Ventilators used in anaesthesia were primarily designed to replace the minute ventilation of a patient with healthy lungs who is paralyzed or has a depressed respiratory drive as a result of anaesthetic agents and opiates.¹ In contrast intensive-care patients are normally encouraged to breathe alongside the mechanical ventilatory support which is gradually reduced during a slow weaning phase. Additionally at the end of anaesthesia, extubation is more precipitous with the rapid recovery of consciousness and return of spontaneous breathing. ICU ventilators, therefore, have to cope with the complexity of both machine- and patient-initiated respiration.

The pulmonary mechanics in terms of airways resistance and total lung compliance in ICU patients is rarely normal. ICU ventilators have, therefore, evolved to support respiration using such techniques as inverse ratio ventilation and high levels of PEEP, which are rarely required during routine anaesthesia. Ventilation for prolonged periods of time may require humidification techniques other than the simple heat and moisture exchanger commonly found in the operative setting. These alternative techniques such as hot water humidifiers are associated with increased accumulation of fluid in the respiratory circuits, which may compromise ventilator function. Large air leaks from the lungs or around airway tubing are more common in the ICU setting, and monitoring facilities on ICU ventilators have to be adapted to function in these unusual circumstances to prevent an excess of false alarms. Nonetheless, the flexibility of modern ICU ventilators is utilizable and sometimes indispensible in the operating theatre and modern anaesthesia ventilators are increasingly equipped with features of an ICU type ventilator.

Driving mechanisms

All ventilators require a driving force to deliver a gas flow into the patient. This force may either deliver the inspiratory gas directly to the patient or indirectly by compressing a bag or bellows containing the inspired gas mixture which in turn delivers the gasses to the patient.² Driving mechanisms which have been used in ventilators include the following:

1. Rotating electric motors. These may be used to:

a. drive a crank connected to a piston which in turn delivers the inspiratory gas. The disadvantage of these systems is that they have difficulty in altering inspiratory flow characteristics as the wheel and interlinked shaft drive will produce a fixed sinusoidal movement of the piston

b. power low pressure blowers. The advantage of this type of driving mechanism is that they can produce very high volumes of gas flow to enable the ability to cope with leaks from the patient circuit, in non-invasive ventilatory systems

c. drive gas compressors producing high pressure gas which may then be manipulated by pneumatic controls.

d. power a piston using a worm drive, this system is not seen in ITU ventilators (see Chapter 9, Dräger E anaesthetic ventilators).

2. Linear electric motors. These can be used to drive either pistons or diaphragms which are used in high frequency oscillators.

3. Tension springs which compress the gas in a storage bellows prior to being delivered to the patient. The disadvantage of this mechanism is that the pressure in the bellows is not constant but varies with the tension in the springs.

4. Weighted bellows. The gravitational force on a mass (the weight) will produce a constant driving pressure within the bellows; however, this mechanism is very susceptible to movement.

5. Pneumatic. These may be of two types:

a. Low-pressure systems using gas supplied from electric blowers or venturi devices.

b. High pressure with the gas supplied from a compressor via a high-pressure gas pipeline or cylinder which is connected to the ventilator.

Microprocessor electronic control

The use of microprocessor control has become virtually ubiquitous in all modern ICU ventilators. At the heart of these is the control microprocessor whose function is to receive data from the analogue to digital converters that measure pressure and flow within the ventilator system. With this information and under software control, the microprocessor provides the commands for the circuit board to precisely control the inspiratory and expiratory flow valves and secondary functions (e.g. nebulizer control or to flush pressure measurement lines). Data are also received from the oxygen and carbon dioxide sensors, if fitted. Information may also be received from external sources and interface boards that allow ventilators to talk to each other for synchrony in ventilator modes such as for independent lung ventilation. At least one current device can use electromyographic input to aid in ‘neural’ control of the ventilator synchronisation, see below NAVA.

Some ICU ventilators use dual microprocessors to control the ventilator function. The advantage of using two processors in parallel is that not only can the function of information display be separated from the control of the valves, but each microprocessor can check the output of the other against its own computations to ensure maximum patient safety and ventilator reliability (Fig. 10.1).

Figure 10.1 Microprocessor controls inside ventilator.

Information display

In the oldest ICU ventilators, the only information available to the operator was the analogue measurements of pressure or volumes (Fig. 10.2). With the same starting point of pressure measurement, microprocessor integration of the data now allows any number of derived variables to be monitored and displayed to inform the clinician of machine function and pulmonary mechanics. Alarm parameters can then be set for many of these values.

Figure 10.2 Analogue pressure display.

Digital information such as pressure values (peak, plateau, PEEP, etc.), calculations of tidal volume, machine or patient triggered breaths, inspired/expired volume differences and the quantity of gas leaking from the patient circuit were available to the user originally only in single-line numeric format (Fig. 10.3).³ With the availability of liquid crystal display (LCD) screens, more of this information, instead of being provided in single numeric displays, is available on a matrix screen, allowing the user not only to see the static numerical data, but also graphical information, such as flow, pressure and volume variations. These may be plotted in parallel against time or as loops against one another. The increasing use of graphical displays allows the user to more easily understand the effect that a change in ventilator controls has on the delivery of gas to the patient. Although LCD screens have the advantage of low-power consumption, they are only readable over a narrow viewing angle and are in turn now being replaced by TFT LCD (thin film transistor LCD) screens that can provide similar information in colour. These are readable over a larger viewing angle and in lower lighting conditions (Fig. 10.4).

Figure 10.3 LED digital display.

Figure 10.4 TFT panel display.

Inspiratory flow valve

Gas flow within ventilators may by controlled by one of two types of solenoid valves. In one type the valve has only two states: either being on or off; in the other type (proportional valves), the amount of opening when activated is directly related to the voltage applied to the solenoid.

High-speed proportional servo controlled (see below) flow valves are used in several manufacturers’ ventilators. They are capable of delivering flows from 20 to 3000 ml s⁻¹ and are adjusted by the ventilator’s microprocessor control (servo control) using an electrodynamic motor similar to that of a loudspeaker. A current flowing in the field coils of the solenoid generates the force to move the piston up and down; connected to the piston is the valve orifice which opens to allow gas to flow. This type of valve has a quick response time of typically 5 ms and with the small internal dead space of the ventilator, the flow rates change almost immediately in the patient circuit allowing precise control of the desired flow pattern and tidal volume. The microprocessor monitors the position of the valve and the pressure drop across it to enable it to continually adjust the flow to the required setting so that its performance characteristics are not affected by back pressure in the circuit (Fig. 10.5).

Figure 10.5 Inspiratory servo valve.

Some manufacturers use high-pressure, high-speed on-off gas solenoids, which control the flow of both oxygen and air. Under the microprocessor’s control these can be rapidly pulsed on and off to create the desired inspiratory flow pattern. Ventilators using these types of inspiratory solenoid no longer require a separate gas blender to create the oxygen air mixture as this is produced directly by the solenoids from the high-pressure gas pipeline. Not having a separate gas blender and mixing chamber allows the ventilator to rapidly change oxygen concentration within the patient circuit in response to altered settings by the operator, facilitating a swift 100% oxygen setting for use prior to tracheobronchial suctioning.

Flow sensors

Flows sensors (see Chapter 2, Measurement of pressure and gas flow) are often placed in both the inspiratory and expiratory limbs of ventilators. These not only allow the ventilator microprocessor control to sense and adjust gas flow, but also provide the user with measurements of inspiratory and expiratory tidal volumes, whether ventilator delivered or patient initiated.

Manufacturers have used a variety of flow sensors. These can be either a wire mesh pneumotachograph as incorporated into ventilators such as the Siemens 900C (Fig. 10.6) or a hot wire anemometer, as used in the expiratory limb of the Dräger Evita ventilators (Fig. 10.7) or the bidirectional variable orifice device that is incorporated into the patient end of the ventilator circuit in the Hamilton G5 ventilators (Fig. 10.8). The Maquet Servo-i uses an ultrasonic flow transducer (see Chapter 2). Flow sensors placed in the expiratory limb are susceptible to condensation hence some manufacturers incorporate a heating arrangement to ensure that these devices are kept free of water droplets.

Figure 10.6 Wire mesh pneumotachograph.

Figure 10.7 Hot wire anemometer. (Note that the very thin filaments of wire are not quite visible on their supports in this sensor photograph)

Figure 10.8 Bidirectional pneumotachograph.

Patient triggering

To facilitate the patient’s spontaneous ventilation alongside mechanically driven breaths in such modes as intermittent mandatory ventilation and pressure support (see below), ICU ventilators have to be able to sense the commencement of the patient’s own respiratory efforts and then provide a gas source from which they can breathe. This can be achieved by sensing a change in either flow or pressure within the circuit, triggering the ventilator to open the inspiratory valve. If the patient has to generate large changes of pressure within the circuit before obtaining any inspiratory gas supply this will add significantly to their work of breathing. Together with the increased pressure changes, a time delay from the commencement of the patient’s inspiratory effort to the start of gas flow will multiply the effect of any pressure drop; further increasing in the patient’s work of breathing.⁴

Pressure triggering

Early ventilators such as the Siemens Servo 900C provided only pressure triggering; the pressure-sensing device being placed upstream within the inspiratory limb inside the ventilator. The pressure drop resulting from the patient’s inspiratory effort activates the trigger whose sensitivity is usually set to −1–2 cm H₂O. When the threshold is reached, the electronic control within the ventilator opens the inspiratory valve allowing sufficient gas to reach the patient.⁵ The disadvantage of this method of sensing is that due to the remote placement of the sensor the patient may have to generate a considerably greater effort at the patient end of the circuit to activate the trigger threshold within the ventilator. This was particularly apparent when thin-walled, highly compliant, disposable plastic tubing was connected to the ventilator. In addition if a hot water humidifier is also connected in the inspiratory limb, the added compliance and resistance of this further increases both the pressure gradient and the patient’s work of breathing before obtaining any inspiratory gas supply.⁶

To overcome this problem, pressure sensing of the patient’s inspiratory effort can be achieved more reliably in the expiratory side of the ventilator, although this is still subject to the effects of condensation accumulating within the expiratory limb of the patient circuit. Other manufacturers, such as Hamilton in their series of ventilators, sense pressure directly at the patient end of the circuit to avoid these problems.

Flow triggering

To overcome the disadvantages of pressure triggering most ICU ventilators now provide a form of flow triggering.⁷ A flow trigger does not directly require change in pressure within the inspiratory circuit to enable a patient-initiated breath. The ventilators provide a continuous bias flow, frequently 10 l min⁻¹, which is introduced into the inspiratory circuit throughout all phases of the machine’s respiratory cycle. When the patient starts to initiate a breath from the circuit, the fall in bias flow in the expiratory limb of the ventilator is sensed and the ventilator opens the inspiratory valve to allow a patient-initiated breath. The change in flow required to trigger a patient breath can be adjusted by the operator and is often set between 1 l min⁻¹ and half the bias flow.⁸ The advantage of this mode is that it is not as affected by humidifiers in the inspiratory limb or condensation in the tubing so patient work of breathing is minimized.^6,^9,¹⁰ Most ventilators allow the user to adjust the sensitivity of the flow triggering to achieve the minimum work of breathing without the occurrence of falsely triggered breaths.¹¹

Neurally adjusted ventilatory assist (NAVA)

Most ventilators rely on changes of either pressure or flow within the respiratory circuit to sense the patient’s own breathing, but the Maquet Servo-i ventilator can use information from the patient’s diaphragm. The electromyography signal from the diaphragm as the patient initiates a breath is sensed by an electrode array on a special nasogastric tube and these signals are used to synchronize the ventilator-assisted breaths more directly with the patient’s own efforts.

Expiratory pressure generation

The treatment of such conditions as adult respiratory distress syndrome (ARDS) requires the ventilators to be able to maintain a pressure in the patient circuit that is above atmospheric during expiration both for ventilator delivered breaths and for patient spontaneous respiration. PEEP/CPAP valves are incorporated into the expiratory limb and can be of two types: either fixed value or variable pressure.

• Fixed pressures can be generated by a spring-loaded valve, an underwater column or a weighted valve leaflet. The latter two types’ function is severely impaired with movement and no longer used. It is worth noting that these types of valves exert a resistance throughout exhalation as a result of the mechanical force of the spring. Electronically controlled valves are usually only activated when the desired PEEP level is reached and do not impede exhalation until this point.

• Variable pressure valves have the advantage that they can be controlled (electronically) by the operator to produce the desired level of expiratory pressure and they can be closed completely by the ventilator to function as the expiratory valve as well.

Exhalation valves

Overpressure valves

To ensure patients are not subjected to excessive airway pressures, ventilators provide protection in the form of an electronically controlled pressure limiter with a secondary mechanical device as back-up. The electronically controlled pressure limit can be set by the operator and this will prevent pressure rising within the circuit even if the desired tidal volume is not delivered. Audible and visual alarms are triggered in any overpressure condition.

Ambient air inlet

In addition, for patient safety, an atmospheric air inlet valve is incorporated into the inspiratory gas pathway. In the event of a gas supply failure or internal electronic dysfunction these valves will open immediately allowing the patient to spontaneously inhale from ambient air.

Nebulizer port

Ventilator manufacturers provide an additional gas source to drive a micro-nebulizer for the delivery of drug into the patient circuit (Fig. 10.13). The advantage of these fixed outlets is that this driving gas contains the same preset oxygen concentration as the respiratory gas, but it can be made to only be operational during the inspiratory phase so the drug is not wasted by being blown down the expiratory limb of the ventilator circuit. Unlike separate external gas flows these will not interfere with the ventilators’ flow triggering sensitivity.

Figure 10.13 (From left to right) Auxiliary pressure, nebulizer and pneumotachograph on G5 ventilator (Hamilton Medical).

Battery back-up

Microprocessor ventilators go though a series of computer self-checks to ensure that the electronic and pneumatics are working correctly on initial power-on; in some older models this checking process could take up to 40 s. To prevent this occurring in the event of a temporary disconnection of the power supply, ventilator manufacturers have provided a battery back-up to ensure the ventilator continues to function. ‘Sleep’ or ‘suspend’ buttons are also provided which enable the ventilator to stop operating, but then resume again with the same settings without the need to go through the initial self checks.

Additional batteries can be provided to allow for more prolonged periods of disconnection from mains electrical supply to facilitate the movement of patients; these batteries are often stored in the base of the ventilator stand (Fig. 10.14). The addition of cylinder supplies of oxygen and air will allow the machine to be used as a transport ventilator.

Figure 10.14 Battery back-up in base of ventilator.

Flow pattern generation/ventilation modes

With the use of sophisticated microprocessors and high-performance pneumatic controls, the gas flow characteristics of such ventilators are no longer primarily determined by the physical characteristics of the machines, as had been the case with anaesthesia ventilators in the past.

The characteristics of the inspiratory and expiratory cycle can now be broken down into its fundamental components and gas delivery characteristics can be preset by the manufacturer or adjusted by the clinician at the bedside. Further advances in microprocessor monitoring are employed in some ventilators in turn allowing the patient’s own breathing characteristics to alter ventilator setting, whose control traditionally had only been accessible to the care provider. This ability allows ventilators to operate in a closed loop automatic fashion.

A classification for mechanical ventilators proposed three types of variables:¹³

1. Control variables, which included pressure, volume, flow and rate

2. Phase variables, which defined how the change over points in the respiratory cycle occur, these being trigger, cycle and limit variables (Fig. 10.15)

3. Conditional variables, which define additional parameters, such as supplementary breaths and sighs.

Figure 10.15 Inspiration showing position of phase variables.

Control and phase variables

When set up for a specific control mode, the ventilator delivers inspiratory gas flow to satisfy the appropriate limit of that variable; either a pre-set tidal volume in the volume control mode or a user defined airway pressure in the pressure control mode. Once initiated, inspiration occurs to the limit of that variable regardless of patient effort.

When a phase variable, such as time cycling (see below), is used in conjunction with the control modes (above) a more reliable delivery of appropriate minute ventilation ensues, especially if the patient is apnoeic or has limited respiratory effort. As the patient’s respiratory function improves the control setting may be reduced to allow the patient to breathe alongside the minute ventilation delivered by ventilator. The control mode can be variably adjusted between full support and zero.

Volume pre-set control mode

In this mode, inhalation proceeds with a pre-set flow rate until the desired tidal volume is delivered. At the end of the predefined inspiratory time (the phase variable) passive exhalation then occurs. Inspiratory time can be set either directly or indirectly by adjusting either the ventilator rate and/or the inspiratory to expiratory ratio (I/E ratio). Inspiratory gas follows a predefined flow pattern and the peak pressure measured in the airways is a function of airways resistance and lung compliance (reflected in the force required for lung distension). Since the volume delivered is constant, peak airway pressure will alter with changing pulmonary compliance and airways resistance. However, true plateau pressure being a reflection of pulmonary system compliance only requires absence of gas movement and hence usually a degree of inspiratory hold to achieve a steady equilibrium. To avoid excessive pressures resulting in pulmonary barotrauma in a volume pre-set mode, most machines have high airway pressure alarms to alert the user to the potentially dangerous situation. In addition many machines have an overpressure release setting at which the ventilator will no longer deliver any additional tidal volume to the patient, but vents the excess to atmosphere. This facility may be governed by the high-pressure alarm limit or may be set independently and results in only partial delivery of the pre-set tidal volume. Warning alarms are triggered to alert staff to this situation.

When this mode of ventilation is used with hot water humidifiers in circuit, the rain out from these devices and subsequent pooling in the tubes may falsely trigger high-pressure alarms. A similar alarm may occur if the patient coughs during the inspiratory cycle.

Pressure pre-set control mode

In this mode, often referred to as pressure-controlled ventilation (PCV), a predefined inspiratory pressure is applied to the airways and the resulting pressure difference between the ventilator and the alveolus results in inflation until there is equilibrium between the two; after a pre-set inspiratory time, passive exhalation follows. In this mode the delivered volume during respiration is dependent on pulmonary and thoracic compliance. A potential disadvantage of this mode is that changes in pulmonary mechanics will result in varying tidal volumes and minute ventilation.¹⁴ With accurate flow sensors within the circuit, close monitoring of tidal ventilation is employed and dramatic changes in tidal volume or minute ventilation will trigger a warning alarm.

As this mode of ventilation pre-sets the pressures within the inspiratory circuit, the flow pattern cannot be greatly influenced by the user, and is usually of a falling flow pattern type as the pressure gradient between the inspiratory circuit and the alveolus declines with lung inflation. This usually results in a more homogenous gas distribution throughout the lung and improved arterial blood oxygenation and may reduce the patient’s work of breathing compared to volume pre-set modes during assisted modes of ventilation.¹⁵

Other ventilator modes

The degree of sophistication of the new generation of ventilators has spawned an increase in the variety of and scope for ventilation strategies. This has led to an impressive increase in terminology to describe these. Unlike the nomenclature for drugs, there is no international standardization of the names attached to different modes of ventilation, even when they use similar control and phase variables throughout the respiratory cycle. Manufacturers have contributed to this confusion by using patents or trademarks to prevent similar names being used on other companies’ machines (Table 10.1).

Table 10.1 Nomenclature and ventilation strategies as termed by various manufacturers

To fully understand what a ventilator will and will not allow a patient to do in each setting, there remains, sadly, the need to read the ventilator manual. For example: many modern ICU ventilators will allow spontaneous respiration, even during CMV mode (see below), and some will even apply pressure support to those breaths, whilst others will deliver only a machine volume or pressure pre-set breath on detecting an inspiratory effort. The principles of the more common terms are explained below.

Conditional variables

Controlled mandatory ventilation (CMV)

Breaths are delivered at pre-set time intervals regardless of patient effort. This mode is similar to that of anaesthesia ventilators and is most often used for the paralyzed or apnoeic patient. Large increases in work of breathing for the patient are created if their respiratory efforts fail to coincide with the ventilator’s inspiratory cycle.

Intermittent mandatory ventilation (IMV)

In this mode breaths are delivered from the ventilator at pre-set intervals. However, patient’s spontaneous respiration is allowed between ventilator-administered breaths.¹⁶ The IMV rate may be reduced allowing increased time for the patient’s spontaneous respiration during the weaning process.

Synchronous intermittent mandatory ventilation (SIMV)

In this mode the ventilator tries to deliver its breaths in conjunction with the respiratory effort of the patient. Spontaneous breathing is also allowed between ventilator-administered breaths (Fig. 10.16).

Figure 10.16 Synchronization of IMV breaths. The second tidal volume (synchronized breath) is delivered early because an inspiratory effort falls within the trigger window. The machine’s next respiratory cycle timing is reset from this point.

Synchronization of ventilator breaths is achieved by the ventilator attempting to detect the patient’s inspiratory effort in a small time window prior to the initiation of its own inspiratory cycle. If patient inspiratory activity is detected the ventilator immediately delivers its mandatory breath. This mode of ventilation improves the comfort of spontaneous respiratory efforts for the patient and reduces the incidences of patient ventilator dyssynchrony where the patient appears to be ‘fighting the ventilator’. Breath stacking may still occur where the patient wishes to exhale, but is then subject to an additional mandatory ventilatory inspiratory tidal volume, potentially over-distending the lungs.

On its own the SIMV mode has not been shown to improve patients’ weaning from ventilatory support.¹⁷ This may be due to the increased work of breathing associated with spontaneous respiration through these mechanical circuits. The work is created by having to generate sufficient pressures and flows within the ventilator tubing to trigger the opening of the ventilator’s inspiratory valve for access to the extra gas flow required.

Pressure support mode/spontaneous assist

Pressure support (PS) ventilation has been shown to decrease the work of spontaneous breathing through ventilator circuits.^18,¹⁹ When triggered to do so, the ventilator produces a pressure in the respiratory circuit to support the patient’s own inspiratory effort. The respiratory effort is detected either by flow or pressure triggering. With this mode of ventilation a user pre-set pressure is generated in the circuit (not a fixed tidal volume) to assist every patient spontaneous effort. This predefined airway pressure is sustained until the patient’s own inspiratory flow falls below a predefined cut off, e.g. 25% of peak inspiratory flow²⁰ (Fig. 10.17). The disadvantage of this mode of ventilation is that if the patient fails to take any respiratory effort, no pressure supported breaths will be initiated. To avoid the potentially disastrous consequences most ventilators have a back-up apnoeic SIMV rate should the patient’s spontaneous respiration cease.

Figure 10.17 Pressure and flow curves when expiration trigger in pressure support mode is set to 25% of peak inspiratory flow.

For patients who have adequate respiratory drive and whose respiratory failure is not severe, PS ventilation may offer the patient considerable advantages, as all the breaths are patient initiated and breath stacking and fighting the ventilator are almost abolished. Even patients who are initially tachypnoeic, may be successful managed in this mode as the supporting pressure can be set sufficiently high to augment their own tidal volume and hence, reduce patient respiratory rate. As this is a pressure pre-set mode of ventilatory support, the risks of barotrauma associated with high airway pressures and fixed tidal volume ventilation are reduced.²¹

For patients who have severe respiratory failure this mode of ventilation is commonly used in conjunction with volume pre-set or pressure pre-set SIMV modes.²²

Traditional ‘control modes’ of ventilation have only allowed the patient to breathe during the ventilator’s expiratory phase. Most modern machines now provide a pressure preset control mode of ventilation that will allow the patient to breathe during any point of the ventilator respiratory cycle; although similar in function this mode is called by several names including BiLevel, Bi-Vent, BIPAP and DuoPAP.

Closed loop controlled ventilatory modes

Historically, adjustment of ventilator settings was made by physicians, nurses or attending bedside staff according to the patient’s requirements. With the advent of microprocessor control it is possible to design ventilators with closed loop control, in which the ventilators themselves can adjust the way they deliver inspiratory gas flow.

The ability of ventilators to automatically wean clinically stable patients off ventilatory support has long been the aim of many manufacturers. Early ventilator models adjusted either tidal volume or inspiratory pressure to achieve a target end tidal carbon dioxide level. However, in intensive care patients, end tidal carbon dioxide does not always correlate with arterial carbon dioxide tension and the target variable of minute ventilation is more often used instead. Most ventilator manufacturers provide a closed loop mode of control implementing different versions of mandatory minute ventilation (MMV) (see Table 10.2) in which the target of minute ventilation is achieved by the ventilator adjusting inspiratory pressure and frequency.²³ Later versions of MMV have incorporated algorithms (Fig. 10.18) that aim to reduce the patient’s work of breathing while still achieving the desired minute ventilation by encouraging the patient to breathe with larger tidal volumes and slower rates, this being more efficient than rapid shallow breathing.²⁴

Table 10.2 Closed loop controlled ventilatory modes

F, rate; VT, tidal volume; MV, minute volume; Pinsp, inspiratory pressure level; Psupp, pressure support level; Ti, Inspiratory time; MMV, mandatory minute ventilation; ASV, adaptive support ventilation

Figure 10.18 Algorithm used in adaptive support ventilation mode (ASV) on the Hamilton Galileo ventilator.

Dual control mode

Newer modes of ventilation referred to as dual control are where the ventilator acting in a closed loop feedback fashion measures parameters during the delivery of the breath and adapts its output in response to these changes according to a predefined algorithm.³ These adaptations can be accomplished within a single breath when the ventilator changes from a pressure to a volume control mode. More commonly dual control breath-to-breath is used by ventilator manufacture to allow incremental changes to occur to achieve the desired outcome.

Volume support (VS)

VS is a mode where the ventilator acting in a pressure support mode either increases or decreases the applied airway pressure to ensure that the operator defined tidal volume is achieved.

Automode

This is a dual control mode in the Servo-I ventilator (Maquet), which supports the patient in a time cycled mode, a pressure regulated mode, a volume control mode or, if capable of spontaneous breathing, a flow cycled volume support mode.

Autoflow

In this mode the Dräger Evita ventilator changes from volume control to pressure control mode, adjusting the pressure to deliver the predefined tidal volume at the minimum pressure possible.

Adaptive support ventilation (ASV)

ASV is based on algorithm using the Otis equation ²⁵ designed to minimize the patient’s work of breathing by targeting tidal volume and respiratory rate. This dual mode works in either a pressure controlled or a pressure support mode, switching between the two modes depending on the patient’s own respiratory activity.

Automatic tube compensation (ATC)

Ventilator manufacturers have tried to produce other additional modes for spontaneously breathing patients which attempt to reduce the resistance inherent with gas flows through an endotracheal tube. With the aim of keeping the pressure in the trachea at the desired level the ventilator adjusts inspiratory pressure according to the gas flow and direction.²⁶

SmartCare

This dual control mode (found in some Dräger ITU ventilators) is designed to wean patients from ventilatory support who are capable of breathing in a pressure support mode by adjusting the level of pressure based on an algorithm with measures respiratory rate tidal volume end tidal carbon dioxide.

Total closed loop ventilation has not found universal favour as appropriate ventilation is not solely about carbon dioxide elimination and oxygenation of blood. It also involves complex interactions with the patient’s neuromuscular effort, and that of their cardiovascular system.

Individual ventilators

900C

The Siemens 900C ceased production in 2004 but is still in use and supported by the manufacturer. It has essentially two parts (Fig. 10.19): a pneumatic section sitting on top, and an electronic section below containing the ventilator controls and electronic displays. The two sections are connected by a cable and could be separated if desired. High-pressure gas enters the pneumatic section from an external blender. A second gas inlet connection is provided that accepts low pressure directly from an anaesthetic machine, if required. The gas then passes through an oxygen analyzer and main bacterial filter before entering a spring-loaded bellows, which stores the gas prior to use.

Figure 10.19 A. Servo 900C series ventilator. B. Servo 900C working principles: diagram of the pneumatic section (see text).

From the bellows, gas passes through the inspiratory flow transducer and then through the inspiratory scissor valve before leaving the unit to enter the patient circuit. Exhaled gas returning from the patient via the expiratory limb of the patient circuit re-enters the ventilator and passes through the expiratory flow transducer and expiratory scissor valve before exiting the unit via a one-way valve to the atmosphere. The inspiratory scissor valve consists of a flexible piece of silicone rubber tubing that is compressed in the jaws of a scissor mechanism using an electric stepper motor to control gas flow. In contrast, the compression of the equivalent tube in the expiratory scissors valve is controlled and varied by a pull of an electro-magnet under the control of the ventilator’s electronics. This force is adjusted to maintain the correct PEEP in the expiratory limb; during inspiration the valve remains shut.

During inspiration, the gas flow is measured at the inspiratory flow transducer and compared in the electronics section to that which is required to achieve the operator’s preset volume. If the actual flow does not match the required value, the stepper motor varies the compression of the inspiratory scissors valve to adjust the flow delivery. The driving pressure for the gas flow is generated by the pre-set tension in the spring attached to the inspiratory bellows, and during high inspiratory flow rates this may be insufficient to deliver the required gas flow. In this situation, the working pressure will have to be increased manually by the turning the key on the front of the pneumatic section (Fig. 10.20).

Figure 10.20 Servo 900C working pressure adjustment.

Servo-i and Servo 300

The Servo-i (Fig. 10.21A) and its predecessor the Servo 300 (last manufactured in 2003) has two units: a patient pneumatic unit and the control unit connected by a cable. The electronic circuit of the control unit both controls and displays the ventilator settings used to operate the pneumatic unit (Fig. 10.21B). Oxygen and air are supplied by pipeline and are blended directly into the patient’s circuit by high-speed gas solenoid valves; unlike their predecessor (the 900C) there is no bellows storage for inspiratory gas and no low-pressure port for the supply of anaesthetic gas.

Figure 10.21 A. Servo-i. B. Working principles showing inspiratory section and detachable expiratory cassette.

Images courtesy of Maquet Critical Care.

The solenoid valves have a response time of 6 ms under microprocessor control and can be rapidly opened or closed to achieve the desired flow rate and pattern in the ventilator circuit. In the Servo 300, the exhaled gas from the patient returns to the unit and passes through the expiratory flow transducer and pressure controlled expiratory valve before exiting out to the atmosphere. The Servo-i has a detachable expiratory cassette containing the entire expiratory gas flow pathway, together with the ultrasonic expiratory flow transducers and the expiratory valve. The expiratory pressure sensor and the actuator for the valve are housed in the body of the ventilator. The expiratory cassette can be autoclaved and the pressure sensor is protected by a bacterial filter.

A version of the Servo-i is manufactured in non-ferrous materials and the electronics shielded from the effect of high magnetic fields to allow it to be used to ventilate patients who are undergoing magnetic resonance imaging (MRI).

Dräger Evita series (2 Dura, 4, XL)

Dräger Evita 4 and XL series of ventilators have three sections; the electronic compartment, which sits directly on top of the pneumatic controls, and a third detachable display unit, which houses the controls and touch-sensitive screen (Fig. 10.22A). The screen displays both the ventilator information and the virtual touch sensitive buttons and dials. The 2 Dura model combines the display and the electronic and pneumatics in a single case.

Figure 10.22 A. Dräger Evita XL. B. Dräger Evita 4 working principles.

Pneumatics (Fig 10.22B)

Gas from the high-pressure pipelines enters the ventilator via a filter and a non-return valve directly into the two proportional valves that control the flow of oxygen and air to be blended directly into the patient circuit. Sensors measure the pressure of the oxygen and air supplied to the ventilator and with this information a central microprocessor is able to adjust the function of the valves to deliver the correct flow into the patient circuit producing the inspiratory breath. The returning expiratory gas leaves the ventilator via a diaphragmatically operated expiratory valve and then through the external hot wire flow sensor finally to atmosphere. The PEEP/CPAP pressure is controlled by a balancing pressure from a separate proportional valve using the regulated oxygen supply applied to the downstream side of the expiratory diaphragm (see also Fig. 4.29). The gas flow of 9 l min⁻¹ required to drive the nebulizer (when used) is also obtained from this oxygen supply. Here, the main two pneumatic valves are automatically re-adjusted to deliver the correct oxygen concentration.²⁷ During an oxygen pipeline failure, a switch-over valve is operated that allows the pneumatics to continue to function normally using the pressure from the air supply.

Non-invasive ventilation

The application of mechanical ventilatory support through a mask or helmet in place of endotracheal intubation is becoming increasingly accepted and utilized in the ICU. This modality of ventilatory support can be used successfully for patients with mild-to-moderate respiratory failure, but the patient must be mentally alert enough to follow commands, as without an endotracheal tube there is no mechanical method of preventing aspiration into the lungs. Clinical situations in which it has proven useful include acute exacerbation of chronic obstructive pulmonary disease (COPD) or asthma, and decompensated congestive heart failure (CHF) with mild-to-moderate pulmonary oedema. Conventional ICU ventilators set in their PSV mode of ventilation with PEEP are commonly use to support non-invasive ventilation through a mask,⁴ although specially designed machines are now available for use in general acute wards (Fig. 10.23). Conventional ICU ventilators have the advantage of sophisticated monitoring and precise control of the oxygen concentration and inspiratory flow pattern, but set in their normal ventilatory modes they are ill adapted to cope with the large leaks of gas that may occur with poorly fitting masks and awake patients.²⁸ Manufacturers now often provide non-invasive modes on standard ICU ventilators that are more tolerant of the gas leakage from the patient circuit.

Figure 10.23 Non-invasive ventilator BiPAP Focus.

Specialist non-invasive machines use an electrically driven blower (Fig. 10.24) to deliver the inspiratory gas supply and are capable of generating flows of 300 L min⁻¹.²⁹ The rotating speed of the electrical blower is adjusted by the ventilator’s microprocessor to achieve the operator’s desired level of pressure for both inspiration and expiration. This bi-level pressure ventilatory mode is time cycled in a similar manner to the CMV mode on conventional ICU ventilator, the only difference being that patient is capable of breathing during the ventilator’s inspiratory and expiratory cycle, as gas is supplied continuously during both phases of respiration. There is no expiratory valve in this type of ventilator, the gas leaving the system through a series of holes or slits close to the patient mask (Fig. 10.25A and B). Alterations in inspired oxygen concentration are achieved by entraining oxygen into the suction side of the blower or adding the oxygen flow directly into the patient circuit just before the mask; as a result it is difficult to achieve high inspired concentrations, particularly when the ventilation is delivering its maximum flow rate.

Figure 10.24 Working principles of a non-invasive ventilator.

Figure 10.25 A. Mask for non invasive ventilation. B. Expiratory holes on mask mount for non-invasive ventilation.

With the addition of a pneumotachograph in the inspiratory limb, by sensing changes of 40 ml s⁻¹ in the flow required to maintain the expiratory pressure, sophisticated non-invasive ventilators enable the patient to trigger the commencement of the inspiratory positive airway pressure. Large and variable leaks from the mask make the detection of patient expiration by flow triggering difficult and, hence, less comfortable for the patient than a time cycled expiratory trigger.³⁰

In the newer models of non-invasive ventilators manufacturers have incorporated pressure sensors and TFT matrix screens enabling the graphic display of pressure waveforms (Respironics BiPAP Vision). In addition, the fitting of oxygen sensors into the flow pathway allows more precise control of F_IO₂ and the ability to display the measured value on the screen.

High frequency oscillators

High frequency oscillators such as the SensorMedics 3100B or the Novalung Vision Alpha (Fig. 10.26A) use a device similar to a loudspeaker as the driving mechanism. The diaphragm (Fig. 10.26B) is made to operate at frequencies of 3–15 Hz or 180–900 breaths per minute, although typical starting settings are in the range of 5–6 Hz for adults.