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What is “dead space”? - Methodological Issues
Michael B. Jaffe PhD
Philips-Respironics, Wallingford, CT
? Dead space measurement provides insight into the distribution
of ventilation and matching of ventilation and perfusion. (1)
The term “Wasted Ventilation” or respiratory dead space (VD)
is considered to be that volume of each breath that is inhaled
but does not participate in gas exchange. As well as the
volume of the airway in which there is no significant exchange
of oxygen and carbon dioxide (2). However, in clinical
settings and in the literature the term is used inconsistently and
may describe all or any combination of the following; total,
physiologic, anatomic, alveolar, and apparatus/equipment dead
space (Figure 1).
1. Hedenstierna G, Sandhagen B. Assessing dead space. A meaningful
variable? Minerva Anestesiol. 2006 Jun;72(6):521-8.
2. Comroe JH. Physiology of respiration. An introductory- text. 2nd ed.
Chicago: Year Book Medical, 1974.
3. Nunn, JF. Applied respiratory physiology, 2nd edn. London,
Butterworths & Co Ltd, 1969.
4. Elliott MW. The interface: crucial for successful noninvasive ventilation.
Eur Respir J. 2004 ;23(1):7-8.
5. Ritter H. Totraumnessungen an Masken. Atemschutz Informationen. 1966;
2: 25-31.
6. Brewer LM, Orr JA, Pace NL. Anatomic dead space cannot be predicted
by body weight. Respir Care. 2008 ; 53(7):885-91.
7. Tang Y, Turner MJ, Baker AB. Systematic errors and susceptibility to
noise of four methods for calculating anatomical dead space from the CO2
expirogram. Br J Anaesth. 2007; 98(6):828-34.
8. Pontoppidan, H., Hedley-Whyte, J., Bendizen, H.H., Laver, M. B. &
Radford Jr, E.P. Ventilation and oxygenation requirements during
prolonged artificial ventilation in patients with respiratory failure. New.
Engl. J. Med. 1965; 273: 401–409.
9. Walsh BK, Crotwell DN, Restrepo RD. Capnography/Capnometry during
mechanical ventilation: 2011. Respir Care. 2011 ;56(4):503-9.
10. ISO 4135, Anaesthetic and respiratory equipment – Vocabulary, 2001.
? Values for dead space used clinically and reported in the
literature need to clearly define what dead space volume is
being measured and how it measured to avoid confusion and
problems with interpretation of values. Improved clarity
through the development of standardized terminology under
the auspices of existing standards organizations (10) is
suggested.
Conclusions
DiscussionDiscussionIntroductionIntroduction
? A partial explanation for the confusion in terminology is that
the volume of VD that is measured is dependent upon a
number of factors including the “type” of VD, the method of
measurement, and patient particulars (e.g. intubated?
spontaneously or mechanically assisted, body position (3)).
? Apparatus dead space (VDapp) or equipment dead space (or
mechanical or instrumental) refers to the dead space introduced
in a breathing circuit used for mechanical ventilation or in
some cases added volume used in the measurement of VDapp.
It has been defined as the “volume of expired gases, contained
within the breathing system of an apparatus, which is re-
inspired without a change in composition” (ISO 4134:
2001,def 4.1.5 ) It may be expressed as a dynamic or effective
dead space (e.g. mask and other breathing circuit components)
to distinguish itself from geometric or physical dead space
(measured volumetrically or estimated dimensionally).
Dynamic dead space can be considered the dead space that the
patient “sees” in terms of rebreathed gases. For masks, a weak
relationship (r2
=0.13) has been shown between physical and
dynamic dead space. (4) The term “dynamic” is appropriate
since this value changes as a function of the flow rate, tidal
volume, frequency and exhaled CO2 concentration (5). Note
that while breathing circuit components such as airway
adapters typically include a dead space value in their
specifications, components such as the wye and ETT
connector also contribute to dead space. In fact the dynamic
dead space of the wye (shown in Figure 2) is larger than it
appears.
(a) (b) (c)
Figure 2 – Dead space of the wye –(a) Apparent “physical”
dead space (b) Expired gas (c) dynamic dead space
ReferencesReferences
Figure 1- Breakdown of Different Deadspace Volumes
1/5/12 MBJ final
Anatomic
Alveolar
Equipment
TotalPhysiologic
?Physiologic dead space (VDphys), the sum of apparatus,
anatomic and alveolar dead space, has been shown as a dead
space fraction to be clinically useful including as a predictor of
outcome in ALI/ARDS. While alveolar CO2 is used in the mass
balance derivation of VDphys, the Enghoff modification which
assumes the near equality of the PCO2 in alveolar gas and arterial
blood is often applied. However, a number of approaches have
used other surrogates for PACO2 including end-tidal CO2 and
extrapolated values from the volumetric capnogram. The
physiologic dead-space ratio (Vd/Vt phys) has recognized as a
marker of lung disease in adult intensive care since the 1960’s
(8). In particular, besides CO2 elimination, two ‘dead space’
related parameters, alveolar ventilation (Valv), and physiological
dead space (Vd/Vt phys) are finding increasing application and
clinical use.
Figure 3 – Commonly Used Fowler Airway Dead Space
Determination – Note that dividing line is moved until areas p
and q are equal
In the AARC’s most recent clinical practice guideline for
capnography during mechanical ventilation (9), it notes
capnography may be indicated for “monitoring severity of
pulmonary disease and evaluating response to therapy,
especially therapy intended to improve the ratio of dead space
to Vd/Vt and the matching of V/Q, and possibly, to increase
coronary blood flow.”
?Anatomic’ dead space methods do not quite measure the same
volume as the morphological definition. The 1 ml per lb. rule of
thumb has been shown to poorly correlate body weight with
measured “anatomic” dead space (r2
= -0002) (6) Instead a
functional definition needs to be adapted based upon the method
used to estimate the anatomic dead space, often referred to as
airway dead space. Methods for estimating airway dead space
include equal area method (e.g. Fowler method- Figure 3), linear
extrapolation of expired CO2 volume, and polynomial curve fits
which have greater numerical and algorithmic complexity (7).

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