Mechanical Ventilatory Support

• Ventilators are special pumps to support ventilatory function of the respiratory system and improve oxygenation by providing high oxygen and positive pressure.

Indications

• Hypoxaemic respiratory failure   :

  • arterial oxygen saturation (Sa0₂) less than 90% inspite of inspired oxygen fraction FI0₂ > 0.6.
  • Causes
  • Pneumonia
  • Pulmonary edema
  • ARDS,
• 
  

   Hypercarbic respiratory failure   :

   

  • arterial PC0₂ >50 mmHg qnd arterial pH <7.30
  • Causes
  • Neuromuscular diseases like myasthenia
  • gravis, myopathies
  • Asthma
  • COPD
  • Restrictive lung diseases
  •  In acute hypercarbic respiratory failure mechanical ventilation must be instituted in all cases.
  • In chronic hypercarbic respiratory failure
  • mechanical ventilator su port mayor may not be given.
  • Mechanical ventilation is also given in cases of raised intracranial pressure, congestive heart failure, myocardial infarction, unconscious patients, post-operative conditions.

Mechanical Ventilation Support in Critical Care

How it works?

• Mechanical ventilators provide humidified gas through an airway opening of a specific volume, pressure, and time-pattern.
• Ventilators serve as energy source to replace the effort of the respiratory muscles.
• Usually inspiration is taken over by the ventilator by providing a positive pressure but expiration is passive and occurs without any effort.
• A positive end-expiratory pressure (PEEP) is delivered which helps to maintain the patency of alveoli and airways and reverses hypoxaemia and atelectasis.
• PEEP levels are between 0-10 cm water.
• A cuffed endotracheal tube is inserted to deliver oxygen gas to the lungs .
• Neuromuscular paralysis may be required and it is usually achieved by succinyl choline, morphine, fentanyl, ketamine or recently by propofo!.
• If ventilator therapy is required for more than 3 weeks. tracheostomy is done to reduce laryngeal injury.

Ventilator Modes

•  Mode means the manner in which ventilator breaths are triggered, cycled, and limited.
•  Trigger is the inspiratory effort or a timed signal.
•  Cycle is the point where inspiration ends by volume,
• pressure or time or flow.
•  Limiting factors are airway pressure or inspiratory flow.

Modes of ventilation

• Assist control mode ventilation (ACMV)
  • An inspiratory cycle is triggered by patient’s inspiratory effort or a timed signal. It is used for initiation of mechanical ventilation.
  • It is also used for weaning patients from mechanical ventilation.

Synchronized intermittent mandatory ventilation (SIMV)

• The patient is allowed to breathe spontaneously without ventilator assistance, with ventilator breaths in between. Mandatory breaths are in synchrony with patients efforts at a fixed frequency.
• If the patient fails to breathe spontaneously the ventilator delivers breaths till the patient breathes spontaneously.
• SIMV allows patients with spontaneous respiration to breathe, supporting the breaths, and helps to wean intubated patients.

 Continuous positive

• airway pressure (CPAP) The ventilator provides fresh gas, gives each inspiration and provides pressure from 0-20 cm water.
• It is useful for patients with intact respiratory function with endotracheal tube for airway protection.

Pressure control ventilation (PCV)

• During inspiration, a given pressure is use to drive the gas in. It is used in patients with severe hypoxaemic respiratory failure.

Pressure support ventilation (PSV)

• This form is patient-triggered, flow-cycled, so that patient receives ventilator assistance only when there is an inspiratory effort by the patient.

 Non-invasive ventilation (NIV)

• This is given without endotracheal intubation through a tight-fitting face mask.
• Commonly used in sleep apnea patients, and as a primary ventilator support in patients with impending respiratory failure.
• It is also called bipap or bi-Ievel positive airway pressure ventilation.
• It is well tolerated by the conscious patient and the most commonly used method of ventilation in reus today.