Mechanical Ventilation

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Contents 1 More Specific Terms 2 Indications 3 Procedure 4 Complications 5 Management 6 More General Terms 7 Additional Terms 8 References

More Specific Terms

• assist-control ventilation (A/CV)
• assisted mandatory ventilation (AMV)
• continuous positive airway pressure (CPAP)
• controlled mechanical ventilation (CMV)
• high frequency ventilation (HFV)
• independent lung ventilation (ILV)
• intermittent mandatory ventilation (IMV)
• intermittent ventilation
• inverse ratio ventilation (IRV)
• pressure supported ventilation (PSV, NIPSV)
• pressure-regulated volume control (PRVC)
• ventilation weaning criteria

Indications

• acute 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
• tidal volume is set by the operator
• patient cannot trigger or override the ventilator

• 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

• 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
  

• 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 [3]

• 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
• generally 8-20/min (8-14/min [2])
• 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

• 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)
  

• Ventilator Management:
• 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

Complications

• 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
• high fiO2
• high tidal volume for volume of lung aerated
  

• Other complications of ventilation
• barotrauma (volutrauma is sometimes used)

• pneumomediastinum
• subcutaneous emphysema
• pneumothorax
• incidence

• 3.5% without PEEP
• 23% with PEEP
• 30% of asthmatics

• 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 [5]

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

• resipiratory 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
• tidal volume of 6 mg/kg IBW for patients with ARDS
• sedation with dexmedetomidine or propofol probably better than continuous infusion of midazolam
• trial of sponteous breathing once or twice daily

• most efficient method of restoring independent ventilation & extubation
  

• 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 [4]
• protocol-driven approaches to spontaneous breathing trials decreases duration of mechanical ventilation [3]

• 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 ()

• methods

• T-tube trial
• IMV, SIMV
• 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

• Tracheostomy for long-term mechanical ventilation reduces duration of mechanical ventilation & ICU days [6]

More General Terms

• clinical procedure

Additional Terms

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

References

1. Jon D. Hirasuna, M.D. Clinical Professor of Medicine, UC Davis, Associate Clinical Professor of Medicine, UCSF, Sept 1997
2. Contributions from Peter Baylor, MD, UCSF Fresno
3. Medical Knowledge Self Assessment Program (MKSAP) 11, 14, 15, American College of Physicians, Philadelphia 1998, 2006, 2009
4. Journal Watch 20(13):101 2000 Kress et al, N Engl J Med 342:1471, 2000
5. Harrison's Principles of Internal Medicine, 13th ed. Isselbacher et al (eds), McGraw-Hill Inc. NY, 1994, pg 1245
6. Journal Watch 23(3):23, 2003 De Jongbe B et al, Paresis acquired in the intensive care unit: a prospective multicenter study JAMA 288:2859, 2002 PMID: [1]
7. Journal Watch 25(15):122, 2005 Griffiths J, Barber VS, Morgan L, Young JD. Systematic review and meta-analysis of studies of the timing of tracheostomy in adult patients undergoing artificial ventilation. BMJ. 2005 May 28;330(7502):1243. Epub 2005 May 18. Review. <PubMed> PMID: [2] <Internet> [3]
8. Jakob SM et al. Dexmedetomidine vs midazolam or propofol for sedation during prolonged mechanical ventilation: Two randomized controlled trials. JAMA 2012 Mar 21; 307:1151. PMID: [4]
9. National Guideline Clearinghouse
   - Best evidence statement (BESt). Clinical utility of neurally adjusted ventilatory assist (NAVA) in decreasing the use of sedation. Cincinnati Children's Hospital Medical Center ngc-guideline: [5]
   - Best evidence statement (BESt). Recruitment maneuvers compared to chest physiotherapy for the mechanically ventilated patient. Cincinnati Children's Hospital Medical Center ngc-guideline: [6]
   - Capnography/capnometry during mechanical ventilation: 2011. American Association for Respiratory Care ngc-guideline: [7]

mechanical ventilation (assisted ventilation)