REVIEW
Clinical measurement of end-expiratory
lung volume on the intensive care unit
nd-expiratory lung volume (EELV) is the volume of gas remaining in the lungs at the end
of a normal expiration. It represents the
relaxation volume when the recoil pressure of the
lung is opposite and equal to the chest wall recoil
pressure. (End-expiratory lung volume and functional residual capacity (FRC) are definitions of
the same subdivision of lung volume.) EELV
measurement is important in the management of
patients requiring mechanical ventilation for
acute lung injury (ALI) and acute respiratory
distress syndrome (ARDS).1
The normal EELV of about 3–4 litres is
reduced in ARDS to less than 1 litre2 and in ALI
to 1.5 litres.3 In contrast, patients with chronic
obstructive pulmonary disease (COPD) often
have an increased EELV.4 Furthermore, ventilator setting can increase EELV by application
of external positive end-expiratory pressure
(PEEP).5 PEEP improves arterial oxygenation
by alveolar recruitment and is thereby associated with an increase in EELV. Therefore, the use
of PEEP is considered part of the standard therapy for patients with ALI requiring mechanical
ventilation.6 Additionally, intrinsic PEEP using
inverse ratio ventilation increases EELV.7
Therefore, the ability to assess and monitor
EELV accurately is essential in the management
of patients requiring mechanical ventilation.
The results assist in diagnosis, optimise mechanical ventilatory support, and predict the likelihood of weaning success from the respirator.8
Therefore, several authors have proposed the
monitoring of EELV as a tool during mechanical
ventilation.8–10 In 1995 the American Thoracic
Society and the European Respiratory Society
published a workshop report on reference values
for EELV.11
This review describes different techniques for
EELV measurement, including advantages, disadvantages and possible bed-side use of these
techniques on the intensive care unit (ICU).
Recently it became obvious that recruitment of
lung volume is a phenomenon that occurs in different regions of the lungs.9,12,13 Therefore, a special focus of this review is on those techniques
that take into account regional measurements.
E
normal inspiration and expiration. In adults it is
about 700 ml (10 ml/kg). Expiratory reserve volume (ERV) is the gas volume additionally exhaled during forced expiration and inspiratory
reserve volume (IRV) is that after a forced inspiration. The sum of VT and IRV is the inspiratory
capacity (IC).
Even after a maximum expiratory effort,
some air is left in the lung; no lung region normally collapses. This persisting gas volume is
called residual volume (RV) and amounts to
2–2.5 litres in adults. Expiration stops at residual volume for two reasons: distal airways of
diameter ≤2 mm will close before the alveoli collapse;14 the chest wall, rib cage and diaphragm
cannot be distorted to the point where all gas in
the lung has been exhaled.
The EELV – the gas volume remaining in the
lungs after an ordinary expiration – amounts to
about 3–4 litres, depending on sex, age, height,
and weight.15 The existence of this persisting gas
volume prevents collapse of the alveoli and
ensures that the variation of gas concentration
inside the alveoli is minimised.
The gas volume in the lung after a maximum
inspiration is called the total lung capacity
(TLC). In adults, typically, it is 6–8 litres. The
volume expired after a maximal inspiration is
called the vital capacity (VC) and is therefore
equal to the difference between TLC and RV.
J Hinz MD,
Department of
Anaesthesiology, Emergency
and Intensive Care Medicine,
Georg-August-University,
Göttingen, Germany
Figure 1. Definitions of lung volumes
in normal breathing and during specific
TECHNIQUES OF MEASURING LUNG VOLUME
manoeuvres such as forced inspiration
The different lung volumes, including the EELV,
can be estimated by various methods: spirome-
IRV
and forced expiration. For abbreviations see text.
Forced
inspiration
IC
VC
TLC
VT
ERV
DEFINITION OF LUNG VOLUMES
The gas volume inside the lung is considered in
different subdivisions. The definitions of lung
volumes in normal breathing and during specific
manoeuvres, such as forced inspiration and
forced expiration, are shown in Figure 1.
Tidal volume (VT) is the gas volume during
Normal
breathing
RV
EELV
(FRC)
Forced
expiration
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Clinical measurement of end-expiratory lung volume
try and radiographic techniques (i.e. chest radiographs or computed tomography of the thorax);
gas dilution techniques; body plethysmography;
respiratory inductive plethysmography; and electrical impedance tomography. The requirements of a technique for lung volume measurement on the ICU are: user-friendly equipment,
accuracy of the technique, no radiation exposure
to the patient and the capacity for bedside use.
Spirometry
Spirometry, introduced by Hutchinson,16 measures lung volume during either normal or forced
breathing. The classic water spirometer has been
replaced by a ‘dry’ spirometer introduced by
Fleisch,17 based on the measurement of gas flow
through a pneumotachograph. Gas flow is derived from pressure drop over a fixed resistance,
which consists of a bundle of parallel capillary
tubes or a metal screen. Following the HagenPoiseuille’s-law flow, it is proportional to the
pressure drop over the resistance. Volumes are
calculated by flow integration over time. Spirometers should be designed to meet the European
Thoracic Societies accepted standards.15
Spirometry is the standard method for measuring VT and VC at the bed-side. However,
measurement of RV, TLC and EELV, which are
not exhaled, is not possible by spirometry. Thus,
a different approach to measurement is required
for these quantities. Furthermore, the expiratory phase of a spirometry measurement is used to
measure obstruction within the lungs. The problem of this method in the intensive care environment is that it requires a co-operative and
motivated patient for accurate results. Such conditions, especially in mechanically ventilated
patients, are unlikely to be met in the ICU.
Body plethysmography
Lung volume is measured by the application of
Boyle’s law, which states that, under isothermal
conditions, when gas in a closed container is
compressed, its volume decreases, while the
pressure inside the container increases. Therefore the product of volume and pressure, at any
given moment, is constant. There are two types
of body plethysmographs: variable volume and
constant volume. In the variable volume plethysmograph, a change in lung volume is measured
direct by a spirometer or pneumotachograph.18
In the constant body plethysmograph, a change
in lung volume is measured as a change in box
pressure.
For the measurement procedure, the patient
sits inside an airtight chamber equipped to measure pressure, gas flow, or volume changes, and
inhales or exhales to a particular volume (usually end-expiratory lung volume). After a shutter
drops across the breathing tube, the patient
makes respiratory efforts against the closed
shutter, causing the chest volume to expand and
decompressing the air in the lungs. The increase
in the chest volume slightly reduces the box vol-
ume and thus slightly increases the pressure in
the box. The most common measurements taken
using the body plethysmograph are thoracic gas
volume and airway resistance.
The advantages of plethysmography are that
recording of lung volume takes only a few
breaths and can be repeated very quickly between each measurement. The technique shows
a high degree of precision indicated by a coefficient of variation of about 5% during repeated
measurements.19 The disadvantages of this technique are that the equipment is expensive and
requires careful maintenance. Note also that
body plethysmography measures the total lung
volume including that volume trapped behind
closed airways. Therefore, it may deliver different lung volume results compared with data
from other measurement techniques. This technique is practicable in spontaneously breathing,
co-operative and motivated patients. Although it
has been effective in anaesthetised patients,
using a specially designed body plethysmograph
that allows the patient to be in a supine
position,20,21 it is difficult to use with critically ill
patients in the intensive care setting and impossible in mechanically ventilated patients. Up to
now, no commercially produced body plethysmograph is available for measurement in the
supine position.
Gas dilution techniques
Any gas that is nontoxic and poorly soluble in
blood and tissue (e.g. helium, xenon, sulphur
hexafluoride (SF6), nitrogen) can be used as tracer gas for measurement of EELV. This technique
is either based on rebreathing the tracer gas in a
closed circuit, or the wash-in/wash-out of the
tracer gas is analysed in a multiple breath procedure. Additionally, information about regional
lung volumes and regional ventilation can be
derived from mathematical compartmental analysis of tracer wash-out curves.22 A major criticism of this approach to assessment of regional
ventilation is that the measurement is not direct
and the calculation of regional ventilation depends on assumptions about ideal tracer washout curves.
Nitrogen is most frequently used as the tracer
gas in multiple breath wash-out investigations.
An open circuit multibreath nitrogen wash-out
manoeuvre was first described by Darling et al.23
and modified later.24,25 The multiple breath nitrogen wash-out technique measures EELV with a
high level of accuracy.24 The necessary change in
nitrogen concentration during the wash-in/washout means that oxygen fraction is altered, so that
the patient cannot be continuously ventilated
with the preselected inspired oxygen fraction.
This is a limitation in severe hypoxaemia. The
tracer gas sulphur hexafluoride (SF6) provides a
solution to this problem by allowing EELV measurements to be taken without interference with
the inspired oxygen fraction.26,27
To overcome the problem of wash-in/wash-out
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tracer gas techniques needing a fast and sensitive gas analyser to allow breath-by-breath measurements of tracer gas, a technique has been
proposed that requires only those analysers frequently used in an ICU. Presuming that air consists mainly of nitrogen, oxygen and carbon dioxide, the tracer gas nitrogen is calculated by
deduction from the measurements of oxygen and
carbon dioxide concentrations.28 An alternative
proposal is a system that collects the expired gas
over a number of breaths in a large bag or
spirometer and records the mixed expired gas
concentration together with expired volume.
This solution requires bulky equipment and is
difficult in the ICU.
A simple helium dilution technique, which
utilises a short period of rebreathing six breaths
from a 1-litre bag, produces consistent estimations of EELV in those patients suffering from
ALI. Xenon has anaesthetic properties, which
limit its use in awake patients. During mechanical ventilation, xenon can be used as an interesting research tool.29 When combined with a
computed tomogram (CT) study it allows the calculation of both total and regional lung volume.30
Rebreathing techniques require a gas-tight
circuit system including the ventilator in order
to reach an equilibrium of the tracer gas concentration.31–33 This may be hard to achieve in intubated patients. Wash-in/wash-out procedures
are less demanding in this respect, but require
as much equipment and effort as rebreathing
techniques.34 Gas dilution techniques, whether
based on rebreathing or wash-in/wash-out, have
the inherent limitation that they can only measure the lung volume that participates in ventilation. Completely closed lung regions behind
collapsed airways or mucus plugs will not be
detected. Poorly ventilated regions with very
long wash-in/wash-out times may also be disregarded. However, ALI appears to be associated
more with lung collapse and no ventilation at all
than with obstructed airways causing poor ven-
Figure 2. The principle of electrical
impedance tomography: for electrical
impedance measurement an alternating
current (5 mA p-p, 50 kHz) is injected
between one pair of adjacent electrodes. The resulting surface voltages
were measured between the remaining
adjacent electrode pairs. All 16 adjacent electrode pairs were used, one
pair after the other, as injecting electrodes with the voltages measured with
the remaining electrodes. One electrical
impedance measurement is completed
when all pairs of adjacent electrodes
had been used once as injecting electrodes.
~
I
Ventral
16
1
2
3
15
14
4
Right
13
5
6
7
11
10
9
Dorsal
8
Chest radiography and computed tomogram
Chest radiography shows frontal and lateral
views of the lungs. It gives qualitative information about aeration, in case of atelectasis or
hyperinflation, but does not allow any quantitative analysis of the EELV. With repeated transverse CT or spiral CT of the chest, EELV can be
quantified from a three-dimensional reconstruction of CT slices. This method enables total and
regional volume calculation.38,39 Furthermore,
information on alveolar recruitment can be obtained from CT scanning.9,40 Unfortunately, CT
is not a bedside method and it is associated with
radiation exposure.
Respiratory inductive plethysmography
Respiratory inductive plethysmography (RIP)
measures chest and abdomen movement (Respitrace Plus, NIMS Inc., Miami, USA).41,42 It
consists of a pair of insulated coils strapped over
the upper abdomen and chest. The change of
cross-sectional area of the coil caused by chest
and abdominal wall displacement modifies the
inductance of the coils proportional to tidal volume and lung volume. For calibration of RIP, a
simultaneous recording of lung volume by
spirometry is necessary. The calibration should
be repeated every time that body posture is even
slightly changed. RIP is a non-invasive technique which can be used at the bedside on the
ICU in mechanically ventilated patients.
Unfortunately, Neumann et al.44 found a high
and unstable baseline drift of 25.4–29.1 ml/min
of the RIP signal from the Respitrace Plus monitor. Although other authors found lower baseline
drifts of 1 ml/min,43 they concluded that these
high drifts raise doubts about this method, especially for long-term measurements. They found
that RIP failed to be consistently accurate
enough for quantitative measurement of tidal
volume and the determination of EELV.44 Improvement in calibration and long-term stability
in RIP may in due course result from a 2002
study by Millard, who introduced more advanced
mathematical algorithms to improve the RIP
signals.45
Electrical impedance tomography
Left
U
12
tilation of lung regions.35 Thus, gas dilution
techniques may be reasonably accurate for assessing EELV in patients with ALI.24,36,37 Unfortunately, no commercial gas dilution techniques are available for bedside measurement on
the ICU.
Barber and Brown developed electrical impedance tomography (EIT) in 1984.46 EIT offers the
possibility of non-invasive, bedside measurement of EELV, regional lung volume and regional ventilation.47–50 The principle of EIT is based
on a small alternate current injection (5 mA, pp) and voltage measurement via surface electrodes placed around the thorax (Figure 2). The
electrical properties of the chest vary depending
on changes in the air content. Therefore, chan-
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routine usage it should be simple and not susceptible to disturbances. The technique should measure the EELV accurately and thereby offer a
rational basis for therapeutic consequences. The
patient should not be endangered by an invasive
method or radiation exposure. To avoid the risk in
volved in transport of the patient to the measurement device, the capacity for bedside use is
important. Therefore, the technique should be
brought to the patient and not the patient to the
technique. Any sudden change of lung volume,
whether due to change of respirator setting or the
progress of lung injury, should be detected immediately: therefore a continuous measurement of
EELV is desirable. Additionally, to recognise heterogeneity in regional lung volume and regional
ventilation a technique is desirable which delivers
information about regional lung volume and
regional ventilation.
As yet no technique exists that fully complies
with all these demands. Tables 1–3 give detailed
information about advantages and disadvantages, possible bedside use and the ability to obtain
regional parameters from the different techniques for lung volume measurement. From the
above, only electrical impedance tomography gets
close to complying with all demands. During the
next few years, electrical impedance tomography
has to prove if it can become a routine tool for the
measurement of EELV, regional lung volume and
regional ventilation on the ICU.
(EELV) measurement by electrical impedance tomography during mechanical
ventilation with identical tidal volume at
CONCLUSION
different positive end-expiratory pres-
Bedside measurement of EELV is desirable, but
not yet standard practice on the ICU. The unfulfilled need for a simple and preferably automated technique has so far limited the acceptance of
measuring EELV in the intensive care setting.
Electrical impedance tomography may become a
simple bedside method for monitoring change in
EELV in the near future.
0.20
DISCUSSION
EELV
1,285 mL
EELV
1,610 mL
EELV
1,885 mL
sures (PEEP 0, PEEP 5, PEEP 10, and
PEEP 15) in one patient. The figure
shows four examples of lung impedance
time course (∆impedance) at different
lung volumes induced by different
PEEP. Note that the end-expiratory lung
impedance (ELI) increased with increasing EELV.
EELV
2,211 mL
0.15
∆ Impedance
It may appear surprising that so many techniques, even some that can be used at the bedside, have been refined for EELV measurement
in mechanically ventilated patients, yet the use
of EELV as a guide in the treatment of the
patient and setting of the ventilator is still not
common in the ICU. The reasons seem to be that
the process of measurement is complicated and
the benefit is not perceived not warrant the
effort. The lack of access to a simple, and preferably automated, technique has so far limited the
use of EELV measurement as accepted practice
in intensive care.
The requirements for an appropriate technique
for lung volume measurement on the ICU are
manifold. In a time of straitened financial
resources the technique used should be cheap. For
Figure 3. End-expiratory lung volume
Tidal volume
ges in lung impedance induced by changes in
lung volume can be reconstructed.47
EIT was validated by comparison of regional
aeration at different tidal volumes and lung volumes derived from computed tomography.51 The
study confirmed that EIT is a suitable, non-invasive method for detecting regional changes in
lung volume and monitoring local effects of artificial ventilation. EIT was compared to ventilation scintigraphy in mechanically ventilated
pigs,52 and the authors concluded that it seems
to allow real time monitoring of regional ventilation distribution at the bedside. The major
advantages of EIT are that it is non-invasive,
easy to use at the bedside and that data collection can be performed with a high time and local
resolution. According to Hahn et al., the minimal
detectable regional lung volume by EIT is in the
range of 9–29 ml.53 Barber and Brown54 found a
spatial resolution of approximately 8% of the
thorax diameter, so that within the thorax a resolution of 8 ml is achieved.
A study performed by Hinz et al. showed that
a PEEP-induced increase of EELV is accompanied by a proportional increase of end-expiratory
lung impedance (ELI).3 An example of end-expiratory lung impedance at different measurements of EELV is shown in Figure 3. However,
use of ELI results in a slight underestimate of
lung volume changes. The reasons are, first, that
impedance changes within the lung are generated either by the air content or changes of the
central blood volume by the pulsatile blood
flow;55 second, that pulmonary air content and
impedance change may not correlate linearly, if
stretching of lung parenchyma itself causes
impedance changes; and, third, that impedance
is linearly correlated to the impedance of a polyacrylamide gel, up to an increase of 20%.56 If gels
with higher impedance were used, the changes
were underestimated by the EIT system. This
result may depend either on the electrical properties of the EIT system57 used or on the image
reconstruction algorithm.58
0.10
ELI
PEEP 15
0.05
0.00
ELI
PEEP 10
-0.05
ELI
PEEP 0
ELI
PEEP 5
Time (not scaled)
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Table 1. Methods for measuring lung volumes, their principles, advantages and disadvantages. The ability to measure regional lung volume and regional ventilation
(regional parameter) and the possibility of bedside use on the ICU are noted
Method
Principle
Advantage/disadvantage
Regional parameter Bed-side
Spirometry
Classic water spirometer has been replaced by ‘dry’
It measures tidal volume and vital capacity. It needs a
spirometer based on the technique of pneumotachography. co-operative and motivated subject, so is unlikely to be
Application of Hagen-Poiseuille’s law
appropriate in the ICU, especially in mechanically
ventilated patients
Body plethysmography
Application of Boyle’s law: patient in closed box,
panting against an occluded shutter with the recording
of airway and box pressures
Reference technique. Requires expensive equipment.
Measures all gases in the lung even behind occluded
airways and gas in poorly ventilated regions. Enables
measurement in rapid sequence
Respiratory inductive
plethysmography (RIP)
RIP measures chest and abdomen movement by a pair
of insulated coils strapped over the upper abdomen and
chest. This displacement modifies the inductance of the
coils proportional to tidal volume and lung volume
High and unstable signal baseline drift
✔
✔
Table 2. Wash-in/wash-out techniques of measuring lung volumes, their principles, advantages and disadvantages. The ability to measure regional lung volume
and regional ventilation (regional parameter) and the possibility of bedside use on the ICU are noted
Method
Principle
Advantage/disadvantage
Regional parameter Bed-side
Helium rebreathing
Rebreathing of the poorly soluble helium until full mixing
between lung volume and spirometer
Requires CO2 absorber in circuit and O2 to compensate
for the uptake
✔
Multiple breath
N2 wash-out
Wash-out of N2 in lung by O2 breathing. Knowing the
initial and final N2 concentrations in lung gas and the
expired amount of N2, lung volume can be calculated
Less reliable if patient is ventilated with high fractions
of O2, because less N2 can be washed out. Some of
expired N2 comes from body stores and must be
subtracted before the calculation of EELV
✔
Xenon wash-in/
wash-out
Wash-in of a tracer gas that does not normally exist in the Xenon attenuates x-rays which may enable regional
lungs and then wash-out similar to N2 wash-out
volume radiographs; poor resolution, anaesthetic
properties
✔
SF6 wash-in/wash-out
Wash-in of a tracer gas that does not normally exist in the Very low concentration of tracer gas, which allows
lungs and then wash-out similar to N2 wash-out
maintenance of inspired oxygen fraction
✔
N2 wash-out
An inspired VC of O2, followed by slow expiration to RV
during the recording of N2
Easy and rapid to perform. Less accurate than multiple
breath wash-out
✔
Helium wash-out
Inspired VC of O2 with 5–10% helium, followed by slow
expiration to RV during the recording of N2
Less reliable than multiple breath techniques. Can be
used with other tracer gases, for example SF6
✔
Multiple breath methods
Single breath methods
Table 3. Imaging techniques of measuring lung volumes, their principles, advantages and disadvantages. The ability to measure regional lung volume and regional
ventilation (regional parameter) and the possibility of bedside use on the ICU are noted
Method
Principle
Advantage/disadvantage
Regional parameter Bed-side
Chest radiography
Frontal and lateral view of the lungs
Gives qualitative information but does not allow any
quantitative analysis
✔
Computed tomogram
Repeated transverse CT scans or spiral CT of the chest
during breath-hold
Not a bedside technique. Enables regional volume
calculation
✔
Isotope techniques 133X Similar to SF6 wash-in/wash-out
Expensive equipment and radiation
✔
Electrical impedance
tomography
Delivers total and regional lung volumes, and regional
ventilation, non-invasive, radiation-free, bedside use,
continuous measurement possible
✔
Reconstruction of impedance change via surface
electrodes placed around the thorax induced by alternate
current injection and voltage measurement
ACKNOWLEDGEMENT
I would like to thank Onnen Moerer, Department of Anaesthesiology, Emergency and Intensive Care Medicine, GeorgAugust-University, Göttingen, Germany for his valuable cooperation and expert advice.
3.
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■
Correspondence to:
José Hinz MD
Department of Anaesthesiology, Emergency
and Intensive Care Medicine
Georg-August-University
Robert-Koch-Str. 40
D-37075 Göttingen, Germany
e-mail: [email protected]
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