Discussion

Introduction:
During spontaneous ventilation gases pass into the lung by passing down a pressure gradient as the thoracic cage expands creating sub-atmospheric pressures. In contrast, ventilatory support during anaesthesia in horses utilises positive pressure ventilation (PPV) where inspired gases are forced into the lungs using supra-atmospheric pressures.

When to use a ventilator:
Any of the indications below justify the use of IPPV in an anaesthetised horse, and having the ability to safely ventilate horses may reduce anaesthetic-related morbidities and mortalities.
However, it is important to appreciate that IPPV is not without its disadvantages, and careful consideration should be made of the pros and cons of IPPV in every case.

1. Horse not breathing at all/apnoea:
Apnoea shortly after induction and during transition to maintenance of anaesthesia is quite common (even with ketamine based inductions) and can be problematic because hypoxaemia is more likely as pre-oxygenation is not often practiced in horses, and, if inhalation agents are being utilised, the horse is not uptaking any agent into its blood/brain if it does not breathe.

2. Horse not breathing sufficiently/hypoventilation:
Under inhalation anaesthesia most horses hypoventilate if spontaneously ventilating. The magnitude of hypoventilation depends on several factors including anaesthetic protocol, depth of anaesthaesia, dorsal vs. lateral recumbency, concomitant disease, etc. As stated above, hypoventilation results in hypercapnia, but often, if horses are receiving high fractional inspired oxygen concentrations (e.g. isoflurane being delivered in
100% oxygen) hypoxaemia is not as common. Many equine anaesthetists tolerate a mild hypercapnia as this has beneficial cardiovascular effects (increased sympathetic tone [increased cardiac output and arterial blood pressures] and local tissue vasodilation). However, hypoventilation can become detrimental if either hypercapnia or hypoxaemia become severe and/or insufficient inhalation agent is being taken up from the alveoli to maintain sufficient depth of anaesthesia. It is important to realise that hypoventilation may occur even if the horse has apparently adequate chest excursions.

3. Thoracic cavity open to atmosphere:
An open thoracic cavity may cause collapse of one or both lungs depending on whether the mediastinum is intact, so be aware of worsening ventilation during deep chest wounds, or during colic surgeries (diaphragmatic rupture/hernia).

4. Use of neuromuscular blocking agents:
Ventilation is mandatory as respiratory muscles will be compromised.

5. Respiratory muscle fatigue:
Often anaesthetised foals require ventilating under anaesthesia partly as a result of the effects of anaesthetic agents on respiratory drive and partly because their compliant chests meaning that expiration is an active process and so they suffer respiratory muscle fatigue more easily.

Other reasons to use a ventilator:

Countering atelectasis/anticipated hypoxaemia:
Many equine anaesthetists feel that if, for any reason, you anticipate that at some stage during anaesthesia you may need to use IPPV (especially to treat hypoxaemia), that for the IPPV to have the best chance of being effective, that you should instigate it from the beginning of anaesthesia.

Cases likely to cause reduced ventilation:
Distended abdomen (large colon torsions), certain positions
(Trendelenberg).

Treating metabolic acidosis using bicarbonate:
Successful treatment of metabolic acidosis using sodium bicarbonate solutions requires that the CO2 produced when bicarbonate associates with hydrogen ions is continually removed from the circulation, otherwise the 'open' buffer system becomes closed. In an anaesthetised horse, this often necessitates the use of IPPV.

Familiarity:
There are just as many anaesthetists that are terrified of anaesthetising horses without ventilators, as there are those that are terrified of using ventilators!

How to use a ventilator
1. MAKE SURE YOU ARE FAMILIAR WITH THE METHOD OF VENTILATION YOU PLAN TO USE, AND IF THIS IS A MACHINE, THAT IT HAS BEEN CHECKED TO MAKE SURE IT IS RUNNING PROPERLY
2. Secure the airway then place an appropriately sized
endotracheal (ET) tube with and inflated cuff before checking the seal does not allow gas to leak at the maximum airway pressures you plan to use
3. Attach the ET tube to the breathing system/ventilator (or demand valve)
4. Apply IPPV at an appropriate rate, tidal volume and maximum airway pressure*

*Most equine anaesthetists utilise a respiratory pattern based on a tidal volume of roughly 1 l/100 kg bwt and then adjust the respiratory rate to maintain normocapnia (or even a mild hypercapnia). It is thought that biotrauma can occur in the lung if airway pressure >70 cmH2O. However, in exceptional cases airway pressure near or in excess of this may be needed to achieve adequate ventilation.
Ventilating a horse can be as technically simple as a person squeezing a rebreathing bag (or using a demand valve attached to an oxygen supply to inflate the lungs), through to using complicated, computer assisted ventilators. In horses, if you are considering using a mechanical ventilator to ventilate anything other than neonatal foals, you will need a specifically designed large animal ventilator. Different ventilator types utilise different ways of controlling factors such as respiratory rate, tidal volume and airway pressures. Examples of different types of ventilators and how to use them will be given in the presentation.

Deleterious physiological consequences of IPPV:
Before contemplating the use of IPPV in any form, the anaesthetist needs to be aware that using IPPV during anaesthesia is not a benign intervention, and has some unavoidable harmful effects as well as some serious potential side effects if misused.

Cardiovascular effects:
- Reduced cardiac output.
- Central venous pressure/ intra-cranial pressure increased.

Pulmonary effects:
- Ventilation-perfusion mismatching.
- Blood gases (hypocapnia to respiratory alkalaemia through over ventilation).
- Ventilation-induced lung damage.

Monitoring ventilation:
Various monitoring aids such as capnographs, blood gas analysis and manometers will allow the effectiveness of ventilation to be monitored.

Summary:
Whether in first opinion or referral practice, it is likely that there are times when it is either desirable or necessary to ventilate a horse during anaesthesia. Being able to do this safely and being aware of the consequences and limitations of ventilation will increase the likelihood of a successful intervention.