WEANING FROM MECHANICAL VENTILATION

By : dr. Tai Al Akawy

Weaning from mechanical ventilation can be defined as the process of abruptly or gradually withdrawing ventilatory support.

Mechanical ventilation can be discontinued abruptly in approximately 75% of mechanically ventilated patients whose underlying cause of respiratory failure has either improved or been resolved. The remaining patients will need progressive withdrawal from mechanical ventilation.

Weaning from mechanical ventilation usually implies two separate but closely related aspects of care, discontinuation of mechanical ventilation and removal of any artificial airway.

a direct method of assessing readiness to maintain spontaneous breathing is simply to initiate a trial of unassisted breathing.

This decision is made on the basis of the patient's mental status, airway protective mechanisms, ability to cough and character of secretions.

If the patient has an adequate sensorium with intact airway protection mechanisms, and is without excessive secretions, it is reasonable to extubate the trachea.

PREDICTIVE WEANING CRITERIA: HOW USEFUL ARE THEY?

Weaning procedures are usually started only after the underlying disease process that necessitated mechanical ventilation has significantly improved or is resolved.

The patient should also have an adequate gas exchange (most studies define this condition as an arterial oxygen tension/fractional inspired oxygen ratio higher than 200), appropriate neurological and muscular status, and stable cardiovascular function.

Weaning indices are objective criteria that are used to predict the readiness of patients to maintain spontaneous ventilation. Some parameters based on respiratory mechanics, gas exchange, and breathing pattern have been proposed as useful predictors of weaning outcome

Several studies have demonstrated that the rapid shallow breathing index (f/VT, where 'f' is the respiratory rate and 'VT' is the tidal volume measured during the first minute of a T-piece trial) is superior to conventional parameters in predicting the outcome of weaning.

Tobin and Yang have shown that the rapid shallow breathing index as reflected by frequency (breaths/minute)/VT (liters) is an accurate predictor of weaning outcome.

Using a threshold of less than 105, the frequency (f)/VT ratio had a positive predictive value of 0.78 and a negative predictive value of 0.95

The advantages of the f/VT ratio as a weaning predictor are that it is easy to measure and not dependent on patient cooperation and effort.

The f/VT ratio was evaluated while patients were breathing spontaneously through an endotracheal tube. A bedside spirometer was used to measure VT.

Simple ventilatory parameters Vital Capacity (VC) Minute ventilationOxygenation parameter PaO2/FIO2Respiratory muscle strength parameters Maximum inspiratory pressure (MIP) Maximum expiratory pressure (MEP)Central respiratory drive parameter Airway pressure developed 100 ms after the beginning of inspiration against an occluded airway (P0.1)

List of commonly used weaning parameters

Respiratory muscle reserve parameters Maximal voluntary ventilation/minute ventilation (MVV/MV) Mean transdiaphragmatic pressure per breath/maximal transdiaphragmatic pressure (Pdi/Pdimax)Pattern of spontaneous breathing parameter Respiratory rate/tidal volume (f/Vt) rapid shallow breathingIntegrated multiple variables index P0.1 X f/Vt Compliance, rate, oxygenation, and pressure

In general, patients who fail weaning trials drop their tidal volumes and increase their

respiratory rates.

Why patients are unable to sustain spontaneous breathing

Concept of Load exceeding Capacity to breathe :

Load on respiratory systemCapacity of respiratory system

Balance Load vs Capacity

Most patients fail the transition from ventilator support to sustain spont. breathing because of failure of the respiratory muscle pump

They typically have a resp muscle load that exceeds the resp neuromuscular capacity

Load on Respiratory System

Need for increase ventilation increased carbon dioxide production increased dead space ventilation increased respiratory drive Increased work of breathing

Causes of Inspiratory respiratory muscle fatigue

Nutrition and metabolic deficiencies: K, Mg, Ca, Phosphate and thyroid hormone

Corticosteroids Chronic renal failure Systemic disceases; protein synthesis, degradation, glycogen stores Hypoxemia and hypercapnia

Capacity of respiratory system

Central drive to breathe Transmission of CNS signal via Phrenic

nerve Impairment of resp muscles to generate

effective pressure gradients Impairment of normal muscle force

generation

IS THE PATIENT ABLE TO SUSTAIN SPONTANEOUS BREATHING?

Once a patient has been considered ready to be weaned, the best method to assess whether the patient is able to breathe on his own is to perform a trial of spontaneous ventilation.

Immediate extubation after successful trials of spontaneous breathing , expedites weaning and reduces the duration of mechanical ventilation as compared with a more gradual discontinuation of ventilatory support.

Pressure-support, continuous positive airway pressure and T-piece trials are the most common methods used to test the readiness for liberation from mechanical ventilation.

The duration of a spontaneous breathing trial has been set at 2 h in most studies

Precise criteria for terminating a weaning trial do not exist, and currently trials are terminated on the basis of the clinical judgement of the physician.

There are two types of criteria used to determine whether a patient passes or fails a spontaneous breathing trial: objective criteria (abnormal arterial blood gas measurements) and subjective criteria (diaphoresis, evidence of increasing effort, tachycardia, agitation, anxiety).

Once a patient is able to sustain spontaneous breathing, a second judgement must be made regarding whether the artificial airway can be removed by assessing the patient's mental status, airway protective mechanisms, ability to cough and character of secretions.

When the patient remains clinically stable with no signs of poor tolerance until the end of the trial, the endotracheal tube should be immediately removed.

If the patient develops signs of poor tolerance, weaning is considered to have failed and mechanical ventilation is reinstituted.

WHAT ABOUT PATIENTS FAILING THE FIRST ATTEMPT AT WEANING?

Weaning attempts that are unsuccessful usually indicate incomplete resolution of the illness that precipitated the need for mechanical ventilation, or the development of new problems.

Failure to wean has been attributed to an imbalance between the load faced by the respiratory muscles and their neuromuscular competence.

Therefore, once a patient fails a spontaneous breathing trial, the clinician must comprehensively evaluate the patient, looking for ways to improve his or her physiologic status.

WEANING MODES OF MECHANICAL VENTILATIONS IS THERE A PREFERRED TECHNIQUE?

Weaning from MV has been described as either a gradual decrease of ventilator support to allow liberation from the ventilator or

determining when the patients will have the ability to be separated from the ventilator

safely.

There is a significant heterogeneity in the population on MV. The weaning of a short-term ventilated patient who is in the ICU for a drug overdose differs markedly from weaning a long-term ventilated patient with COPD, pneumonia, or ARDS.

Nevertheless, the commonly used techniques of weaning are T-piece, synchronized intermittent mandatory ventilation (SIMV), or PSV.

It generally is accepted that SIMV weaning prolongs the duration of MV.

Daily T-piece trials consistently have been superior to the SIMV mode in weaning, and at least equivalent to PSV weaning.

PSV provides a progressive unloading of inspiratory muscles compared with SIMV.

Noninvasive positive pressure ventilation (NPPV) also has been used as a method to support ventilation following early extubation

this technique ,compared with standard oxygen therapy, averted respiratory failure after extubation and decreased ICU mortality among patients at increased risk.

With recent advances in technology, new features on ventilators like automatic tube compensation (ATC) have been developed

ATC compensates for the pressure drop across the endotracheal or tracheostomy tube by delivering exactly the amount of pressure necessary to overcome the resistive load imposed by the tube for the flow measured at the time

ATC has been shown to decrease the work of breathing necessary to overcome endotracheal tube resistance more effectively than PSV or CPAP

It is possible, however, that ATC could allow more marginal patients to tolerate a breathing trial, who then would develop ventilatory failure after extubation

PROTOCOLIZED WEANING FROM MECHANICAL VENTILATION

The initial step in any protocol-driven ventilator weaning is daily screening for readiness to wean using several weaning parameters.

To do so, every appropriate patient in the ICU also should undergo a daily interruption of sedation to be in optimal neurological condition for the screening

Daily Screening Resolution/improvement of patient’s

underlying problem Adequate gas exchange (SaO2 > 90%,

PaO2/FiO2 >200) Respiratory rate < 35/ min Absence of fever, temperature < 38C Adequate haemoglobin concentration, >

8-10 g/dl Stable cardiovascular function: heart

rate < 140/min, 180>SBP>90

Daily Screening (cont.(

Indices suggesting an adequate capacity of the ventilatory pump: respiratory rate of less than 30/ min, Maximum inspiratory pressure < -20 to -30 cmH2O

Correction of metabolic and electrolyte disorders

Normal state of consciousness

Candidates for a weaning protocol are patients who have adequate oxygenation (PaO2 >60 mm Hg with FIO2 ≤0.5 and PEEP ≤8 cm H2O).

The screening for readiness may include calculation of the rapid shallow breathing index (RSBI), which is the frequency to tidal volume ratio(f/Vt) measured after 1 minute of spontaneous breathing.

In general, patients who fail weaning trials drop their VTs and increase their respiratory rate.

Yang and Tobin have shown that the RSBI is an accurate predictor of weaning outcome. Using a threshold of less than 105, the f/VT ratio had a positive predictive value of 0.78 and a negative predictive value of 0.95.

After manifesting adequate coughing during suctioning to ensure intact airway reflexes, those patients passing the rapid shallow breathing trial will be subject to spontaneous breathing trials using PS, CPAP, or T-piece for up to 120 minutes.

The spontaneous breathing trial (SBT) is terminated if The patient successfully tolerates SBT from 30

minutes to 2 hours or if starts showing signs and symptoms of failing :

(respiratory rate > 35 for >5 minutes, SaO2 <90% for >30 seconds, 20% increase or decrease in heart rate for >5 minutes, systolic blood pressure SBP >180 or SBP <90 for 1 minute with repeated measurements, agitation, anxiety, or diaphoresis as a change from the baseline).

In general, these protocols are driven by the unit respiratory care practitioners and/or nurses.

The next improvement of protocolized weaning may be computer-driven.

The computerized protocol used in a recent trial included an automatic gradual reduction in pressure support, automatic performance of SBTs, and the generation of an incentive message when an SBT was passed successfully

Summary

MV is a life-sustaining therapy fraught with side effects. The successful removal of MV at any time is associated with a higher survival rate. Therefore, removing the patient from the ventilator as soon as possible is in the patient’s best interest.

The best approach to weaning patients from MV involves a team approach of all caregivers (physician, nurses, respiratory therapist, physical therapist, and nutritionists).

The team uses a weaning protocol to start a daily screening of ventilated patients.

If patients meet certain criteria, a trial of spontaneous breathing (positive pressure flow mode or T-piece) is undertaken.

If patients pass the trial, they are extubated. Patients who do not pass the SBT will be reassessed to identify and treat any reversible factors and undergo daily SBTs if they continue to meet the criteria.

The daily screening of patients who are on mechanical ventilation with the aim of identifying those able to breathe spontaneously is, possibly, the best approach to reduce the duration of ventilatory support.

Standard weaning criteria were used to identify patients who were able to resume spontaneous breathing, and patients who did not meet such criteria remained on mechanical ventilation.

The ability to breathe spontaneously is adequately tested by performing a trial with either T-tube or pressure support of 7 cmH2O. A duration of 2 h has been extensively evaluated, but weaning outcome is the same when the duration is reduced to 30 min.

Patients failing the initial spontaneous breathing trial need a gradual withdrawal of ventilatory support.

It is known that SIMV is the most ineffective method of weaning those patients.

With respect to the use of pressure support or T-tube, clinicians should choose the method they feel most comfortable with and individualize the strategy to meet the patient's needs.

References

1-Chao DC, Scheinhorn DJ. Weaning from mechanical ventilation. Crit Care Clin 1998;14:799–817.

2-Lellouche F, Mancebo J, Jolliet P, et al. A multicenter randomized trial of computer-driven protocolized weaning from mechanical ventilation. Am J Respir Crit Care Med 2006;174:894-900.

3-Manthous CA, Schmidt GA, Hall JB. Liberation from mechanical ventilation. A decade of Progress. Chest 1998;114:886–901.

4-Epstein SK, Ciubotaru RL. Independent effects of etiology of failure and time to reintubation on outcome for patients failing extubation. Am J Respir Crit Care Med 1998;158:489-93.

5-Esteban A, Frutos F, Tobin MJ, e t a l.: A comparison of four methods of weaning patients from mechanical ventilation.

N Eng l J Me d 1995, 332:345-350.6-Brochard L, Rauss A, Benito S, e t a l.: Comparison of

three methods of gradual withdrawal from ventilatory support during weaning from mechanical ventilation. Am J Re s p ir Crit Ca re Me d 1994, 150:896-903.

7-Esteban A, Alía I, Gordo F, e t a l.: Extubation outcome after spontaneous breathing trials with T-tube or pressure support ventilation. Am J Re s p ir Crit Ca re Me d 1997, 156:459-465.

THANK YOU

Dr. Tai Al Akawy