Weaning from Mechanical Ventilation

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Weaning from Mechanical Ventilation

• Mazen Kherallah, MD, FCCP
• Consultant Intensivist
• King Faisal Specialist Hospital Research Center
• Assistant Professor
• University of North Dakota, USA
• www.icumedicus.com
• mkherallah_at_msn.com

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Objectives

• Discuss physiologic variables that are used to
  indicate readiness to wean from mechanical
  ventilation
• Contrast the approaches used to wean patients
  from mechanical ventilation
• Discuss the use of protocols to wean patients
  from ventilatory support
• Discuss the criteria used to indicate readiness
  for extubation
• Describe the most common reasons why patients
  fail to wean from mechanical ventilation

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Introduction

• 75 of mechanically ventilated patients are easy
  to be weaned off the ventilator with simple
  process
• 10-15 of patients require a use of a weaning
  protocol over a 24-72 hours
• 5-10 require a gradual weaning over longer time
• 1 of patients become chronically dependent on MV

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Readiness To Wean

• Improvement of respiratory failure
• Absence of major organ system failure
• Appropriate level of oxygenation
• Adequate ventilatory status
• Intact airway protective mechanism (needed for
  extubation)

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Oxygenation Status

• PaO2 60 mm Hg
• FiO2 0.40
• PEEP 5 cm H2O

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Ventilation Status

• Intact ventilatory drive ability to control
  their own level of ventilation
• Respiratory rate lt 30
• Minute ventilation of lt 12 L to maintain PaCO2 in
  normal range
• VD/VT lt 60
• Functional respiratory muscles

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Intact Airway Protective Mechanism

• Appropriate level of consciousness
• Cooperation
• Intact cough reflex
• Intact gag reflex
• Functional respiratory muscles with ability to
  support a strong and effective cough

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Function of Other Organ Systems

• Optimized cardiovascular function
• Arrhythmias
• Fluid overload
• Myocardial contractility
• Body temperature
• 1? degree increases CO2 production and O2
  consumption by 5
• Normal electrolytes
• Potassium, magnesium, phosphate and calcium
• Adequate nutritional status
• Under- or over-feeding
• Optimized renal, Acid-base, liver and GI
  functions

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Predictors of Weaning Outcome
Predictor Value
Evaluation of ventilatory drive P 0.1 lt 6 cm H2O
Ventilatory muscle capability Vital capacity Maximum inspiratory pressure gt 10 mL/kg lt -30 cm H2O
Ventilatory performance Minute ventilation Maximum voluntary ventilation Rapid shallow breathing index Respiratory rate lt 10 L/min gt 3 times VE lt 100 lt 30 /min
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Maximal Inspiratory Pressure

• Pmax Excellent negative predictive value if less
  than 20 (in one study 100 failure to wean at
  this value)
• An acceptable Pmax however has a poor positive
  predictive value (40 failure to wean in this
  study with a Pmax more than 20)

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Frequency/Volume Ratio

• Index of rapid and shallow breathing RR/Vt
• Single study results
• RR/Vtgt105 95 wean attempts unsuccessful
• RR/Vtlt105 80 successful
• One of the most predictive bedside parameters.

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Measurements Performed Either While Patient Was
Receiving Ventilatory Support or During a
BriefPeriod of Spontaneous Breathing That Have
Been Shown to Have Statistically Significant LRs
To Predict theOutcome of a Ventilator
Discontinuation Effort in More Than One Study
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Refertences

• 2 Tobin MJ, Alex CG. Discontinuation of
  mechanical ventilation. In Tobin MJ, ed.
  Principles and practice of mechanical
  ventilation. New York, NY McGraw-Hill, 1994
  11771206
• 4 Cook D, Meade M, Guyatt G, et al. Evidence
  report on criteria for weaning from mechanical
  ventilation. Rockville, MD Agency for Health
  Care Policy and Research, 199910 Lopata M, Onal
  E. Mass loading, sleep apnea, and the
  pathogenesis of obesity hypoventilation. Am Rev
  Respir Dis 1982 126640645
• 16 Hansen-Flaschen JH, Cowen J, Raps EC, et al.
  Neuromuscular blockade in the intensive care
  unit more than we bargained for. Am Rev Respir
  Dis 1993 147234236
• 18 Bellemare F, Grassino A. Effect of pressure
  and timing of contraction on human diaphragm
  fatigue. J Appl Physiol 1982 5311901195
• 20 Roussos C, Macklem PT. The respiratory
  muscles. N Engl J Med 1982 307786797
• 24 Le Bourdelles G, Viires N, Boezkowski J, et
  al. Effects of mechanical ventilation on
  diaphragmatic contractile properties in rats. Am
  J Respir Crit Care Med 1994 14915391544

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Approaches To Weaning

• Spontaneous breathing trials
• Pressure support ventilation (PSV)
• SIMV
• New weaning modes

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Do Not Wean To Exhaustion
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Spontaneous Breathing Trials

• SBT to assess extubation readiness
• T-piece or CPAP 5 cm H2O
• 30-120 minutes trials
• If tolerated, patient can be extubated
• SBT as a weaning method
• Increasing length of SBT trials
• Periods of rest between trials and at night

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Frequency of Tolerating an SBT in Selected
Patients and Rate of Permanent Ventilator
DiscontinuationFollowing a Successful SBT
Values given as No. (). Pts patients. 30-min
SBT. 120-min SBT.
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Criteria Used in Several Large Trials To Define
Tolerance of an SBT
HR heart rate Spo2 hemoglobin oxygen
saturation. See Table 4 for abbreviations not
used in the text.
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Pressure Support

• Gradual reduction in the level of PSV
• PSV that prevents activation of accessory muscles
• Gradula decrease on regular basis (hours or days)
  to minimum level of 5-8 cm H2O
• Once the patient is capable of maintaining the
  target ventilatory pattern and gas exchange at
  this level, MV is discontinued

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SIMV

• Gradual decrease in mandatory breaths
• It may be applied with PSV
• Has the worst weaning outcomes in clinical trials
• Its use is not recommended

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New Modes

• Volume support
• Automode
• MMV
• ATC

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Protocols

• Developed by multidisciplinary team
• Implemented by respiratory therapists and nurses
  to make clinical decisions
• Results in shorter weaning times and shorter
  length of mechanical ventilation than
  physician-directed weaning

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Mechanical Ventilation
Low level CPAP (5 cm H2O), Low levels of
pressure support (5 to 7 cm H2O) T-piece
breathing
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Failure to Wean

• Weaning to exhaustion
• Auto-PEEP
• Excessive work of breathing
• Poor nutritional status
• Overfeeding
• Left heart failure
• Decreased magnesium and phosphate leves
• Infection/fever
• Major organ failure
• Technical limitation

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Weaning to Exhaustion

• RR gt 35/min
• Spo2 lt 90
• HR gt 140/min
• Sustained 20 increase in HR
• SBP gt 180 mm Hg, DBP gt 90 mm Hg
• Anxiety
• Diaphoresis

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Work-of-Breathing

• Pressure Volume/compliance flow X resistance
• High airway resistance
• Low compliance
• Aerosolized bronchodilators, bronchial hygiene
  and normalized fluid balance assist in
  normalizing compliance, resistance and
  work-of-breathing

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Auto-PEEP

• Increases the pressure gradient needed to inspire
• Use of CPAP is needed to balance alveolar
  pressure with the ventilator circuit pressure
• Start at 5 cm H2O, adjust to decrease patient
  stress
• Inspiratory changes in esophageal pressure can be
  used to titrate CPAP

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0
-5
0
-5
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0
-5
Auto PEEP 10
-15
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PEEP 10
5
Auto PEEP 10
-5
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Left Heart Failure

• Increased metabolic demands that are associated
  with the transition from mechanical ventilation
  to spontaneous breathing
• Increases in venous return as that is associated
  with the negative pressure ventilation and the
  contracting diaphragm which results into an
  increase in PCWP and pulmonary edema
• Appropriate management of cardiovascular status
  is necessary before weaning will be successful

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Nutritional/Electrolytes

• Imbalance of electrolytes causes muscular
  weakness
• Nutritional support improves outcome
• Overfeeding elevates CO2 production due to
  excessive carbohydrate ingestion

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Infection/Fever/Organ Failure

• Organ failure precipitate weaning failure
• Infection and fever increase O2 consumption and
  CO2 production resulting in an increase
  ventilatory drive

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Points to Remember

• The primary prerequisite for weaning is reversal
  of the indication of mechanical ventilation
• Adequate gas exchange should be present with
  minimal oxygenation and ventilatory support
  before weaning is attempted
• The function of all organ systems should be
  optimized, electrolytes should be normal, and
  nutrition should be adequate before weaning is
  attempted
• The most successful predictor of weaning is RSBI
  lt 100
• Maximum inspiratory pressure is the best
  predictor of weaning failure
• Ventilatory discontinuation should be done if
  patient tolerates SBT for 30-120 minutes
• Patients who fail an SBT should receive a stable,
  non-fatiguing, comfortable form of ventilatory
  support
• Use of liberation and weaning protocol
  facilitates the process and decreases the
  ventilator length of stay