space is an often misunderstood and overlooked aspect of veterinary
anesthesia patient management. Dead space is always present as a component
of the patient’s airway and, to a variable degree, as a component of the
anesthetic system. Ignoring dead space increases induced by anesthesia
systems or airway adaptors can have a significant clinical impact on very
are three different types of dead space; anatomic, alveolar, and
equipment/mechanical. Dead space ventilation involves that component of
the respiratory gases that does not participate in gas exchange.
Increasing the proportion of dead space to alveolar ventilation will lead
to retention of carbon dioxide by the patient. If mechanical dead space volume equals or exceeds alveolar
ventilation volume the patient will not be able to clear carbon dioxide at
dead space is comprised of the upper airway
structures that do not participate in gas exchange. This includes the
gases in the nasal passages, nasopharynx, larynx, trachea, and in the
larger airways. Alveolar dead space represents those alveoli that
are ventilated with fresh gas but not perfused by the pulmonary
circulation. Together, anatomic and alveolar dead space is referred to as physiologic
dead space. Physiologic dead space gases do not participate in CO2 and
tidal volume is 10 to 15 ml/kg in the normal unanesthetized patient. Physiologic dead space
volume, 3.5 to 5.25 ml/kg, makes up about 35% of this tidal volume while the remainder of the tidal volume, 6.5 to 9.75 ml/kg, is the portion
of the tidal volume that actually participates in gas exchange (alveolar
ventilation volume). During anesthesia, however, patient tidal volume
decreases and, to a small degree, alveolar dead space increases. As a
result, alveolar ventilation volume is reduced to 3.5 to 5.25 ml/kg (50%
of tidal volume)
in a normal anesthetized patient during spontaneous ventilation.
an example, a 2.0 kg patient would normally have a tidal volume of 20 to
30 ml. Awake, patient physiologic dead space would be 7 to 10.5 ml,
leaving 13 to 19.5 ml to participate in alveolar ventilation.
Anesthetized, alveolar ventilation drops to 7 to 10.5 ml. Thus, 10.5 ml is
the maximum volume of gas available for alveolar ventilation during
spontaneous respiration if there is no mechanical dead space
associated with the anesthetic system and airway adaptors.
or equipment dead space is made up of the
endotracheal tube extending beyond the patient’s incisors, patient
monitor adaptors (ETCO2, apnea alert, etc.), any adaptors used to
facilitate patient/system positioning (right-angle or swivel adaptors used
to reduce the risk of tracheal trauma during patient rotation), the volume
within a mask, humidification management exchangers (HME), and the “Y”
piece (defined as the terminal end of an F circuit or noncircle system and
the inhalation/exhalation hose connector in a circle system).
soda lime or malfunctioning one-way valves can also contribute to
increasing mechanical dead space. Dead space also increases in a non-rebreathing
system when fresh gas flows are inadequate or when certain defects are
present in the system (for instance, when the center tube of a Bain system
or F circuit is cracked or broken). These dead space contributors can all
be controlled through proper system inspection and maintenance.
dead space gas is the first gas inhaled at the beginning of the each
respiratory cycle. As the mechanical dead space volume increases, less fresh gas can move into the patient’s alveoli to participate in gas
dead space is never zero. As a minimum, the anesthetic system’s
contribution to mechanical dead space is the dead space present in the
“Y” piece or terminal segment of an F circuit or noncircle system.
Norman elbow contains the least amount of mechanical dead space of any
conventionally used anesthetic system due to the fresh gas inlet tubing
being positioned directly at the ET tube adaptor opening. An Ayre T piece,
Jackson-Rees modified Ayre T piece, or Bain noncircle system contain 3 to
4 ml of dead space. For our example 2.0 kg patient, a Bain system could
reduce its alveolar ventilation volume from 10.5 ml to 6.5 ml.
circle systems have “Y” piece dead space that varies from 8 ml for an
adult Y-piece down to 4 ml for a pediatric hose Y-piece. Interestingly,
the pediatric F circuits can possess significantly greater terminal dead
space (15 ml) than the adult size F circuits (8 ml) offsetting their
inherent advantage of reduced system volume.
tube dead space includes any of the tube extending past the patient’s
incisors. The ET tube adaptor alone adds about 2 ml of dead space. The
added dead space from the ET tube itself extending past the incisors is
relatively negligible; a few tenths of an ml per cm excess tube for a 4 or
5 mm OD ET tube. For our 2.0 kg patient on a Bain system, having the ET
tube adaptor extending beyond the incisors reduces our alveolar
ventilation volume from 6.5 ml to 4.5 ml.
ETCO2 monitor adaptors, apnea alert monitor adaptors, and positional
facilitation adaptors can add 7 to 8 ml of dead space each. If our 2.0 kg
patient (on Bain system with exposed ET tube adaptor) had one adult ETCO2
adaptor placed between patient endotracheal tube and anesthetic hoses,
this would effectively eliminate its alveolar ventilation (4.5 ml – 7 ml
= -2.5 ml).
ETCO2 adaptors are available for most ETCO2 monitors, and should be a
requisite if using an ETCO2 monitor on very small patients. A typical
pediatric adaptor reduces dead space volume from 7 ml to 2 ml. We could add 5 ml to our patient’s alveolar ventilation by using
a pediatric rather than adult adaptor giving us, at best, 2.5 ml for
alveolar ventilation; better than negative numbers but still far less than
normal alveolar ventilation. Add in an apnea alert or a positional adaptor
and we again completely eliminate alveolar ventilation during normal
facemasks also contribute mechanical dead space. This effect is
exaggerated if the mask is of large volume and, interestingly, if there is
a tight seal around the patient’s muzzle. A poor mask seal would reduce
this dead space effect but would subject the staff to unwanted waste gas
exposure and create more difficulties regulating anesthetic levels.