Predictors of weaning
outcome
Muhammad Asim Rana
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.



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.
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.

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

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.
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.
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.


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.
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).

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.
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 <55 mmHg when the FiO2 is
>0.40), although this approach has not been
validated in clinical studies.



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
<350 mmHg



Another index that was proposed as a
predictor of weaning outcome is the
arterial/alveolar oxygen tension ratio
(PaO2/PAO2).
A PaO2/FiO2 ratio >200 mmHg
The PaO2/PAO2 is more stable when the
FiO2 changes than the A-a oxygen gradient.
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.
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.

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.
Compliance


Respiratory system compliance is estimated
during condition of zero gas flow by:
Compliance = VT / (plateau pressure - PEEP)

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.
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.


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.
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.


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, >1.0 J/L or >13 J/min)
predicts weaning failure.
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.
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 ÷ PAO2)] ÷ Respiratory
Rate

in which Cdyn is dynamic compliance, PImax
is maximal inspiratory pressure, and PaO2 ÷
PAO2 is a measure of gas exchange. A
CROP index >13 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.
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.
Integrative index of Jabour



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incorporates these measures:
Integrative Index = PTI x (VE40 ÷ VTsb)
where VTsb is the tidal volume during spontaneous
breathing.
An integrative index <4 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.
USING PREDICTORS

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.




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.

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.


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.
SUMMARY AND
RECOMMENDATIONS


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.
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.

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



We do not consider weaning until patients are
hemodynamically stable and have an arterial
oxyhemoglobin saturation (SaO2) >90 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.
Clinical variables used to predict
weaning success

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Oxygenation: PaO2 of 60 mmHg with FiO2 of 0.35
Alveolar-arterial PO2 gradient of <350 mmHg
PaO2/FiO2 ratio of >200
Ventilation:
Vital capacity of >10-15 ml/kg body weight
Maximum negative inspiratory pressure <-30 cmH2O
Minute ventilation <10 L/min
Airway occlusion pressure (P0.1) <4-6 cmH2O
Frequency to tidal volume ratio (f/VT) <100 b/min/liter
CROP index >13 ml/breath/min
Integrative index of Jabour et al <4/min
Poor respiratory system compliance
Increased work of breathing
Shukran Alhamdullilah