Mechanical ventilation

Introduction

Many anaesthetic agents depress respiration and this can lead to the production of hypercapnia, hypoxia and acidosis. To maintain blood carbon dioxide and oxygen concentrations within normal levels, it is often necessary to assist ventilation. If the thoracic cavity is opened, the normal mechanisms of lung inflation are disrupted and it is usually necessary to ventilate the animal’s lungs artificially. It is not necessary to use a mechanical ventilator provided a suitable anaesthetic breathing system is in use, but using a ventilator will often be more convenient than manually assisting ventilation. A mechanical ventilator will often allow the precise control of the duration of inspiration and expiration, the volume of gas delivered to the lungs and the pressure reached in the airway during inspiration. It is not necessary to administer a NMB agent in order to carry out artificial ventilation but, unless the animal is deeply anaesthetised or is hyperventilated to produce hypocapnia, spontaneous respiratory movements may occur and these may interfere with ventilation and surgery.


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Mechanical ventilators

Ventilators for use with animals may have either been specifically designed for these species, or may be adapted from their original use as ventilators for human subjects. Ventilators are designed to achieve controlled ventilation of the animal’s lungs by applying ntermittent positive pressure to the airway. This may be achieved either by delivering gas directly to the anaesthetic breathing system or, indirectly, by compressing a rebreathing bag or bellows, which in turn delivers gas to the animal.

A variety of techniques have been devised to control the delivery of gas to the patient and to determine the patterns of gas flow and gas pressure that occur during ventilation and full details of the mechanisms used can be found in standard anaesthetic texts.

When selecting a ventilator for use in laboratory animals, the most important factor to be considered is the ability to ventilate a wide range of animal species. It should be emphasized that the successful delivery of small tidal volumes (e.g. less than 50 ml) often requires a leak-proof anaesthetic breathing system and minimal compliance of breathing system components such as connecting tubing. Additional features that may be needed are the ability to apply PEEP (positive end expiratory pressure) and a facility for humidification of gases. It is also important to select a machine that is simple to use and is reliable and easy to maintain. 

An important practical consideration is that some ventilators require a source of compressed gas to provide the driving power for the ventilator. If a piped gas supply is available, then this does not represent a particular problem. If small gas cylinders are used to drive the ventilator, then large numbers may be required during a prolonged anaesthetic, even when used on a relatively small animal. Since the driving gas does not reach the animal’s lungs, a compressor delivering medical air is one possible solution. Alternatively a large cylinder of compressed air can be provided as the driving gas. If none of these solutions are thought practicable, then a mechanically driven ventilator is required. Most of these ventilators are designed to ventilate the animal either with room air or with gas provided from an anaesthetic machine. If gas is supplied from an anaesthetic machine, then it is important that a pressure relief valve is incorporated into the breathing system between the fresh gas inflow and the ventilator to prevent over-inflation of the animal’s lungs. Most ventilators designed for clinical use incorporate this highly desirable feature.

Practical considerations when ventilating animals

Controlled ventilation usually requires placement of an endotracheal tube, or if the animal is not intended to recover, a tracheostomy can be performed and the trachea cannulated. Ventilating using a face mask is not usually successful, because of the difficulty of providing a suitable fitting mask, and the risk of inflating the stomach, which can interfere with respiratory movements. 

The ventilator needs connecting to the animal using a suitable breathing system. In larger species, a circle system will be preferred because of the much lower gas flows needed. Small animals (less than 500g) are often connector to ventilators using a simple “Y” connector and plastic tubing connected to the inspiratory and expiratory ports of the ventilator. The ventilator is supplied with fresh gas either simply from room air or, preferably, is supplied with a gas mixture from an anaesthetic machine. When using ventilators designed for small rodents, check the manufacturer’s instructions regarding the methods of connecting the fresh gas supply.  It is possible to over-pressurise some ventilators unless a separate pressure relief valve is placed between the anaesthetic gas supply and the ventilator. One easy way of achieving this is to place a standard pop-off valve and rebreathing bag on the anaesthetic machine gas outlet, and connect this to the “gas-in” port of the ventilator. The flow rate of gas from the anaesthetic machine needs only be equal to the animal’s minute volume (see below), but it is often difficult to set this accurately so slightly more gas than needed is supplied. This will result in gradual inflation of the reservoir bag. This can be prevented from over-filling by minor adjustments to the flow rate, or by partly opening the pop-off valve. 

Remember that if an inhalant anaesthetic is being used, changing the vaporiser setting will not produce an immediate change in the concentration of anaesthetic delivered to the animal, because of mixing with gas in the rebreathing bag. To speed this process, fully open the pop-off valve, empty the bag by squeezing it, then close the valve again. This same technique should be used with larger animals and circle breathing systems. 

To ventilate an animal, calculate the required tidal volume (approximately 7–10 ml/kg body weight) and select a suitable respiratory rate. Generally, a rate slightly lower than the normal resting rate, when conscious, is adequate. 

Table: Suggested Ventilation Rates for Laboratory Animals
SpeciesBreaths/min
Pig, dog (<20 kg)15–25
Pig (>20 kg), sheep (>20 kg), dog (>20 kg)10–15
Primates (>5 kg)20–30
Marmosets40–50
Cat and rabbit (1–5 kg)25–50
Guinea pig50–80
Rat60–100
Other small rodents80–100
Tidal volumes of 10 ml/kg are normally required. Whenever possible the adequacy of ventilation should be assessed by monitoring the end-tidal carbon dioxide concentration or by arterial blood gas analysis.

Separate controls for inspiratory and expiratory time may not be provided; some ventilators have only a control for the inspiratory time, and one for the inspiratory:expiratory (I:E) ratio. During IPPV, the heart and large veins in the thorax are compressed during inspiration, in contrast to the negative pressure that develops in the thorax during inspiration with spontaneous ventilation. The positive pressure produced during IPPV can reduce cardiac performance and cause a fall in blood pressure. To reduce this effect, inspiration should be completed in as short a period as possible, but must not be too rapid as this could result in high airway pressure. Conventionally, I:E ratios are set to be 1:2, but ratios of 1:3 and 1:4 will often cause less cardiac depression, while maintaining inflation pressures below 20 cm water.

After setting the rate and tidal volumes, and I:E ratio (if possible), set the maximum inspiratory pressure – this should be less than 15 cm water for small animals and should not exceed 25 cm water in most circumstances.

To monitor inflation pressures, if the ventilator is not equipped with a pressure monitor, place a needle in the inspiratory side of the anaesthetic breathing system and attach it to a pressure transducer. Besides checking that excessive pressures do not develop, by setting appropriate limits on the pressure monitor, it can act as an alert should the animal become disconnected from the breathing system or the ventilator malfunction.

When the chest is open, the lungs collapse completely, and to prevent this many ventilators allow a positive pressure to be maintained at the end of expiration (PEEP). Only very low pressures, ranging from 1–5 cm of water, are normally required for small animals. PEEP can be applied either via a specific feature on some ventilators, or by attaching a PEEP valve onto the ventilator. In some models of rodent ventilator (e.g. the Harvard volume cycled model), PEEP can be achieved simply by immersing the end of the tubing from the expiratory gas port into a few centimetres of water.

If a muscle relaxant is not being used to prevent spontaneous respiratory movements, then these can occur and interfere with the breathing cycles produced by the ventilator. One technique that can often reduce or eliminate these spontaneous movements is to increase the respiratory rate by approximately 50%, and slightly over-ventilate the animal for a few minutes. The respiratory rate can then be reduced slowly, and in many animals any spontaneous movements will remain suppressed, or will be occurring in synchrony with the ventilator. Setting up and managing intermittent positive pressure ventilation (IPPV) can seem daunting, but it is particularly useful in long anaesthetics (>1 hour). Many veterinary or medically qualified anaesthetists should be able to provide expert advice. Once some simple protocols have been established, IPPV should be a relatively easy technique to master. 

Next Article : Use of neuromuscular blocking agents for surgical or prolonged procedures

Updated on 12th May 2020

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