IPPV - assessment of weaning

Prior to weaning, it is important that the cause of respiratory failure and any complications arising have been corrected.

• Sepsis should be eradicated as should factors that increase O2 demand.

• Attention is required to nutrition, fluid and electrolyte balance.

• The diaphragm should be allowed to contract unhindered by choosing the optimum position for breathing (sitting up unless the diaphragm is paralysed) and ensuring that intra-abdominal pressure is not high. Adequate analgesia must be provided.

• Sedatives are often withdrawn by this point, but may still be needed in specific situations, e.g. residual agitation, raised intracranial pressure.

Weaning should start after adequate explanation has been given to the patient. Factors predicting weaning success are detailed in the table opposite. Spontaneous (pressure-supported) breathing should generally start as soon as possible to allow a reduction in sedation levels and to maintain respiratory muscle function. Weaning with the intention of removing mechanical support is unlikely to be successful while FIO2 >0.4.

Continuous pulse oximetry and regular clinical review are essential during weaning. Arterial blood gases should be taken after 20–30min of spontaneous breathing. After short-term ventilation, extubate if arterial gases and respiratory pattern remain satisfactory, the cough refl ex is adequate, and the patient can clear sputum. Patients being weaned from longer-term ventilation (>1wk) should generally be allowed to breathe spontaneously for at least 24h before extubation.

Indications for re-ventilation

If spontaneous respiration is discoordinate or the patient is exhausted, agitated, or clammy, the ventilator should be reconnected. Successful weaning is more easily accomplished if excessive fatigue is not allowed to set in. Tachypnoea (>30/min), tachycardia (>110/min), respiratory acidosis (pH <7.2), rising PaCO2, and hypoxaemia (SaO2 <90%) should all prompt reconnection of the ventilator.

Factors associated with weaning failure

Failure to wean is associated with:

• Increased O2 cost of breathing.

• Muscle fatigue (malnutrition, peripheral neuropathy or myopathy, drugs, (e.g. muscle relaxants, aminoglycosides), and electrolyte abnormalities (e.g. low Mg 2+, K+, and perhaps phosphate).

• Inadequate respiratory drive (alkalosis, opiates, sedatives, malnutrition, cerebrovascular accident, coma).

• Inadequate cardiac reserve and heart failure.

In the latter case, monitor cardiac function during spontaneous breathing periods. Any deterioration in cardiac function should be treated aggressively (e.g. optimal fluid therapy, vasodilators, inotropes). Early muscle fatigue due to prolonged disuse requires regular and controlled exercise after correction of nutritional deficit. Weaning should be stopped during periods when exercise is provided. Patients who are building up muscle on an exercise programme may also benefit from rest periods at night so that the time free of mechanical ventilator support is gradually increased.

Factors predicting weaning success

• PaO2 >11kPa on FIO2 = 0.4 (PaO2/FIO2 ratio >27.5kPa).

• Minute volume <12L/min.

• Vital capacity >10mL/kg.

• Maximum inspiratory force (PImax) >20cmH2O.

• Respiratory rate/tidal volume <100.

• Qs/Qt <15%.

• Dead space/tidal volume <60%.

• Haemodynamic stability.

• A ratio of respiratory rate to tidal volume (f/VT, shallow breathing index) ≤105 has a 78% positive predictive value for successful weaning.