VENTILATOR WEANING

VENTILATOR WEANING – Respiratory Parameters (Huba), Signs of Fatigue, Methods of Fatigue, Methods of Weaning, Causes of Failure to Wean and Patient Who is Fighting the Ventilator

Weaning is the process by which the patient is gradually allowed to assume responsibility for regulating ad performing his own ventilation. There are many factors which should be considered before attempting to wean the patient

• The initial pulmonary pathology that indicated intubation and mechanical ventilation should be resolving
• Cardiovascular function should be stable with minimal or no need for vasopressors
• Fluid balance is maintained
• Nutritional state maintained without any nitrogen imbalance

Signs of intolerance to spontaneous breathing (weaning failure)

PaO₁ <50-60 mm Hg and FiO₁ >0.5

SaO₁ <88-90% and FiO₁ >0.5

PaCO₁ >50 mm Hg or increased by more than 8 mm Hg

pH <7.32 or reduced by more than 0.07

RR >35 breaths/min or increased by more than 50%

HR >140 bpm or increased by more than 20%

SBP >180 mm Hg or <90 mm Hg

Uncontrollable psychomotor agitation

Reduced level of consciousness

Excessive sweating and cyanosis

Evidence of increased respiratory muscle effort

RESPIRATORY PARAMETERS AS CRITERIA FOR WEANING (Huba)

• Ability to oxygenate: the arterial tension during mechanical ventilation should be greater than 70 mm of Hg with a FiO₂ of 40%. The above condition should exist with a level of PEEP that is less that 10 cm H₂O. When PEEP is greater than 10 cm H₂O weaning is contraindicated. Arterial blood gas PaO₂ >55 mm Hg with FiO₂ <0.4 and CPAP <5 cm H₂O Oximetry – Hb. Saturation >90% corresponds to PaO₂ >55mm of Hg
• Spontaneous resting ventilatory needs: ventilation is a given amount of gas exchange required to eliminate carbon dioxide adequately

the necessary level of minute ventilation (VT multiply RR) is determined by CO₂ production and dead space ventilation. The dead space to tidal volume ratio defines the percentage of tidal volume that does not participate in CO₂ elimination and can be considered as wasted ventilation. This dead space ratio is increased in disease that affect the lung parenchyma and distribution of gas flow as in adult respiratory distress syndrome, pulmonary embolism, chronic obstructive pulmonary disease and hypovolemia. If VD/VT is 0.6, i.e. 60% of tidal volume or more, the minute volume required to eliminate CO₂ is sufficient to allow total wearning. Minute ventilation of 10 liters/minute or less during mechanical ventilation indicates that patient can be safely weaned

• Respiratory mechanical capability to sustain spontaneous respiration, the patient’s mechanical function is assessed by measuring the vital capacity, inspiratory force and the spontaneous rate. The vital capacity should be 10-15 ml/kg of actual body weight to institute weaning. Inspiratory force (the amount of negative pressure generated against occluded airway) is the measurement of muscle strength. The respiratory rate should not exceed 35 breaths/minute as it results in fatigue, CO₂ retention and respiratory acidosis

SIGNS OF FATIGUE

• Tachypnea – respiratory rate >20/min
• Hemodynamic – tachycardia, hypotension
• Minute ventilation <6 L/min or >10 L/min
• General appearance – restlessness, sweating, cyanosis
• Inability to generate inspiratory force
• Arterial blood gas – PaO₂ <55 – 70 mm not maintained (7.35 – 7.45)

At each step of weaning, these parameters are assessed

Psychological readiness: psychologically patient must be prepared to wean. The patient’s anxiety is from fear of ventilator malfunction, suffocation and loss of control. To overcome these nurses should explain the safety mechanism like alarm and keep the Ambu bag in patient’s vicinity and give verbal reassurance. Another problem, they face is the inability to communicate. Alternate method of communication available should be utilized

An important aspect of successful weaning is the patients trust in the staff responsible for care. A team effort by both medical and nursing staff can pave away for gaining patients confidence. Added with related explanations this helps in achieving a successful weaning

METHODS OF WEANING

There are two methods of weaning.

1. The conventional method is the episodic ventilator with T piece or CPAP. Briggs T piece technique is used. The patient is disconnected from ventilator for a specific period of time and allowed to breathe spontaneously using the Briggs T piece or CPAP. Weaning starts with shorter intervals such as 5-10 minutes every hour or more. The patient requires rest period. Weaning should not be attempted during night until patient can maintain spontaneous breathing. The vital capacity and the inspiratory force along with vital signs and signs of fatigue are monitored closely during the weaning period
2. Intermittent mandatory ventilation – weaning: intermittent mandatory ventilation is a technique by which patients can breathe spontaneously and in addition receive mechanically ventilated breaths at specific preselected rates. Set rate, interval and keep sensitivity at maximum setting. If tolerance is not indicated disease mandatory rate. Record at each weaning interval heart rate, blood pressure and respiratory rate and arterial blood gas and pulse oxygenation while IMV is used. The spontaneous rate should not exceed 30 breaths/minutes as these results in fatigue, CO₂ retention and respiratory acidosis. Rates greater than 30 indicate a need to reduce weaning time. Weaning can continue as long as patient’s condition is stable and arterial pH is 7.32 – 7.35

Patient is positioned in sitting or Fowler’s position during weaning. All respiratory and other parameters are monitored. Patient is supported emotionally during weaning process.

CAUSES OF FAILURE TO WEAN

• Patient factors: inadequate spontaneous breathing, intrinsic pulmonary disease resulting in atelectasis, consolidation, edema, bronchospasm which can be managed with PEEP and chest physical therapy and bronchodilators

Another factor is derangement of chest wall function which include chest wall trauma. Abdominal distension muscle weakness

Abnormal cardiac functioning can limit weaning

Starvation, protein loss cause break down of muscle mass resulting in decreased respiratory muscle function which may affect weaning process

• Ventilatory system factors: ventilatory design and PEEP devices are a major source of weaning problems, meticulous attention should be paid to be appropriate setting of flow and sensitivity when IMV is used. Continuous positive pressure should be produced with a system that provides a minimum of external work for patient
• Airway factors: the artificial airway also may produce weaning problems. It is noticed that endotracheal tube of small inter-diameter requires increased patient effort during spontaneous ventilation obstruction of tube can be a cause of sudden and marked change in weaning ability

PATIENT WHO IS FIGHTING THE VENTILATOR

The major causes of respiratory compensation in a ventilated patient are pneumothorax endotracheal tube slipping into the main bronchus, mucus or hemorrhage plug obstructing endotracheal tube, ventilator and aspirations systemic abnormality such as sepsis, pulmonary embolus or congestive heart failure. In the patient is in severe distress, he should be disconnected from ventilator and manually ventilated with 100% oxygen. Patient is closely observed, suctioning done to rule out obstruction. A chest X-ray and arterial blood  gas taken and ventilator checked to ensure adequate functioning

Once it is ensured that there is no serious life-threatening problem then fighting the ventilator can be managed. It means that the patient is out of phase with the machine. The patient will be exhaling while the machine is delivering the breath. Intermittent mandatory ventilation or synchronized intermittent mandatory ventilation is beneficial in handling patients who are fighting the ventilator. In some cases, patient has to be paralyzed with pancuronium to be ventilated effectively