Tuesday, September 1, 2009

Assessment for extubation

Despite advances in mechanical ventilation and respiratory support, the science of determining if the patient is ready for extubation is still very imprecise. A lot depends on the clinician’s threshold for reduction in ventilatory support, than does the modes of ventilatory wean.

Patients who can be extubated safely can be divided into three phases-
1. Freedom from the primary problem that prompted mechanical ventilation
2. Identifying whether the patient will sustain spontaneous respiration and ventilation and also when the patient is failing the assessment
3. What to look for immediately before extubation.

These assessments are multifaceted and usually include the overall patient condition, hemodynamic stability, neurological and muscular status and adequacy of gas exchange.

Commonly used clinical parameters that predict successful weaning from
mechanical ventilation.

Parameter with Desired value
Respiratory rate Less than 30-38 breaths/minute
Tidal volume 4-6 mL/kg
Minute ventilation 10-15 L/minute
Negative inspiratory force -20 to –30 cm H2O
Maximal inspiratory pressure (MIP) -15 to –30 cm H2O
Mouth occlusion pressure 100 msec after the
onset of inspiratory effort (P0.1) divided by MIP 0.3
Rapid shallow breathing index (RSBI)
(respiratory rate divided by tidal volume) 60-105
Rapid shallow breathing index rate
[(RSBI2 – RSBI1)/RSBI1] x 100 Less than 20%
CROP score (an index including compliance,
rate, oxygenation and pressure) 13
PaO2/FiO2 ratio >150-200
Despite high sensitivity, however, these parameters lack specificity.

All the patients who are deemed fit for extubation should undergo some means of assessment to see if they will tolerate extubation. It is routine to perform Spontaneous breathing trial (SBT), by means of – CPAP of 5 cm of H2O, T –Piece trial, or Pressure support of 7 cm of H2O. Once the patient is on SBT we should watch for indicators of failure, which are –

1. Inadequate gas exchange
Arterial oxygenation saturation (SaO2) <85% - 90% PaO2 <50 – 60 mmHg pH < 7.32 Increase in PaCO2 >10 mmHg

2. Unstable ventilatory/respiratory pattern
Respiratory rate >30 – 35 breaths/minute
Respiratory rate change over 50%

3. Hemodynamic instability
Heart rate >120 – 140 beats/minute
Heart rate change greater than 20%
Systolic blood pressure >180 mmHg or <90 mmHg Blood pressure change greater than 20% Vasopressors required 4. Change in mental status
Agitation
Anxiety
Somnolence
Coma

5. Signs of increased work of breathing
Nasal flaring
Paradoxical breathing movements
Use of accessory respiratory muscles

6. Onset of worsening discomfort ± diaphoresis

Another parameter that is widely used is the Rapid shallow breathing index (RSBI), from which we can calculate the RSBI rate which is the measure of change of RSBI over time, and may offer more predictive value .A RSBI rate of less than 20% is 90 % sensitive and 100% specific for predicting weaning success .It has a positive predictive value of 100% and negative predictive value of over 81 %.

Finally, once it is confirmed that patient can sustain spontaneous breathing , we can extubate the patient provided—
1 .Presence of a patent airway –assessed with “Cuff leak test”( dividing expiratory volume by inspiratory volume and multiplying with 100)
2. Patients’ ability to consistently protect the airway and clear secretions –assessed by the presence of adequate cough and gag reflex.
3. Mental status compatible with maintenance of airway and secretion clearance.
4. Absence of any other reasons for potential post-extubation failure—
- severe pain
- presence of apnea
- poorly controlled seizures
- risk of massive upper GI bleeding.

Thanks to Dr Harjit Mahay for contributing this wonderful article.