Predictors of weaning outcome

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Predictors of weaning outcome

• Muhammad Asim Rana

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INTRODUCTION

• Weaning is the progressive decrease of the amount
  of support that a patient receives from the
  mechanical ventilator. However, it is more
  commonly used to describe the entire process of
  decreasing the amount of support that a patient
  receives from the mechanical ventilator,
  assessing the patient's clinical response, and
  discontinuing mechanical ventilation.

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• Discontinuation of mechanical ventilation is a
  two-step process.
• 1) First, patients who may be ready to wean are
  identified using various predictors of weaning
  outcome.
• 2) Weaning is then initiated in those patients.

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IMPORTANCE OF PREDICTORS 

• It is desirable to have accurate, objective
  predictors of weaning outcome that can be applied
  early in a patient's clinical course because
  clinicians tend to underestimate readiness to
  wean. In several randomized, controlled trials
  that compared weaning techniques, most patients
  were able to tolerate discontinuation of
  mechanical ventilation on the same day that their
  ability to wean was first assessed.

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• When assessed early in a patient's clinical
  course, predictors of weaning outcome can help
  prevent unnecessary prolongation of mechanical
  ventilation by identifying the earliest time that
  a patient is able to resume and sustain
  spontaneous ventilation

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• Conversely, by identifying patients who are
  likely to fail weaning, predictors of weaning
  outcome can prevent a premature weaning attempt
  that could result in cardiovascular, respiratory,
  or psychological distress. Finally, the
  predictors may provide insight into the reasons
  for ongoing ventilator dependence.

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PREDICTORS 

• Numerous measures have been proposed as
  predictors of weaning outcome.
• These predictors are assessed during spontaneous
  breathing and used to decide whether a trial of
  weaning is warranted.

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Rapid shallow breathing index (RSBI) 

• The ratio of respiratory frequency (f, also
  called the respiratory rate) to tidal volume (VT)
  is called the rapid shallow breathing index
  (RSBI). In other words, RSBI f/VT. Measurements
  of f and VT can be obtained using a hand-held
  spirometer attached to the endotracheal tube,
  while the patient breathes room air spontaneously
  for one minute.

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• Using the RSBI as a predictor of weaning outcome
  is based on the observation that f increases and
  VT decreases immediately following
  discontinuation of ventilator support in patients
  who fail weaning.
• The likelihood of weaning failure increases as
  the RSBI increases.

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Physical examination 

• one of the most helpful methods of judging the
  likelihood of successful weaning is to conduct a
  careful physical examination when the patient is
  breathing spontaneously.
• Evidence of increased effort includes nasal
  flaring, accessory muscle recruitment, recession
  of the suprasternal and intercostal spaces, or
  paradoxic motion of the rib cage and abdomen (ie,
  abdomen moves inward during inspiration).

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• The chest should be auscultated to detect new
  wheezing or crackles. Dyspnea and changes of
  mental status, blood pressure, heart rate,
  cardiac rhythm, or respiratory rate should be
  identified. An elevated respiratory rate is a
  sensitive sign of respiratory distress if it is
  carefully counted over a one minute period
  however, bedside estimation of tidal volume is
  inaccurate. Finally, the patient should be
  evaluated for cyanosis, although this is not an
  accurate sign.

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Arterial oxygenation 

• Measurements of gas exchange are frequently
  considered when deciding whether to initiate
  weaning.
• Do not consider discontinuation of ventilator
  support if a patient has significant hypoxemia
  (eg, PaO2 lt55 mmHg when the FiO2 is gt0.40),
  although this approach has not been validated in
  clinical studies.

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• Several indices derived from an arterial blood
  gas (ABG) have been proposed as predictors of
  weaning success
• An arterial oxygen tension (PaO2) 60 mmHg with
  a fraction of inspired oxygen (FiO2) 0.35
• An alveolar-arterial (A-a) oxygen gradient of
  lt350 mmHg

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• Another index that was proposed as a predictor of
  weaning outcome is the arterial/alveolar oxygen
  tension ratio (PaO2/PAO2).
• A PaO2/FiO2 ratio gt200 mmHg
• The PaO2/PAO2 is more stable when the FiO2
  changes than the A-a oxygen gradient.

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Minute ventilation 

• A minute ventilation below 10 L/min was
  considered a predictor of weaning success. It has
  since proven to be a poor predictor of weaning
  outcome, with a high rate of false positive and
  false negative results. As an example, one
  prospective cohort study found that a minute
  ventilation less than 10 L/min had positive and
  negative predictive values of 50 and 40 percent,
  respectively, suggesting that flipping a coin
  could more accurately predict weaning outcome.

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Maximal inspiratory pressure 

• Maximal inspiratory pressure (PImax) is a global
  assessment of the strength of all the respiratory
  muscles. It was considered a predictor of weaning
  outcome after a study reported that a PImax of
  -30 cmH2O or less predicted successful weaning
  and a PImax value higher than -20 cmH2O predicted
  weaning failure.

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• However, subsequent studies have demonstrated
  poor sensitivity and specificity, probably
  because PImax assesses the strength of the
  respiratory muscles without considering the
  demands being placed upon them.

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Compliance

• Respiratory system compliance is estimated during
  condition of zero gas flow by
• Compliance VT / (plateau pressure - PEEP)

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• In a prospective cohort study, a respiratory
  system compliance of 33 mL/cmH2O (normal 60 to
  100 mL/cmH2O) had a positive and negative
  predictive value of only 60 and 53 percent,
  respectively.

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Occlusion pressure 

• The airway pressure that is measured 0.1 sec
  after the initiation of an inspiratory effort
  against an occluded airway is called the airway
  occlusion pressure (P0.1).
• It is a measure of respiratory drive whose
  usefulness as a predictor of weaning outcome is
  uncertain due to conflicting data.

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• In normal subjects, P0.1 values are less than 2
  cmH2O. Several studies have demonstrated that
  patients who have a P0.1 greater than 4 to 6
  cmH2O usually fail weaning, whereas patients with
  a lower P0.1 usually wean successfully.
• In contrast, other studies have found P0.1 to be
  an inaccurate predictor of weaning outcome.

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Work of breathing 

• The mechanical work of breathing can be
  calculated from the intrathoracic pressure that
  is generated by contraction of the respiratory
  muscles (or a ventilator) and the VT.
• There are insufficient data to recommend that it
  be measured routinely as a predictor of weaning
  outcome.

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• In healthy subjects who are breathing at rest,
  the average work per liter is 0.47 J/L and the
  average work per minute of ventilation is 4.33
  J/min.
• Several studies have reported that increased work
  of breathing (eg, gt1.0 J/L or gt13 J/min) predicts
  weaning failure.

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Integrative indices 

• Weaning failure is usually multifactorial
  therefore it is not surprising that single
  measures tend to be unreliable. Indices that
  integrate several physiologic functions were
  developed to improve predictive accuracy. There
  are insufficient data to recommend any index be
  used routinely to predict weaning outcome.

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The CROP index

• The CROP index integrates thoracic compliance
  (C), respiratory rate (R), arterial oxygenation
  (O), and maximal inspiratory pressure (P). Thus,
  it considers both demands on the respiratory
  system and the capacity of the respiratory
  muscles to handle them
•      CROP index Cdyn  x  PImax  x  (PaO2    PA
  O2)    Respiratory Rate

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• in which Cdyn is dynamic compliance, PImax is
  maximal inspiratory pressure, and PaO2 PAO2 is
  a measure of gas exchange. A CROP index gt13
  mL/breath per min is generally considered to
  predict weaning success. When the CROP index was
  prospectively examined, the positive and negative
  predictive values were 71 and 70 percent,
  respectively.

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The pressure-time product Index (PTI)

• This is the time integral of respiratory muscle
  pressure. It is a measure of ventilatory
  endurance. The minute ventilation needed to bring
  PaCO2 to 40 mmHg (VE40) is a measure of
  ventilatory endurance and an estimate of the
  efficiency of gas exchange.

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Integrative index of Jabour

• incorporates these measures
• Integrative Index PTI x (VE40    VTsb)
• where VTsb is the tidal volume during spontaneous
  breathing.
• An integrative index lt4 units per minute is
  generally considered to predict weaning success.
  In a retrospective study, this integrative index
  had a positive predictive value of 96 percent and
  a negative predictive value of 95 percent.

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USING PREDICTORS 
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• It should be emphasized that predictors of
  weaning outcome are intended to identify patients
  in whom weaning can begin. They should not be
  used to justify immediate extubation when
  successful weaning is forecast.

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• In other words, predictors of weaning outcome
  should be used in the first step of a two-step
  approach to discontinuation of mechanical
  ventilation
• Identify patients who may be ready to wean using
  predictors of weaning outcome.
• Wean those patients whose predictors of weaning
  outcome forecast success.
• This includes performing a weaning trial,
  assessing the patient's response during the
  trial, and extubating the patient if the trial is
  successful.

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• This approach is consistent with the cardinal
  precept of diagnostic testing - begin with a
  screening test and follow with a confirmatory
  test. Evaluation of predictors of weaning outcome
  can be considered the screening test, while a
  weaning trial can be considered the confirmatory
  test.

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• The goal of screening is to not miss anybody with
  the condition under consideration (in this case,
  the ability to sustain spontaneous ventilation).
  Thus, a good screening test has a high
  sensitivity (ie, a low false negative rate). A
  high false positive rate is acceptable. The RSBI
  fulfills these criteria, with a sensitivity of
  90 percent in some studies
• . The RSBI also satisfies the desire that a
  screening test be simple, expeditious, and safe.
  Measurement of the RSBI requires only a minute or
  so to perform.

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SUMMARY AND RECOMMENDATIONS
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• Weaning is the progressive decrease of the amount
  of support that a patient receives from the
  mechanical ventilator.
• However, it is more commonly used to describe the
  entire process of decreasing the amount of
  support that a patient receives from the
  mechanical ventilator, assessing the patient's
  clinical response, and discontinuing mechanical
  ventilation.

36
Discontinuation of mechanical ventilation is a
two-step process.

• First, patients who may be ready to wean are
  identified using various predictors of weaning
  outcome.
• Weaning is then initiated in those patients.

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• Proposed predictors of weaning outcome include
  the rapid shallow breathing index (RSBI),
  physical examination, arterial oxygenation,
  minute ventilation, maximal inspiratory pressure,
  respiratory system compliance, occlusion
  pressure, work of breathing, and integrative
  indices

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• We do not consider weaning until patients are
  hemodynamically stable and have an arterial
  oxyhemoglobin saturation (SaO2) gt90 percent while
  receiving a fraction of inspired oxygen (FiO2)
  40 percent and positive end-expiratory pressure
  (PEEP) 5 cm H2O.
• Once these goals are achieved, we suggest that
  weaning be initiated in most patients when the
  RSBI is 100 breaths/min per L (Grade 2C).
• The threshold can be increased (eg, 115 to 125
  breaths/min per L) if the risk of complications
  due to continued mechanical ventilation outweighs
  the risks of reintubation, or decreased (eg, 80
  to 90 breaths/min per L) if the risk of
  reintubation outweighs the risks of continued
  mechanical ventilation.

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Clinical variables used to predict weaning
success

• Oxygenation PaO2 of 60 mmHg with FiO2 of 0.35
• Alveolar-arterial PO2 gradient of lt350 mmHg
• PaO2/FiO2 ratio of gt200
• Ventilation
• Vital capacity of gt10-15 ml/kg
  body weight
• Maximum negative inspiratory
  pressure lt-30 cmH2O
• Minute ventilation lt10 L/min
• Airway occlusion pressure
  (P0.1) lt4-6 cmH2O
• Frequency to tidal volume ratio
  (f/VT) lt100 b/min/liter
• CROP index gt13 ml/breath/min
• Integrative index of Jabour et al lt4/min
• Poor respiratory system compliance
• Increased work of breathing

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Shukran Alhamdullilah