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VENTILATOR VARIABLES

The main controls available on an anaesthetic ventilator relate to the following equation:

EQ

UIP

ME

NT

THIS article discusses some of the commonly available machines that can be used for ventilatory support of anaesthetised patients. An article in the last issue of In Practice (April 2007, volume 29, pp 186-193) reviewed physiological aspects of mechanical ventilation. An appreciation of these factors is important for understanding the features and limitations of the ventilator in use.

The ins and outs of ventilation

2. Mechanical ventilators ALEX DUGDALE

Cat about to undergo surgical repair of a ruptured diaphragm. Its lungs are being ventilated using a Newton valve attached to a Pneupac Ventipac 5 ventilator

In Practice (2007) 29, 272-282

Alex Dugdale graduated from Cambridge in 1990. She spent six years in mixed practice in Lancashire before undertaking a three-year residency in anaesthesia and critical care at the Animal Health Trust in Newmarket. She is currently head of the Division of Veterinary Anaesthesia at Liverpool veterinary school. She holds the RCVS and European College of Veterinary Anaesthesia and Analgesia diplomas in veterinary anaesthesia.

Variables determined directly or indirectly using an anaesthetic ventilator

■ Breathing frequency■ Tidal volume (commonly inspiratory, but sometimes expiratory)■ Ratio of inspiratory time:expiratory time (I:E ratio)■ Inspiratory time■ Inspiratory flow rate■ Expiratory time■ Peak inspiratory pressure■ Positive end expiratory pressure (PEEP)

It follows that it is only possible to preset two of these variables because the third is dependent on the first two. Hence, a ventilator will not normally have controls for all three variables. For example, the Manley Pulmovent MPP and Manley MP3 models (see later) have controls for tidal volume and minute ventilation (determined by setting the fresh gas flow); the breathing rate is deter-mined by the ventilator.■ TIDAL VOLUME is derived as follows:

Inspiratory tidal volume

=Inspiratory

timex

Inspiratory flow rate

Minute ventilation (minute respiratory

volume)=

Breathing rate

x Tidal volume

Breathing rate(breaths per

minute)

=60

Time (seconds) for a complete respiratory cycle

Breathing rate(bpm)

=60

Inspiratory time + Expiratory time

Minute ventilation

=60

x (Inspiratory time

x Inspiratory flow rate )Inspiratory

time + Expiratory time

Therefore, if there is no breathing (ventilation) rate control knob, there should be controls for inspiratory time and expiratory time (or I:E ratio) instead.

Overall, this equates to:

VENTILATOR FUNCTION

For a ventilator to deliver intermittent breaths, it must be able to provide the four basic phases of breathing:■ End of expiration and beginning of inspiration;■ Delivery of inspiration;■ End of inspiration and beginning of expiration;■ Expiratory phase (exhalation is usually passive).

These are controlled by the following phase variables:■ Triggering variable, which ends expiration and begins inspiration;

If the ventilator does not have a control knob for tidal volume, it should have controls for inspiratory time and inspiratory flow rate instead.■ BREATHING RATE is derived from the cycle time, which equates to the number of complete cycles per minute:

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■ Cycling variable, which ends inspiration;■ Limiting variable, which places a maximum (limit) value on a control variable during delivery of a breath.

The ventilator-dependent control variables, which can be used to control the phase variables, are:■ Pressure;■ Volume;■ Time;■ Flow.

DESCRIPTION OF VENTILATORSMany different classification systems have been described but, with the advent of modern highly sophisticated venti-lators, some older systems of classification are not easily applicable. Some simple ways of distinguishing between ventilators are given below; of these, the general mode of operation provides a convenient means of discussing ven-tilators that might be encountered in the practice setting (see later). The key, however, is to get to know how the particular ventilator available within the practice operates.

Triggering variableVentilators can be:■ TIME-TRIGGERED;■ PATIENT-TRIGGERED. Such systems are activated:– When a patient makes an inspiratory effort, and the ventilator senses a drop in pressure (pressure-triggered), where the operator chooses the trigger sensitivity;– After a certain volume has been exhaled (volume-trig-gered), where the trigger volume is set by the operator;– When the patient finishes an exhalation and the venti-lator senses the decrease in flow (flow-triggered), where the operator chooses the trigger flow rate;■ OPERATOR-TRIGGERED.

The type of triggering helps to determine whether the breath is spontaneous, assisted or mandatory (see table on page 191, Part 1).

Cycling variable The cycling variable refers to the changeover from the inspiratory phase to the expiratory phase, and can be:■ VOLUME-CYCLED, where the changeover occurs when a predetermined volume has been delivered. Note that the volume leaving the ventilator may not necessarily be the volume entering the animal’s lungs due to some expan-sion of compliant tubing/compression of gases and leaks;■ PRESSURE-CYCLED, where the changeover occurs when a predetermined inspiratory pressure has been reached;■ TIME-CYCLED, where the changeover occurs when a predetermined inspiratory duration has been reached;■ FLOW-CYCLED, where the changeover occurs when the inspiratory flow falls to a predetermined flow rate.

Limiting variableVentilators can be:■ PRESSURE-LIMITED;■ VOLUME-LIMITED;■ FLOW-LIMITED.

Although these limiting variables can be used to limit the inspiratory phase, where their value cannot be exceeded, this does not necessarily mean that inspiration is terminated – that is, the cycling that terminates inspi-ration may be controlled by another variable.

Breath waveformsDelivered breath waveforms (see Part 1), especially for

ventilators in the intensive care setting, are described as either volume- (flow-) controlled or pressure-controlled, whereby inspiration depends on either the delivery of a fixed tidal volume over a given time, or on the main-tenance of a given airway pressure for a given time, respectively.

Method of operation■ PRESSURE GENERATORS produce inspiration by gener-ating a predetermined pressure. The maximum pressure is lower than that achievable from flow generators (see below) and, hence, these are sometimes called low pow-ered ventilators. Weighted bellows tend to produce a con-stant inflation pressure, while bellows attached to a weak spring or those using gases stored in a distensible bag produce a decreasing pressure as the bellows/bag empties.■ FLOW GENERATORS produce inspiration by deliver-ing a predetermined flow of gas. They deliver gas under high pressure (either compressed gas or gas from a bel-lows compressed by a heavy weight or powerful spring) through a variable orifice flow-restrictor proximal to the patient, which determines the flow and pressure wave-forms delivered. These generators are also referred to as high powered ventilators.

Source of powerVentilators can be powered by:■ Mains electricity;■ Compressed gas (ie, pneumatic);

Guidelines for ventilator settings

■ Inspiratory time should be around one second for small animals or just long enough to allow delivery of tidal volume (longer if the lungs are diseased)■ Inspiratory flow usually needs to be between 0·25 and 1 litre/second for small animals (and more for horses)■ Peak inspiratory pressure (PIP) for healthy animals should be ≤15 to 25 cmH2O (at the lower end for cats and cattle). Higher pressures may be required for animals with diseased lungs or where compliance is low or resistance is high■ PEEP should be 0-5-15 cmH2O, if necessary. It is best to start at about 2 to 3 cmH2O■ The fraction of inspired oxygen (FiO2) should be at least 0·3 for anaesthe-tised animals

Basic requirements of ventilators

■ Simple to use, and easy to clean and sterilise■ Robust and portable■ Efficient/economical to use■ Versatile and compatible with different (anaesthetic) breathing systems, and able to be used for a wide range of patient sizes and needs■ Able to deliver any gas/vapour mixture, as required■ The facility for humidification of gases, if intended for intensive care use■ The option of different modes of ventilation, if intended for intensive care use (see Part 1)■ Suitable alarms, especially for ‘patient disconnect’ and ‘high airway pressure’

Aim for normocapnia■ Tidal volume should be around 6-10-20 ml/kg (lower if there is pulmonary parenchymal disease)■ Breathing rate should be eight to 20 breaths per minute (higher if the lungs are diseased)■ I:E ratio of 1:2 to 1:3

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■ A combination of the two – that is, pneumatically powered, but electronically or microprocessor controlled (which requires electrical power).

General mode of operation■ MECHANICAL THUMB;■ MINUTE VOLUME DIVIDER;■ BAG SQUEEZER. This can be further classified in terms of the type of bellows used (ie, how the bellows move during exhalation):– Standing/ascending;– Hanging/descending;– Horizontal;

Ascending bellows are said to be safer than descend-ing bellows if a leak is present because they will fail to fill, while descending bellows will hang down and seem to be filling;■ INTERMITTENT BLOWER.

VENTILATORS FOR VETERINARY ANAESTHESIA

MECHANICAL THUMBSIf an Ayre’s T-piece (Mapleson E system) is attached to the common gas outlet of an anaesthetic machine and connected to a patient’s endotracheal tube, then simply occluding the expiratory limb produces inflation of the patient’s lungs by continuing fresh gas flow from the anaesthetic machine. Removal of the occlusion allows the lungs to deflate. By rhythmically repeating this occlusion/relief procedure, intermittent positive pressure ventilation (IPPV) can be achieved. This action of the anaesthetist’s thumb has been mechanised over the years – primarily in ventilators for use in baby care units. The Newton valve can function in this mode.

Newton valve The Newton valve is attached to/driven by another ventilator (eg, Nuffield series 200 or Pneupac Ventipac). The patient port is connected via hosing to the expira-tory limb of a Jackson Rees modified Ayre’s T-piece, where it replaces the bag. The normal fresh gas flow for the breathing system is delivered from the anaesthetic machine to the inspiratory limb. It is suitable for use in patients up to 10 kg bodyweight.

The Newton valve acts in one of three modes, depend-ing on the flow rate delivered by the ventilator it is attached to:■ PARTIAL THUMB OCCLUDER. When the ventilator cycli-cally delivers low flows to the Newton valve, the pressure developed inside the valve is only low because there is continual leakage through the orifice outlet. Therefore, it cyclically only partially dams back gases in the expiratory limb of the T-piece, thus pro-ducing small tidal volumes depending on the continued fresh gas flow;■ THUMB OCCLUDER. With a

higher driving gas flow from the ventilator, the expira-tory limb is more effectively occluded and, thus, the device acts like a thumb occluder, cyclically damming back gases in the expiratory limb and resulting in infla-tion of the patient’s lungs, again consequent to the continued fresh gas flow;■ BAG SQUEEZER. When the ventilator delivers an even greater gas flow to the Newton valve, not all of it can

Newton valve for use with Pneupac Ventipac ventilators

Newton valve for use with Nuffield series 200 ventilators

Newton valve (red arrow) connected to a Pneupac Ventipac 5 ventilator and T-piece hosing. The ventilator requires a pressurised gas supply, which in this case is provided via a mini-Schrader socket oxygen supply from the anaesthetic machine (white arrow)Operation of a Newton valve

Pressure relief valve

Drive gas from ventilator

To the patient

To air/scavenge

Control orifice

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exit through the orifice. Some gas enters the expira-tory limb of the T-piece so that cyclical inflation of the patient’s lungs now depends on gases being driven back-wards up the expiratory limb rather than on the fresh gases continuing to enter while the expiratory limb is blocked off.

Note that the driving gas from the ventilator should not enter the patient’s lungs, despite mixing with gases in the expiratory limb. In the bag-squeezer mode, an elongated expiratory limb on the T-piece may be neces-sary to prevent rebreathing of carbon dioxide.

Vetronic Small Animal Mark 3 (SAV03) ventilatorFor patients up to 10 kg bodyweight, an SAV03 uses a T-piece but with a solenoid valve interposed between the inspiratory and expiratory limbs. The valve allows continued flow of fresh gases into the patient’s lungs by occluding the expiratory limb until a chosen preset pres-sure is reached, at which point exhalation is allowed. The operator sets the expiratory time to between one and 30 seconds. The respiratory rate depends on inspira-tory flow (fresh gas flow) as well as expiratory length. Ventilation is thus pressure-limited, pressure-cycled and time-triggered. An adequate seal must be made around the endotracheal tube for the pressure to increase. If uncuffed endotracheal tubes are employed, use of a throat pack can help to reduce ‘leakage’. Alternatively, the fresh gas flow can be increased to cater for this leakage.

A low dead space ‘Y’ piece is used for very small animals (~10 g), with the ventilator again functioning to block off the expiratory limb until the preset pressure is reached, at which point exhalation is allowed. A selec-

tor switch on the back panel allows the expiratory length set by the front dial to be reduced by a factor of 5, thus providing much higher respiratory rates.

MINUTE VOLUME DIVIDERSMinute volume dividers collect a continuous flow of gas into a reservoir before delivery to the patient under positive pressure. The reservoir is ‘pressurised’ either by a weight, a spring or due to the elastic recoil of the material compris-ing the reservoir. The fresh gas flow delivered is set to be the intended minute respiratory volume, which is simply divided up into the required number of breaths per minute. These systems are relatively expensive in terms of fresh gas flow (which may also include anaesthetic vapour).

Some examples still in use in the veterinary setting are described below.

Manley Pulmovent MPP and Manley MP3 modelsManley Pulmovent MPP and Manley MP3 models are the last versions of a series of minute volume dividers that are robust, simple and easy to use, and found favour in both the anaesthetic and intensive care settings. They can provide ‘manual’ or ‘automatic’ ventilation. In man-ual mode, spontaneous ventilation is allowed, with the system acting as a Mapleson D system, which requires appropriate high fresh gas flows to prevent rebreathing. In this mode, the pop-off valve may be closed transient-ly and the bag squeezed manually to deliver a breath. In automatic mode, the bag port and normal pop-off valve are bypassed. The fresh gas flow delivered (eg, from the anaesthetic machine) is the estimated minute ventilation (around 200 ml/kg/minute).

The tidal volume (eg, 10 to 20 ml/kg) in the MPP model is adjusted by turning the tidal volume knob; in the MP3 model it is determined by the position of the catch on the lever arm of the main bellows. The tidal volume, along with the fresh gas flow, helps to deter-mine the ventilation rate. Each full turn of the knob increases the tidal volume by 100 ml. The range of tidal volumes available is 100 to 1000 ml for the MPP model, and 200 to 1000 ml for the MP3 model.

Vetronic SAV03 set up for (above) a patient weighing about 5 kg and (below) a very small patient (eg, small bird, rodent or snake)

Manley Pulmovent MPP (above) and Manley MP3 (below) ventilators

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The inspiratory pressure in the MP3 model can be altered by adjusting the position of the weight on the bellows; the manometer is observed to help adjust the weight position to avoid delivering excessive pres-sures. The position of the weight also helps to deter-mine inspiratory flow, although there is a knob to help control this. These settings can affect inspiratory time and therefore the I:E ratio and, to some extent, the ventilation rate.

The storage bellows (inside the box) fills during inspiration when the main bellows empties towards the patient’s lungs. Inspiration ends when the storage bellows is full so that, during expiration, the storage bellows empties into the main bellows. The main bellows is visible in the MP3 model, but not in the MPP model.

Cycling depends on filling of the storage bellows. This is established by the time taken to reach the predetermined volume, which can be affected by the fresh gas flow (a fast flow means the bellows fills quicker, which shortens the inspiratory time). The position of the inspiratory flow con-trol can also affect the duration of the inspiratory phase. It is therefore sometimes difficult to ascertain the principal determinant of the cycling, although some authors pro-nounce that volume cycling is the main mechanism!

Expiration ends and inspiration begins when the main bellows has filled to a predetermined volume (adjusted using the tidal volume knob or lever) and is therefore

Manley MPP with manual mode selected (red arrows). If the pop-off valve is closed, manual compression of the bag leads to inflation of the patient’s lungs. Scavenging is carried out from the pop-off valve (white arrow)

Manley MP3 with manual mode selected (red arrows). The pop-off valve must be closed (white arrow) to deliver a breath to the patient’s lungs when the bag is squeezed. Scavenging is accomplished from the side port (green arrow) in both manual and automatic modes, as gases are ducted here from the visible pop-off valve, as well as the ventilator’s internal valve

Manley MPP with automatic mode selected (red arrows). The pop-off valve and bag are bypassed, so scavenging must occur via a different valve (green arrow)

Manley MP3 with automatic mode selected (red arrows). Hoses and a bag are attached, and scavenging is carried out via the side port (green arrow)

Manley MPP ventilator set up for automatic ventilation with a scavenging system attached

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considered primarily to be volume-triggered. Although the tidal volume chosen determines the degree of filling of the main bellows as well as the storage bellows, these volumes are not exactly the same because some of the fresh gas flow enters the main bellows too.

FlomastaThe Flomasta is a minute volume divider that attaches to the common gas outlet of an anaesthetic machine. The main control knob (housing several valves) allows the operator to choose between spontaneous, manual or automatic modes. Using the automatic mode, the opera-tor can choose one of five positions that essentially determine the inflation pressure and therefore volume of the bag. The reservoir bag is enclosed in a harness partly to avoid overdistension and partly to ensure that adequate pressures/volumes are reached.

In automatic mode, the bag fills with fresh gas until the pressure is sufficient to open a valve (against a spring that is tensioned to predetermined positions 1 to 5). Inspiration ends when the inspiratory pressure falls low enough to allow the spring tension to close this valve, thus initiating expiration. The tidal volume and ventila-tion rate depend on the fresh gas flow and the control position (1 to 5). At the same fresh gas flow (minute ven-tilation), the tidal volume and ventilation rate are both affected by altering the position of the control knob. However, the tidal volume delivered to the patient’s lungs also depends on the compliance/resistance of the patient’s respiratory system and the compliance of the reservoir bag, so these factors can indirectly affect the ventilation rate too. The fresh gas flow itself may influ-ence the tidal volume as the fresh gases can push against the valve during inspiration.

The pressure/volume characteristics of black rubber bags change as they become more compliant with use. Therefore, with time, more fresh gases are required to fill the bag to a sufficient pressure to open the inspira-tory valve, and so the delivered tidal volume tends to be increased as well. Eventually, if the bag becomes too compliant, such that the pressure within it fails to increase sufficiently, IPPV will cease to occur. Consequently, the bag’s expansion is limited to about 5 litres by the enclosing harness, which prevents it over-stretching and rapidly increasing its compliance. Also, even if its compliance is increased, the harness (which is minimally stretchy) will allow an eventual pressure increase within the bag to ensure continued ventilation.

The changeover from inspiration to expiration is a composite of pressure- and flow-cycling. Inspiration is said to be time-triggered, but not in the traditional sense because the patient’s respiratory system characteris-tics also affect the delivered tidal volume and hence the ventilation rate.

BAG SQUEEZERSBag squeezers are the most common type of ventilators used for anaesthetised patients. The ventilator is con-nected to the expiratory limb of a Mapleson E system, or to the bag port of either a Mapleson D non-rebreathing system (Bain) or circle rebreathing system.

The most familiar form of these ventilators is the ‘bag (bellows) in a bottle’ arrangement. The bellows is placed into an airtight perspex cylinder and high-pressure driv-ing gas is forced into the space between the canister and bellows. The bellows may be ascending, descending or

horizontal. Ascending bellows, which rise during exha-lation, are said to alert the operator to a bellows leak by virtue of the bellows failing to fill. However, rising bellows are also thought to add resistance to exhalation, but whether this additionally provides some beneficial PEEP is controversial. Descending bellows do not allow easy detection of leaks in the bellows and are also full of air before patient connection, which should ideally be purged from the system before use.

The bellows may otherwise be inflated and deflated by some form of pneumatic piston or by mechanical squeez-ing/releasing using a motor-driven piston with linear or rotary (cam-driven) linkage.

Rather than being used to drive a pneumatic pis-ton, which, in turn, drives the bellows, the bellows can be removed and the drive gas itself used to force gases into the patient’s breathing system and lungs. A suitable length of wide-bore hosing is needed to link the ventilator to the bag port of the patient’s breathing system so that drive gases and patient gases do not mix. Although there is no physical separation of the gases, if used correctly, the drive gases do not mix with, and potentially dilute, the patient’s gases. The Newton valve, when set at high ventilator flow rates, operates in this manner – hence, its bag-squeezer mode of action. Pneupac Ventipac 5 and 10, and the Nuffield series 200 ventilators connected to adult valves both function in this way. However, they are also referred to as intermittent blowers (see below).

Hallowell EMC ventilatorsHallowell Engineering and Manufacturing Corporation (EMC) Small Animal Anesthesia Ventilator mod-els 2000 and 2002 differ mainly in that the 2002 sys-tem includes fine and coarse controls for tidal volume. Otherwise, they function in a similar fashion. IPPV is time-cycled and pressure-limited. The pressure limit can be set to between 10 and 60 cmH2O to ensure patient safety. Should the set pressure be exceeded, an audible alarm sounds. This alarm also sounds if 6 cmH2O pressure is not reached (ie, if the patient’s breathing system becomes disconnected).

Three sizes of ascending bellows and bellows hous-ings are available, all of which are easily interchange-able via a twist lock on/off system. The smallest bellows provides tidal volumes of 20 to 300 ml and is suitable for animals weighing between 1 and 25 kg. The interme-

This anaesthetised dog’s lungs are being ventilated using a Flomasta. Picture, Liz Leece

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diate-sized bellows provides tidal volumes of between 300 and 1600 ml and is suitable for animals weighing between 20 and 120 kg. The largest bellows provides tidal volumes of up to 3000 ml and is suitable for animals of up to about 200 kg bodyweight.

The tidal volume is determined by turning a knob on the control panel, which also includes a dial that adjusts respiratory rates to between six and 40 breaths per minute.

The bellows can be easily attached, via a piece of hosing, to the bag port of a circle rebreathing system or a Bain non-rebreathing system.

JD-Bird Equine VentilatorBird ventilators are intermittent blowers. However, the JD-Bird Equine Ventilator may be used to drive bag-squeezer type ventilators. This is a pressure-cycled, pressure-limited pneumatically driven flow generator

that can be used to provide mandatory IPPV or assisted ventilation (see below).

INTERMITTENT BLOWERSIntermittent blowers need a pressurised gas source to drive them. An electronically timed and activated pro-portional flow valve or a pneumatically timed oscillator divides the driving gas up into tidal volumes of a set size and rate. Anaesthetic ventilators tend to use pneumatic oscillators, while more sophisticated intensive care ven-tilators use proportional flow valves. Pneupac Ventipac 5 and 10 systems, and Nuffield series 200 ventilators are examples of the former. Control knobs usually include inspiratory flow and time, which determine tidal vol-ume, as well as expiratory time, which helps to deter-mine ventilation rate and I:E ratio. They are time-cycled, with the tidal volume delivered depending on the chosen

(above) Medium-sized bellows connected to a Hallowell EMC 2000 system to provide IPPV via a circle. (below) The same set-up viewed from behind. Note that the ventilator hose passes from the bellows to the bag port of the circle (yellow arrow). The pop-off valve (red arrow) is closed and scavenging is carried out via the exhaust port (green arrow). In this case, oxygen is being used as the drive gas (white hose)

Small bellows connected to a Hallowell EMC 2000 ventilator via the bag port of an anaesthetic breathing system (eg, Bain)

Bellows mounting

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inspiratory time and inspiratory flow. Ventilation rate is determined by the inspiratory and expiratory times. The side panels on the Ventipac ventilators display charts that allow ‘ready-reckoning’ and, if necessary, adjustment of values to set for required tidal volumes and I:E ratios.

Merlin Small Animal VentilatorThe Merlin ventilator can be time-, volume- or pressure-cycled. IPPV can be pressure-limited during volume-cycled ventilation. This ventilator has a microprocessor-controlled precision piston, which is driven by solenoid valves.

Tidal volumes of between 1 and 800 ml can be deliv-ered making it suitable for patients from 50 g to 70 kg. Inspiratory and expiratory times can be altered inde-pendently and without the need to change fresh gas flow or tidal volume. Peak inspiratory pressure can be limited to between 1 and 57 cmH2O in 1 cmH2O increments.

Circle showing the bag disconnected, and the bellows in a chamber connected instead. The pop-off valve of the circle is closed, so scavenging is achieved via the exhaust valve (green arrow) above the bellows chamber

Equine ventilator using a Bird modified Mark 7 to drive bellows in a bottle. The one-way valves of the circle are concealed within the Y piece

Burtons LDS 3000 large animal anaesthesia machine being used for spontaneous ventilation. Picture, Burtons

Burtons Equivent large animal anaesthesia system with LDS 3000 and DHV 1000. Picture, Burtons

Equivent system set up for use. This is a pneumatically powered, microprocessor-controlled, pressure-limited, time-cycled ventilator. Controls for inspiratory time and flow determine the tidal volume delivered, which is limited by pressure. Picture, Burtons

Circle in use for spontaneous breathing, where the bag is connected instead of the bellows. Scavenging is achieved using the pop-off valve (green arrow)

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The inspiratory flow rate can be set to between 100 ml/minute and 25 litre/minute. As the airway pressure and volume delivered are measured, compliance is calculated and displayed. A PEEP valve is also easy to use with this ventilator.

An inspiratory assist control, which covers a range of pressures between –1 and –10 cmH2O, allows weaning from the ventilator. The system also has audible alarms and on-screen displays to alert the operator in the case of disconnection or overpressure. In the event of power failure, all valves open to allow manual IPPV.

Bird ventilatorsFirst generation Bird Mark 7 ventilators are one of a series of pneumatically powered ventilators that can be used to ventilate intensive care patients directly, or anaes-thetised patients by driving a bellows in a bottle in the same way as the JD-Bird Equine Ventilator. Bird venti-lators can be time-, pressure- or manually triggered and are pressure-cycled. Peak inspiratory pressure is set to cycle and limit the inspiratory phase and, therefore, along with the patient’s respiratory system characteris-tics, determines the delivered tidal volume. A pneumati-cally driven expiratory timing device helps to control the ventilation rate. Trigger sensitivity can be set for patient-triggered breaths. Inspiratory flow can be controlled and will also affect the ventilation rate and I:E ratio.

When the air–mix control is pushed in, oxygen from the anaesthetic machine is delivered to the patient. A rec-tangular waveform showing a constant flow pattern is pro-duced throughout inspiration with an ascending pressure waveform (see Part 1). With the air–mix control pulled out, air can be entrained via a Venturi, so that less than 100 per cent oxygen is delivered to the patient. A descending flow pattern waveform is produced, with a variable (ascend-ing to rectangular) pressure waveform depending on the patient’s respiratory system compliance and resistance.

SUMMARY

This two-part article has aimed to demystify some of the medical terminology surrounding ventilation and mechanical ventilators. Hopefully, the information will help general practitioners build up their confidence in recognising those patients requiring ventilatory support and selecting the most appropriate ventilators and venti-lator settings.

Pneupac Ventipac 5 ventilator with adult valve attached (blue arrow). The hose (yellow arrow) is connected to the bag port of an anaesthetic breathing system. Scavenging is achieved by a different port (green arrow)

(above) Nuffield series 200 ventilator with an adult valve attached (blue arrow). Sufficient wide-bore hosing (yellow arrow) should connect the ventilator to the bag port of the breathing system to prevent drive gas from diluting the anaesthetic gases. The green arrow indicates the port from which scavenging is achieved. (below) Nuffield series 200 ventilator with an adult valve attached via wide-bore tubing to the bag port of the circle. The circle’s pop-off valve is closed, and scavenging is achieved via the exhaust valve (green arrow) of the ventilator

Bird modified Mark 7 ventilator. The inspiratory pressure control lever (blue arrow) is used to set the pressure to cycle and limit inspiration, which helps to determine the tidal volume delivered. The sensitivity control (black arrow) can be set to determine what effort the patient must make before the ventilator assists the breath and can be used to determine whether ventilation is assisted or fully controlled. The inspiratory flow rate control (yellow arrow) sets the flow to zero when fully rotated ‘in’, which effectively turns the ventilator off. Another dial (red arrow), which controls expiratory time, will also effectively switch the ventilator off if the expiratory time is greatly lengthened

Merlin Small Animal Ventilator. Picture, Vetronic Services