This is a very useful presentation for respiratory therapists and ICU and Emergency physicians. Intended to teach how to assess you patient's readiness for weaning from mechanical ventilator and successful separation from machine.

1. Predictors of weaning
outcome
Muhammad Asim Rana
MBBS, MRCP, SF-CCM, FCCP, EDIC
Department of Critical Care Medicine
King Saud Medical City
Riyadh, Saudi Arabia

2. INTRODUCTION

Weaning is the progressive decrease of the
amount of support that a patient receives
from the mechanical ventilator.

3. 
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.

4. 
Discontinuation of mechanical ventilation is a
two-step process.
1) Assessment to identify patients who may be
ready to wean using various predictors of
weaning outcome.
2) Weaning is then initiated in those patients.

5. Types of Ventilator
Discontinuance

Rapid and routine discontinuance




Post op
Overdose
Acute illnesses
Gradual reduction of support



Chronic or severe illnesses
Severe trauma
Ventilator dependent patients

6. Readiness testing

During readiness testing, objective clinical
criteria are evaluated to determine whether a
patient is ready to begin weaning.

Some clinicians also consider physiological
tests, known as weaning predictors.

7. PURPOSE OF READINESS
TESTING

Readiness testing has two major purposes.


The first is to identify patients who are ready to
wean from mechanical ventilation.
The second major purpose of readiness testing is
to identify patients who are not ready for weaning.

8. CLINICAL CRITERIA

The criteria can be segregated into
1)Required criteria
2)Optional criteria

9. Required Criteria





The cause of the respiratory failure has
improved.
Adequate oxygenation(PaO2>60,FiO2<35%)
Arterial pH >7.25.
Hemodynamic stability, without myocardial
ischemia.
The patient is able to initiate an inspiratory
effort.

10. Optional Criteria



Hemoglobin level ≥7 to 10 mg/dL
Core temperature ≤38 to 38.5 ºC
A mental status that is either awake and alert,
or easily arousable

11. 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.

12. Why predictors

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.

13. Why predictors

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

14. Why predictors

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,
psychological distress.

15. 
Finally,
the predictors may provide insight into the
reasons for ongoing ventilator dependence.

16. 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.

17. 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.

18. 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).

19. 

The chest should be auscultated to detect new
wheezing or crackles.
Patient should be checked for







Dyspnea
changes of mental status,
blood pressure,
heart rate,
cardiac rhythm,
respiratory rate
Cyanosis

20. WHEANS NOT & WEANS NOW









wheezes
heart disease
electrolytes
anxiety
neuromuscular disease
sepsis
nutrition
opiates
thyroid disease

21. 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).
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.

22. 

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.

23. Arterial oxygenation
Several indices derived from an arterial blood
gas (ABG) have been proposed as predictors
of weaning success:
1) An arterial oxygen tension (PaO2) ≥ 60
mmHg with a fraction of inspired oxygen
(FiO2) ≤ 0.35
2) An alveolar-arterial (A-a) oxygen gradient of
<350 mmHg
3) A PaO2/FiO2 ratio >200 mmHg


24. Minute ventilation



The total minute ventilation estimates the demand on
the respiratory system.
5 to 6 liters/min in healthy individuals at rest
increases among patients who are mechanically
ventilated or have increased carbon dioxide
production






fever,
hypermetabolic states
metabolic acidosis
Hypoxemia
increased dead space
increased central respiratory drive.

25. 

Since elevated total minute ventilation is
indicative of increased respiratory demand, it
is reasonable to expect that elevated total
minute ventilation might predict weaning
failure.
However, a systematic review found that
minute ventilation is a poor predictor of
weaning outcome

26. 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.

27. Maximal inspiratory pressure


The pooled LR+ ranged from 1.15 to 1.57, while
the pooled likelihood ratio negative (LR-) ranged
from 0.31 to 0.65.
These results indicate that there is little or no
increase in the probability of weaning success
among patients with a normal MIP, but a small
increase in the probability of weaning failure
among patients with a reduced MIP.

28. Compliance


Static respiratory system compliance (Cst,rs)
is an indirect quantification of the work of
breathing that is required to overcome the
elastic forces of the respiratory system. It is
estimated during a condition of zero gas flow:
Compliance = VT / (plateau pressure - PEEP)
Also Cst,rs = VT / (plateau pressure - PEEP)

29. 
In a prospective cohort study, a respiratory
system compliance of 33 mL/cmH2O (normal
60 to 100 mL/cmH2O) had a poor predictive
capacity

30. 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.

31. 


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.

32. 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.
measured using an esophageal balloon
it tends to be higher among patients who fail
weaning compared to those who successfully
wean

33. 

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.

34. Gastric mucosal acidosis



Blood flow may be diverted from the
splanchnic vascular bed to the respiratory
muscles during weaning in order to meet the
oxygen demands of the respiratory muscles.
This is most severe during weaning failure
Thus, gastric mucosal acidosis may be an
indicator of weaning failure

35. Oxygen cost of breathing


The difference between total O2 consumption
during spontaneous breathing and during
relaxed mechanical ventilation.
Its measurement requires special equipment
(ie, a metabolic cart) that is not routinely
available in most intensive care units.

36. 


The O2 cost of breathing is <5 percent of the
total O2 consumption in most healthy
subjects.
It can exceed 50 percent in patients who are
being weaned and tends to be highest among
patients who are failing weaning.
Studies are trying identify a threshold value
that accurately discriminates patients who are
at increased risk for weaning failure

37. 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.

38. Inspiratory effort quotient (IEQ)

IEQ = [(0.75VT/Cdyn) x (TI/TTOT)] / MIP
An IEQ >0.15 has been suggested as the
fatiguing threshold that predicts weaning
failure.





VT-tidal volume
Cdyn-dynamic compliance
TI-inspiratory time
TTOT-respiratory duty cycle
MIP-maximal inspiratory pressure

39. The CROP index
CROP (ml/breath/min) = [Cdyn * MIP * (PaO2/PAO2)] / R.
 It considers both demands on the respiratory system
and the capacity of the respiratory muscles to handle
them
 A prospective cohort study found that a CROP of 13
ml/breath/min predicted weaning success with a positive
and negative predictive value of 71 and 70 percent
Cdyn - dynamic compliance
PImax -maximal inspiratory pressure
PaO2 /PAO2 is a measure of gas exchange

40. Weaning Index (WI)
WI = PTI*(VE40/VTsb)




PTI-pressure time index
VE40-minute ventilation needed to bring PaCO2
to 40 mmHg
VTsb-tidal volume during spontaneous breathing
In a post-hoc analysis that used a threshold
of 4 min-1, the WI was highly accurate in
predicting weaning outcome

41. Integrative weaning index (IWI)


IWI = [(Cst,rs)*SaO2] / [f/VT]
An IWI ≥25 ml/cmH2O/breaths/min/liter
predicted successful weaning with a
sensitivity and specificity of 0.97 and 0.94,
respectively. The LR+ and LR- were 16 and
0.03, respectively.
The IWI was more accurate than other
weaning predictors
Integrative index of Jabour

42. USING PREDICTORS
APPROACH TO
READINESS TESTING

43. 
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.

44. 



This approach is consistent with the cardinal
precept of diagnostic testing
begin with a screening test and follow with a
confirmatory test.
Thus, a good screening test has a high
sensitivity (ie, a low false negative rate).
The RSBI fulfills these criteria, with a
sensitivity of ≥ 90 percent in some studies

45. SUMMARY AND
RECOMMENDATIONS
The goal is to identify patients who are
not ready to wean in order to avoid the
risks of failed weaning.

46. 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.


Weaning is the progressive decrease of the amount of
support that a patient receives from the mechanical
ventilator.
Weaning may involve either a period of breathing
without ventilator support (ie, a spontaneous breathing
trial [SBT]) or a gradual reduction in the amount of
ventilator support. An SBT is generally preferred, but
the gradual reduction may be better in certain
situations.

47. Screen patients daily for
readiness to wean





The cause of the respiratory failure has
improved
The patient is oxygenating adequately
The arterial pH is >7.25
The patient is able to initiate an inspiratory
effort
The patient is hemodynamically stable,
without myocardial ischemia

48. Clinical variables used to predict
weaning success




PaO2 60 mmHg on FiO2 of 0.35
Alveolar-arterial PO2 gradient of <350 mmHg
PaO2/FiO2 ratio of >200
Ventilation:


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RSBI(f/VT) <100 b/min/liter
PImax <-30 cmH2O
Minute ventilation <10 L/min
Airway occlusion pressure (P0.1) <4-6 cmH2O
CROP index >13 ml/breath/min
IWI ≥25 ml/cmH2O/breaths/min/liter
Respiratory system compliance
Work of breathing

49. Finished!!!!