Invasive mechanical ventilation (IMV)

Invasive mechanical ventilation (IMV) is a form of advanced airway management used to deliver oxygen and/or to regulate ventilation at the level of the trachea through 3 main routes:

• Oropharyngeal intubation
• Nasopharyngeal intubation
• Tracheostomy

Common indications for IMV

• Hypoxemia refractory to noninvasive oxygenation (e.g., ARDS)
• Hypercapnia with altered mental status (e.g., chronic? obstructive pulmonary disease (COPD) exacerbation)
• GCS < 8 (e.g., cardiac arrest)
• Inability to protect the airway (e.g., anaphylaxis?)
• Surgical procedures (e.g., neuromuscular blockade)
• Progressive neuromuscular disorders (e.g., amyotrophic lateral sclerosis? (ALS))

Considerations

There are several modes of IMV that are managed jointly based on:

• Preferences of the intensivist
• Preferences of the respiratory therapist
• The individual’s needs

Invasive mechanical ventilation is not a benign? intervention:

• Ventilator-induced lung injury (VILI) is common.
• VILI may compound the specific etiology of respiratory failure.

General Concepts

Background ventilation physiology

Ventilator mode is described based on 3 characteristics, namely, trigger, cycle, and limit:

• Trigger is the type of signal that initiates the inspiratory phase by the ventilator:
  • Patient-triggered signal: The individual’s inspiratory effort triggers the inspiratory phase by the ventilator.
  • Time-triggered signal: A time interval set by the operator determines when the ventilator initiates the inspiratory phase.
• Cycle: the type of signal that ends the inspiratory phase delivered by the ventilator
  • Volume-cycled ventilation: The inspiratory phase ends once a preset volume exits the ventilator.
  • Other types include:
    • Time-cycled ventilation
    • Pressure-cycled ventilation
• Limit: a value (e.g., pressure or time) that should not be exceeded and that is specified by the operator to protect the lungs

Basic settings controlled by the operator

• PEEP:
  • Pressure remaining in the distal airways (i.e., the alveolus) of an individual at the end of expiration
  • Keeps the alveoli open to participate in oxygenation
  • Can be increased to improve oxygenation (↑ PaO₂)
  • Can be lowered to decrease oxygenation (↓ PaO₂)
• FiO₂ is the percentage of O₂ delivered directly to the individual:
  • Increasing FiO₂ increases oxygenation and decreasing FiO₂ lowers oxygenation.
• Tidal volume is the volume (in mL) delivered with each breath:
  • Can be increased to increase ventilation (↓ PaCO₂)
  • Can be lowered to reduce ventilation (↑ PaCO₂)
• Respiratory rate (RR) is the value set for breaths/min:
  • Can be increased to increase ventilation (↓ PaCO₂)
  • Can be lowered to reduce ventilation (↑ PaCO₂)
• Flow rate is the maximum flow (in L/min) that a ventilator can deliver a set tidal volume:
  • Can be increased with a possible increase in ventilation effect (↓ PaCO₂)
  • Can be lowered with a possible reduction in ventilation effect (↑ PaCO₂)

Common Modes

Controlled mode

• Commonly used in critically ill individuals who have a significantly suppressed or absent respiratory drive
• All spontaneous patient breaths sensed by the ventilator are assisted with a preset volume or pressure specified by the operator.
• Patient-/time-triggered and volume-/pressure-cycled modes
• The 2 most common modes are:
  • Assist-control (AC) volume-control (VC) ventilation
  • AC pressure-control (PC) ventilation

Spontaneous/supported mode

• Used after a significant improvement of the critical state of individuals who can breathe spontaneously and are being considered for weaning
• Patient-triggered and flow-cycled modes
• Pressure-support (PS) ventilation is the most common: All spontaneous patient breaths sensed by the ventilator are supported with a preset pressure specified by the operator.

Combined (controlled + spontaneous/supported) mode

• Commonly used for individuals on maintenance ventilation and for weaning
• A preset number (not all) of patient breaths are assisted by the ventilator, as described for controlled mode; the remaining spontaneous patient breaths are supported as described for spontaneous/supported mode PS ventilation.
• Also known as synchronized intermittent mandatory ventilation (SIMV)

Assist-control volume-control ventilation

• Most commonly used initial mode of ventilation
• Assists every sensed inspiratory effort made by the individual, thereby reducing the work of breathing
• Trigger:
  • Time triggered, if the individual’s inspiratory effort is not sensed: The ventilator delivers a preset number of mandatory breaths per minute.
  • Patient triggered, if the individual’s inspiratory effort is sensed: All inspiratory efforts are assisted by the ventilator.
• Volume-cycled ventilation:
  • Inspiratory phase ends when a preset volume exits the ventilator.
  • Inspiratory volume is an independent variable because it is set by the operator and does not vary between breaths.
  • Inspiratory pressure is a dependent variable because it is not set by the operator and varies between breaths.
• Pressure-limited ventilation:
  • Preset by the ventilator to abort the inspiratory phase if dangerous levels of airway pressure are reached
  • Helps prevent VILI
• Examples:
  • In an individual on AC/VC ventilation who has spontaneous breaths of 20/min and with RR set at 20/min:
    • All breaths are efficient enough to be sensed by the ventilator.
    • All breaths are assisted immediately after patient initiation.
    • Thus, the ventilator frequency is 20/min.
  • If the individual’s spontaneous breaths increase to 30/min while on AC/VC:
    • The ventilator frequency becomes 30/min.
    • The additional 10 breaths are assisted by the ventilator.
  • An individual has 4 spontaneous breaths/minute on AC/VC while the RR is set at 12/min:
    • The ventilator assists with 4 breaths of the individual.
    • The ventilator adds 8 breaths every minute to reach the minimum number set by the operator.
• Problems:
  • Respiratory alkalemia in individuals with tachypnea due to anxiety, pain?, or airway irritation
  • Auto-PEEP: dynamic hyperinflation of lungs when air builds up due to insufficient expiratory time

Assist-control pressure-control ventilation

• Appropriate when the control of peak airway pressures is important (i.e., consider for individuals with previous barotrauma or after thoracic? surgery)
• Time-triggered, time-cycled, and pressure-limited ventilation
• Inspiratory pressure is an independent variable because it is set by the operator and does not vary between breaths.
• Inspiratory volume is a dependent variable because it is not set by the operator and varies between breaths:
  • Increasing volume at constant settings reflects improved pulmonary compliance.
  • Decreasing volume at constant settings reflects worsening pulmonary compliance.

Synchronized intermittent mechanical ventilation/VC ventilation

• More commonly used in surgical ICUs as opposed to medical ICUs
• Previously thought to be best for weaning individuals off the ventilator
• Allows the individual to practice unassisted breathing between assisted synchronized mandatory breaths
• Trigger:
  • Patient-triggered process if the individual’s inspiratory effort is sensed:
    • Some of the sensed inspiratory efforts are assisted by the ventilator.
    • Number of assisted breaths/minute is determined by the operator.
    • Patient breaths in excess of the operator’s settings remain unassisted.
  • Time-triggered process if the individual’s inspiratory effort is not sensed: The ventilator delivers a preset number of mandatory breaths/minute.
• Volume-cycled ventilation:
  • Inspiratory phase ends when a preset volume exits the ventilator.
  • Inspiratory volume is an independent variable because it is set by the operator and does not vary between breaths.
  • Inspiratory pressure is a dependent variable because it is not set by the operator and varies between breaths.
• Pressure-limited ventilation:
  • Preset by the ventilator to abort the inspiratory phase if dangerous levels of airway pressure are reached
  • Helps prevent VILI
• Example for SIMV mode:
  • An individual has 20 spontaneous breaths while intubated, and the ventilator frequency is set to 12/min.
  • The ventilator delivers each of the 12 mandatory breaths in a synchronized fashion when the individual initiates a spontaneous breath.
  • Of the individual’s total 20 spontaneous breaths/minute, 12 are assisted by the ventilator and 8 remain unassisted.
  • The total number of breaths remains 20/min.
• Problems:
  • Reduced minute ventilation:
    • When tachypneic, the individual exhales during the ventilator inspiratory phase.
    • Causes a rapid rise in airway pressure beyond the pressure limits set by the operator
    • Will lead to premature cessation of ventilator-delivered breaths
  • A switch to AC ventilation mode would increase the number of assisted breaths and minute ventilation.
  • SIMV is usually delivered in combination with PS ventilation to support unassisted breaths with a preset pressure.

Pressure-support ventilation

• Commonly used in combination with SIMV to ensure volume-cycled backup.
• Patient-triggered, flow-cycled, pressure-limited ventilation
• Operator sets the pressure that must be reached during each inspiratory phase by the ventilator.
• PS ventilation is always patient triggered: All sensed inspiratory efforts are supported by the ventilator to reach the pressure set by the operator.
• Once the flow in the airway reaches a level below a minimum threshold, the PS is terminated.

Complications

Ventilator-associated pneumonia (VAP)

• Requires a minimum of 3 days on mechanical ventilation
• Risk factors:
  • Advanced age
  • Higher degree of comorbidities
  • Prolonged length of hospital/ICU stay
  • Prolonged length of IMV
  • Exposure to invasive procedures
• Typically diagnosed via imaging along with:
  • Increased respiratory secretions
  • Culture data (bronchoalveolar lavage or tracheal aspirate)
  • Presence of sepsis (worsening or new)
• Most common organisms:
  • Staphylococcus aureus
  • Pseudomonas aeruginosa
  • Klebsiella pneumoniae
  • Enterobacter species
  • Actinobacter species
• Treatment is with antibiotics.

Subtypes of VILI

• Barotrauma:
  • High lung inflation pressure that results in:
    • High transpulmonary pressure
    • Lung overdistension
    • Air leakage
• Atelectrauma: high shear forces that open and close recruitable atelectatic lung units
• Volutrauma:
  • Alveolar overdistension causing epithelial tendon?. সহজ বাংলা: মাংসপেশি/টেনডনে টান।" data-rx-term="strain" data-rx-definition="A strain is injury to a muscle or tendon. সহজ বাংলা: মাংসপেশি/টেনডনে টান।">strain? and lipid mobilization
  • Lipid mobilization causes cellular detachment of vascular endothelial cells:
    • Alveolar epithelium breaks down.
    • Results in alveolar and interstitial edema?
• Biotrauma: inflammatory response from mechanical injury via cytokines and other inflammatory mediators
• Practices preventing VILI include:
  • Low tidal volumes
  • Moderate amount of PEEP
  • Prone positioning
  • Neuromuscular blocking agents to prevent patient/ventilator dyssynchrony

Oxygen toxicity

• Results from reactive O₂ species that cause:
  • Cellular damage
  • infection?, or irritation, often causing pain?, swelling?, heat, or redness. সহজ বাংলা: শরীরের প্রদাহ; ব্যথা, ফোলা বা লালভাব হতে পারে।" data-rx-term="inflammation" data-rx-definition="Inflammation is the body’s response to injury, infection, or irritation, often causing pain, swelling, heat, or redness. সহজ বাংলা: শরীরের প্রদাহ; ব্যথা, ফোলা বা লালভাব হতে পারে।">Inflammation?
  • Genetic alterations
• To avoid toxicity, it is recommended to use the lowest FiO₂ to achieve targeted oxygenation.

Auto-PEEP

• Results of persistent end-expiratory airflow that compounds after each breath, contributing to increasing pulmonary hyperinflation
• Also known as “air stacking” or “intrinsic PEEP”
• Most commonly seen in asthma, COPD, and ARDS
• Results in alveolar overdistension and reduced lung compliance (barotrauma)
• Increases intrathoracic pressure
• Increases in intrathoracic pressure can result in:
  • Decreased venous return
  • Reduced cardiac preload
  • Increased left ventricular afterload
  • Reduced cardiac output leading to hypotension?
• Suspect when the individual has:
  • Patient/ventilator dyssynchrony
  • Pulsus paradoxus
  • Hypoxia
  • High plateau pressure
  • Hypotension?
• Therapeutic interventions depend on the ventilator mode, but include:
  • Lowering PEEP
  • Reducing inspiratory time to allow more expiratory time
  • Reducing respiratory rate
  • Disconnecting from the ventilator and pressing down on the chest briefly
  • Hemodynamic support for hypotension?, if needed

Peptic ulcers

• Common in mechanically ventilated individuals becasue of gastric colonization
• Can be associated with upper GI bleeding
• Prevention strategies (note: may increase the risk of VAP):
  • Histamine-2 receptor antagonist
  • Proton pump inhibitors

Ventilator bundles

Commonly used to prevent ventilator-associated complications.

• 30–45 degrees head-of-bed elevation and semirecumbent position: reduces the risk of gastric reflux, aspiration, and VAP
• General infection?-control measures (vary by hospital)
• Peptic ulcer? prophylaxis
• Deep venous thrombosis? prophylaxis
• Daily spontaneous breathing trials
• Spontaneous awakening trials/periodic interruption of sedation
• Maintenance of endotracheal cuff pressure between 20 and 30 cm H₂O
• Frequent subglottic suctions
• Oral care with chlorhexidine gluconate
• Pressure care for wound prevention
• Early mobilization

Weaning and Extubation

Approach to weaning

• Ventilator weaning should be approached based on the following metrics:
  • Has the underlying issue been addressed and improved?
  • Is the individual headed toward hemodynamic stability?
  • Is the O₂ saturation consistently improving?
  • Is the individual able to breathe comfortably on minimal ventilator settings?
• If all the above questions are answered affirmatively, it may be time to start a spontaneous breathing trial (SBT).

Spontaneous breathing trial

An SBT consists of minimal ventilator settings that can be used to predict the chances of successful extubation.

• PS ventilation with:
  • PS of 8–5 cm H₂O
  • PEEP of 5–0 cm H₂O
  • FiO₂ < 40%
• T-piece trial:
  • Use of an adapter piece that disconnects the individual from the ventilator
  • Allows the endotracheal tube to remain in place
• Should be trialed from 30–120 minutes on these minimal settings
• Sedation should be weaned off or set at minimum prior to SBT (exception is during dexmedetomidine use, as it does not suppress respiratory drive).

When to consider extubation

• Improvement in original cause of respiratory distress or when limited airway protection is required
• No severe tachypnea or apneic episodes
• SpO₂ > 92%
• Tidal volumes consistently > 325 mL
• No tachycardia?
• Normotensive
• Mental status: capable of following 1-step commands
• Cuff leak (checks for subglottic edema?)
• Minimal secretions
• Plan for the day (consider waiting after procedures or imaging studies).
• Rapid shallow breathing index:
  • Respiratory rate/tidal volume < 105
  • Most reliable predictor

Post-extubation considerations

When extubating, consider trialing higher levels of supplemental O₂:

• High-flow nasal cannula in individuals with hypoxic respiratory failure (ARDS)
• BiPAP in hypertensive individuals with left heart failure or mitral regurgitation