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# (p. 419) Design and function of mechanical ventilators

Design and function of mechanical ventilators
Chapter:
Design and function of mechanical ventilators
DOI:
10.1093/med/9780199600830.003.0092
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date: 17 October 2021

## Key points

• A mode of ventilation is a predefined pattern of interaction between the ventilator and the patient.

• There are nearly 300 commercial names for modes of ventilation and no standardized vocabulary nor classification system. There is a great need to distinguish modes based on tags (classifications) instead of names.

• A taxonomy or hierarchical classification system suitable for comparing modes can be based on 10 fundamental concepts described in this chapter.

• The taxonomy based on these concepts allows description of modes in terms of control variable, breath sequence, and targeting schemes.

• Once modes can be classified, their ability to serve the goals of ventilation can be compared and matched appropriately to patient needs. This is analogous to the way drugs are prescribed.

## Introduction to ventilator design

A mechanical ventilator is an automatic machine designed to provide all or part of the work required to move gas in and out of the lungs. There is a huge variety of ventilator designs, but no standardized classification system. To understand such a vast subject in such a small chapter requires that we depart from the traditional approach of describing schematics and specific design components (e.g. drive mechanisms, valves, pneumatic circuits, etc.) [1]‌. Indeed, the internal operations of modern ventilators are largely unknowable and unimportant to most clinicians. What distinguishes ventilators is their range of technological capabilities, expressed mainly by the ‘modes of ventilation’ they offer. There are nearly 300 commercial names for modes of ventilation and no standardized vocabulary nor classification system. To address this problem, we use a simple taxonomy which is based on the fundamental concepts of pulmonary physiology and ventilator design [2,3,4]. What follows is a description of this approach. These concepts build on one another to yield a practical taxonomy that may be used to simplify the task of comparing and contrasting the features of ventilators, and ultimately, to select the most appropriate mode for a given clinical situation. This didactic approach is informed by over 30 years of experience teaching mechanical ventilation and the data from an international survey [4]. After describing these 10 concepts, we demonstrate how the resulting taxonomy can be used to guide the selection of modes.

## Defining a breath

The most basic function of a ventilator is to deliver a breath. A breath is defined in operational terms using a graph of flow versus time (Fig. 92.1). A breath is defined as one cycle of inspiratory flow followed by a matching expiratory flow, yielding approximately the same volumes. These flows are paired by size, not necessarily by timing. For example, in Airway Pressure Release Ventilation there is a large inspiration (transition from low pressure to high pressure) possibly followed by a few small inspirations and expirations, followed finally by a large expiration (transition from high pressure to low pressure). These inspirations and expirations represent several small spontaneous breaths superimposed on one large mandatory breath. In contrast, during high frequency oscillatory ventilation, small mandatory breaths are superimposed on larger spontaneous breaths.

Fig. 92.1 A breath is defined as one cycle of inspiratory flow followed by a matching expiratory flow. (Left) Pressure control ventilation. (Right) Volume control ventilation.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Ventilators shape the flow waveform using a wide variety of physical systems comprised of a source of high pressure gas (e.g. a compressor), flow control valves, and electronic control systems. A detailed description of these components is beyond the scope of this chapter. However, a generalized schematic of a ventilator is shown in Fig. 92.2. The two most important valves are shown in Fig. 92.3. They are similar in design in that an electronic signal adjusts the position of the actuator such that the orifice diameter changes and thus controls the flow through the valve [5]‌. They work in synchrony, meaning that during inspiration the output flow control valve opens and the exhalation valve closes, forcing gas into the lungs. During expiration, the output flow control valve closes and the exhalation valve opens, allowing gas to flow from the lungs to the atmosphere. In the case of the output flow control valve, the input source is at high pressure (e.g. 10–20 psi) and the output is adjusted by the targeting scheme software to achieve a desired waveform for either inspiratory flow or inspiratory pressure. In contrast, the exhalation valve operates at patient circuit pressures (<1 psi) and generally controls the end expiratory pressure level, rather than the expiratory flow waveform.

Fig. 92.2 Simplified schematic of a modern intensive care ventilator. High pressure gas enters the ventilator through the gas inlet connections for oxygen and air (1,2). Mixing takes place in a reservoir (5) and is controlled by two valves (3,4). Oxygen content is monitored by an oxygen sensor (16). Inspiratory flow from the reservoir is controlled by a separate inspiratory flow control valve (6). On the inspiratory circuit there is a safety valve (7) and two non-return valves (8,9). In normal operation the safety valve is closed so that inspiratory flow is supplied to the patient‘s lungs. When the safety valve is open, spontaneous inspiration of atmospheric air is possible through the emergency breathing valve (8). The emergency exhalation valve (9) provides a second channel for expiration when the exhalation valve (17) is blocked. Also on the inspiratory circuit are an inspiratory pressure (P) sensor (11) and a pressure sensor calibration valve (10). The exhalation circuit consists of the exhalation valve (17), expiratory pressure sensor (13) with its calibration valve (12), and an expiratory flow (F) sensor (18). The exhalation valve is a proportional valve and is used to adjust the pressure in the patient circuit. Conversion of mass flow to volume (barometric temperature and pressure saturated, BTPS) requires knowledge of ambient pressure, measured by another pressure sensor. Pressure in the patient circuit is measured with two independent pressure sensors (11,13). Oxygen flow to the nebulizer port (19) is controlled by a pressure regulator (14) and a solenoid valve (15).

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Fig. 92.3 Schematics of output flow control valve and exhalation valve.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

## Assisting a breath

The definition of a ventilator implies assistance with the patient’s work of breathing. Work is a function of pressure and volume; either the patient’s inspiratory muscles or the ventilator generates an increase in the pressure difference across the lungs (which we will refer to simply as inspiratory pressure; a detailed discussion of the subject is provided in a recent textbook [6]‌) so that their volume increases during inspiration. In terms of ventilator graphic displays, an assisted inspiration can be recognized as one for which airway pressure rises above baseline (positive end expiratory pressure) during inspiratory flow. On the other hand, if airway pressure drops below baseline pressure during inspiration, the patient is doing some work on the ventilator and the breath is ‘loaded’, rather than assisted. Some loading is unavoidable for several reasons—ventilators cannot control airway pressure perfectly so pressure always drops a little with patient inspiratory effort, some pressure drop may be necessary for triggering a breath, and the presence of electrical/mechanical delays between sensing a patient effort and the start of inspiratory flow [7].

## Assistance with volume control versus pressure control

A ventilator provides assistance either by manipulating inspiratory pressure (called pressure control) or by manipulating inspiratory flow (called volume control). To understand this, we employ a mathematical model of patient-ventilator interaction known as the equation of motion for the respiratory system[8]‌:

$Display mathematics$
[eqn 1]

where P(t) is inspiratory pressure generated by the ventilator as a function of time (t), E is respiratory-system elastance, V(t) is volume as a function of time, R is respiratory-system resistance, and $V ˙ ( t )$is flow as a function of time. This is a simplified version that assumes a passive respiratory system; otherwise patient inspiratory effort is represented as muscle pressure, Pmus(t), on the left side of the equation. The equation indicates that the ventilator can either control the left side of the equation (pressure control, implying preset inspiratory pressure, either as a specific target value or in proportion to patient effort) or the right side (volume control, implying preset tidal volume and inspiratory flow). With pressure control, for a given pressure waveform, the inspired volume and flow are functions of elastance, resistance, and time. In contrast, with volume control, for a given flow waveform, inspiratory pressure is a function of elastance, resistance, and time. Idealized waveforms for volume and pressure control are shown in Fig. 92.4.

Fig. 92.4 Idealized ventilator output waveforms. (a) Pressure-controlled inspiration with a rectangular pressure waveform. (b) Volume-controlled inspiration with a rectangular flow waveform. (c) Volume-controlled inspiration with an ascending-ramp flow waveform. (d) Volume-controlled inspiration with a descending-ramp flow waveform. (e) Volume-controlled inspiration with a sinusoidal flow waveform. The short dashed lines represent mean inspiratory pressure, and the long dashed lines represent mean pressure for the complete respiratory cycle (i.e. mean airway pressure). Note that mean inspiratory pressure is the same as the pressure limit in (A). These waveforms were created as follows: (1) defining the control waveform using a mathematical equation (e.g. an ascending-ramp flow waveform is specified as flow = constant × time); (2) specifying the tidal volume for flow- and volume-control waveforms; (3) specifying the resistance and compliance; (4) substituting the preceding information into the equation of motion for the respiratory system; and (5) using a computer to solve the equation for the unknown variables and plotting the results against time.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

For completeness, we mention the case where neither pressure nor volume are controlled and the only preset values are inspiratory and expiratory times. This case is called time control and while rare, does exist in the modes called High Frequency Oscillatory Ventilation and Intrapulmonary Percussive Ventilation.

## Starting and stopping inspiration

To assist a breath, the ventilator must know when to start and stop the inspiratory time (see Fig. 92.1). It does this by monitoring one or more variables and taking action when a preset threshold value is met. Starting inspiration is called triggering. Common trigger variables include time, airway pressure or flow change and the electrical signal from the diaphragm. Stopping inspiration is called cycling. Common cycle variables are the same as for triggering. The amount that the trigger or cycle variables must change before triggering or cycling occurs is called trigger or cycle sensitivity.

## Machine versus patient triggering and cycling

Machine triggering or cycling is the initiation or termination of inspiratory flow, independent of the patient determined components of the equation of motion, i.e. Pmus (effort), elastance, or resistance. Common examples of machine trigger variables are preset frequency and minimum minute ventilation. Common examples of machine cycling variables are preset inspiratory time and tidal volume. Patient triggering or cycling is the initiation or termination inspiratory flow based on one of the patient determined components of the equation of motion, i.e. Pmus, elastance, or resistance. Common examples of patient trigger variables are airway pressure drop below baseline and inspiratory flow due to patient effort. Common examples of cycling variables are peak inspiratory pressure and percentage of peak inspiratory flow. A detailed description of trigger and cycle variables has been presented elsewhere [9]‌.

## Mandatory versus spontaneous breaths

A spontaneous breath is one for which the patient both initiates and terminates inspiration, independent of machine settings, e.g. inspiratory time. Thus, the patient both triggers and cycles inspiration. If the machine triggers or cycles inspiration, the breath is defined as mandatory.

## Breath sequences

There are only two classes of breaths—mandatory and spontaneous—which can be combined in only three basic sequences. If every breath is spontaneous, we call the sequence continuous spontaneous ventilation (CSV). If spontaneous breaths may exist between mandatory breaths (whether or not they actually happen) we call the sequence intermittent mandatory ventilation (IMV). Finally, if spontaneous breaths are not permitted between mandatory breaths, we call the sequence continuous mandatory ventilation (CMV).

## Ventilatory patterns

Combining the concepts of control variable and breath sequence, we can construct a simple means for classifying modes of ventilation. With two main control variables and three possible breath sequences, there are only five basic ventilatory patterns: volume control continuous mandatory ventilation (VC-CMV), volume control intermittent mandatory ventilation (VC-IMV), pressure control continuous mandatory ventilation (PC-CMV), pressure control intermittent mandatory ventilation (PC-IMV), and pressure control continuous spontaneous ventilation (PC-CSV). Volume control continuous spontaneous ventilation (VC-CSV) is not valid because presetting the tidal volume implies machine cycling, which makes spontaneous breaths impossible.

Again, for completeness we note the rare case of time control intermittent mandatory ventilation (TC-IMV) is a ventilatory pattern that describes High Frequency Oscillatory Ventilation and Intrapulmonary Percussive Ventilation.

## Targeting schemes

Ventilatory patterns can be used to sort the large number of mode names into just a few categories. This is very useful for many purposes, but when it becomes necessary to distinguish among modes within the same category, we need to consider the underlying feedback control or targeting schemes used to create the mode. We define a targeting scheme as a model of the relationship between operator inputs and ventilator outputs to achieve a specific ventilatory pattern. Targeting schemes are being developed all the time, but currently, there are only seven that are used for most commercially available ventilators [10]. These targeting schemes are described in Table 92.1.

Table 92.1 Explanation of how targeting schemes transform operator inputs into ventilator outputs

Name

Description

Example mode name

Ventilator

Manufacturer

Set-point

The operator sets all parameters of the pressure waveform (pressure control modes) or volume and flow waveforms (volume control modes)

Simplicity

Changing patient condition may make settings inappropriate

Volume Control Continuous Mandatory Ventilation

Evita Infinity 500

Dräger

Dual

The ventilator can automatically switch between volume control and pressure control during a single inspiration

Can adjust to changing patient condition and assure either a preset tidal volume or peak inspiratory pressure, whichever is deemed most important

May be complicated to set correctly and may need constant readjustment if not automatically controlled by the ventilator

Volume Control

Servo-i

Maquet

Servo

The output of the ventilator (pressure/volume/flow) automatically follows a varying input

Support by the ventilator is proportional to inspiratory effort

Requires estimates of artificial airway and/or respiratory system mechanical properties

Proportional Assist Ventilation Plus

PB 840

Covidien

The ventilator automatically sets target(s) between breaths in response to varying patient conditions

Can maintain stable tidal volume delivery with pressure control for changing lung mechanics or patient inspiratory effort

Automatic adjustment may be inappropriate if algorithm assumptions are violated or they do not match physiology

Pressure Regulated Volume Control

Servo-i

Maquet

Bio-variable

The ventilator automatically adjusts the inspiratory pressure or tidal volume randomly

Simulates the variability observed during normal breathing

Manually set range of variability may be inappropriate to achieve goals

Variable Pressure Support

Evita Infinity 500

Dräger

Optimal

The ventilator automatically adjusts the targets of the ventilatory pattern to either minimize or maximize some overall performance characteristic (e.g. work rate of breathing)

Can adjust to changing lung mechanics or patient inspiratory effort

Automatic adjustment may be inappropriate if algorithm assumptions are violated or they do not match physiology

G5

Hamilton Medical

Intelligent

Targeting scheme that uses artificial intelligence programs such as fuzzy logic, rule based expert systems, and artificial neural networks

Can adjust to changing lung mechanics or patient inspiratory effort

Automatic adjustment may be inappropriate if algorithm assumptions are violated or they do not match physiology

IntelliVent ASV

G5

Hamilton Medical

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

## A taxonomy for modes of ventilation

A mode of ventilation may be generally defined a predetermined pattern of interaction between the ventilator and the patient [3]‌. Referring to a mode using the name coined by the manufacturer has become problematic with the proliferation of new modes. The solution is a hierarchical classification system, a taxonomy that allows us to describe and compare modes at any level of detail. The previous nine theoretical constructs support a practical taxonomy to achieve these goals [11,12,13,14,15,16]. The taxonomy has four levels as follows:

1. 1. Control variable (pressure or volume).

1. A. Breath sequence (CMV, IMV, or CSV).

1. i. Primary breath targeting scheme.

1. a. Secondary breath targeting scheme (only for IMV).

In CMV and CSV there is only one type of breath; mandatory for CMV and spontaneous for CSV. We refer to those breaths as primary breaths when specifying their targeting schemes. For IMV we will consider mandatory breaths primary and spontaneous breaths secondary. An example of how this taxonomy can be used to compare the modes of two common intensive care ventilators is shown in Table 92.2. This table illustrates two very important points. First, modes that are essentially the same or very similar are given very different names by manufacturers and, secondly, because of this, modes are most practically compared using their tags (taxonomic attribute groupings), rather than by their names or ad hoc pseudo-classifications (e.g., VC-CMV is often referred to as ‘assist/control’ in the adult literature, but it means PC-CMV in the paediatric literature).

Table 92.2 Classification of mode names on two common intensive care ventilators

Manufacturer

Model

Manufacturer‘s mode name

Control variable

Breath sequence

Primary breath target scheme

Secondary breath target scheme

Tag

Covidien

840

Volume control plus assist/control

Pressure

CMV

N/A

PC-CMVa

Maquet

Servo i

Pressure-regulated volume control

Pressure

CMV

N/A

PC-CMVa

Covidien

840

Pressure control assist control

Pressure

CMV

Set-point

N/A

PC-CMVs

Maquet

Servo i

Pressure control

Pressure

CMV

Set-point

N/A

PC-CMVs

Covidien

840

Volume support

Pressure

CSV

N/A

PC-CSVa

Maquet

Servo i

Volume support

Pressure

CSV

N/A

PC-CSVa

Covidien

840

Tube compensation

Pressure

CSV

Servo

N/A

PC-CSVr

Covidien

840

Proportional assist ventilation plus

Pressure

CSV

Servo

N/A

PC-CSVr

Maquet

Servo i

Pressure

CSV

Servo

N/A

PC-CSVr

Covidien

840

Pressure support

Pressure

CSV

Set-point

N/A

PC-CSVs

Covidien

840

Spontaneous

Pressure

CSV

Set-point

N/A

PC-CSVs

Maquet

Servo i

Pressure support/CPAP

Pressure

CSV

Set-point

N/A

PC-CSVs

Covidien

840

Volume control plus synchronized intermittent mandatory ventilation

Pressure

IMV

Set-point

PC-IMVa,s

Maquet

Servo i

Synchronized intermittent mandatory ventilation (pressure-regulated volume control)

Pressure

IMV

Set-point

PC-IMVa,s

Covidien

840

Volume ventilation plus synchronized intermittent mandatory ventilation

Pressure

IMV

PC-IMVa,a

Maquet

Servo i

Automode (pressure-regulated volume control to volume support)

Pressure

IMV

PC-IMVa,a

Covidien

840

Pressure control synchronized intermittent mandatory ventilation

Pressure

IMV

Set-point

Set-point

PC-IMVs,s

Covidien

840

Bi-level

Pressure

IMV

Set-point

Set-point

PC-IMVs,s

Maquet

Servo i

Synchronized intermittent mandatory ventilation (pressure control)

Pressure

IMV

Set-point

Set-point

PC-IMVs,s

Maquet

Servo i

Bi-vent

Pressure

IMV

Set-point

Set-point

PC-IMVs,s

Maquet

Servo i

Automode (pressure control to pressure support)

Pressure

IMV

Set-point

Set-point

PC-IMVs,s

Covidien

840

Volume control assist/control

Volume

CMV

Set-point

N/A

VC-CMVs

Maquet

Servo i

Volume control

Volume

IMV

Dual

Dual

VC-IMVd,d

Maquet

Servo i

Synchronized intermittent mandatory ventilation (volume control)

Volume

IMV

Dual

Set-point

VC-IMVd,s

Maquet

Servo i

Automode (volume control to volume support)

Volume

IMV

Dual

VC-IMVd,a

Covidien

840

Volume control synchronized intermittent mandatory ventilation

Volume

IMV

Set-point

Set-point

VC-IMVs,s

a, adaptive; d, dual; r, servo; s, set-point.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Fig. 92.5 provides an algorithm for recognizing the control variable of a mode. Fig. 92.6 shows an algorithm for determining the breath sequence. Table 92.1 provides the information necessary for identifying targeting schemes.

Fig. 92.5 Algorithm for recognizing the control variable of a mode.

Paw, airway pressure.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Fig. 92.6 Algorithm for determining the breath sequence of a mode.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

## Comparing modes

The previous section provides a means for identifying modes. This is useful to clinicians, educators, researchers, and manufacturers alike. However, clinicians are faced with the task of not only identifying modes, but comparing them and selecting the one that best serves the needs of the patient. Yet, trying to compare modes by simply reading the operator’s manuals is impractical. We propose an approach that first identifies the general goals of patient care and then matches them to specific technological capabilities of the available modes [14]. First, we assert that there are only three main goals of mechanical ventilation, safety (optimizing both gas exchange, and the risk of ventilator associated lung injury Fig. 92.7), comfort (optimizing synchrony between patient and ventilator Fig. 92.8), and liberation (optimizing the duration of mechanical ventilation Fig. 92.9) [10]. Next we realize that these general goals and their specific objectives lead naturally to clinical aims for individual patients. Once the patient’s needs are assessed, treatment options in terms of mechanical ventilation modes can be identified using the general technical capabilities of ventilators and design features of specific modes.

Fig. 92.7 Objectives, clinical aims, and ventilator technological capabilities for mechanical ventilation goal of safety.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Fig. 92.8 Objectives, clinical aims, and ventilator technological capabilities for mechanical ventilation goal of comfort.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

Fig. 92.9 Objectives, clinical aims, and ventilator technological capabilities for mechanical ventilation goal of liberation.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

In Table 92.3, we have identified the technological capabilities of currently available targeting schemes (Table 92.1) and grouped them according to the three goals of ventilation. Then we have listed the mode names from Table 92.2 and noted which technological capabilities they offer. Note that some columns have no check marks because they represent capabilities of modes on other ventilators.

Table 92.3 Comparison of technological capabilities for the modes in Table 92.2

Mode name

Mode tag

Automatic adjustment of minute ventilation target

Automatic adjustment of support in response to changing respiratory mechanics

Automatic adjustment of minute ventilation parameters (f, VT)

Manual adjustment of minimum minute ventilation parameters (f, VT)

Automatic adjustment of end expiratory lung volume

Automatic adjustment of ventilation parameters within lung protective limits

Minimize tidal volume

Safety Capabilities

All breaths are spontaneous with patient effort

Trigger/cycle based on signal representing chest wall/diaphragm movement

Coordination of mandatory and spontaneous breaths

Automatic limitation of autoPEEP

Unrestricted inspiratory flow

Automatic adjustment of flow based on frequency

Automatic adjustment of support to maintain specific breathing pattern

Automatic adjustment of support proportional to patient demand

Comfort Capabilities

Ventilator initiated weaning of support

Ventilator recommends liberation

Automatic reduction of support in response to increased patient effort

Liberation Capabilities

Automode (Pressure Regulated Volume Control to Volume Support)

PC-IMVa,a

3

3

1

Automode (Volume Control to Volume Support)

VC-IMVd,a

3

3

1

Volume Support

PC-CSVa

1

2

1

Pressure Regulated Volume Control

PC-CMVa

2

1

1

Synchronized Intermittent Mandatory Ventilation (Volume Control)1

VC-IMVd,s

1

2

0

Proportional Assist Ventilation Plus

PC-CSVr

0

3

0

Volume Control Synchronized Intermittent Mandatory Ventilation

VC-IMVs,s

1

1

0

Pressure Support

PC-CSVs

0

2

0

BiLevel

PC-IMVs,s

0

2

0

Pressure Control Synchronized Intermittent Mandatory Ventilation

PC-IMVs,s

0

2

0

Automode (Pressure Control to Pressure Support)

PC-IMVs,s

0

3

0

Pressure Control Assist Control

PC-CMVs

0

1

0

Volume Control Assist Control

VC-CMVs

1

0

0

1 Maquet Servo i ventilator

PC, pressure control; VC, volume control; CMV, continuous mandatory ventilation; IMV, intermittent mandatory ventilation; CSV, continuous spontaneous ventilation Targeting scheme; s, setpoint; d, dual; r, servo; a, adaptive.

Courtesy of Mandu Press Ltd., Cleveland Heights, Ohio.

## Conclusion

In this chapter we introduce a new approach to understanding the design and function of mechanical ventilators. These devices have become so complex that a formal classification system (taxonomy) is required to compare and contrast treatment options. We have shown how such a taxonomy can be constructed from 10 fundamental maxims of ventilator design. Finally, we have demonstrated how a formal description of ventilator technology can be used to match specific patient needs identified from clinical assessment to particular mode capabilities. This approach highlights the requirement for a thorough patient evaluation, identification of the goals of care and an understanding of ventilator technology.

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