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Ventilator Monitoring 

Ventilator Monitoring
Chapter:
Ventilator Monitoring
Author(s):

Yuan Lei

DOI:
10.1093/med/9780198784975.003.0011
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date: 24 July 2021

11.1 Introduction

During mechanical ventilation, gas pressure, volume, and flow in the ventilator system fluctuate. We cannot see the pneumatic signals directly with our eyes, but we can observe them with special monitoring devices.

Ventilator monitoring refers to a group of specific ventilator functions that involves measuring the pneumatic and non-pneumatic signals at defined locations and displaying them in various ways.

Ventilator monitoring devices may be as old as the ventilator itself. Fig.11.1 shows the monitoring devices integrated in ventilators that are 150, 65, and 5 years old. The red frames show the displayed monitoring results.


Fig. 11.1 Evolution of ventilator monitoring.

Fig. 11.1 Evolution of ventilator monitoring.

Ventilator monitoring is also one of the fastest evolving areas of ventilator technology. It has expanded in three directions. First came the monitoring of new signal types, such as O2, PetCO2 (end-tidal partial pressure of CO2), gas temperature, and SpO2; second, new monitoring information that is extracted from directly monitored signals; and third, the improved visualization of the monitoring for easier understanding.

A good example of the third is the ‘dynamic lung’ on the HAMILTON-G5 ventilator (Fig.11.2). Monitored respiratory compliance and airway resistance are typically presented numerically. This new feature shows the monitoring results graphically. The user gets much valuable information at a glance from the shape of the airway tree and the lungs.


Fig. 11.2 The dynamic lung graphic on the HAMILTON-G5 ventilator.

Fig. 11.2 The dynamic lung graphic on the HAMILTON-G5 ventilator.

This chapter will focus on the basics of ventilator monitoring, including:

  • General monitoring concepts;

  • The ventilator monitoring system;

  • Conditions required for monitoring;

  • Presentation of monitoring results;

  • Common ventilator monitoring parameters.

11.2 General monitoring concepts

Before beginning our discussion of ventilator monitoring, let’s clearly define a few general concepts related to monitoring technology.

11.2.1 Definition of four common terms

When talking about monitoring, the technical staff often use some special terms:

  • The monitoring is accurate;

  • The monitoring is reproducible;

  • The monitoring has a high resolution;

  • The monitoring is precise.

What do they really mean? Are they the same or different? Let’s discuss them one by one.

Accuracy

Accuracy refers to the deviation between the truth and the monitored results. If the deviation is sufficiently small, the monitoring is accurate. Otherwise, the monitoring is not accurate.

Reproducibility

If we measure the same object repeatedly under the same conditions, we may still get different results. Reproducibility refers to the variability of the measurement results. The smaller the variability, the more reproducible the measurements are.

Precision

Precision is generally synonymous with reproducibility. A measurement system can be (a) accurate but not precise, (b) precise but not accurate, (c) neither accurate nor precise, or (d) both accurate and precise.

Resolution

Resolution is the ability to ‘resolve’ the differences of measurement results. A measurement device with higher resolution uses more gradations to report its measured results. A good way to understand resolution is to look at the scale of a watch (Fig.11.3). Resolution is unrelated to accuracy—a watch with low resolution can be very accurate, while a watch with high resolution can be inaccurate.


Fig. 11.3 Two examples to show the meaning of resolution.

Fig. 11.3 Two examples to show the meaning of resolution.

We can see the differences between accuracy, reproducibility or precision, and resolution by using a game of darts as an example (Fig.11.4). The blue centre represents the truth, while the red dots represent the measurement results.


Fig. 11.4 The relationship between accuracy, reproducibility, and resolution.

Fig. 11.4 The relationship between accuracy, reproducibility, and resolution.

11.2.2 Technical tolerance

Ideally, measurements should always be absolutely accurate and precise. In many cases, however, absolute accuracy is unfeasible, unaffordable, and unnecessary. For instance, if the true tidal volume of a breath is 500 ml, yet the monitored result is 501 ml or 499 ml, does the tiny difference matter clinically? In practice, ‘sufficient’ accuracy is typically satisfactory. The term ‘tolerance’ or technical tolerance refers to an acceptable range of values within which a measurement is considered to be accurate.

Let’s look at an example. A ventilator manual states that the pressure monitoring tolerance is ‘±2 cmH2O or ±10%, whichever is greater’. What does this mean?

In Fig.11.5, the x-axis shows actual airway pressure, and the y-axis shows measured airway pressure. The light blue line represents the absolute accuracy.


Fig. 11.5 Absolute accuracy and technical tolerances.

Fig. 11.5 Absolute accuracy and technical tolerances.

The tolerance described in the manual is the combination of two ranges, ±2 cmH2O and ±10%. The range of ±2.0 cmH2O is defined by two red lines parallel to the blue line. The range of ±10% is shown by the two yellow lines. The range widens as the pressure increases.

The light blue area is the tolerance defined by the two ranges. All measurements inside the light blue area are considered to be sufficiently accurate. The measurements outside that area are not considered to be sufficiently accurate.

When we discuss tolerance, we need to consider the relevant range of the measurement. For instance, the normal range of airway pressure is 0 to 40 cmH2O. Here, the tolerance of ±2 cmH2O may be appropriate. However, this same tolerance may be inappropriate for the measurement of a gas cylinder where the pressure range is between 0 and 200 bars.

The concept of tolerance applies not only to ventilator monitoring, but also to gas delivery by a ventilator system.

11.2.3 Calibration

A ventilator has various sensors, such as flow sensors, an oxygen sensor, and a CO2 sensor. They measure the magnitude or concentration of intended signals in specific locations.

Ventilator manufacturers often recommend periodic calibration of these sensors in order to ensure monitoring accuracy. What is calibration? Why is it necessary?

Calibration is the act of checking or adjusting the accuracy of a measuring instrument. Calibration involves comparing the measuring instrument with a standard. Proper calibration can improve the monitoring accuracy of a ventilator system, so it is sensible to conduct calibration as recommended.

11.3 Ventilator monitoring system

Inside a ventilator, the monitoring system should be functionally independent from the pneumatic operations so that the displayed readings tell the truth.

The material foundation of ventilator monitoring is a subsystem with three parts: (a) a sensor, (b) a signal processing device, and (c) a display device. They are typically connected in series.

11.3.1 Sensor

A sensor is a device to sense the magnitude of an intended signal, to convert it into an electronic signal, and to feed the converted signal to the signal processing device.

Conventionally, gas pressure, flow, and oxygen concentration are measured in all ventilators. A modern intensive care unit (ICU) ventilator may measure even more types of signal, such as PetCO2, gas temperature, and SpO2. The Maquet SERVO-i ventilator uses a newer technique called NAVA (neutrally adjusted ventilatory assist), where a special oesophageal catheter sensor captures neural (bioelectric) signals.

A flow sensor is a common device to measure pneumatic signals, typically gas flow and pressure, in the gas passageway of a ventilator system. Flow sensors may use various operating principles, such as a fixed or variable orifice, a heating wire, or ultrasonic techniques. Each operating principle has its own strengths and weaknesses. As clinical ventilator operators, we may not need to know these sensor principles in detail. A wealth of information is available elsewhere for anyone who wants to know more.

What we do need to know, however, is that a sensor may behave very differently under laboratory conditions than under clinical conditions. For instance, a flow sensor can operate perfectly when the passing gas is clean and dry. However, if the passing gas contains moisture, secretions, and medication aerosols, the same flow sensor may not work as well or at all. This explains why monitoring quality worsens over time for a flow sensor installed in the airway or the expiratory limb of the circuit.

In most cases, a sensor has a specific location. For instance, for FiO2 monitoring the oxygen sensor (cell) should be in the inspiratory limb, and for PetCO2 monitoring the CO2 sensor should be at the airway.

Typically, a ventilator has two flow sensors. The inspiratory flow sensor is always in the inspiratory limb. The second may be positioned at the expiratory valve or the airway (Fig. 11.6). Conventionally, a flow sensor at the expiratory valve is called a distal flow sensor and a flow sensor at the airway is a proximal flow sensor. The position of the second flow sensor may noticeably affect the monitoring. Table 11.1 summarizes the differences.

Table 11.1 Comparison of distal and proximal flow sensors

Distal flow sensor

Proximal flow sensor

Applications

Most ventilators

Most neonatal ventilators and Hamilton Medical ventilators

Advantages

  • Integrated into the ventilator

  • No need for a proximal flow sensor, which is perceived as an extra accessory with associated potential problems

  • Directly measures airway flow and pressure

  • Much better signal-to-noise ratio

  • Less sensitive to circuit leaks

  • In volume modes, VT monitoring is minimally affected by circuit compliance

Weaknesses

  • Airway flow is not directly measured, but is calculated from the difference between the simultaneously measured inspiratory flow and expiratory flow

  • Inferior signal-to-noise ratio under comparable conditions

  • Susceptible to circuit leaks

  • In volume modes, the patient may receive a lower VT than that delivered, due to circuit compliance (refer to section 5.3.4). Additional correction is necessary

  • Perceived to be an extra accessory

  • Increased probability of airway disconnection

  • Increased airway dead space (refer to section 5.3.5)

A flow sensor can only measure the flow of the gas that passes through it at its assigned location, which may confuse the clinician. For instance, let’s say we are ventilating a neonate in P-SIMV mode. There is a leak around the endotracheal tube (ETT), which is typically uncuffed in neonates. The ventilator reports that the monitored VT is a surprisingly low 5 ml, and it activates a low VT alarm. We expected a VT of approximately 20 ml, and we fear that the baby cannot survive with such a low tidal volume. Then we start to question whether the ventilator is defective.

To our surprise, our technician tells us that the ventilator is working properly. The reported VT is so low, because much of the exhaled gas has leaked, while only a fraction passed through the flow sensor and was (correctly) measured (Fig.11.7). In this case, the ventilator monitoring is functioning properly, but the sensor senses only a part of the flow, not the whole. The flow and volume readings return to normal after the gas leak is eliminated.


Fig. 11.7 A flow sensor can only detect the gas flow that passes through it.

Fig. 11.7 A flow sensor can only detect the gas flow that passes through it.

11.3.2 Signal processing device

A modern ICU ventilator displays a number of monitoring parameters. These may be grouped into three categories:

  1. a. Direct measurements;

  2. b. Monitoring parameters that are extracted from direct measurements;

  3. c. Monitoring parameters that are calculated from direct measurements.

There are just a small number of signal types that are directly measured. These commonly include gas flow, gas pressure, and the O2 concentration of the inspiratory gas. Other possible signal types include PetCO2, gas temperature, and SpO2. This direct measurement is continuous and in real time. The results may be displayed graphically, such as in waveforms. The original signals are often weak and full of noise, so that signal amplification and filtering are often the first steps required.

The ventilator processes these direct measurements to create additional monitoring parameters through extraction and calculation (Fig.11.8):

  • For extracted parameters, the ventilator takes certain pieces of information from the measured results in defined ways. A typical example is peak airway pressure.

  • For calculated parameters, the ventilator computes results using special mathematical models. A typical example is expiratory tidal volume.


Fig. 11.8 Overview of signal sensing, processing, and displaying.

Fig. 11.8 Overview of signal sensing, processing, and displaying.

The extracted and calculated parameters are typically displayed numerically.

11.3.3 Display device

A display device is designed to visualize the monitoring data. The device can be a gauge scale, a digital display, or liquid crystal display (LCD) in various shapes, colours, and sizes. In rare cases, the monitoring data takes the form of audible signals.

A modern ventilator typically has an LCD screen, which provides versatility for the data display. In section 11.5, we will discuss in depth the presentation of monitoring results.

11.4 Required conditions for ventilator monitoring

Ventilator monitoring is conditional: it functions as expected only when all required conditions are fully satisfied. If the displayed monitoring results are outside the expected range, there are three possibilities:

  1. A. The monitoring system is malfunctioning, because:

    • One or more components are missing, incompatible, or inoperative;

    • Components are incorrectly or insecurely connected;

    • The sensor is positioned incorrectly;

    • The inner surface of a flow sensor is coated with moisture, secretions, or aerosolized medication;

    • The monitoring system is not calibrated as recommended.

  2. B. The monitoring system is functioning properly, but the ventilator system is malfunctioning, because:

    • One or more required parts are missing, incompatible, or inoperative;

    • Components are incorrectly or insecurely connected;

    • The system has a noticeable leak;

    • The system has a noticeable occlusion in its gas passageway.

  3. C. Both A and B

11.5 Presentation of monitoring results

The results of ventilator monitoring must be presented so that a human being can see and understand them. Conventionally, ventilator monitoring results are presented numerically and graphically.

11.5.1 Numerical parameters

Numerical parameters are the most common way to present monitoring results. The displayed parameters are extracted or calculated from direct measurements. Parameters are typically updated after every mechanical breath.

For each monitoring parameter, there are three components: the term, the unit of measurement, and the current value (the signal magnitude or concentration), as shown in Fig.11.9.


Fig. 11.9 The monitoring window of the GALILEO ventilator.

Fig. 11.9 The monitoring window of the GALILEO ventilator.

The major advantage of the numerical presentation is digital precision, which does not necessarily mean high accuracy.

Its major disadvantage is that it is difficult to link the displayed value to the patient’s clinical condition. Furthermore, a numerical parameter is just a snapshot: it does not show change over time. For instance, a tidal volume of 500 ml in an adult might appear to be normal or acceptable. It can signal a problem if it was 800 ml 10 minutes ago, provided that the ventilator settings were not changed.

Be aware that some ventilator controls and monitoring parameters share the same terms (e.g. PEEP, FiO2, rate, and tidal volume). VT as a control parameter means the desired or targeted tidal volume, while VT as a monitoring parameter means the actual tidal volume. For this reason, if someone tells you that the tidal volume for a patient is 600 ml, always ask whether they are referring to the set tidal volume or the monitored tidal volume.

11.5.2 Graphic presentations

Graphic presentations of monitored data take three common forms: waveforms, dynamic loops, and trend curves.

Waveforms

Waveforms are the most common way to graphically display directly measured parameters. A typical ventilator displays waveforms of gas pressure, flow, and (calculated) volume (Fig.11.10). PetCO2, SpO2, gas temperature, and others may optionally be displayed.


Fig. 11.10 Pressure, flow, and volume waveforms displayed on the Evita 4 ventilator from Dräger (left) and the GALILEO ventilator (right).

Fig. 11.10 Pressure, flow, and volume waveforms displayed on the Evita 4 ventilator from Dräger (left) and the GALILEO ventilator (right).

The waveform is a running curve on a chart with time as the x-axis and the monitored parameter as the y-axis. Typically, two or three waveforms share the same time scale so that we can observe them simultaneously.

To read and understand a waveform, we must identify four basic elements: (a) the displayed parameter; (b) the unit of measure; (c) the x and y-axes; and (d) the scales of these axes.

By comparing these side by side, we can identify four differences between the waveforms:

  • The graphical presentations are different, with different colour schemes and with the waveforms in the Evita 4 filled in.

  • The two ventilators display these same three parameters in different orders.

  • The volume waveform of the Evita 4 is in litres (L), while that of the GALILEO waveform is in millilitres (ml).

  • The two waveforms have slightly different time scales: 16 s for the Evita 4, and 10 s for the GALILEO.

Note that scaling of the x or y-axis may cause the same waveforms to look very different. The two waveforms shown in Fig.11.11 are identical, but have different y-scales. The y-scales of the left-hand graph range from 0–16 cmH2O, while that of the right-hand graph range from 0–64 cmH2O.


Fig. 11.11 A waveform can look very different with different Y scales.

Fig. 11.11 A waveform can look very different with different Y scales.

Provided that the monitoring is accurate, the shape or profile of a waveform is mainly determined by four factors:

  • The patient’s respiratory mechanics (e.g. respiratory compliance and airway resistance);

  • The functional status of the ventilator system;

  • The ventilator settings, mainly the ventilation mode and control parameters;

  • The patient’s breathing activities.

Any change in the four factors results in a corresponding change in the waveforms.

Waveforms are a rich source of useful information. Waveform analysis and interpretation require deep understanding of mechanical ventilation, the ventilator system, and their interactions. Details on waveform analysis are beyond the scope of this book, but they can be found in many readily available information sources.

Waveforms are running curves. They are constantly updated, and the old waveform data is continuously discarded unless it is saved with special equipment.

Shapes of normal waveforms

To recognize an abnormality in a waveform, we must be truly familiar with the shapes of normal waveforms. These shapes are directly related to the types of mechanical breaths, which we discussed in section 7.3 and which are summarized in Table 11.2. Fig.11.12 shows some normal types of waveform.

Table 11.2 Eight breath types and their essential variables

Type

Basic breath type

Trigger variable

Cycle variable

Control variable

A

Volume control

Time

Time

Volume

B

Pressure control

Time

Time

Pressure

C

Volume assist

Patient

Time

Volume

D

Pressure assist

Patient

Time

Pressure

E

Pressure support

Patient

Flow

Pressure

F

Adaptive control

Time

Time

Adaptive

G

Adaptive assist

Patient

Time

Adaptive

H

Adaptive support

Patient

Time

Adaptive


Fig. 11.12 Waveforms of normal breath types in passive and active patients.

Fig. 11.12 Waveforms of normal breath types in passive and active patients.

Dynamic loops

Monitoring data, mainly directly measured parameters, can also be presented as dynamic loops (Fig.11.13). The common signal types shown include airway pressure, flow, and volume.


Fig. 11.13 Two common dynamic loop types.

Fig. 11.13 Two common dynamic loop types.

A dynamic loop is shown on a two-dimensional graph. One monitoring parameter is plotted as the x-axis, and another monitoring parameter as the y-axis. As the monitoring data continuously becomes available, the loop is dynamically displayed in real time.

There are several possible combinations of the three signals. In clinical practice, however, only two loop types are widely used and studied: pressure-volume loops and flow-volume loops.

Pressure-volume loop

In a pressure-volume loop, monitored airway pressure is plotted on the x-axis, and volume is plotted on the y-axis. The inspiratory curve goes upward, and the expiratory curve goes downward.

Spontaneous breaths without pressure support go clockwise, and positive pressure breaths go counter-clockwise.

The bottom of the loop is at the set positive end-expiratory pressure (PEEP) level, which may be zero or a positive value. If we draw a line down the middle of the loop, the area to the right represents inspiratory resistance, and the area to the left represents expiratory resistance.

Flow-volume loop

In a flow-volume loop, airway flow is plotted on the y-axis, and volume is plotted on the x-axis. Inspiration is above the horizontal line, and expiration is below. The shape of the inspiratory curve matches the ventilator settings. The shape of the expiratory flow curve represents passive exhalation. The shape of the pressure-volume loop differs in different breath types. With spontaneous breaths, the flow-volume loop looks circular.

Just like waveforms, dynamic loops are a rich source of useful information. Analysis and interpretation of the loops require deep understanding of mechanical ventilation and ventilators.

Trend curves

Both waveforms and dynamic loops are displayed for a short period of time: one breath or several breaths. By contrast, trending curves show events over a much longer period. However, they do not necessarily suggest with accuracy what will happen in the future.

As we have seen, a number of numeric monitoring parameters are displayed and updated after every breath. These parameters may or may not be identical to those of previous breaths.

A trend curve is a two-dimensional graph with time as its x-axis and the monitoring parameter as its y-axis. It contains numerous tiny bars (Fig. 11.14). Each bar represents one reading or the average of several consecutive readings of the monitoring parameter.


Fig. 11.14 A trend curve is composed of a number of vertical bars. Every bar represents the value of the monitoring parameter during that unit of time.

Fig. 11.14 A trend curve is composed of a number of vertical bars. Every bar represents the value of the monitoring parameter during that unit of time.

As new readings are continuously available, the trend curve moves very slowly but continuously from right to left. The right-most bar shows the most recent reading, while the left-most bar shows the oldest reading.

The trend curve shows how the monitoring parameter has changed over a defined period of time, such as 1 hour, 6 hours, 12 hours, or 24 hours.

Typically, you select and define a trend curve in four steps:

  1. a. Activate the trend function;

  2. b. Select the monitoring parameters to display as a trend curve;

  3. c. Set the desired trending duration; and

  4. d. Confirm the selection.

Some ventilators allow multiple trend curves to be displayed simultaneously.

Trend curves are particularly useful for a number of reasons. They allow a clinician to understand what happened to a ventilated patient the previous night. They provide hints about how the ventilator settings changed over the period of surveillance. They can also show how the patient responded to a special therapy (e.g. the effects of a bronchodilator or tracheal suction on respiratory mechanics).

Do not confuse trend curves with waveforms!

Freeze and cursor measurement

Waveforms and dynamic loops are continuously updated. The updating may happen so fast that you can barely take a close look at an interesting detail before it disappears forever.

The solution is freeze and cursor measurement functions, which are two interrelated technical features.

The freeze function lets you temporarily stop the running waveforms or loops while mechanical ventilation continues in the background. This allows you to study the waveforms at your leisure. The ‘freezing’ status can be cancelled manually or automatically.

Cursor measurement is a graphic tool primarily to analyse the frozen waveforms. The cursor is an indicator on the frozen waveforms, as shown in Fig.11.15. Once the cursor is activated, you can move it back and forth and read the numerical values of the waveform at selected time points.


Fig. 11.15 Freeze and cursor measurement in the HAMILTON-G5 ventilator.

Fig. 11.15 Freeze and cursor measurement in the HAMILTON-G5 ventilator.

The freeze and cursor measurement functions are strictly monitoring functions: they have no effect upon mechanical ventilation.

11.6 Common pressure monitoring parameters

Now let’s shift our attention to common ventilator monitoring parameters.

Pressure parameters displayed on a ventilator are extracted or calculated from the continuously monitored circuit or airway pressure. Under normal conditions, both circuit pressure and airway pressure are almost identical, so that circuit pressure is often used as a synonym for airway pressure.

Common pressure monitoring parameters include peak inspiratory pressure (PIP), plateau pressure (Pplateau), PEEP, and mean airway pressure (Pmean); see Fig.11.16. These parameters are updated after every breath.


Fig. 11.16 Waveforms for peak inspiratory pressure, plateau pressure, and positive end-expiratory pressure (PEEP).

Fig. 11.16 Waveforms for peak inspiratory pressure, plateau pressure, and positive end-expiratory pressure (PEEP).

11.6.1 Peak inspiratory pressure (PIP)

Peak inspiratory pressure is the highest airway/circuit pressure measured during a mechanical breath. It is also called peak pressure, positive inspiratory pressure, maximum inspiratory pressure, maximum airway pressure, and peak circuit pressure. Common abbreviations are PIP and Ppeak.

Peak inspiratory pressure has three components:

  • PEEP;

  • The maximum inspiratory pressure applied by the ventilator;

  • Abnormal pressure overshoot or spikes, if present.

PIP has a different clinical significance in pressure and volume breaths.

PIP in a volume breath

In a volume mode, the operator sets the tidal volume and the inspiratory flow or inspiratory time. The ventilator delivers the desired volume at the set inspiratory flow within the set Ti. PIP varies depending on (a) the settings (VT, Ti, or peak flow), (b) the current respiratory mechanics, and (c) the patient breathing activity. A change to any of the three factors may cause a corresponding change to PIP, provided all other factors remain unchanged.

PIP in a pressure breath

In a pressure mode, the operator sets the inspiratory pressure, which is the difference between PIP and PEEP. The ventilator delivers the inspiratory gas into the circuit in order to achieve the set target pressure. The tidal volume varies depending on (a) the set inspiratory pressure, (b) the current respiratory mechanics, (c) the patient’s breathing activity, and (d) in rare cases, the set Ti. Any change in the four factors can cause a corresponding change in the resultant tidal volume, provided that other factors remain unchanged.

An abnormally high PIP takes two forms: pressure overshooting and a pressure spike (Fig.11.17). Pressure overshooting appears at the beginning of inspiration due to circuit pressurization that is too fast. A pressure spike occurs at the end of inspiration, in active patients only. It is a form of patient-ventilator asynchrony.


Fig. 11.17 Two forms of abnormally high peak inspiratory pressure (PIP).

Fig. 11.17 Two forms of abnormally high peak inspiratory pressure (PIP).

Common causes of increased or decreased PIP are summarized in Table 11.3.

Table 11.3 Common causes of increased and decreased PIP

In a volume breath

In a pressure breath

What causes PIP to increase

  • The set PEEP increases

  • The set tidal volume increases

  • The airway resistance increases

  • The respiratory compliance decreases

  • The set inspiratory time decreases

  • The set inspiratory flow increases

  • The patient is active

  • The set PEEP increases

  • The set inspiratory pressure (Pcontrol or Psupport) increases

  • A pressure overshoot or pressure spike occurs

  • Tube resistance compensation (TRC) is active

  • The patient is active

What causes PIP to decrease

  • The gas passageway has a leak or disconnection

  • The set PEEP decreases

  • The set tidal volume decreases

  • The airway resistance decreases

  • The respiratory compliance increases

  • The set inspiratory time increases

  • The set inspiratory flow decreases

  • The patient is active

  • The gas passageway of the ventilator system has a noticeable gas leak or disconnection

  • The set inspiratory pressure decreases

  • The set PEEP decreases

  • The patient is active

PEEP: positive end-expiratory pressure; PIP: peak inspiratory pressure.

11.6.2 Plateau pressure (Pplateau)

Plateau pressure is the end-inspiratory pressure at zero flow. Under this condition, the Pao and the Palv equilibrate. The unit of measurement for Pplateau is cmH2O or millibars. Plateau pressure is also called Ppause, end-inspiratory pressure, Pplat, Pplateau, PI END, and PPL.

Plateau pressure is clinically important because it represents the alveolar pressure when the lungs are inflated. It can be used to estimate the current static respiratory compliance.

Plateau pressure can be monitored in passive patients only. Conventionally, plateau pressure is measured in volume modes during an inspiratory pause (Fig.11.18). Less conventionally, plateau pressure can also be measured in pressure breaths if the inspiratory time is sufficiently long so that the airway flow reaches zero. In this case, PIP is equal to the plateau pressure.


Fig. 11.18 Plateau pressure is typically monitored in a volume breath during an inspiratory pause (left). However, it is also possible in a pressure breath if Ti is long enough (right).

Fig. 11.18 Plateau pressure is typically monitored in a volume breath during an inspiratory pause (left). However, it is also possible in a pressure breath if Ti is long enough (right).

During mechanical ventilation, we should do everything possible to lower the plateau pressure in order to avoid barotrauma of the lungs.

11.6.3 PEEP

PEEP (positive end-expiratory pressure) is the monitored pressure at the end of expiration. It is expressed in cmH2O or millibars.

It is important to clearly differentiate the set PEEP from the measured PEEP. The set PEEP is the intended or desired PEEP, while the latter is the actual PEEP. Under normal conditions, both should be equal or very close to each other.

If the measured PEEP differs significantly from the set PEEP, troubleshooting is indicated. The common causes for the difference include a significant gas leak, patient-ventilator asynchrony, or a ventilator malfunction.

11.6.4 Mean airway pressure (MAP)

Mean airway pressure is the average pressure applied over one mechanical breath. It is expressed in cmH2O or millibars. It is also called mean pressure, mean circuit pressure, Pmean, PMEAN, and mPaw.

Mean airway pressure is calculated in two steps (Fig.11.19). First, the breath cycle time (i.e. the duration of one breath) is divided into a number of equal slices. Second, the pressure values for all time slices are totalled, and this total is divided by the number of the slices. In other words, peak airway pressure is the average of the applied pressure over a breath. PEEP is part of the mean airway pressure.


Fig. 11.19 Mean area pressure (MAP) is the average airway pressure throughout a mechanical breath.

Fig. 11.19 Mean area pressure (MAP) is the average airway pressure throughout a mechanical breath.

The heart, large blood vessels, and the lungs are soft structures sitting inside the thoracic cavity. A positive pressure applied to the airway and lungs compresses the circulatory organs. A high MAP can cause elevated pulmonary vascular resistance, decreased cardiac output, and even decreased system blood pressure. Mean airway pressure is a good indicator of how much these intrathoracic structures are compressed. To minimize this unwanted effect, mean airway pressure should be kept as low as clinically acceptable.

In the past, mean airway pressure was manually calculated in volume modes. This tedious work is automated in modern ventilators. The displayed mean airway pressure is updated after every breath.

11.7 Common flow monitoring parameters

When we talk about ‘flow’ monitoring in ventilation, we are referring to the airway gas flow. As we discussed earlier, airway flow is either measured directly by a proximal flow sensor or calculated from the difference in the simultaneously measured flows through the inspiratory and expiratory limbs of the breathing circuit.

Monitored airway flow is often displayed as a flow-time waveform. In rare cases, peak inspiratory and expiratory flows are shown as numeric parameters. Both parameters are expressed in litres per minute or L/min, and updated after every breath. Airway flow monitoring is also the basis for calculating tidal volume.

Peak inspiratory and expiratory flows are useful and sensitive indicators of changes in the ventilator system pneumatics, and are especially useful for troubleshooting. The trend curves for peak flow are the most helpful, because they show how the peak flow has changed over time.

Fig.11.20 shows pressure and flow waveform displays.


Fig. 11.20 Typical pressure-time waveform (yellow) and flow-time waveform (pink).

Fig. 11.20 Typical pressure-time waveform (yellow) and flow-time waveform (pink).

11.7.1 Peak inspiratory flow (Insp flow)

Peak inspiratory flow refers to the highest (most positive) reading of the inspiratory flow monitored over a mechanical breath. Peak inspiratory flow rate and inspiratory flow pattern are predefined in a volume breath, but variable in a pressure breath. In pressure breaths, peak inspiratory flow is determined by several factors:

  • The set inspiratory pressure;

  • The set rise time;

  • The patient’s current airway resistance and respiratory compliance;

  • The patient’s breathing activities;

  • Inspiratory limb or airway occlusion;

  • Any large gas leak in the ventilator system;

  • Tube resistance compensation.

11.7.2 Peak expiratory flow (Exp flow)

Peak expiratory flow refers to the lowest (most negative) reading of the monitored expiratory flow over a mechanical breath.

In a passive patient, expiration is usually a passive process, driven by the elastic recoil force of the chest wall and lungs. Expiratory flow typically drops sharply to its negative maximum and then exponentially returns to zero. The process is the same for both volume and pressure breaths.

If a ventilated patient is active, the patient can influence expiration by use of the expiratory muscles, resulting in variable peak expiratory flows.

Peak expiratory flow can be influenced by:

  • The actual tidal volume or the inspiratory pressure;

  • The patient’s current airway resistance and respiratory compliance;

  • The patient’s breathing activities;

  • Airway or expiratory limb occlusion;

  • Any large gas leak in the ventilator system;

  • Tube resistance compensation.

Clinically, peak expiratory flow is an excellent indicator of airway patency. Its reading is much lower than normal in patients with obstructive diseases such as chronic obstructive pulmonary disease (COPD) and asthma. Trending this parameter may prove more helpful than simply reading the values for single breaths.

11.8 Common volume monitoring parameters

As we know, lung ventilation is achieved by moving a certain volume of gas into and out of the lungs in a series of natural or artificial breaths.

Monitored tidal volume is just one of several key parameters of lung ventilation. Fig.11.21 shows the relationship between tidal volume, respiratory rate, dead space, alveolar tidal volume, (minute) alveolar ventilation, and minute ventilation.


Fig. 11.21 The relationship between tidal volume, rate, and dead space. Note: Minute volume is represented by the combined light and dark grey areas.

Fig. 11.21 The relationship between tidal volume, rate, and dead space. Note: Minute volume is represented by the combined light and dark grey areas.

CO2 removal is directly related to alveolar ventilation. Under normal conditions, increased alveolar ventilation leads to decreased PaCO2, and vice versa.

Common volume monitoring parameters include tidal volume and minute volume.

11.8.1 Expiratory tidal volume (VTE)

In respiratory physiology, tidal volume refers to the volume of gas that one inhales (inspiratory tidal volume) or exhales (expiratory tidal volume) during one breath. Normally, both tidal volumes are almost identical. The tidal volume displayed by the ventilator is an estimation of this physiological tidal volume. This monitored tidal volume can be very close to the actual tidal volume if all required conditions are satisfied; otherwise, they may differ considerably. An example of this situation is shown in Fig. 11.7.

There are two types of monitored tidal volume: inspiratory tidal volume and expiratory tidal volume.

Inspiratory tidal volume (VTI) is the monitored volume of gas going into the lungs. It represents the maximum tidal volume that the ventilated patient can possibly receive.

Expiratory tidal volume (VTE) is the monitored volume of gas leaving the lungs. It represents the minimum tidal volume that the ventilated patient can possibly receive.

VTE is commonly displayed, because it is considered to be more clinically relevant.

Designators for expiratory tidal volume include VTE, VTE, and TVexp. It is expressed in millilitres (ml) or litres (l or L).

Tidal volume, both VTI and VTE, is not directly measured, but calculated from the measured airway flow over time, as shown in Fig.11.22.


Fig. 11.22 Tidal volume is typically calculated from measured flow over time.

Fig. 11.22 Tidal volume is typically calculated from measured flow over time.

If a ventilator system leaks, the displayed VTE can be much lower than expected, because (a) the leak lowers the applied positive pressure, and (b) the flow sensor may detect just a part, but not all, of the exhaled gas, as shown in Fig.11.7. In this case, both the low tidal volume alarm and the low airway pressure alarm may be activated as the direct consequence.

11.8.2 Expiratory minute volume (MVexp)

Expiratory minute volume is the sum of the monitored VTE of all mechanical breaths within one minute. It represents the minimum cumulative volume that the patient exhales within that minute. Technically, MVexp is a moving average of the monitored VTE of the last 5 to 10 consecutive breaths, extrapolated to a minute volume. The display of MVexp is updated after every breath.

Designators for expiratory minute volume include ExpMinVol, VE TOT, MV, Ve, MVe, and MVexp. It is expressed in litres per minute (L/min).

Expiratory minute volume indicates the minimum level of the patient’s lung ventilation. In a passive patient, expiratory minute volume is the product of monitored expiratory tidal volume and the respiratory rate. In an active patient, the monitored expiratory minute volume may vary as both the respiratory rate and the expiratory tidal volume can change dynamically.

Like tidal volume, the monitored expiratory minute volume also contains an ineffective portion due to dead space, which is not involved in gas exchange. Do not forget this portion when interpreting the monitored expiratory minute volume.

11.9 Common time-related monitoring parameters

During mechanical ventilation, all pneumatic parameters, such as pressure, flow, and volume, change over time. Common time-related parameters include total rate, inspiratory time, expiratory time, I:E ratio, and spontaneous breath rate.

It is important for us to distinguish the time parameters used as controls from the monitoring parameters. Both may share the same name, such as rate, I:E ratio, and Ti. The former are the operator’s commands to the ventilator system, while the latter are the monitored results. For this reason, if we hear ‘a rate of 15’, we need to ask whether this refers to a ventilator setting or a monitored value.

11.9.1 Inspiratory time (Ti) and expiratory time (Te)

As with other parameters, it is also important to differentiate Ti and Te settings from Ti and Te monitored values. In this discussion, we are referring to the monitored values only. In mechanical ventilation, monitored Ti is defined as the time between a valid triggering point and the following valid cycling point, while Te is the time between a valid cycling point and the following valid triggering point. Both Ti and Te are expressed in seconds.

The best way to identify Ti and Te is with a flow-time waveform where Ti is the part with positive flow and Te is the part with negative flow (Fig.11.23). Identifying these parameters on a pressure-time waveform may be confusing or misleading when the ventilated patient is active.


Fig. 11.23 In spontaneous breaths, we can recognize Ti and Te more easily on a flow waveform rather than a pressure waveform.

Fig. 11.23 In spontaneous breaths, we can recognize Ti and Te more easily on a flow waveform rather than a pressure waveform.

Lung inflation and lung deflation take time. If Ti is too short, inspiration is incomplete. In a pressure mode, this results in a lower-than-expected tidal volume. Similarly, if Te is too short, the expiration cannot complete, resulting in autoPEEP.

In a natural breath, Ti is always longer than Te. During mechanical ventilation, we can also intentionally set Ti longer than Te. This strategy, known as inverse ratio ventilation (IRV), is thought to improve oxygenation in patients with acute respiratory distress syndrome (ARDS).

11.9.2 Total frequency (fTotal)

The monitored total breath rate is sometimes called total frequency (fTotal) to distinguish it from the set rate. Other designators for total frequency include RR, Rate, and fTOT.

The total frequency, expressed in breaths per minute, is typically calculated as a moving average over the latest 8 to 10 breaths, updated after every breath.

In an actively breathing patient, the monitored total breathing frequency is often higher than the set rate.

The monitored frequency may be abnormally low if the ventilator fails to detect the patient’s inspiratory efforts in a (pressure or volume) support mode. It may be abnormally high if auto-triggering is present.

11.9.3 Spontaneous breath rate (fSpont) and mandatory breath rate (fControl)

The mandatory breath rate (fControl) is the number of control breaths and assist breaths in one minute. It is calculated from a moving average of 8 to 10 breaths.

The spontaneous breath rate (fSpont) is the number of support breaths in one minute. It is also calculated from the moving average of 8 to 10 breaths. Other designators include RRspont and Spon Rate.

f T o t a l f C o n t r o l + f S p o n t

In a passive patient, fTotal and fControl are equal, and fSpont is zero.

In an active patient, fTotal equals fSpont, and fControl is zero.

In a partially active patient, fTotal is the sum of fSpont and fControl. Both are greater than zero.

Fig.11.24 shows the relationship between total breath frequency, spontaneous breath frequency, and mandatory breath frequency.


Fig. 11.24 The relationship between total breath frequency, spontaneous breath frequency, and mandatory breath frequency.

Fig. 11.24 The relationship between total breath frequency, spontaneous breath frequency, and mandatory breath frequency.

Clinically, fSpont is an excellent indicator of the patient’s breathing activity and possibly also of ventilator demand. The displayed monitored breath frequencies may provide a false picture if (a) auto-triggering is present, (b) the patient signals are too weak, or (c) the ventilator monitoring is problematic.

11.10 Oxygen concentration

Oxygen concentration refers to the oxygen concentration of the gas that the patient is inhaling. It is also called FiO2, O2, or O2%. It is expressed as a percentage.

Again, we need to differentiate the set FiO2 from the monitored FiO2. Normally, both are, or should be, very close to each other, if not identical. They may differ temporarily immediately after the FiO2 setting is readjusted.

The key component of FiO2 monitoring is an oxygen cell or sensor, which has a limited lifetime. Calibrate the oxygen cell periodically to assure its accuracy, and replace it if the calibration fails more than once. Note that pure oxygen gas is required for oxygen cell calibration.