Predictors of weaning outcome

Muhammad Asim Rana MBBS, MRCP, SF-CCM, FCCP, EDICDepartment of Critical Care Medicine

King Saud Medical CityRiyadh, Saudi Arabia

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) Assessment to identify patients who may be ready to wean using various predictors of weaning outcome.

2) Weaning is then initiated in those patients.

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

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.

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.

CLINICAL CRITERIA  The criteria can be segregated into

1)Required criteria  2)Optional criteria 

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.

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

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.

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.

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

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.

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.

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.

Patient should be checked for Dyspnea changes of mental status, blood pressure, heart rate, cardiac rhythm, respiratory rate Cyanosis

WHEANS NOT & WEANS NOW

wheezes heart disease electrolytes anxiety neuromuscular disease sepsis nutrition opiates thyroid disease

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.

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.

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

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.

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

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.

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.

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)

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

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.

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

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.

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

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.

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

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.

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

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 compliancePImax -maximal inspiratory pressurePaO2 /PAO2 is a measure of gas exchange

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

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

USING PREDICTORS 

APPROACH TO READINESS TESTING

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 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

SUMMARY AND RECOMMENDATIONS

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

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.

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

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:

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

Finished!!!!Do not try to ask

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