Mechanical Ventilation Support in Critical Care

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 in Critical Care

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.