Mechanical ventilation is similar to providing manual intermittent positive-pressure ventilation (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 from ventilators used in the intensive care unit (ICU) as the former 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. The advantages and disadvantages of mechanical ventilation during anaesthesia are listed in Figure 1.

Figure 1. Advantages and disadvantages of mechanical ventilation during anaesthesia.

 Advantages:

 Respiratory variables such as tidal volume, respiratory rate, inspiratory time and pressure can accurately be adjusted

 Regular consistent respiratory pattern results in a more stable delivery of inhalational agent

 Regular respiratory rhythm improves operating conditions and depresses the patient's own respiratory drive

 Frees the nurse/anaesthetist for other jobs

 Special ventilatory functions such as positive end- expiratory pressure (PEEP) can be employed

 Disadvantages:

 Can impair venous return and therefore reduce blood pressure

 Can cause barotrauma to the lungs if inappropriate inflation pressures or tidal volumes are used

 Equipment and maintenance costs

 Knowledge of ventilator is necessary

 In patients unable to ventilate spontaneously unnoticed disconnection or equipment failure can result in death

 Some ventilators may not be suitable for all sizes patient

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 1 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.

 Atelectasis is the collapse of all or part of a lung.

 Positive end-expiratory pressure (PEEP) is the application of positive pressure at the end of expiration so the patient is breathing against a set pressure preventing the lungs from fully collapsing. PEEP can reduce atelectasis associated with intrathoracic procedures and prolonged recumbency. It increases the volume available for gas exchange.

Practical Tips for Using Ventilators

 Normal physiological respiratory rate, inspiratory time, inspiratory:expiratory ratio and tidal volume should be used.

 The ventilator should be set up and checked prior to use to allow detection and correction of any problems before induction of anaesthesia.

 End-tidal carbon dioxide (ETCO₂) should be maintained in the normal range of 4.6–6.0 KPa or 35–45 mmHg.

 The thorax should be examined to look at the degree of chest wall movement. There should be noticeable, but not excessive chest wall movement.

 Ventilators should be serviced regularly by the manufacturer and kept clean and in good working order.

Classification of Ventilators

Most veterinary ventilators are volume-controlled ventilators, meaning a constant flow (or volume) is delivered to the patient. Volume-controlled ventilators may be either time cycled, volume cycled or pressure cycled. This refers to the method used to change from the inspiratory to expiratory phase, i.e., the ventilator changes from inspiration to expiration when either a set time, volume or pressure is reached. To use a volume-cycled ventilator, the patient's tidal volume must be set, inspiration will occur and the lungs will inflate until the set tidal volume is delivered. With a pressure-cycled ventilator a peak inspiratory pressure is set. The lungs will inflate until this set pressure is reached. The respiratory rate can also be set and some ventilators also allow the inspiratory to expiratory ratio to be adjusted. Most ventilators require either an electricity supply or an additional gas source to drive the ventilator.

Specific Ventilators Used in Small Animal Anaesthesia

Minute volume divider ventilators work differently and do not fit into the above classification system. The operator sets the tidal volume; the ventilator uses the fresh gas flow from the anaesthetic machine and divides it into individual breaths of the set tidal volume which are delivered to the patient. Gas from the fresh gas flow is stored within the ventilator until a large enough volume is reached for the ventilator to deliver the next breath. The ventilator stores gas from the fresh gas flow, when enough gas for the next breath is obtained it delivers it to the patient. The Manley ventilator is an example of a minute volume divider. The main limitation with the Manley ventilator is that it cannot be used in animals weighing less than approximately 13 kg, as small enough tidal volumes cannot be delivered. Although old, the main advantages of these ventilators are that they do not need a power supply or an additional driving gas and that they are easy to use and very reliable.

The Vetronic 'Merlin' ventilator is a ventilator specifically designed for use in small animals. It is very versatile and can deliver tidal volumes varying between 1 and 800 ml, with the manufacturer stating that it can be used on patients from 50 g to 70 kg. Its working mechanism is volume controlled and can either be volume cycled, pressure cycled or time cycled. It has many controls and variables which provide flexibility and a wealth of information including compliance. This may appear confusing to operators not accustomed to using ventilators; however, it comes with a comprehensive instruction manual. It can deliver PEEP and audible alarms are available to warn of patient disconnection, high airway pressures and a blocked inlet. It can be connected to a rebreathing or a non-rebreathing system. As it is electronically driven, a power supply is required.

The Pneupac Ventilator is a volume-controlled, time-cycled ventilator. Ex-hospital Pneupac ventilators can be sourced secondhand relatively easily and cost effectively. An additional gas source such as medical air or oxygen is required as a driving force. The standard valve can be replaced with a Newton valve for use in small animals below approximately 5–7 kg and it functions as a 'mechanical thumb'.

Weaning Off the Ventilator

This refers to the process of changing over from the provision of IPPV to spontaneous ventilation. In most healthy patients this occurs easily; however, a pause in ventilation is normally necessary to allow the concentration of carbon dioxide in the blood to increase. Chemoreceptors located in the carotid and aortic bodies detect this increase in blood carbon dioxide levels; information is passed to the respiratory centre in the medulla within the central nervous system, stimulating spontaneous ventilation. Some people suggest providing several manual breaths in quick succession after switching off the ventilator to break the rhythmic ventilation cycle, in order to encourage spontaneous ventilation. The patient's chest excursions and ETCO₂ should be monitored to ensure that the patient is ventilating adequately.

Complications (Figure 2) are rare if the working mechanism of the ventilator is understood and applied correctly.

Figure 2. Potential complications of mechanical ventilation.