mechanical ventilation (assisted ventilation)

Indications

• acute respiratory failure
  • hypoxemic respiratory failure
  • hypercapneic respiratory failure
• respiratory fatigue in patients with obstructive lung disease
  • respiratory rate > 35/min
• decreased ventilatory drive
  • patients with neuromuscular disease
    • vital capacity < 10 mL/kg
    • maximum inspiratory pressure < 30 cm H2O
  • neuromuscular blockade
• inability to protect airway
  • loss of consciousness
    • drug overdose
    • anesthesia
  • neuromuscular blockade
  • narrowed upper airway
  • excessive secretions
• laboratory parameters
  • severe respiratory acidosis (pH < 7.20)
  • severe hypoxemia (pO2/fiO2 < 200)
  • rise in pCO2 of > 10 mm Hg

Procedure

Modes of Mechanical Ventilation

• Controlled mechanical ventilation (CMV)
  • breaths are delivered at a preset time interval or rate
    • all breaths are supported
  • tidal volume is set by the operator
  • patient cannot trigger or override the ventilator
  • patient may trigger additional supported breaths above the preset frequency*
• Assist-control ventilation (A/CV)
  • tidal volume is set by the operator
  • A/C rate is the minimum # of breaths patient will receive per minute
  • patient can trigger ventilator by making an inspiratory effort, raising the ventilation rate above the set rate
• Intermittent mandatory ventilation (IMV)
  • IMV rate is set
  • tidal volume is set
  • Between mandatory ventilations, patient may take additional breaths without ventilatory assistance or with pressure support from ventilator
  • Synchronized intermittent mandatory ventilation (SIMV)
    • same as IMV except that mandatory ventilations are synchronized with spontaneous respirations
    • without SIMV dyssynchrony may occur in which ventilator respirations do not match patient respirations contributing to lung injury*
• Continuous positive airway pressure (CPAP)
  • CPAP is the pressure in the airway the ventilator attempts to maintain
  • purpose is to increase lung volumes at end-expiration
  • tidal volume is determined by patient effort without ventilatory assistance or with pressure support from ventilator
  • NO backup ventilation
• Pressure supported ventilation (PSV)
  • can be added to CPAP & non-mandatory ventilations of IMV
  • level is pressure generated by ventilator to augment otherwise non-ventilator assisted breaths
  • NO backup ventilation
• Inverse ratio ventilation (IRV)
  • inspiration time is prolonged to the point that inhalation is longer than exhalation
  • allows longer time for alveoli to open up
  • generally used for ARDS
  • contraindicated in patients with obstructive lung disease
• pressure-regulated volume control (PRVC)
  • gaurantees a specified volume of air will be delivered
  • pressure adjusted accordingly to maintain delivery of tidal volume
  • allows for lowest pressure to deliver specified volume

* volume-controlled continuous mandatory ventilation is the initial mode of choice in patients with respiratory failure

• explanation in is quite confusing; CMV seems to be confused with SIMV
• pressure-controlled ventilation more likely to be associated with lung injury than volume-controlled ventilation
• limit initial tidal volume to 6 mL/kg
• limit plateau pressure to 30 cm H20

Settings

• Tidal volume (Vt)
  • alveolar volume + dead space
  • Vt(set) = Vt(delivered) + tubing expansion volume
  • Vt(delivered) = 5-10 mL/kg(lean body weight)
    • less with severe COPD, ARDS, or high PEEP
      • < 6 mL/kg reduces mortality in patients with ARDS
    • excessive tidal volume &/or minute ventilation can
      • result in auto-PEEP & high alveolar pressures
      • result in barotrauma, respiratory alkalosis, decreased cardiac output
    • a low tidal volume (Vt) may result in:
      • hypercapnia (increased CO2)
      • may be indicated in order to keep alveolar pressure low (permissive hypercapnia)
      • hypoventilation, hypoxemia, atelectasis
  • Vt of 600-800 mL is typical
• respiratory rate
  • anticipated minute ventilation
  • adjust based upon pH & pCO2, relative to patient's baseline
    • target pH = 7.30-7.45
  • generally 8-20/min (8-14/min)
  • respiratory rate too high can result in respiratory alkalosis & auto-PEEP
  • respiratory rate too low can result in
    • hypoventilation
    • respiratory acidosis
    • hypoxemia
    • patient discomfort
• fraction of inspired oxygen (FiO2)
  • select high FiO2 initially
  • adjust FiO2 for pO2> 60-70 mm Hg
  • target SaO2 of 88-92% (mean = 93% achieved) not inferior to target SaO2 of > 95% (mean = 97% achieved)
• flow rate: generally 40-100 L/min
• Alarms:
  • low pressure - should detect air leaks
    • disconnects
    • ET tube cuff deflation
  • high pressure (generally < 30 cm H2O)
    • prevents excessive pressures
    • when reached, undelivered portion of tidal volume is vented, not delivered
    • signals worsened airway resistance or lung/chest wall compliance

Positive end-expiratory pressure (PEEP)

• Indications
  • diffuse disease
  • stiff lungs
  • inability to oxygenate on a non-toxic fiO2 (<50%)
• Mode of action
  • opens up atelectic or fluid-filled alveoli
  • decreases ventilation-perfusion mismatch
  • improves oxygenation
• Goals
  • decrease fiO2 to non-toxic level (<50%)
  • maintain cardiac output (PEEP can reduce cardiac output by reducing preload)
• target PEEP to plateau pressure < 30 cm H2O*
• Complications: auto-PEEP

* if plateau pressure remains > 30 cm H2O, consider pneumothorax

• see other complications below

Ventilator Management

• Mean alveolar pressure
  • reflected by mean airway pressure (MAP)
  • increased by:
    • increased minute ventilation
    • increased PEEP
    • alterations in the inspiratory flow pattern
  • determines alveolar recruitment
  • affects pulmonary blood flow
• Conventional ventilation
  • tidal volumes about 10-12 mL/kg of ideal body weight
  • Inspiration:Expiration period considerably < 1:1
  • PEEP used to:
    • recruit nonaerated alveoli
    • prevent airway closure & collapse at end-expiration
  • normalize pH & pCO2
  • high airway pressures commonly result
• Ventilator manipulations to reduce peak airway pressure
  • decrease tidal volume
    • recommendation of 10-12 mL/kg based on limited data
    • with patchy disease in most cases of ARDS, lung is functionally small
    • can allow pCO2 to increase & pH to decrease
  • sedation or paralysis
    • indications:
      • high airway pressures
      • asynchrony with the ventilator
      • refractory hypoxia
    • benefits:
      • increased FRC
      • ventilation:perfusion matching improved
      • decreased O2 consumption & CO2 production
  • pressure control mode
    • square pressure wave produces decelerating flow
    • tidal volume vary with changes in resistance
    • may improve gas exchange & work of breathing
    • square pressure wave increases potential of shear force problems
  • extended inspiratory time
    • inverse ratio ventilation (IRV) is an extreme form
    • greater tidal volume with lower peak pressure
    • alveolar recruitment
    • decreased dead space
    • may be used with pressure control mode
    • may be used with volume control mode
      • decreased inspiratory flow
      • may use inspiratory pause
      • may use decelerating inspiratory flow pattern
    • potential problems
      • air-trapping causing auto-PEEP
      • auto-PEEP reduces cardiac output
      • need for heavy sedation or paralysis
      • when pressure is limited, tidal volume may be reduced
• decrease arterial pCO2 for respiratory acidosis
  • increase respiratory rate
  • increase tidal volume
    • in pressure-control mode, increase inspiratory pressure support
• increase arterial pCO2 for respiratory alkalosis

Complications

• bedside ultrasonography for acute respiratory failure in critically ill patients

Pressure injury

• Acute respiratory distress syndrome (ARDS) can be induced in experimental animals by moderately high peak airway pressure
  • pigs, sheep, rats
  • 30-45 cm H2O
• ARDS mechanics
  • distribution patchy
  • gravitationally-dependent areas affected first
  • lung stiffness may be due to fewer functional alveoli
• increased airway resistance may reflect fewer functional airways
  • remaining lung may receive entire volume delivered by ventilator, resulting in:
    • higher airway pressures
      • increased peak inspiratory pressure
    • high fiO2
    • high tidal volume for volume of lung aerated
  • bronchospasm, secretions in airway, endotracheal tube or ventilator tubing, mucus plug obstructing airway, agitation resulting in respiratory dyssynchrony with respirator

Other complications of ventilation

• barotrauma (volutrauma is sometimes used)
  • pneumomediastinum
  • subcutaneous emphysema
  • pneumothorax (even small pneumothorax may need treatment if fiO2 > 50%)
    • needle decompression of affected side if hemodynamically unstable
    • small-bore tube thoracostomy if hemodynamically stable
  • incidence
    • 3.5% without PEEP
    • 23% with PEEP
    • 30% of asthmatics
• auto-PEEP
• hypotension
  • decreased venous return with positive pressure breathing
  • increased pulmonary vascular resistance due to alveolar capillary compression
• respiratory acidosis, respiratory alkalosis
• oxygen toxicity
• confusion in volume status, especially with high PEEP
• complications resulting from the presence of endotracheal tube
  • glottic edema
  • tracheomalacia
  • tracheal stenosis
• polyneuropathy & myopathy above & beyond deconditioning
• reactivation of oropharyngeal herpes simplex virus
  • no benefit of prophylactic acyclovir

• disease interaction(s) of pneumothorax with mechancal ventilation

Management

• respiratory acidosis
  • decrease pCO2 (pCO2 is high)
  • increase respiratory rate (avoid auto-PEEP)
  • increase tidal volume
    • in assist-control mode, directly increase volume
    • in pressure support mode, increase inspiratory pressure to increase tidal volume
  • permissive hypercapnia for ARDS rather than increase tidal volume > 6 mL/kg
• respiratory alkalosis
  • increase pCO2 (pCO2 is low)
  • decrease respiratory rate
    • this will not work if patient is breathing faster than the ventilator setting
  • decrease tidal volume
  • identify & treat cause of respiratory alkalosis
    • sepsis
    • pulmonary embolism
    • liver disease
    • pain
• tissue hypoxia
  • increase pO2 (pO2 is low)
  • increased FiO2
  • increase PEEP

General considerations

• H2-receptor agonist
  • reduce risk of stress gastric ulceration
• elevation of head of bed to 30-45 degrees reduces risk of ventilator-associated pneumonia
• tidal volume of 6 mg/kg IBW for patients with ARDS
  • maintain plateau pressure < 30 cm of H2O
• sedation with dexmedetomidine or propofol probably better than continuous infusion of midazolam
• light sedation vs heavy sedation reduces ICU-associated PTSD, time on ventilator & mortality
• hold sedation & anesthesia in unresponsive patients rather than taper
• treat pain with interrupted opioid infusion
• nebulization as indicated (on-demand) with acetylcysteine & salbutamol is noninferior to preventive nebulization in ventilator-free days in ICU patients
• trial of spontaneous breathing once or twice daily
  • most efficient method of restoring independent ventilation & extubation
• prone positioning may help to aerate lung regions dependent in a supine position
• selective decontamination of the GI tract may reduce mortality in ICU patients receiving mechanical ventilation
• early mobilization during mechanical ventilation more harmful than beneficial
• target oxygenation of SaO2 90%, 94% or 98% did not differ in ventilator-free days, death, cardiac arrest, arrhythmia, myocardial infarction, stroke, or pneumothorax
• low-calorie, low-protein enteral nutrition may reduce ICU stay in ventilated patients (see intensive care unit)
• probiotics of no benefit

Weaning from an endotracheal tube

• considerations
  • patient alert, cooperative
  • ability to clear secretions
  • ability to protect airway
  • reversal of condition for which patient was initially intubated
  • no PEEP & reasonable arterial blood gas (ABG) for fi02 <40%
  • pH & pCO2 at baseline for COPD patients
  • daily interruption of continuous sedation until patient is either awake or clearly needs the sedation resumed decreases duration of mechanical ventilation
  • protocol-driven approaches to spontaneous breathing trials decreases duration of mechanical ventilation
  • volume status
    • diuresis in patients with LV systolic dysfunction
    • diuresis guided by serial serum BNP
• parameters
  • tidal volume > 5 mL/kg
  • vital capacity >10 mL/kg or 1 liter
  • minute ventilation < 10 L/min
  • maximum voluntary ventilation > twice the minute ventilation
  • peak inspiratory pressure < -20 cm H2O (NIF)
  • SaO2 > 90%, fiO2 < 0.5%, PEEP < 5 cm H2O, arterial pH > 7.30
• methods
  • T-tube trial
  • IMV, (avoid SIMV as a weaning mode, increases time to extubation)
  • pressure support
• Failures
  • increased ventilatory demands
    • increased CO2 production
      • sepsis
      • excess carbohydrates in diet
    • increased dead space
  • inadequate ventilation
    • continued use of sedatives
    • weak or discoordinated muscles
    • increased work of breathing
      • small endotracheal (ET) tube
      • bronchospasm
      • pulmonary edema
      • bronchial secretions
    • chronic CO2 retainer who has been hyperventilated on ventilator
    • metabolic alkalosis
      • may be induced by loop diuretics
      • reduces respiratory drive
  • non-respiratory factors
    • congestive heart failure
    • malnutrition
    • anxiety or depression
    • hypophosphatemia

Post-extubation

• evaluation:
  • ability to clear secretions
  • ability to protect airway
• non-invasive positive pressure ventilation shortly after extubation for 24 hours reduces need for reintubation in patients with heart failure, COPD or hypercapnia

Tracheostomy

• Tracheostomy for long-term mechanical ventilation reduces duration of mechanical ventilation & ICU days

Notes

• in bed cycle ergometry of no benefit

More general terms

• clinical procedure

More specific terms

• assist controlled ventilation (ACV)
• assist-control ventilation (A/CV)
• assisted mandatory ventilation (AMV)
• biphasic cuirass ventilation
• controlled mechanical ventilation; continuous mandatory ventilation (CMV)
• high frequency ventilation (HFV)
• high-frequency oscillatory ventilation
• independent lung ventilation (ILV)
• intermittent mandatory ventilation (IMV)
• intermittent ventilation
• inverse ratio ventilation (IRV)
• lung protective ventilation; low tidal volume ventilation; permissive hypercapnia
• pressure supported ventilation (PSV, NIPSV)
• pressure-regulated volume control (PRVC)
• prolonged mechanical ventilation

Additional terms

• bag-mask ventilation
• benefits of mechanical ventilation
• mechanical ventilator
• ventilation weaning
• ventilator-associated pneumonia

Patient information

mechanical ventilation patient information