Ventilator Settings

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Part of: Mechanical Ventilation

MSS Assessment

Perform an MSS assessment after changing ventilator settings or modes:

Chest movement and capnography

Breath sounds

Saturation

Mode
Fraction of inspired oxygen $F_{i} O X_{2}$
Tidal volume ( $V_{T}$ )
Respiratory rate (RR)
Positive End-Expiratory Pressure (PEEP)
Pressure Support (PS)
Flow shape/contour
Inspiratory:Expiratory (I:E) ratio

Control Variables

The variable which the ventilator uses as feedback signal for controlling inspiration

P_{vent} + P_{muscles} = elastance \times volume + resistance \times flow

Flow cannot be a control variable because flow is a volume over time and thus when volume is controlled, flow is controlled indirectly
Pressure control maintains a stable pressure in the face of fluctuating respiratory performance, which prevents lung injury from excess pressure but doesn’t give a consistent minute ventilation
- Advantages
  - Increased mean airway pressure ⇒ improves oxygenation
  - Increased duration of alveolar recruitment ⇒ square pressure waveform causes alveoli to open earlier and remain open for longer allowing for better gas exchange
  - Prevents excessive airway pressures thereby protecting against barotrauma
  - Avoids regional alveolar overdistention
  - May lead to earlier liberation from mechanical ventilation
  - Allows for significant air leak (e.g. during bronchoscopy or bronchopleural fistula)
- Disadvantages
  - Tidal volume is variable and dependent on respiratory compliance
  - Uncontrolled volume may result in “volutrauma” (overdistension)
  - A high early inspiratory flow may breach the pressure limit if airway resistance is high

Volume control gives a more stable minute ventilation, keeping $P_{a} CO X_{2}$ at the desired level
- Advantages:
  - Guaranteed tidal volumes produces a more stable minute volume
  - The minute volume remains stable over a range of changing pulmonary characteristics
  - The initial flow rate is lower than in pressure-controlled modes, avoiding a high resistance-related early pressure peak
- Disadvantages:
  - The mean airway pressure is lower with volume control ventilation
  - Recruitment may be poorer in lung units with poor compliance
  - In the presence of a leak, the mean airway pressure may be unstable
  - Insufficient flow may give rise to patient-ventilator dyssynchrony

Targeting Scheme

Set point: the ventilator will try to achieve the parameter (control variable chosen)
Dual targeting: the ventilator switches from targeting one control variable to another in the middle of the breath
- For example a breath may start with a pressure control variable using a decelerating flow waveform, then reach the pressure limit mid breath and change to volume control until the target volume is reached
Adaptive targeting:
- For example in PRVC, the inspiratory pressure is automatically adjusted to achieve an average tidal volume target; this varies from breath to breath adapting to the changing compliance (guarantees a prescribed volume while maintaining a square pressure waveform)

Phase Variables

Trigger Variable

Determines when a breath is delivered, distinguishing ‘mandatory’ and ‘spontaneous’ modes of ventilation
Sensitivity affects:
- Work of breathing
- Patient-ventilator synchrony

Time Triggered

Mandatory ventilation
Guarantees a minute volume offering predictable $CO X_{2}$ removal and decreased work of breathing
Less comfortable and sedation requirements are higher

Flow Triggered

Patient effort changes circuit flow
Most comfortable but can be over-sensitive leading to dyssynchrony
Ventilation is triggered when flow is diverted to the patient when the patient begins inspiration
- The exact threshold value can be altered but is generally 1-2 L/min¹
Some machines indicate spontaneous respiratory effort by colouring the waveform

Advantages:
- Generally quite sensitive meaning that patient’s work of breathing is not wasted on triggering the ventilator
- Allows the patient to have control over their minute volume
- May decrease the work of breathing
- More comfortable
- Permits a lower level of sedation
Disadvantages:
- May be too sensitive, giving rise to auto-triggering (this is probably the only use case for pressure trigger)
- Does not guarantee a minute volume — therefore unsuitable for patients with a diminished or unreliable respiratory drive

Pressure Triggered

Patient generates negative pressure
A typical pressure trigger threshold would be 1 cm $H X_{2} O$

Disadvantages
- Requires more effort to trigger the ventilator
- Represents a wasted respiratory effort
- Less comfortable for the patient
Use cases
- Can be used to decrease auto-triggering (e.g. circuit leak, bronchopleural fistula or hyperdynamic circulation)
- Can be used to test the power of respiratory musculature in the context of an assessment of readiness for extubation (e.g. a patient who is able to trigger the ventilator by generating a negative intrathoracic pressure of -20 cm $H X_{2} O$ is unlikely to fail extubation due to weakness of their respiratory muscles)

Shape-Signal Triggering

May decrease wasted effort by “predicting” the next respiratory effort but not widely available

Phase Variables

Limit/Target Variable

Refers to the maximum value a variable can reach during inspiration but importantly, it does not end inspiration
Common limit variables include pressure, flow and volume
- For example, continue holding this pressure or flow value (don’t exceed it) during the rest of inspiration

Phase Variables

Cycling Variable

Time-Cycled

Feature of mandatory modes
Usually set by setting a respiratory rate and the I:E ratio
Advantages:
- Careful control of minute volume, with obvious advantages for scenarios where tight PaCO2 control is desirable (e.g. traumatic brain injury)
- Ventilation is unaffected by changes in lung compliance or airway resistance because the timing of the breath is unrelated to any respiratory system parameters
- Minute ventilation is not affected by an unreliable respiratory drive, making this method suitable for paralysed or deeply unconscious patients
Disadvantages:
- Unsuitable for lightly sedated and awake patients
- May result in patient-ventilator dyssynchrony particularly if the patient tries to exhale before the cycle time runs out

Flow Cycled

The ventilator cycles into the expiratory phase once the flow has decreased to a predetermined value during inspiration
- Can be expressed as a fixed value in litres per minute or a percentage fraction of the peak flow rate

In patients with restrictive lung disease (poor compliance), flow rate drops quickly → lower tidal volumes
In patients with emphysema (high compliance), flow rate drops slowly → higher tidal volumes
Advantages:
- More comfortable for the patient
- Limited by changes in lung compliance and airway resistance, preventing inadvertent ventilator-induced lung injury
Disadvantages
- Tidal volumes may be poor in patients with poor lung compliance, resulting in inadequate minute volume
- Patient comfort depends on intelligent settings; inappropriately low or high settings could result in uncomfortably deep inspiration or “double triggering”

Pressure Cycled

Largely obsolete in the modern era
Advantages: Safety from pressure-related lung injury; decelerating ramp flow pattern; compliance determines cycling
Disadvantages: Volume is determined by compliance; respiratory rate may fluctuate; may increase respiratory effort; pressure cannot also be a control variable

Volume Cycled

Largely obsolete in the modern era
Inspiratory phase ends when the specified volume has been delivered
Major disadvantage: propensity to generate high peak airway pressures when lung compliance decreases

Phase Variables

Fraction of Inspired Oxygen $F_{i} O X_{2}$

$F_{i} O X_{2}$ should be titrated to the lowest value which still maintains adequate oxygenation
In common practice, set at 100% after the patient is first intubated, then titrated downward over one to several hours as indicated by pulse oximetry and/or serial ABGs
$F_{i} O X_{2} > 60%$ leads to oxygen toxicity; if adequate oxygenation requires $F_{i} O X_{2} > 60%$ , additional strategies are required:
- ↑ PEEP
- Recruitment manoeuvres
- Trial of a different mode

Phase Variables

Tidal Volume $V_{T}$

The tidal volume is 7 mL/kg in a normally breathing patient
Most applicable to volume cycled modes (AC, SIMV)
Initial values should be weight based:

Patient	$V_{T}$
Healthy lungs (e.g. neurological catastrophe, drug overdose)	10 mL/kg IBW
COPD	8 mL/kg IBW
ARDS	6 mL/kg IBW

Higher $V_{T}$ leads to ↓ $P_{a} CO X_{2}$ , ↑ pH and ↑ $P_{plateau}$ and vice versa

Phase Variables

Respiratory Rate

Typical respiratory rate is 10-20 breaths/min in order to provide 7-10 L/min of minute ventilation
The normal resting minute volume is 70-100 mL/kg/min; therefore to produce tidal volumes of 6-8 mL/kg, a respiratory rate of between 12-16 breaths per minute
Higher RR leads to ↓ $P_{a} CO X_{2}$ , ↑ pH and higher risk of auto-PEEP and vice versa

Phase Variables

Positive End-Expiratory Pressure

Continuous positive pressure present throughout all of ventilation
Physiologic effects:
- ↑ oxygenation
  - ↑ alveolar recruitment
  - ↑ alveolar surface area
- ↑ cardiac output in CHF patients but can ↓ BP in non-CHF patients
  - ↓ preload
  - ↓ LV afterload
- ↑ RV afterload → ↑ R-L shunts if present

O X_{2} Delivery \propto S_{a} O X_{2} \times [Hb] \times Cardiac Output

**Figure 5.** Relationship between oxygen delivery and PEEP. There exists a sweet spot where an optimal PEEP delivers optimised oxygen delivery

In clinical practice: PEEP is set to the lowest value that allows $F_{i} O X_{2}$ to be ≤ 60% with a minimum value of 5 cm of $H X_{2} O$
The healthy lung should be ventilated with 5-8 cm of $H X_{2} O$ of PEEP
ARDS patients will require higher PEEP (usually > 12 cm of $H X_{2} O$ )
With bronchospasm, low PEEP or zero PEEP (ZEEP) is often warranted (< 5 cm of $H X_{2} O$ )

Phase Variables

Pressure Support

Amount of additional positive pressure beyond PEEP that is provided during inspiration. Important in PSV, BiPAP and almost always used in SIMV
Simple estimate of optimal PS is:

P S_{optimal} \approx P_{plateau} - PEEP

However in practice, PS is typically set to twice PEEP

Phase Variables

Flow Shape/Contour

Describes the pattern of airflow during inspiration; set in volume-targeted ventilator modes
- Always decelerating shape in pressure-targeted modes as a consequence of lung mechanics

Contour	$P_{peak}$	$P_{mean}$	Dead space ( $V_{D}$ )	Auto-PEEP
Decelerating	↓	↑	↓	More
Constant	↑	↓	↑	Less

Phase Variables

Inspiratory : Expiratory Ratio

Ratio between the amount of time spent in inspiration and expiration
Normal I:E ratio is 1:2; deviations from this are uncomfortable, often requiring deep sedation

Change	Effects
↑ Inspiratory time	↑ Mean airway pressure → better oxygenation (but ↓ CO₂ clearance, ↑ haemodynamic instability, ↑ gas trapping)
↑ Expiratory time	↑ CO₂ clearance → better ventilation (but ↑ probability of atelectasis)

In AC and SIMV: usually set indirectly via $V_{T}$ and flow rate/pattern
In PCV: usually set directly
In PSV: generally outside of clinician control
A higher ratio (higher inspiratory time) results in ↑ $P_{mean}$ and higher risk of auto-PEEP and vice versa

Typical Initial Ventilator Settings

Option	Typical Settings
Mode	Intrinsic hyperventilation → SIMV; No intrinsic hyperventilation → AC or SIMV
FiO₂	Start at 100%; Taper as able to 35–60% to keep $P_{a} O X_{2}$ >60–80 mmHg
Tidal Volume ( $V_{T}$ )	~10 cc/kg normal; ~8 cc/kg COPD; ~6 cc/kg ARDS (Use IBW); Lower $V_{T}$ if $P_{plateau}$ > 30 cm $H X_{2} O$
Rate	10–20 breaths/min to achieve MV of 7–10 L/min; adjust based on pH
PEEP	Start at 5 cm $H X_{2} O$ ; Titrate up if $P_{a} O X_{2}$ <60 on >60% FiO₂; May start with no PEEP in pure hypoventilation
Pressure Support (n/a for AC)	5–20 cm $H X_{2} O$ ; Optimal $PS \approx P_{plateau} - PEEP$ ; (Minimum 5 cm $H X_{2} O$ PS always to overcome ETT resistance)

Typical Initial Ventilator Alarm Settings

High pressure limit: 10–15 cm H₂O above PIP
Low pressure limit: 5–10 cm H₂O below PIP
Low PEEP limit: 3–5 cm H₂O below set PEEP
Low exhaled tidal volume: 100 mL or 50% below set VT
Low minute ventilation: 2–5 L/min or 50% below baseline
High minute ventilation: 50% above baseline
FiO₂ alarm: ±5% from set oxygen concentration
Temperature alarm: ±2°C from set temperature
Apnea delay: About 20 seconds

Footnotes

Normal mean inspiratory flow rate at rest is around 15 L/min with a peak of around 30-35 L/min

Ventilator Settings

Control Variables

Targeting Scheme

Phase Variables

Trigger Variable

Time Triggered

Flow Triggered

Pressure Triggered

Shape-Signal Triggering

Limit/Target Variable

Cycling Variable

Time-Cycled

Flow Cycled

Pressure Cycled

Volume Cycled

Fraction of Inspired Oxygen Fi​OX2​

Tidal Volume VT​

Respiratory Rate

Positive End-Expiratory Pressure

Pressure Support

Flow Shape/Contour

Inspiratory : Expiratory Ratio

Typical Initial Ventilator Settings

Typical Initial Ventilator Alarm Settings

Footnotes

Fraction of Inspired Oxygen $F_{i} O X_{2}$

Tidal Volume $V_{T}$