Lung Ventilators

Although artificial ventilation of the lungs is less used in veterinary than medical anesthesia, it is frequently useful and its use is occasionally mandatory. The primary indication for artificial ventilation is pulmonary hypoventilation, which may be caused by central depression, muscle weakness or paralysis (for example, during the use of neuromuscular blocking drugs), and thoracic trauma or surgery. It must be noted that IPPV has profound effects on the distribution of pulmonary ventilation and blood flow, and systemic hemodynamics. For example, IPPV results in a significant reduction in cardiac output, particularly in anesthetised horses, but further discussion of these effects is beyond the scope of this article.
   The most commonly used method of artificial ventilation in veterinary anesthesia is Intermittent Positive Pressure Ventilation (IPPV) which, as its name suggests, consists of the intermittent application of postive pressure to the airway. (Other forms of lung ventilation include subatmospheric pressure applied to the chest wall, as in the 'iron lung', and high frequency oscillation, but these are little used in veterinary practice.)
   Although IPPV may easily and effectively be performed by manually squeezing the breathing bag and occluding the pop-off valve or other gas outlet, this is inconvenient for long periods and is physically exhausting when used in large animals such as horses and cattle. Many mechanical devices have been developed that aid the anesthetist by performing this function automatically.

Types of Ventilator

The classification of ventilators is somewhat complicated, and this is exacerbated by the fact that the same ventilator may function differently depending on how the controls are set. However, two of the most commonly used types of ventilator used in veterinary anesthesia are bag-squeezers and minute volume dividers. Ventilators designed for long-term ventilation in intensive care units are not usually suitable for use in anesthesia.

Bag Squeezers

As their name suggests, bag-squeezers replace the breathing bag, and usually also the pop-off valve, in the breathing circuit. Although some have been designed that compress the bag or bellows mechanically, using, for example, an eccentric cam or crank arrangement, most are of the bag-in-bottle type. Here, the breathing bag is enclosed in a rigid container and is squeezed by gas pressure applied to the inside of the container. The original bag has now usually been replaced by a concertina-type bellows.

Operation of a typical bag-squeezer (below) is as follows. During inspiration, the inspiratory valve (V1) is opened, allowing gas from the low-pressure system to enter the container and squeeze the bellows. The expiratory valves (V2 and V3) are closed, so gas from the bellows is directed, via the breathing circuit, into the lungs. During expiration, the inspiratory valve (V1) closes, the expiratory valve (V2) opens and gas flows from the patient circuit into the bellows. Once the bellows is full, the pop-off valve (V3) opens and any excess gas is vented from the circuit.

Many older ventilators of this type are of the descending bellows design, in which the bellows hangs from the top of the assembly (as above). The problem with this arrangement is that if the ventilator becomes disconnected or the circuit develops a leak, the ventilator will continue to cycle as normal (since, during inspiration, the bellows will be squeezed and empty as usual and, during expiration, the bellows will expand under its own weight and fill with gas from the atmosphere).
    More modern ventilators are of the ascending bellows type. Here, exhalation from the patient during expiration causes the bellows to expand upwards. If a leak or disconnection occurs, the bellows will not fill during the expiratory phase since there is nothing to make it expand. This arrangement is markedly safer than the older descending bellows design since it will be immediately obvious if disconnection has occurred.

Bag squeezers are used by substituting them for the breathing bag in circle absorber systems (with the pop-off valve on the circle closed) and Mapleson D systems (Modified Bain or T-Piece). They cannot be used in Mapleson A systems (Magill or Lack) owing to the different position of the pop-off valve in these circuits.
   It will be noted that the minute volume of ventilation (as determined by the ventilator) is completely independent of the fresh gas flow rate delivered to the breathing circuit.
   The controller unit, which determines the pattern of ventilation, may be mechanical, fluidic or, more recently, electronic.

Minute Volume Dividers

In contrast with bag-squeezers, which simply replace the breathing bag on an existing circuit, minute volume dividers are self-contained circuits which take the fresh gas flow rate, divide it up and deliver it to the patient in individual breaths. It follows, again in contrast with bag-squeezers, that the minute volume of ventilation is equal to the fresh gas flow rate.
   The operation of a typical conventional minute volume divider is illustrated below. Fresh gas from the common gas outlet of the anesthesia machine flows continuously into the circuit. During inspiration, the inspiratory valve (V1) opens, the expiratory valve (V2) closes, the weighted or spring-loaded bellows empties and the lungs are inflated. During expiration, the inspiratory valve closes, the expiratory valve opens, and the patient exhales, expired gas being vented from the system.

A variation on this theme is to replace the entire anesthesia machine with a microprocessor-controlled minute volume divider. The machine is connected directly to a low-pressure oxygen supply (there are no external flowmeters). During inspiration, the inspiratory valve (V1) opens, the expiratory valve (V2) closes, and gas flows through the vaporizer into the lungs. During expiration, the inspiratory valve closes, the expiratory valve opens and the patient exhales. The operation of the valves is controlled by a microprocessor which can calculate the required tidal volume and respiratory rate (and, therefore, minute volume) from the patient's body weight. By partially opening the inspiratory valve and opening the expiratory valve, the system can also operate as a Mapleson E circuit.

This provides a compact and convenient ventilator for small animal use. The very high flow through the vaporizer during inspiration (up to 60 l/min) may lead to inaccurate output and leaks.

Controls

Every ventilator needs a means of controlling tidal volume and respiratory rate. However, the method by which these are controlled varies widely between makes and models of ventilator. For example, tidal volume may be controlled:

• Directly, by adjustment of the excursion of the bellows.
• By adjustment of the peak inspiratory pressure.
• By adjustment of the inspiratory time.
• More than one of the above.

Similarly, respiratory rate may be controlled or influenced:

• Directly, by a respiratory frequency control.
• By adjustment of the inspiratory and/or expiratory time.
• By the tidal volume or peak inspiratory pressure.
• By the inspiratory and expiratory flow rate.
• By the fresh gas flow rate (in the case of minute volume dividers).

Settings that may be adjusted on different ventilators may include:

• Tidal volume
• Peak inspiratory pressure
• Respiratory rate
• Inspiratory/expiratory time ratio
• Inspiratory time
• Expiratory time
• Inspiratory flow rate
• Expiratory flow rate
• Inspiratory hold
• Inspiratory trigger effort
• Positive end expiratory pressure (PEEP)

In view of the large variety of controls on different ventilators, it is impossible in the space available to give specific instructions on the use of every specific make and model of ventilator: users should become familiar with the operation of the ventilators that they have in their facility. However, some general guidelines can be offered:

1. Look at the patient to make sure that the respiratory pattern looks reasonable.
2. A good starting point for tidal volume is around 10 ml/kg body weight, although this may need to be adjusted.
3. The peak inspiratory pressure should be kept as low as possible in order to avoid pulmonary barotrauma and adverse hemodynamic effects. Peak pressure should generally not exceed 1.5 kPa (15 cmH2O) in small animals (dogs and cats), which have highly compliant chest walls. Higher peak inspiratory pressure (3 kPa, 30 cmH2O or more) will usually be required in large animals (horses and cattle). If the chest wall is opened, lower pressures will be necessary in all species (since the stiffness of the chest wall has been removed).
4. Respiratory rate of 10 - 15 breaths per minute in small animals and 8 - 10 breaths per minute in large animals.
5. The optimal inspiratory to expiratory time ratio reflects a compromise between the beneficial effects of a longer inspiratory time on the distribution of ventilation within the lung and the adverse hemodynamic effects of prolonged positive intrathoracic pressure. An inspiratory:expiratory time ratio of between 1:2 and 1:3 is usually satisfactory.
6. The definitive criteria for adequacy of pulmonary ventilation are arterial blood gas analysis and capnography (measurement of the end-tidal CO2 concentration). Blood gas analysis is still rather expensive for general practice, but capnography is becoming increasingly affordable.

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