Ventilator Settings

Part of: Mechanical Ventilation

• Mode
• Fraction of inspired oxygen $F_{\text{i}}\mathrm{O}{}_2$
• Tidal volume ($V_{\text{T}}$)
• 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

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

• 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 $P_{\text{a}}\mathrm{CO}{}_2$ 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 $\mathrm{CO}{}_2$ 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 $\mathrm{H}{}_2\mathrm{O}$

• 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 $\mathrm{H}{}_2\mathrm{O}$ 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

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

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

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Fraction of Inspired Oxygen $F_{\text{i}}\mathrm{O}{}_2$

• $F_{\text{i}}\mathrm{O}{}_2$ 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
• $F_{\text{i}}\mathrm{O}{}_2>60\%$ leads to oxygen toxicity; if adequate oxygenation requires $F_{\text{i}}\mathrm{O}{}_2>60\%$, additional strategies are required:
  • ↑ PEEP
  • Recruitment manoeuvres
  • Trial of a different mode

Tidal Volume $V_{\text{T}}$

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

• Higher $V_{\text{T}}$ leads to ↓ $P_{\text{a}}\mathrm{CO}{}_2$, ↑ pH and ↑ $P_{\text{plateau}}$ and vice versa

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 ↓ $P_{\text{a}}\mathrm{CO}{}_2$, ↑ pH and higher risk of auto-PEEP and vice versa

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

$$\mathrm{O}{}_2\text{Delivery}\propto S_{\text{a}}\mathrm{O}{}_2\times [\mathrm{Hb}]\times \text{Cardiac Output}$$

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 $F_{\text{i}}\mathrm{O}{}_2$ to be ≤ 60% with a minimum value of 5 cm of $\mathrm{H}{}_2\mathrm{O}$
• The healthy lung should be ventilated with 5-8 cm of $\mathrm{H}{}_2\mathrm{O}$ of PEEP
• ARDS patients will require higher PEEP (usually > 12 cm of $\mathrm{H}{}_2\mathrm{O}$)
• With bronchospasm, low PEEP or zero PEEP (ZEEP) is often warranted (< 5 cm of $\mathrm{H}{}_2\mathrm{O}$)

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:

$$P{S}_{\text{optimal}}\approx P_{\text{plateau}}-\text{PEEP}$$

• However in practice, PS is typically set to twice PEEP

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

Figure 6. Decelerating flow contour

Figure 7. Constant flow contour

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

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 $V_{\text{T}}$ and flow rate/pattern
• In PCV: usually set directly
• In PSV: generally outside of clinician control
• A higher ratio (higher inspiratory time) results in ↑ $P_{\text{mean}}$ 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 $P_{\text{a}}\mathrm{O}{}_2$ >60–80 mmHg
Tidal Volume ($V_{\text{T}}$)	~10 cc/kg normal; ~8 cc/kg COPD; ~6 cc/kg ARDS (Use IBW); Lower $V_{\text{T}}$ if $P_{\text{plateau}}$ > 30 cm $\mathrm{H}{}_2\mathrm{O}$
Rate	10–20 breaths/min to achieve MV of 7–10 L/min; adjust based on pH
PEEP	Start at 5 cm $\mathrm{H}{}_2\mathrm{O}$; Titrate up if $P_{\text{a}}\mathrm{O}{}_2$ <60 on >60% FiO₂; May start with no PEEP in pure hypoventilation
Pressure Support (n/a for AC)	5–20 cm $\mathrm{H}{}_2\mathrm{O}$; Optimal $\text{PS}\approx P_{\text{plateau}}-\text{PEEP}$; (Minimum 5 cm $\mathrm{H}{}_2\mathrm{O}$ 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

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