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VENTILATION STRATEGIES :

2. SUPPORTED SPONTANEOUS BREATHING
Assisted modes of ventilation are those in which part of the breathing pattern is contributed or initiated by the child. The work of breathing performed by the child is never abolished.

2.1 Pressure Support Ventilation (PSV) : Pressure support ventilation (PSV) is designed to support spontaneous breaths during inspiratory phase. It is primarily designed to assist spontaneous breathing and therefore the patient should have an intact respiratory drive (20-22).

The patient triggers each breath by opening the demand valve of the ventilator. A supplementary gas flow is delivered to the inspiratory circuit to produce positive inspiratory pressure at a pre-set value. Cycles are pressure limited and there is no pre-set tidal volume. The patient triggers assisted breathing and regulates respiratory rate, inspiratory and expiratory time and tidal volume.

Advantages:

• minimizes work of breathing
  
  
• reduces respiratory muscle fatigue and oxygen consumption
  
  
• hemodynamic stability favored as breathing is triggered spontaneously
  
  
• can be used to compensate for extra work produced by endotracheal tube and demand valve (23).

Disadvantages

Tidal volume is uncontrolled and variable and depends on respiratory mechanics, cycling frequency and synchrony between patient and ventilator.

• If pressure support is high, the patient tends to reduce his respiratory rate and tidal volume. The risk of baro- and volutrauma is increased and ventilated gases may not be adequately warmed and humidified.
  
  
• If pressure support is low, patient tends to increase respiratory frequency and reduce tidal volume. In such case oxygen consumption and work of breathing are increased.
  
  
• In cases of inhomogeneous lung pathology, PSV tends to favor ventilation of better-aerated areas without affecting collapsed or atelectatic areas.
  

PSV needs continuous and careful adaptation of respiratory support to avoid the aforementioned undesirable effects (increased support from health workers).

Indications:

- Intensive care

• Weaning from ventilation after improvement in lung pathology
  
  
• Weaning from long term ventilation
  
  
• Weaning of patients with chronic obstructive pulmonary disease e.g. infants with severe BPD
  
  
• To promote respiratory muscle training
  
  
• To compensate for high resistance of endotracheal tubes during CPAP.
  
  

- Postoperative care

1. To preserve or reactivate spontaneous breathing
2. To resolve atelectasis after surgery.

Contraindications:

1. Deep sedation and muscle paralysis
2. Severe neurological disorders
3. Hypoventilation syndromes
4. Patients who may be unable to activate trigger demand valve.

2.2 Volume Support Ventilation (VSV) : VSV is a new means of assisting spontaneous breathing which avoids the disadvantages deriving from pressure support ventilation which needs frequent adaptation by medical staff. The Ventilator, breath by breath, adapts inspiratory pressure support to changes in the mechanical properties of the lung and the thorax in order to ensure that the lowest possible pressure is used to deliver pre-set tidal and minute volume that remain constant. Inspiratory pressure is constant and flow is decelerated (4, 24, 25).

When the patient is able to ventilate pre-set tidal volume, the ventilator does not support the breath. At this stage, extubation may be performed with safety. In cases of apnea the ventilator automatically switches to PRVC. The initial values for expected tidal and minute volume should be set as should all parameters to be used in PRVC in the presence of apnea ventilation. Indications and contraindications are similar to PSV. The main advantages of VSV vs. PSV is the possibility of maintaining stable tidal volume, being protected should apnea occur and being able to recognize when the patient no longer requires pressure support to ventilate pre-set tidal volumes (extubation can safely be performed).