Mechanical Ventilation

Mechanical ventilation is similar to providing manual intermittent positive pressure ven- tilation (ippV) except the breathing is controlled by a ventilator. the use of a ventilator instead of manual ippV under anaesthesia means the nurse is free to monitor the patient more effectively, however a good understanding of the ventilator being used is necessary. Ventilators used during anaesthesia are different to ventilators used in iCu as they are designed for healthy lungs and are relatively basic and simple to use. the role of mechanical ventilation during anaesthesia is normally to correct hypoventilation and maintain normal arterial carbon dioxide tension. Most anaesthetic agents cause dose dependent hypoventilation which is often mild enough to not require ippV however, ippV is necessary in many situations, including: the use of neuromuscular blocking agents, thoracotomies, thoracoscopy or chest wall surgery/trauma, the use of respiratory depressant drugs such as potent opioids, intracranial disease, cervical spinal cord injury or disease involving the phrenic nerve, myasthenia gravis and other neuromuscular conditions causing an inability to ventilate.

Definitions
Tidal volume is the volume of air that moves in or out of the lungs during one breath. in small animals tidal volume is about 10-15 mL/kg.

Minute volume is the amount of air breathed in or out in one minute and is therefore the respiratory rate multiplied by the tidal volume.

Compliance is a measure of the elasticity of the lungs. respiratory disease can alter lung compliance. Decreased compliance is seen in patients with pulmonary fibrosis when the lungs become ‘stiff’ and increased compliance is seen in patients with emphysema.

Inspiratory:expiratory ratio is the ratio between the inspiratory and expiratory phases of ventilation. a normal inspiratory: expiratory ratio is 1: 2 with inspiration lasting for approximately 1 second. [...]