulate blood pH, contains receptors for smell, filters inspired air, produces sound, and rids the body of some water and heat in exhaled air

1.1 Respiratory Failure Thresholds

Learning level: Foundation

1.2 Oxygen

Learning level: Foundation

• Toxic substance
• Moves from high to low concentrations (diffusion)
• Slow diffusion time compared to other gases
• Carried by red blood cells: 1 Hb molecule carries 4 O2 molecules
• Capillary transport time: if < 0.25 seconds, adequate oxygenation in adults is impaired

1.3 Carbon Dioxide

Learning level: Foundation

• By-product of metabolism
• Mainly removed as HCO3⁻ (70%)
• CO2 has a higher water solubility than O2 and penetrates membranes 24 times faster

Cross-reference: The 2019 Fundamentals deck covers the same CO2 and oxygenation principles. See 01-fundamentals-of-ventilation.md, Section 6. The 2025 deck adds the specific capillary transport time threshold of 0.25 seconds and the 70% HCO3⁻ figure.

1.4 Factors Affecting Diffusion

Learning level: Intermediate

2. Anatomy

Learning level: Foundation

2.1 Upper and Lower Respiratory System

Structural classification:

1. Nostril
2. Nose
3. Nasal cavity
4. Nasopharynx
5. Oropharynx
6. Larynx
7. Trachea
8. Bronchus
9. Lung

• Upper respiratory system: 1-5
• Lower respiratory system: 6-9

Functional classification:

• Conducting zone: Interconnecting cavities and tubes outside and within the lungs — filter, warm, and moisten air and conduct it to the lungs
• Respiratory zone: In the lungs where gas exchange occurs — respiratory bronchioles, alveolar ducts, sacs, and alveoli

2.2 Diaphragm

Learning level: Foundation

• Dome-shaped skeletal muscle
• Forms the floor of the thoracic cavity
• Innervated by phrenic nerves (C3/4/5)
• Contraction responsible for approximately 75% of air entering the lungs
• Diaphragm descent during contraction: normal breathing = 1 cm, laboured breathing = up to 10 cm

3. Mechanics of Breathing

3.1 Negative Pressure Breathing

Learning level: Foundation

Cross-reference: The 2019 Fundamentals deck (01-fundamentals-of-ventilation.md, Section 1) provides an identical detailed description of negative pressure breathing. The 2025 deck reinforces the same three pressures and three pressure gradients.

Three pressures:

Important in determining airflow through the bronchial tree and balancing the chest wall’s tendency to recoil outward and the alveoli’s tendency to collapse inward.

Three pressure gradients:

Boyle’s Law: Increasing volume will decrease pressure.

Transpulmonary pressure detail (2025 addition):

• Ppl is always negative
• Palv is slightly negative during inspiration and slightly positive during expiration
• TPP is always positive
• When diaphragm contracts, it decreases Ppl and Palv to allow air to move in from the atmosphere
• During expiration, Ppl becomes more positive to move air out of lungs

Inspiration/Expiration cycle:

• Identical to the 2019 deck description — see 01-fundamentals-of-ventilation.md, Section 1.4 for the full step-by-step

3.2 Forces

Learning level: Intermediate

Elasticity

• The lung’s ability to recoil after being stretched
• Generated by lung tissue and surface tension
• Influenced by elastic fibres and surfactant
• Directly impacts compliance
• Elastic recoil = the lung’s tendency to pull away from the chest wall

Compliance

• How much effort is required to stretch the lungs and chest wall
• High compliance = will expand easily
• Related to elasticity and surface tension
• Lungs normally have high compliance unless pathology exists
• If compliance doubles, tidal volumes double

Dynamic Compliance vs Static Compliance (2025 specific distinction):

Cross-reference: The 2019 deck mentions compliance measured under static conditions but does not explicitly distinguish dynamic from static compliance as the 2025 deck does.

Alveolar compliance detail:

• When alveoli are deflated: high compliance (more room to stretch)
• When alveoli are inflated: low compliance
• Each alveolus has a different compliance

Airway Resistance

• Opposition to flow caused by forces of friction
• Depends on: laminar vs turbulent flow, airway diameter, viscosity of gas
• Decreases as lung volume increases (wider airways = lower resistance)
• Combined resistance of smaller airways is less than bigger airways (parallel pathways)
• CO2 has a lower viscosity than O2

Flow types (reinforced from 2019 deck):

• Laminar flow: organised and smooth, usually in smaller airways
• Turbulent flow: chaotic, usually in larger airways, higher resistance
• Changes in resistance do not alter VTs

Surface Tension

• Must be overcome to expand lungs during inhalation
• Accounts for 2/3 of elastic recoil during exhalation
• Surfactant reduces surface tension

Cross-reference: The 2025 deck specifies that surface tension accounts for 2/3 of elastic recoil — a detail not explicitly stated in the 2019 deck.

3.3 Anatomical Dead Space

Learning level: Intermediate

• 70% of the VT actually reaches the respiratory zone
• 30% remains in the conducting zone = anatomic dead space
• Mechanical ventilation will increase dead space by up to 32%

Note: Alveolar dead space (where there is air but no perfusion) is a separate concept not covered in detail in this deck.

4. Positive Pressure Breathing

Learning level: Intermediate

4.1 Mechanism

• Forces air into the lungs to expand them
• Passive emptying of the lungs
• Need: gas supply, inspiratory valve, expiratory valve, breathing circuit, power supply, machine
• Driven by flow (air and oxygen mix to deliver a volume or pressure to the patient)

How it works:

1. Cylinder/bellows create high pressure, pushing air through tubing into the lungs
2. Alveoli are pushed open by positive pressure (unlike negative pressure which pulls from outside)
3. Diaphragm does not contract (paralysed)
4. Natural recoil of chest wall causes passive expiration
5. Ventilator stops positive pressure; thoracic wall pushes inward; alveoli collapse

4.2 Mountain Analogy for Ventilator Settings

Learning level: Foundation

This analogy (from the 2025 IST) is a useful teaching tool:

Imagine walking across a mountain range:

• Up the mountain = inspiration
• Down the mountain = expiration
• Starting altitude (not sea level) = PEEP
• Peak of the mountain = Ppeak
• Height from start to peak = PS/PC (pressure support/pressure control)
• Time to reach the top = Ti (inspiratory time)
• Total rock/mass of the mountain = volume of air
• Speed of climbing = flow

Key insights from the analogy:

• You can reach the same peak height with different combinations of PEEP and Ppeak:
  • PEEP 5, Ppeak 10 vs PEEP 8, Ppeak 7
• A high, narrow mountain (high pressure, short Ti) might be less total volume than a wider, moderate one (moderate pressures, long Ti)
• Volume controlled: wanting a bigger mountain in the same time = faster climbing (higher flow)
• Pressure controlled: wanting a bigger mountain = higher pressure = bigger volume
• Longer Ti at the same flow to the same Ppeak: wider mountain = higher volume

4.3 Decelerating Flow Pattern

Learning level: Intermediate

• Highest flow at the beginning of inspiration, decreasing over time
• Start of breath is usually when patient flow demand is greatest
• Improves synchrony

5. Modes of Ventilation

Learning level: Intermediate to Advanced

5.1 Overview

Cross-reference: The 2025 deck covers SIMV and CPAP/PS modes with additional detail on sequencing, triggering, and cycling compared to the 2019 deck. The 2019 deck covers VCV, PCV, PRVC, SIMV, PS, and CPAP. The 2025 deck omits VCV, PCV as standalone modes and instead focuses on the interplay between mandatory and spontaneous breaths within SIMV.

Modes not covered in detail in this deck (see 2019 deck):

• CMV (rarely used)
• HFOV (separate teaching — see 03-hfov-and-nitric-oxide.md)
• NAVA

CPAP/PS:

• All patient-triggered breaths (cycled by flow; consider trigger level)
• Patient decides RR and Ti
• PS can be provided as needed (need minimum of 4-5 cmH2O above PEEP to overcome ETT resistance)
• Need a backup in case of apnoea
• Good: preserves respiratory drive, reduces muscle atrophy, reduces need for sedation, aids weaning, potentially more comfortable

5.2 Respiratory Cycle in Detail

Settings example (pressure mode):

• PC = 20, PS = 20, PEEP = 5, Ppeak = 25
• Ti = 1 second (fixed for mandatory breaths, variable for spontaneous)

Calculating RR from settings:

• If RR set at 20 and patient is paralysed:
  • 1 breath every 3 seconds (60 seconds / 20)
  • I:E ratio generally 1:2
  • Ti = 1 second, therefore Te = 2 seconds (passive, not set)
  • Entire respiratory cycle = 3 seconds
  • RR = 60 / 3 = 20

5.3 Breaths: Mandatory vs Spontaneous

Mandatory breaths:

• Programmed by the ventilator
• If patient is paralysed: no issues, vent delivers as programmed
• If patient is NOT paralysed: patient can help/interfere
• Set window time for patient to synchronise with mandatory breaths
• If patient triggers in the window: vent synchronises and delivers set pressure/volume
• If patient does not trigger in the window: vent delivers as programmed
• When patient triggers: negative pressure drop (as per normal breathing) is detected by the vent

Spontaneous breaths:

• Initiated by the patient and finished by the patient
• PS kicks in if patient cannot reach set pressure/volume levels
• PS can be weaned as patient improves
• Set RR directly affects: number of mandatory breaths, time available for spontaneous breathing, and level of support
• Does not account for patient’s WOB

5.4 Sequencing: Trigger and Cycle

How the ventilator interacts with the patient:

Types of breath defined by how they start (trigger) and end (cycle).

Trigger (initiation of breath):

Low trigger number = easier for patient to trigger.

Cycle (transition from inspiration to expiration):

5.5 Inspiratory Rise Time

Learning level: Advanced

• Time taken to reach Ppeak
• Covered in greater detail under ventilator graphics in the 2019 deck (01-fundamentals-of-ventilation.md, Section 10.1)

6. Part 2 Preview (To Be Confirmed)

The following topics are flagged for Part 2 of this IST:

• Plateau pressures
• Loops and curves (pressure-volume, flow-volume)
• Troubleshooting ventilator graphics
• Stress and strain

Cross-reference: The 2019 Fundamentals deck (01-fundamentals-of-ventilation.md, Sections 10.1-10.8) already covers ventilator graphics, PV loops, and troubleshooting in detail.

7. Key Differences Between the 2019 and 2025 Decks