Flow-Triggered Ventilation and Sensitivity

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Questions and Answers

What pressure condition initiates a mechanical breath in the pressure-trigger mechanism?

-3 cm H₂O (correct)

3 cm H₂O

0 cm H₂O

-5 cm H₂O

Which of the following statements about flow-triggered ventilation is correct?

It solely depends on pressure variations.

It can only be used in pressure-controlled modes.

It requires no continuous flow through the ventilator circuit.

It is more responsive to patient effort than pressure triggering. (correct)

What happens to the flow during flow-triggered ventilation as a patient initiates a breath?

Delivered flow decreases while returned flow increases.

Delivered flow equals returned flow.

Both delivered and returned flow remain constant.

Delivered flow increases while returned flow decreases. (correct)

How does ventilator sensitivity affect a patient's ability to trigger a breath?

Higher sensitivity reduces the effort required to trigger a breath. Signup and view all the answers

What physiological parameters are increased during flow-triggered ventilation?

Volume, pressure, and inspiratory flow Signup and view all the answers

In which modes can the flow-triggered ventilation feature be applied?

Continuous Mandatory Ventilation (CMV), Synchronized Intermittent Mandatory Ventilation (SIMV), and Pressure Support Ventilation (PSV) Signup and view all the answers

Which of the following best describes the concept of 'limit variable' in flow-triggered ventilation?

The time interval during which volume, pressure, and flow increase Signup and view all the answers

What is the primary advantage of using flow-triggered ventilation compared to pressure-triggered ventilation?

It provides better responsiveness to patient effort. Signup and view all the answers

What happens to the flow in the ventilator circuit during flow-triggered ventilation when the patient initiates a breath?

Delivered flow exceeds returned flow, triggering the ventilator. Signup and view all the answers

In terms of ventilator sensitivity, how does a higher sensitivity affect patient effort during breath initiation?

It allows the patient to trigger breaths with less effort. Signup and view all the answers

What defines the term 'limit variable' in flow-triggered ventilation?

The time interval during which inspiratory values rise. Signup and view all the answers

What is a characteristic of flow-triggered ventilation compared to pressure-triggered ventilation?

It is more responsive and reduces the patient's inspiratory effort. Signup and view all the answers

Which mode of ventilation is NOT typically associated with flow-triggered ventilation features?

Assist-Control Mode (AC) Signup and view all the answers

How does the pressure in the airway and ventilator tubing change as inspiration begins in flow-triggered ventilation?

Pressure drops to -3 cm H₂O. Signup and view all the answers

Which aspect of ventilation does flow-triggering primarily influence for the patient?

The effort required to initiate a breath. Signup and view all the answers

What happens to the inspiratory time during flow-triggered ventilation?

It increases due to rising pressure and volume. Signup and view all the answers

Which clinical conditions are associated with low static compliance?

Retained secretions, Tension pneumothorax, Bronchospasm, Kinking of ET tube Signup and view all the answers

What is the main characteristic of high compliance lungs?

Incomplete exhalation due to air trapping Signup and view all the answers

How is static compliance measured?

By dividing volume by plateau pressure when airflow is absent Signup and view all the answers

Which factor is NOT reflected in dynamic compliance measurements?

Static lung resistance Signup and view all the answers

What compensatory mechanism might patients with low compliance lungs exhibit?

Increased respiratory frequency Signup and view all the answers

Which of the following is true regarding static compliance?

Indicates elastic resistance of the lung and chest wall Signup and view all the answers

What is a common problem associated with extreme high compliance in the lungs?

Chronic air trapping Signup and view all the answers

What treatments are typically employed to manage low lung compliance?

PEEP, FiO2, Positioning Signup and view all the answers

What is the formula for calculating lung compliance?

$C = rac{ ext{Volume Change}}{ ext{Pressure Change}}$ Signup and view all the answers

Which condition is associated with low lung compliance?

Acute respiratory distress syndrome (ARDS) Signup and view all the answers

What does high lung compliance indicate in relation to lung function?

The lungs can expand easily but may trap air during exhalation. Signup and view all the answers

Which step is NOT involved in measuring static compliance?

Measure total lung capacity Signup and view all the answers

How is dynamic compliance calculated?

$ = rac{ ext{Corrected Tidal Volume}}{( ext{Peak Inspiratory Pressure - PEEP})}$ Signup and view all the answers

How does excessive airway resistance affect ventilation and oxygenation?

It can lead to respiratory muscle fatigue. Signup and view all the answers

What does increased bowing of the pressure-volume loop indicate?

Increased airflow resistance. Signup and view all the answers

In which scenario might patients with restrictive lung disease exhibit their breathing pattern?

Shallow but faster breathing. Signup and view all the answers

What indicates that a patient is experiencing ventilatory failure?

Inability to maintain adequate minute ventilation relative to CO2 production. Signup and view all the answers

Which patient condition might lead to increased expiratory resistance?

Chronic airway obstruction. Signup and view all the answers

What happens to the flow during expiration according to the described stages?

Flow is only outgoing. Signup and view all the answers

In pressure-controlled ventilation, what might happen to the tidal volume delivered to the patient?

It can be less than expected. Signup and view all the answers

Which condition is associated with the inability to reach peak inspiratory pressure?

Airway obstruction. Signup and view all the answers

What does a variable peak inspiratory pressure (PIP) indicate in volume-controlled ventilation?

The pressure is adjusted to meet tidal volume. Signup and view all the answers

Which of the following examples is NOT a condition limiting volume delivery?

Controlled ventilatory breathing. Signup and view all the answers

What is the role of the peak inspiratory pressure (PIP) during mechanical ventilation?

It sets the maximum pressure during inspiration. Signup and view all the answers

Which of the following factors can lead to unnecessary increases in PIP?

Kinking of the ET tube. Signup and view all the answers

What is generally the timing for measuring peak inspiratory pressure (PIP)?

At the end of inspiration. Signup and view all the answers

What effect does low lung compliance have on the pressure needed for mechanical ventilation?

Increases the required positive inspiratory pressure Signup and view all the answers

What is mean airway pressure (mPaw) primarily influenced by?

Inspiratory time and respiratory frequency Signup and view all the answers

Which strategy is primarily used to prevent the adverse effects of positive pressure ventilation on the cardiovascular system?

Monitoring cardiac function Signup and view all the answers

What effect does positive end-expiratory pressure (PEEP) have on cardiac output?

It exerts a negative effect on cardiac output Signup and view all the answers

What happens to stroke volume when there is a decrease in venous return during positive pressure ventilation?

Stroke volume is reduced Signup and view all the answers

Which of the following conditions is associated with extremely low lung compliance?

Atelectasis Signup and view all the answers

What is the relationship between tidal volume and peak inspiratory flow rate in the context of lung mechanics?

Directly related while inversely related to compliance Signup and view all the answers

Which of the following best describes pulsus paradoxus during spontaneous breathing?

Decrease in arterial blood pressure during inspiration Signup and view all the answers

What effect does positive pressure ventilation have on arterial blood pressure during inspiration?

It results in reverse pulsus paradoxus, increasing arterial blood pressure. Signup and view all the answers

What can signify hypovolemia in a patient undergoing positive pressure ventilation?

Significant reverse pulsus paradoxus with an increase of systolic pressure >15 mm Hg Signup and view all the answers

Which of the following mechanisms contributes to reduced oxygen delivery during positive pressure ventilation?

Reduced stroke volume due to decreased venous return Signup and view all the answers

What physiologic change occurs during spontaneous inspiration that contrasts with positive pressure ventilation?

Temporary decrease in arterial blood pressure Signup and view all the answers

How does positive pressure ventilation affect the left ventricular afterload?

It decreases left ventricular afterload. Signup and view all the answers

Which factor decreases during positive pressure ventilation, directly affecting oxygen delivery?

Cardiac Output Signup and view all the answers

What is the primary consequence of increased intrathoracic pressure during positive pressure ventilation?

Decreased venous return to the heart Signup and view all the answers

Which condition could exacerbate the decrease in arterial blood pressure during spontaneous inspiration?

Cardiac tamponade or acute asthma exacerbation Signup and view all the answers

What primary feedback signal is used by a volume controller to manage output?

Measured volume Signup and view all the answers

Which method is NOT typically used to measure flow in a flow controller?

Pressure monitors Signup and view all the answers

Which statement best describes the function of time controllers in ventilation?

They allow pressure to vary without direct measurement. Signup and view all the answers

Which component is primarily used by a volume controller to displace air or fluid?

Bellows or piston Signup and view all the answers

How do time controllers affect the inspiratory and expiratory phases of ventilation?

They measure and control inspiratory and expiratory times. Signup and view all the answers

What initiates a time-triggered breath on a ventilator?

A pre-set time interval elapsing Signup and view all the answers

What is the sensitivity level in pressure-triggered ventilation?

The range of negative pressure necessary for triggering Signup and view all the answers

How does increasing the sensitivity from -3 cm H₂O to -5 cm H₂O affect triggering in pressure-triggered ventilation?

It increases the patient's effort necessary to trigger the ventilator Signup and view all the answers

Which factor must be overcome for a patient to successfully trigger a ventilator in cases of auto-PEEP?

Both auto-PEEP level and ventilator's sensitivity settings Signup and view all the answers

What is the respiratory frequency if a ventilator is set to deliver 15 breaths per minute?

3 seconds per breath Signup and view all the answers

Which of the following best describes a pressure-triggered breath?

It is started when spontaneous inspiratory flow is detected Signup and view all the answers

If the sensitivity is set at -4 cm H₂O, what does this indicate about the patient's required effort?

The patient has to generate more than -4 cm H₂O Signup and view all the answers

Which variable can act as a trigger for initiating a ventilator-supported breath?

Both volume and time Signup and view all the answers

Study Notes

Flow-Triggered Ventilation

In flow-triggered ventilation, the ventilator delivers a breath when the patient's inspiratory flow reaches a specific value.
Flow triggering is more responsive to patient effort than pressure triggering.
Flow-triggered ventilation combines continuous and demand flow
Flow-triggered ventilation requires less inspiratory work than pressure triggering.
During flow triggering, continuous flow passes through the ventilator circuit and returns to the ventilator.
The ventilator detects the difference between the delivered flow and the returned flow and instantaneously supplies enough flow to meet the patient's mechanical or spontaneous tidal volume.
This feature applies to CMV, SIMV, and PSV modes.

Ventilator Sensitivity

The effort required by the patient to initiate a breath is the ventilator sensitivity.
A more sensitive ventilator (higher sensitivity, in terms of pressure, flow, or volume) will be easier for the patient to trigger.

Limit Variable

During flow-triggered ventilation, volume, pressure, and inspiratory flow all increase above their baseline values.
The inspiratory time is the time interval during which these values rise.

Pressure-Trigger Mechanism

Mechanical breath is not initiated when the pressure in the airway and ventilator tubing is 0 cm H₂O
A mechanical breath is initiated when the pressure drop in the airway and ventilator tubing is -3 cm H₂O

Flow-Triggered Ventilation Strategy

Ventilators measure inspiratory and expiratory flows to initiate a breath
More responsive to patient effort than pressure triggering
Combines continuous and demand flow
Continuous flow passes through the ventilator circuit and returns to the ventilator
When patient initiates a breath, a portion of the flow goes to the patient and the return flow to the ventilator reduces. Ventilator detects this flow differential and supplies enough flow to meet the patient's mechanical or spontaneous tidal volume
This feature applies to CMV, SIMV, and PSV modes

Ventilator Sensitivity

The effort required by the patient to initiate a breath is the ventilator sensitivity.
A more sensitive ventilator (higher sensitivity, in terms of pressure, flow, or volume) will be easier for the patient to trigger.

Limit Variable

During flow-triggered ventilation, volume, pressure, and inspiratory flow all increase above their baseline values.
The inspiratory time is the time interval during which these values rise.

Clinical Conditions and Lung Compliance

Low Static Compliance:
- Conditions: ARDS, Atelectasis, Tension pneumothorax, Obesity, Retained secretions, Bronchospasm, Kinking of ET tube, Airway obstruction
- Low static compliance results from conditions that reduce the patient's functional residual capacity (FRC).
- Patients with low static compliance often have:
  - Restrictive lung defect
  - Low lung volumes
  - Low minute ventilation
- The body compensates for low compliance by increasing respiratory rate.
High Static Compliance:
- Conditions: Emphysema
- High compliance means a large volume change for a small pressure change.
- In extreme cases, exhalation may be incomplete due to reduced elastic recoil.
- Emphysema leads to:
  - Chronic air trapping
  - Destruction of lung tissue
  - Enlargement of terminal and respiratory bronchioles
- High static compliance results from conditions that increase the patient's FRC and Total Lung Capacity (TLC).
Static Compliance Measurement:
- Calculated by dividing volume by plateau pressure.
- Airflow is stopped momentarily during measurement, removing airway resistance as a factor.
- Reflects the elastic resistance of the lungs and chest wall.
Dynamic Compliance Measurement:
- Calculated by dividing volume by peak inspiratory pressure.
- Airflow is ongoing during measurement, meaning airway resistance is a factor.
- Reflects the condition of the airway, both elastic resistance and non-elastic airway resistance.

Treatment

PEEP: Positive End Expiratory Pressure
FiO2: Fractional Inspired Oxygen
Positioning: Optimize lung function.

Key Insights

Low Lung Compliance: Increased work of breathing
High Lung Compliance: Incomplete exhalation due to reduced elastic recoil
Static Compliance: Reflects elastic properties (elastic resistance) of the lung and chest wall
Dynamic Compliance: Reflects airway resistance (nonelastic resistance) and the elastic properties of the lung and chest wall (elastic resistance).

Lung Compliance

Lung compliance is the measurement of how much the lungs expand for a given change in pressure.
Calculation: Lung Compliance (C) = Change in Volume ((\Delta V)) / Change in Pressure ((\Delta P)).
Low lung compliance (high elastance) indicates stiff lungs, requiring more effort to breathe. Conditions like acute respiratory distress syndrome (ARDS) can lead to low lung compliance.
High lung compliance indicates lungs that expand easily, potentially leading to incomplete exhalation and difficulty eliminating carbon dioxide.

Measuring Lung Compliance

Static compliance measures lung expansion during a held breath, while dynamic compliance measures lung expansion during active breathing.
Static Compliance: Corrected Tidal Volume / (Plateau Pressure - PEEP)
Dynamic Compliance: Corrected Tidal Volume / (Peak Inspiratory Pressure - PEEP)
Plateau Pressure - PEEP = Pressure during a held breath, after the lungs have fully expanded
Peak Inspiratory Pressure - PEEP = The highest pressure reached during a normal breath.

Airway Resistance and Ventilation

Airway resistance is inversely proportional to minute ventilation.
Patients with chronic airway obstruction experience deeper but slower breathing due to highly compliant lung parenchyma.
Restrictive lung diseases lead to shallower but faster breathing due to low lung compliance.
Sustained excessive airway resistance can lead to respiratory muscle fatigue, resulting in ventilation and oxygenation failure.
Ventilatory failure occurs when minute ventilation cannot keep up with CO2 production.
Oxygenation failure typically follows ventilatory failure as the cardiopulmonary system becomes unable to adequately provide oxygen.

Airflow Resistance Monitoring

Airflow resistance in a patient-ventilator system can be assessed using a pressure-volume (P-V) loop display.
A bowed P-V loop indicates increased airflow resistance.
Bowing of the inspiratory limb signals excessive inspiratory flow.
Bowing of the expiratory limb suggests increased expiratory resistance.

Airway Pressures during Mechanical Ventilation

Airway pressures are measured during mechanical ventilation to monitor lung function and adjust ventilator settings.
Flow of air into and out of the lungs is measured during inspiration and expiration.
Inspiration - Air flows into the lungs, Expiration - Air flows out of the lungs
Peak Inspiratory Pressure (PIP) is the maximum pressure reached during inspiration, usually measured at the end of inspiration.
Two main types of mechanical ventilation are Pressure-controlled ventilation and Volume-controlled ventilation.

Pressure-controlled ventilation

The peak inspiratory pressure (PIP) is preset, and the inspiratory phase ends when the preset pressure is reached.
This method may deliver smaller tidal volumes than expected, especially in patients with low lung compliance or high airway resistance.

Volume-controlled ventilation

The tidal volume is preset, and the ventilator adjusts the pressure to deliver that volume.
The PIP will vary depending on lung compliance and airway resistance.

Factors Affecting Volume Delivery

Conditions that can limit volume delivery include:
- Airway obstruction
- Kinking of the endotracheal tube (ET tube)
- Bronchospasm
- Low lung compliance
- Pressure limit set too low
- ET tube cuff leak
- Ventilator circuit leak

Compliance and Mechanical Ventilation

In lungs with normal compliance, the pressure transmitted to the thoracic cavity is 50% of the airway pressure.
In noncompliant lungs (e.g., atelectasis, ARDS), the pressure transmitted to the thoracic cavity is less due to the dampening effect of nonelastic lung tissues.
High levels of PIP and PEEP may be necessary to ventilate and oxygenate patients with low compliance.
The decrease in cardiac output due to excessive PIP or PEEP is less severe in patients with normal lungs.
Mechanical ventilation creates airflow by generating a pressure gradient.
Pressure changes occur in the airways, thoracic cage, and pulmonary blood vessels during mechanical ventilation.
Cardiovascular functions should be monitored to prevent adverse effects of positive pressure ventilation on the heart and blood vessels.

Positive Airway Pressure (PEEP)

PEEP is an airway pressure strategy used in ventilation.
PEEP has a more negative effect on cardiac output compared to other pressure strategies.
The effect of PEEP can be detrimental to cardiac output because PEEP is the end-expiratory pressure during mechanical ventilation, while CPAP includes only airway pressure during spontaneous breathing.

Mean Airway Pressure (mPaw)

mPaw is the average pressure in the airways during a complete respiratory cycle.
mPaw is influenced by inspiratory time, respiratory frequency, peak inspiratory pressure, and PEEP.
Low airway resistance leads to a higher PIP.
High compliance leads to a lower PIP.

Factors Affecting PIP and Cardiovascular Function

Tidal volume, airway resistance, and compliance directly impact peak inspiratory flow rate and inversely affect compliance.
These factors influence major organ system function, including the thorax, relying on adequate blood flow and perfusion.
Decreased venous return leads to reduced stroke volume and cardiac output.
Stroke volume is the blood volume output delivered by one ventricle during contraction.
Total oxygen delivery is the product of oxygen content and cardiac output.

Pulsus Paradoxus

During spontaneous inspiration, a temporary decrease in arterial blood pressure, known as pulsus paradoxus, may occur.
A significant reverse pulsus paradoxus (increase in systolic pressure >15 mm Hg) during positive pressure ventilation may indicate hypovolemia.

Positive Pressure Ventilation and O₂ Delivery

Mean airway pressure: Higher in PEEP compared to CPAP due to its additional pressure application
Pulsus paradoxus:
- During spontaneous inspiration, arterial blood pressure temporarily decreases.
- This decrease is amplified in conditions like cardiac tamponade or acute asthma exacerbations (>10 mm Hg).
- Reverse pulsus paradoxus: Arterial blood pressure is slightly higher during positive pressure ventilation compared to spontaneous breathing.
- This is due to increased ventricular emptying and reduced left ventricular afterload.
- A significant increase in systolic pressure (>15 mm Hg) during positive pressure ventilation indicates hypovolemia.
Effect on cardiovascular function:
- Positive pressure ventilation and PEEP can reduce venous return, potentially compromising cardiovascular function in individuals with limited cardiovascular reserve.
- This is because increased intrathoracic pressure compresses vessels, reducing venous return to the heart.
- Decreased venous return reduces stroke volume and cardiac output, leading to reduced oxygen delivery.
Oxygen Delivery:
- Oxygen delivery is the product of oxygen content and cardiac output.
- Positive pressure ventilation can negatively affect oxygen delivery by decreasing cardiac output.

Figures (A) and (B)

Figure A: Illustrates CPAP (Continuous Positive Airway Pressure)
Figure B: Depicts PPV + PEF (Positive Pressure Ventilation + Positive End-Expiratory Pressure)
Treatment: Includes PEEP (Positive End-Expiratory Pressure) and FIO₂ (Fraction of Inspired Oxygen), along with positioning.
Patient: A patient suffering from low lobe pneumonia and experiencing end-up hypoxemia, with various treatments, positions, etc., recorded before and after the treatment.

Volume Controller

Uses measured volume as feedback signal
Delivers constant volume output
Pressure may fluctuate due to changes in resistance and compliance
Uses piston or bellows to displace air or fluid
Controls volume by controlling displacement

Flow Controller

Measures and controls flow directly
Uses various methods for measurement including vortex sensors, heated wire grids, venturi pneumotachometers, and strain gauge flow sensors
Flow signal is used as feedback for ventilator control

Time Controller

Measures inspiratory and expiratory times
Allows pressure and volume to vary with compliance and resistance changes
Neither pressure nor volume is directly used as a control signal
Controls inspiratory and expiratory phases through time

Phase Variables

Ventilator-supported breaths are divided into four phases: inspiration, transition from inspiration to expiration, expiration
Phase variables refers to the changes in flow and time during a specific phase of a breath

Trigger Variables

The trigger variable initiates inspiration
Pressure, volume, flow, or time can act as trigger variables
Most ventilators use either time or pressure as the trigger variable

Time-Triggered

With time-triggered ventilation, inspiration begins after a pre-set time interval has elapsed.
Ventilator frequency (breaths per minute) sets the time interval for each full respiratory cycle
For example, a frequency of 12 breaths per minute results in a 5-second interval per breath (60 seconds / 12 breaths = 5 seconds/breath)

Pressure-Triggered

Pressure-triggered ventilation begins when the ventilator detects the patient's inspiratory effort, specifically a change in pressure gradient
Sensitivity level refers to the amount of negative pressure (below baseline airway pressure) needed to trigger the ventilator
Sensitivity levels for pressure-triggering range from -1 to -5 cm H₂O
Increasing sensitivity level requires more effort from the patient to trigger ventilation
Auto-PEEP (positive end-expiratory pressure) can increase the required triggering pressure, as the patient needs to overcome both auto-PEEP and sensitivity settings

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Description

This quiz covers flow-triggered ventilation and ventilator sensitivity, focusing on how ventilators deliver breaths based on patient effort. Participants will learn about the advantages of flow triggering over pressure triggering and the implications for various ventilatory support modes. Test your understanding of these critical concepts in mechanical ventilation.