Continued...

VENTILATION STRATEGIES:

3. SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION - SIMV
SIMV combines a pre-set number of ventilator-delivered mandatory breaths of predetermined tidal volume with the facility for intermittent patient-generated spontaneous breaths. The ventilators on the market offer the possibility of delivering pressure-or volume-targeted breaths during mandatory cycles. Child with SIMV can activate mandatory breaths but if child trigger is not efficacious, ventilator can deliver a mandatory breath. Pressure or Volume Support can be applied during non-mandatory support.

Advantages:

• possibility to maintain spontaneous breathing and avoid muscle atrophy, especially in neuromuscular   patients who rapidly lose ability to    breath spontaneously
  
  
• progressive reduction of controlled breaths can favor return to complete spontaneous breathing.

The method is difficult to apply in cases of acute and severe lung pathology, in compromised clinical status and in cases of neurological disorders than can interfere with ventilation drive. In these cases, children could adapt completely to control breathing furnished from ventilator that could be insufficient to maintain adequate gas exchange (26-29).

4. CONTROLLED MECHANICAL VENTILATION - CMV
This mode of ventilation controls the patient's respiratory activity completely. Introduction of gases into the lung, inspiration, is obtained using positive pressure, which pushes gases into the lung (30). This method is completely different from spontaneous breathing in which introduction of gases into the lung occurs as a result of negative pressure inside the lung. As a consequence, venous return and cardiac output can be reduced.

The expiratory phase is passive and is regulated by the opening of expiratory valve at end of inspiration. This valve is opened when a prefixed pressure or tidal volume is achieved in the lung.

4.1. Constant Pressure Generators (Pressure Controlled Ventilators): The pressure ventilator delivers a volume of gas at constant pressure to the lung during set inspiratory time. Volume received by patient is determined by set inspiratory pressure, respiratory rate and inspiratory time. Pressure remains constant during inspiration, while flow decelerates.

Advantages:

• flow rate is geared to reach Peak Inspiratory Pressure (PIP) as quickly as possible and flow will exceed patient's demand, improving    patient ventilator synchrony and decreasing work of breathing;
  
  
• using decelerating flow pattern and square-wave pressure pattern; distribution of gas within the lung can be improved.
  

Disadvantages:-

• pressure remains constant while tidal volume will vary, according to changes in compliance and airway resistance
  
  
• any thorax and diaphragmatic compression reduces tidal volume, which can be insufficient to maintain adequate gas exchange.
  

This mode of ventilation has been proposed to protect the lung from barotrauma but instability of tidal volume and PEEP can create inhomogeneity of lung. Hypoventilation or hyperventilation may occur: e.g. in infants with RDS, with low compliance and normal airway resistance, pressure ventilation may lead to insufficient tidal volume and hypoventilation.

4.2. Constant Flow Generators (Preset Volume Controlled Ventilators): The generator delivers the same preset tidal volume with each breath. During inspiratory time pressure will be slowly increased while flow pattern produces a square wave.

Advantages of volume controlled ventilation:

• supply of constant minute volume
  
  
• maintenance of preset tidal volume with variation of compliance and/or pulmonary resistance.

Disadvantages:

• atelectatic areas are not re-ventilated, as they require higher pressure in order to be re-ventilated
  
  
• opened
  - less damaged areas tend to be hyperventilated
  
  
• using uncuffed tubes, a large part of preset tidal volume can be lost leading to inadequate ventilation.
  

This ventilatory mode, using small tidal volume and high respiratory frequency to maintain stable minute volume, has been demonstrated to be effective in reducing Ventilation Associated Lung Injury (VALI), morbidity and mortality in ARDS. The best results have been obtained when PEEP levels have been maintained over inferior flex point and lung expansion under superior inflection point of volume/pressure curve (31-33).

4.3 Pressure Regulated Volume Controlled Ventilation -PRVC: PRVC ventilation delivers controlled tidal and minute volume in a pressure-limited manner using lowest possible pressure, kept constant during inspiratory phase. Gas flow is decelerated and pressure and flow constantly vary, breath by breath, in order to achieve pre-set tidal volume at minimum peak inspiratory pressure. The ventilator tests the first breath at 5 cm H2O above PEEP and calculates pressure-volume ratio. Inspiratory pressure changes breath by breath until preset tidal volume is reached at a maximum of 5cm H2O below set upper pressure limit. At this stage, measured tidal volume corresponds to preset value and pressure remains constant. If measured tidal volume increases above preset level, inspiratory pressure is reduced until set tidal volume is reached (34-37).

Advantages of PRVC ventilation

• improvement of respiratory mechanics and gas exchange
  
  
• reduction of barotrauma connected with Peak Inspiratory Pressure (PIP)
  
  
• reduction of oxygen toxicity due to possibility of using reduced FiO2 to maintain adequate gas exchange as compared with
  conventional mechanical ventilation
  
  
• opening of closed areas of lung connected with use of decelerated and laminar flow
  
  
• immediate reduction of PIP in presence of a rapid change of compliance and resistance as   surfactant, bronchodilators, nitric oxide,
  etc. are used.

Indications:

• when lung compliance and resistance vary rapidly
  
  
• If there is an initial requirement of high flow in order to re-open closed pulmonary areas (e.g.   atelectasis, etc.)
  
  
• to reduce high ventilatory peak pressure (e.g. in premature infants, interstitial emphysema, etc.)
  
  
• to control ventilatory pressures from the moment non-ventilated alveoli and bronchioles are re-  opened (e.g. surfactant, theophylline or nitric oxide administration, etc.)
  
  
• in presence of broncho- and bronchiole-spasms (e.g. asthma, bronchiolitis, etc.)
  
  
• in all patients in whom PEEP levels must be reduced in order to avoid hemodynamic complications.
  
  

Clinical controlled trials are required to evaluate real benefits of PRVC ventilation in acute phase of lung pathology (need of peak pressure to reopen non ventilating areas), in ventilation of healthy lungs (i.e., neurosurgical patients) and during weaning from ventilator.