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How to choose a Ventilator System Purpose.......................................................................................................................................................... 2 Basics ............................................................................................................................................................ 3 Inspiratory and expiratory valves and output variable control .................................................................... 7 Phase variables: trigger, cycling and limit ..................................................................................................... 8 Modes of mechanical ventilation ................................................................................................................11 Control system.............................................................................................................................................13 User interface ..............................................................................................................................................14 Safety and alarm systems............................................................................................................................15 Monitoring system ...................................................................................................................................... 16

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Purpose

The purpose of this guide: A range of Ventilators systems is available to the intensive care unit (ICU) physician, something that may in theory enable him or her to treat different forms of respiratory failure more effectively. New ventilator modes are frequently introduced so whether there is a actual need of these modes or these modes just used by the manufactures for business purpose? These grievances would be addressed by this guide.

New modes might to some extent assist the clinician in daily practice, scientific evidence proving their effectiveness is often lacking .When purchasing a ventilator, clinicians rely on personal experience and on the Results of small observational trials showing positive effects on Physiological variables, such as oxygenation and work of breathing; And on subjective variables, such as patient comfort .

Because manufacturers’ specifications alone are of limited importance, they should not be a prominent factor in decision making. A basic knowledge of the principles of ventilator functioning may be helpful when choosing a mechanical ventilator, enabling the purchaser to consider its technical performance in relation to the clinical characteristics of the patients to be treated, the healthcare environment and the financial resources available.

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Basics

Basic principles of ventilator function Briefly, a mechanical ventilator can be considered as a series of consecutive functions that turn an input (energy) into an output (ventilator variable), such as, pressure, flow or volume. It can transfer energy by applying positive pressure to the airways, acting as a positive pressure ventilator (PPV), or by applying sub atmospheric pressure externally to the chest, acting as a negative pressure ventilator.

Table 1 Fundamental elements of a positive pressure ventilator 1. Pneumatic system 2. Inspiratory and expiratory valves and output variable control 3. Phase variables: trigger, cycling off and limit 4. Modes of mechanical ventilation 5. User interface 6. Safety and alarm systems 7. Monitoring system

1. Pneumatic system: Based on the gas source, ICU PPVs can practically be divided into two categories: 1) those that work with oxygen and air at high pressure (4 atm (400 kPa)); 2) those that work with oxygen at high pressure (4 atm (400 kPa)) and atmospheric air.

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Category 1. Oxygen and air at high pressure PPVs: High-pressure air and oxygen are the external source of energy for this type of ventilator Below are few of the category 1 Ventilator Systems

Fig1.eVent Inspiration LS category 1 ventilator

2. Hamilton Medical Galileo, G5 and Raphael XTC category 1 ventilator

Figure 8. Viasys Avea category 1 ventilator

Figure 7. GE Engstrom Carestation category 1 ventilator 4

Category 2 Oxygen at high pressure (4 atm) plus atmospheric air PPVs :

In these ventilators, a piston or turbine sucks atmospheric air from the environment. Although piston turbine driven ventilators are usually used for homecare positive pressure ventilation the simultaneous use of high-pressure oxygen and a sophisticated user interface make these PPVs suitable for critical care use.

Internal battery Most category 1 PPVs have an internal battery, usually with a short life per charge.

Interestingly, some category 1 PPVs have a “plug and play” battery system (e.g.

Maquet Servo i, Dräger Evita XL, Hamilton Medical G5) to increase battery life.

Some category 1 ventilators (e.g. eVent Inspiration LS, Viasys Avea) have an internal battery that can power an internal compressor. Some PPVs, especially those of category 2, have a very long-lasting (4–9 h) internal battery (e.g. Covidien Airox Supportair, ResMed Elisée 350, Viasys Vela). Others may be equipped with a supplemental battery providing energy for up to 4–7 h (e.g. Dräger Savina, VersaMed iVent 201 IC, Hamilton Medical C2). This can be very useful when transporting critical patients inside the hospital.

Below are some images of category 2 Ventilator System

ResMed Elisée 350 category 2 ventilator

Respironics Esprit category ventilator

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Recommendations to the buyer

1. The buying team must be aware of the differences in terms of gas input and type of control system when choosing a critical care ventilator. In areas where compressed air is not available, a turbine-driven ventilator with additional oxygen (category 2) may replace compressed air.

2. In all category 2 ventilators that also have a high-pressure oxygen inlet preset oxygen tension remains constant regardless of the minute volume.

3. Some turbine-driven ventilators also have the option of a lowpressure oxygen inlet. When using this option, oxygen tension is never constant.

4. The charge-life of the internal battery as well as the possibility of using a supplemental battery must be taken into account in areas where mains power is frequently interrupted and where electrical back-up is inconsistent.

5. When choosing category 2 PPVs in areas where oxygen is provided only in tanks, ventilator oxygen consumption for a given minute ventilation must be taken into account. In some markets, oxygen may be a real cost issue.

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Inspiratory and expiratory valves and output variable control The inspiratory valve is meant to control respiratory cycle phases, along with the expiratory valve. In category 1 PPVs and some category 2 PPVs, the valve manages the output of the ventilator (set point, auto-set point, servo, adaptive and optimal control with controlled proportional valve).

In many category 2 PPVs, the inspiratory valve has only an on-off function: pressure and flow both depend on the mechanical system In current-generation category 1 ventilators and in some category 2 ventilators, three waveforms are available: rectangular; descending ramp in volume-targeted mode; and exponential decay in pressure targeted mode Manufacturers refer to pressure limited ventilation that allows patients to breathe spontaneously throughout the respiratory cycle by various names (e.g. assisted positive-release ventilation (APRV) as well as BIPAP in the Dr채ger Evita 2 dura, 4 and XL and in the BIPAP Savina; BiLevelTM in the Covidien PB 840 and GE Engstrom Carestation; BIVENT in the Maquet Servo i; BiPhasic in the Viasys Vela and Avea, etc.) [8].

Recommendations to the buyer

1. Control of output variables and valve functioning are very important when choosing a PPV, regardless of its pneumatic system.

2. The use of microprocessor controlled expiratory valves or electromagnetic valves operated by an actuating shaft can reduce exhalation system inefficiencies

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Phase variables: trigger, cycling and limit Three phase variables define inspiration.The trigger that begins inspiration (pressure-, volume-, flow- and time-dependent).

• The limit that cannot be exceeded during inspiration (pressure, volume, and flow).

• The cycling-off criteria.

Trigger variable During mechanical ventilation a breath can be initiated by [6,11]: • time (timed mandatory breaths set by the operator); • pressure (assisted breath pressure-triggered); • flow (assisted breath flowtriggered); • volume (assisted breath volumetriggered);or • diaphragmatic activity (Neurally Adjusted Ventilatory Assist (NAVA)).

The term “mandatory breath” defines a breath that is initiated and cycled by the ventilator. During partial ventilator assistance (assisted breath), the inspiratory synchronisation system (trigger) detects any patient inspiratory effort and activates a mechanical act. The goal of a good inspiratory trigger is to reduce as much as possible the duration and intensity of the muscular effort that comes before mechanical support, while avoiding auto trigger effects. While pressure triggering allows detection of a pressure drop within the circuit (at the airway opening or inside the ventilator) due to the patient’s inspiratory effort, flow triggering is achieved with the measurement of flow by using a pneumotachograph at the airway opening (e.g. Hamilton Medical ventilators, Viasys Pulmonetic LTV 1200) or inside the ventilator (most category 1 and category 2 ventilators).

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Cycle variable A breath can be pressure-, time-, volume- or flow-cycled. A breath in a flowcycled ventilation mode (PSV, inspiratory positive airway pressure (IPAP) or assisted spontaneous breathing (ASB) for Dräger ventilators) is cycled when inspiratory flow reaches a given threshold value (default at 25% of peak flow in most PPVs, but adjustable in others). The inspiratory flow threshold value, also called “expiratory trigger”, thus controls the inspiration-to expiration switch in these modalities.The aim is to detect the very end of patient inspiration through inspiratory flow, measurement. Its goal is to optimize synchronisation between spontaneous patient inspiratory time and ventilator inspiratory time. 09 HOW TO CHOOSE AN INTENSIVE CARE UNIT VENTILATOR

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IRecommendations to the buyer

1. All commercial PPVs, except ones dedicated mainly to NIV (e.g. Respironics Vision) offer both pressure- and volume-targeted ventilation.

2. In spite of clear evidence that a flow trigger performs better than a well-set pressure trigger [6] the sensitivity of pressure or flow trigger setting is mandatory to avoid patient–ventilator dyssynchrony.

3. When combining different kinds of inspiratory trigger, manufacturers should specify their ranking logic clearly. One ventilator (Maquet Servo i) offers the option of using a neural trigger through NAVA. 4. Almost all category 1 and category 2 PPVs enable the user to set the sensitivity of the expiratory trigger threshold (expiratory trigger). The wider the range (e.g. 1– 90% of peak expiratory flow), the higher the chance of matching the patient’s neural inspiratory time.

5. When using turbine-driven or piston ventilators, the buyer must be aware of the set-up governing peak inspiratory pressure above PEEP/EPAP during pressurelimited ventilatory modes.

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Modes of mechanical ventilation Various modes of ventilators are: PAV, PAV+ and PPS PAV can be viewed as a ventilation mode in which the pressure within each breath is titrated by the ventilator n proportion to patient inspiratory flow, which is used as an estimate of patient respiratory muscle effort . The proportionality between flow and airway pressure is determined by a “gain” setting, which is adjusted by the clinician to determine the proportion of the total work of breathing to be performed by both ventilator and patient.

NAVA NAVA is a new form of assisted ventilation that takes into account most principles of proportional ventilation. Pressure is applied by the ventilator in proportion to the electrical activity of the diaphragm, recorded with a dedicated nasogastric tube with a multiple array of electrodes placed in the distal oesophageal portion .

ATC In order to reduce patient work of breathing arising from endotracheal tube resistance, some ventilators (e.g. Hamilton Medical ventilators) keep tracheal pressure constant during inspiration and expiration. The output delivered by the ventilator adapts automatically and immediately to the inspiratory effort of the patient. This control is named ATC.

Dual modes To generate further confusion among buyers, the so-called “dual modes of ventilation” (e.g. PRVC or VAPS) have recently been introduced in the critical care area. The buyer must be aware that even if the target of the dual mode is a given tidal volume, this volume can be reached by increasing flow or by increasing pressure to the airway to reach the preset volume.

ASV ASV may be thought of as an “electronic ventilator protocol” that incorporates the most recent and sophisticated measurement tools and algorithms in an attempt to make ventilation safer, easier, and more consistent.This mode is designed to accommodate not only ventilated patients who are passive, but also those who are breathing actively.

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Recommendations to the buyer

1. The buyer should determine which ventilatory modes will be used regularly in order not to purchase sophisticated ventilatory mode.

2. The buyer must be aware of the limitations of dual modes of ventilation such as PRVC/Auto Flow, etc 3. Evidence is lacking that any ventilation mode provides better outcomes, though there are different potential advantages.

4. Should result in cost savings.

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Control system The control system controls or servo-controls output variables and activates safety systems. It can work with either a pressure or a flow feedback. All category 1 and 2 ventilators use a closed-loop control that maintains a constant ventilator output by using output as a feedback signal that is compared with the operator-set input Recommendation to the buyer

1. The buyer must be aware of the different places where the feedback signal is recorded.

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User interface The control panel allows the user to interact with the ventilator in order to set ventilator parameters and verify through monitoring that they are correctly applied.

Some parameters are set directly whereas other are derived from measurements.

With the current generation of PPVs, the user interface is commonly a touch pad and/or rotary encoder with or without a touch-screen control. Some ventilators use an intelligent interface to decrease the demand on the user's cognitive resources (e.g. "dynamic lungs" Hamilton Medical G5). Only two category 2 PPVs can be removed from their conventional user interface (as docking stations of a PC) to be used as transport ventilators with their built-in user interface

Recommendations to the buyer

1. User interfaces vary significantly among PPVs. Unfortunately there is no consistency among manufacturers in terms of labeling ventilator functions, particularly ventilation modes.

2. The large number of ventilator modes, settings and monitoring can result in systems that are poorly designed, non-intuitive and almost impossible to remember, with the subsequent need for clinicians to refer to manual ventilator instructions in order to interpret poorly labeled controls or to navigate multiple levels of software.

3. The buyer must choose the ventilator according to current need (number of beds, emergency room area, different operators rotating in the shifts, etc.).

4. Last but not least, another practical consideration is whether the monitor displaying the settings, curves, alarms, etc., is easily readable.

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Safety and alarm systems The ventilator safety system aims at avoiding any damage to the patient due to ventilator malfunction. In case of electrical failure, there is a room-air inlet that will let the patient breathe through a simultaneous opening of the inspiratory and expiratory valves. This may be of little or no help, however, in patients who are sedated and paralyzed. Another safety system is the presence of an overpressure valve, positioned between the inspiratory and expiratory valves, which can unload any excess pressure in the circuit. In most ventilators, it is usually set to open above a 100 cmH2O threshold and thus it does not limit the risk of barotrauma.

All ventilators in a critical care area should have alarms for: • Electrical failure; • Compressed gas supply problems; • Patient disconnection from the ventilator; • Changes in airway pressure (in volume-targeted mode); • Changes in tidal volume (in pressure-targeted mode); • Changes in minute ventilation (in volume- or pressure-targeted modes); and • Changes in Fi, O2.

Recommendations to the buyer

1. It is imperative that they respond in a safe manner in caseof ventilator malfunction (pneumatic or electrical failure).

2. In spite of the safety system now provided in all PPVs, the buyer must be aware of the presence in the hospital of electrical power generators in case of main power blackout. Users must be informed of the units attached to the emergency generator.

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Monitoring system The monitoring system is not a part of the ventilator itself and its absence will not jeopardize proper ventilator functioning. However, it is of the utmost importance in optimizing ventilator assistance. Some turbine-driven ventilators can be connected to a full-size screen to provide additional monitoring. Thus, even though clinical and instrumental monitoring based on patient vital signs is very relevant, frequent control of both preset parameters on the ventilator and their actual application through machine-integrated systems is crucial in the ICU setting. In any ventilator, directly measurable variables are the pressure applied to the airway and the flow, while other parameters can be derived from the analysis of these signals.Continuous display of airway pressure, flow, and/or volume curves is available with all current ICU ventilators. Volume is usually calculated through the integration of flow. Although there is still a lack of adequate easy-to-use technologies, this type of monitoring may provide useful information on the respiratory mechanics of spontaneously breathing patients at the bedside .

Recommendations to the buyer 1. The buyer should determine which monitoring capabilities will be used regularly in order not to purchase sophisticated monitoring systems.

2. In adult patients, there may be no significant differences between the two sites of measurement: distal (inside the ventilator) or proximal (at the airway opening), except for a better detection of start of inspiration and end of inspiratory effort.

3. Regarding the monitoring of respiratory mechanics, pressure–volume curves (low inflation technique) are still a point of discussion with regard to strategies for setting mechanical ventilation in patients with acute respiratory distress syndrome.

However, it should be kept in mind that interpreting the information provided by these curves and drawing therapeutic conclusions may require some experience or at least clear strategies to deal with the results.

4. Finally, it must be borne in mind that the sensor accuracy of many ventilators lacks proper validation, and thus measured data can differ (even substantially) from real values.

5. Should result in cost savings

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