HOW TO WEAN AND WHEN?

A patient is considered fit for weaning from ventilator when he is conscious responsive and hemodynamically stable and when the pathology which necessitated mechanical ventilation has been resolved adequately.Improved patient outcomes and decreased costs are two benefits of implementing a protocol for early weaning from ventilator.A simplified approach to weaning off ventilator is described below.

1).Criteria to determine whether a patient can be given a trial of spontaneous breathing.
Repiratory criteria:

• PaO2 > 60 mmHg on Fio2 40-50% and peep less than 5-8 cmH2O
• PaCO2 normal or baseline
• Adequate inspiratory effort

Cardiovascular criteria:

• No evidence of myocardial ischemia
• Heart rate less than 140/mt.
• Blood pressure normal without inotrops or minimal inotropic support eg: dopamine <5 mic/kg/mt

Adequate mental status:

• GCS >13 and arousable

Absence of correctable comorbid conditions:

• Afebrile
• No significant electrolyte abnormalities

2) Criteria to determine whether patients can tolerate spontaneous breathing trial

• Tidal volume               5-7 ml/kg                  threshold    >4-6 ml/kg
• Respiratory rate          10-18beats/mt          threshold    <30/mt.
• RR/VT ratio                20-40 /LTR                threshold 100/LTR
• Max insp. pressure     -90to -120 cm H2O     threshold  -15 to -30 cm H2O

The RR/VT ratio or rapid shallow breathing index is a useul predictor. Value above 105/LT, 95% of the attempts are successfull.


3) Spontaneous breathing trials (SBT)

• Breathing through ventilator: The advantage is TV and RR can be monitored and apnea ventilation mode is utilised to detect apnea.The disadvantage is extra work of breathing offered by the circuits and valves.
• Pressure support ventilation: A positive pressure of 5-7 cm of H2O is kept to overcome the problems due to increased resistance and increased work of breathing (helps to gradually assume more more work of breathing)
• Breathing through T Piece: Known as the T piece trial,The advantage is reduced work of breathinge the disadvantage is the inability to monitor TV and RR and to provide the desired FIO2
• Airway pressure release ventilation: The patient breaths with a high continuous positive airway pressure and at a point the expiratory valve opens and pressure drops down to a lower level or baseline.It remains there for 1-2 seconds and again the original CPAP is  reapplied.It is claimed that APRV provides good gas exchange at lower mean airway pressures and thus weaning is thought to be easy.

Protocol for weaning

• Initial trial  shuold be for  30-120 mts and if tolerates higher chance for permanent weaning
• For short term ventilatory suport, ie for post op ventilation  one hour is enough
• In case of prolonged ventilatory support for eg: more than 'days'  a trial for upto 8-24 hours may be given

Failure to wean

• Dependance: The only possible way to treat ventilator dependancy is by frequent weaning trials
• Low cardiac output:  Due to high intrathoracic pressure causing reduced venous return or more negative intrathoracic pressure preventing ventricular emptying.CNS blood supply and  diaphragmatic blood supply and thereby oxygenation are affected due to low cardiac output
• Over feeding:  Excess CO2 produced due to metabolism of carbohydrates stimulate ventilation and increase the work of breathing So calorie intake should match calorie requirement.
• Neuromuscular weakness: due to hypoproteinemia or critical illness neuromyopathy may affect the intercostal muscles or diaphragm
• Metabolic: Hypomagnesemia or hypocalcemia may affect the proper functioning of respiratory muscles

Other points to note:

• Optimum hemoglobin
• Absence of arrhythmias
• PaO2/FiO2 >150
• Normal ABG  values.

References:
1). Esteban A, Alia I, Gordo F, et al. Extubation outcome after spontaneous breathing trials with T-tube or pressure support ventilation. The Spanish Lung Failure Collaborative Group. Am J Respir Crit Care Med. 1997;156(2 pt 1):459-465
2) Manual of  Intensive Care Medicine, James Ripple
3) Paul L Marino The ICU Book 3rd edition
4) Oxford Hand Book of Critical Care
5)CHEST:  120/ Number 6/ Dec 2001/suppl evidence based guidelines for weaning

http://ccforum.com/content/4/2/72   discontinuing ventilatory support