Overview
Mechanical ventilation is any means by which physical devices or machines are used to either assist or replace spontaneous respiration. It is one of the main interventions offered in the ICU.
- Goals of mechanical ventilation
- Maintain oxygenation
- Maintain ventilation
- Reduce work of breathing
- Protect the lungs from ventilator-induced injury
- Buy time while the underlying disease is being treated
Ventilation vs oxygenation
| Term | Definition | Nota bene |
|---|---|---|
| Ventilation | Ventilation is the removal of CO2. It is controlled by minute ventilation and assessed using PaCO2. | For a high PaCO2 → increase minute ventilation. For a low PaCO2 → reduce minute ventilation |
| Oxygenation | Oxygenation is the delivery of oxygen. It is mainly controlled using FiO2 and PEEP and assessed using SpO2 and PaO2 | If there is hypoxia increase FiO2 first then PEEP if needed. Prolonged hyperoxia should be avoided by reducing FiO2 as soon as safely possible. |
Indications for mechanical ventilation
| General indications | Examples |
|---|---|
| Airway protection | GCS ≤ 8, loss of airway reflexes, severe head and neck trauma, and upper airway obstruction |
| Hypoxaemic (type 1) respiratory failure | ARDS, pneumonia, pulmonary oedema, pulmonary embolism, and pulmonary fibrosis |
| Hypercapnic (type 2) respiratory failure | COPD exacerbation, neuromuscular disease, asthma, drug overdose, and respiratory muscle fatigue |
| Increaesd work of breathing | Severe sepsis, septic shock, severe metabolic acidosis, and respiratory fatigue |
Initial setting in PAC
| Parameter | Initial Setting |
|---|---|
| Inspiratory pressure | 10 – 20 cmH20 adjusted to achieve and adequate VT |
| RR | 10–20 breathes/min (commonly 12–16 breathes/minute) |
| Inspiratory time | |
| FiO₂ | Initially 100%, then rapidly titrate (often to ~40%) |
| PEEP | 5 cmH₂O |
Ventilation Variables
Mechanical ventilation can be described using 3 variables: trigger, control, and cycling.
Trigger
This is how the ventilator determines when to initiate a controlled breath.
Trigger variables and typical settings
| Options | Typical setting |
|---|---|
| Time triggered | RR of 14-20 |
| Pressure triggered | ~ 2cm H20 |
| Volume triggered | 2-3L/min |
Control
This is the aspect the ventilator controls during inspiration. It can be pressure or volume (flow).
Control variables and typical setting
| Control | Typical settings |
|---|---|
| Pressure controlled | VT of 6-8 mL/kg |
| Volume (Flow) controlled | Indirect by selecting Vt, RR and I:E ratio. |
Cycling
This is what signals the ventilator to stop inspiration
Cycling variables and typical settings
| Cycling variable | Typical settings |
|---|---|
| Volume cycled | < 25% peak flow |
| Flow cycled | VT of 6-10 mL/kg |
| Time cycled | Provide RR of 14-20 |
Ventilation Modes
This is how the ventilator delivers the breath.
NB: AC and SIMV are identical in patients with no spontaneous breaths
Types of breaths
| Source of trigger | Level of support | |
|---|---|---|
| Mandatory breath | Ventilator | Full support |
| Synchronized (assisted) breath | Patient | Full or partial support |
| Non-synchronized breath | Patient | No support |
Assist Control (AC) Mode
The ventilator provides a fixed tidal volume or pressure once a breath has been triggered. It has both mandatory and assisted breaths.
It is also known as CMV on some machines.
AC mode
| Component | Settings |
|---|---|
| Breath type | Pressure cycling, volume cycling, volume-targeted |
| Universal settings | FiO2 and PEEP |
| Major settings | Tidal volume or Pressure Control, and Respiratory rate |
| Other settings | Sensitivity, inspiratory time or flow rate, and flow pattern |
| Assist control | Description | Monitoring |
|---|---|---|
| Volume assist-control (V-AC) | The ventilator delivers a set tidal volume with every mandatory or patient-triggered assisted breath. | Monitoring the plateau pressure is important since the same tidal volume delivered to a lung with worse compliance can lead to higher airway pressures. |
| Pressure assist control (P-AC) | The ventilator delivers breaths up to a set inspiratory pressure. | The tidal volume must be monitored closely since less volume is delivered in a lung with poor compliance, and more is delivered in a lung with more compliance. |
- Indications
- Initial mode for an intubated patient
- Critically ill patients requiring full ventilatory support
- In patients where fluctuations in tidal volume are undesired
- Advantages
- A predictable minute ventilation is possible since tidal volume is fixed
- P-AC limits inspiratory pressure
- Low work of breathing
- Disadvantage
- In V-AC, Airway pressure can rise with increased compliance or airway resistance
- In P-AC, tidal volume can fall substantially if compliance worsens
- Respiratory alkalosis
- Auto-PEEP
- Hypotension in hyperventilating patients
- Hypotension in hyperventilating patients
Synchronised Intermittent Mandatory Ventilation (SIMV)
The ventilator provides a set number of mandatory breaths synchronised with the patient’s spontaneous efforts.
It is also known as IMV on some machines.
It is usually combined with pressure support (PS) for spontaneous breaths (SIMV-PS)
SIMV Mode
| Component | Settings |
|---|---|
| Breath type | Pressure control, volume control, volume-targeted |
| Universal settings | FiO2 and PEEP |
| Major settings | Tidal volume or Pressure Control,, respiratory rate, additional spontaneous breath |
| Other settings | Sensitivity, inspiratory time or flow rate, and flow pattern |
- indications
- Useful for weaning
- Critically ill patients who are hyperventilating
- Patients who are prone to auto-PEEP or high airway pressures
- Advantages
- Guarantees a minimum minute ventilation like assist control
- Has a lower mean airway pressure than assist-control mode
- It can provide a wide range of respiratory support
- Disadvantages
- Increased work of breathing
- Spontaneous breaths between mandatory breaths can result in inadequate ventilation if the patient is week
Pressure Support (PS)
Pressure support only has spontaneous breaths. The patient initiates the breath, triggers the ventilator, receives a preset amount of pressure/volume assistance, then continues to breathe spontaneously.
It is also known as pressure support ventilation (PSV), volume support ventilation (VSV), or continuous positive airway pressure (CPAP).
The typical pressure support (PS) is 5 – 15 cmH20 for spontaneous breaths in SIMV/PSV.
Inspiration is terminated when the inspiratory flow falls to a preset level. The default expiratory threshold is 25 – 30% in most machines. Decreasing the expiratory threshold increases the inspiratory time, and vice versa.
SV mode
| Component | Settings |
|---|---|
| Breath type | Pressure support or volume-targeted (volume support) |
| Universal settings | FiO2 and PEEP |
| Major settings | Volume support or pressure support |
| Other settings | Sensitivity and expiratory threshold |
- Indications
- Weaning
- Spontaneous breathing trials
- PS is used to support spontaneous breaths in SIMV
- Advantages
- Comfortable for conscious patients
- Disadvantages
- Each breath is triggered by the patient
- A minimum minute ventilation cannot be guaranteed
- Poor quality of sleep
- It cannot provide full ventilatory support
Breath Types
Either volume or pressure can be used for each breath.
Volume vs Pressure targeted ventilation
| Description | Effect of high lung compliance | Effect of low lung compliance | |
|---|---|---|---|
| Volume Targeted Ventilation | Any mode that delivers a specific tidal volume | Low airway pressure | High airway pressure |
| Pressure Targeted Ventilation | Any mode that ensures pressure does not exceed a maximum preset value | High lung volume | Low lung volume |
Volume Control
The ventilator controls the volume delivered. The tidal volume is set, and the pressure varies depending on compliance and airway resistance.
Pressure Control
The ventilator controls the inspiratory pressure. The inspiratory pressure is set, and the tidal volume variesaccording to compliance and airway resistance.
Volume-targeted
Pressure-Regulated Volume Control (PRVC)
PRVC combines features of volume and pressure control. A target tidal volume is set, then the ventilator adjusts inspiratory pressure to deliver the set volume.
Airway Pressure Release Ventilation (APRV)
APRV is a form of prolonged high airway pressure with brief pressure releases using:
- P-high (high airway pressure)
- P-low (low airway pressure during release)
- T-high (how long the patient stays at P-high)
- T-low (how long the pressure is released)
The patient can breathe spontaneously during the high-pressure phase.
The prolonged high-pressure phase helps maintain alveolar recruitment, while the brief release allows exhalation and CO₂ clearance.
Ventilator Settings
Universal Settings
These settings are encountered across most conventional ventilator modes
| Setting | Description | Initial value/target |
|---|---|---|
| Fraction of inspired oxygen (FiO2) | This controls the concentration of oxygen delivered. | Initially 100%, then rapidly titrate (often to ~40%) |
| Positive End-Expiratory Pressure (PEEP) | This is the pressure remaining in the airway at the end of expiration. PEEP prevents alveolar collapse, increases functional residual capacity, promotes alveolar recruitment, and improves oxygenation. | 5 cm H20 |
Major Settings
Basic ventilation parameters and their initial settings
| Setting | Description | Initial value/target |
|---|---|---|
| Tidal volume (VT) | The volume delivered with each breath. It is set using the ideal/predicted body weight since lung size correlates to height and sex more than actual body weight. | 6 – 8 ml/kg (4 – 6 ml/kg in ARDS) |
| Respiratory Rate (RR) | This is the number of mandatory breaths per minute. It can be increased when a greater minute ventilation is needed. Increasing RR increases minute ventilation and CO2 elimination, while reducing PaCO2. In COPD/asthma, increasing RR can shorten expiratory time → cause air trapping and increase auto-PEEP | 12 – 16 per minute. |
| Peak Inspiratory Pressure (PIP) | PIP is the highest airway pressure reached during inspiration. It reflects airway resistance, lung/chest-wall compliance, flow, and tidal volume. | < 35 cmH20 |
| Pressure Support (PS) | PS is the amount of pressure added during a patient-triggered spontaneous breath. Increasing PS increases tidal volume and reduces work of breathing | 5 – 15 cm H20 |
| FiO2 | Start at 100% then rapidly titrate often to ~ 40% | |
| Inspiratory flow | 40 – 60 L/min | |
| Trigger | Pressure 2 cmH2O or flow trigger |
Minor Settings
| Settings | Description | Initial value/target |
|---|---|---|
| I:E Ratio/Inspiratory Time | I:E = Inspiratory time:Expiratory time. Prolonging the expiratory time prevents air trapping → auto-PEEP → dynamic hyperinflation. | 1:2 – 1:3 in normal lungs; 1:3 – 1:4 in COPD and asthma |
| Trigger | Trigger determines how the ventilator detects that the patient wants to take a breath. It can be a pressure triggered (negative presure detected in the circuit) or flow trigger (a change in flow in the circuit). Flow trigger is more patient friendly. It is useful when auto-PEEP is present since the patient may need to generate considerable effort before the ventilator detects a breath in pressure trigger. | Pressure trigger – -2cmH20; Flow trigger ~ 2L/min. |
| Inspiratory Flow | This is how rapidly the ventilator delivers the inspiratory volume. increasing flow shortens the inspiratory time and prolongs the expiratory time. | 40 – 60 L/min |
Initial ventilator settings
| Parameter | Initial Setting |
|---|---|
| FiO₂ | Initially 100%, then rapidly titrate (often to ~40%) |
| PEEP | 5 cmH₂O |
| Tidal volume (VT) | 6–8 mL/kg of the ideal body weight |
| RR | 10–20 breathes/min (commonly 12–16 breathes/minute) |
| PIP | Aim < 35 cmH2O |
| PS | 5 – 15 cmH2O if spontaneous breaths are being supported |
| I:E | 1:2 – 1:3; adjusted according to disease |
| Trigger | Pressure ~ -2 cmH2O or flow ~ 2L/min |
| Flow | 40–60 L/min |