Page 128 - Basic Monitoring in Canine and Feline Emergency Patients

Table 6.3. Most common causes of decreased ETCO values (less than 30 mmHg).
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VetBooks.ir Cause Physiologic explanation
● ● Decreased metabolic and cellular function due to hypothermia, metabolic acidosis leading to
Cellular/metabolic
decreased CO formation
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Variations in ● ● Pain or anxiety-induced tachypnea
alveolar ● ● Excessive manual ventilation or increased respiratory rate during mechanical ventilation
ventilation ● ● Bronchospasm, asthma, chronic bronchitis causing decreased gas exchange in the alveoli
● ● Total airway obstruction blocking gas exchange in the alveoli
● ● Apnea/respiratory arrest
Alterations in ● ● Sampling line leak or obstruction in sidestream capnographs
pulmonary ● ● Total airway obstruction or disconnected system
perfusion ● ● Malfunctioning ETT cuff (deflated or too small for patient’s size) causing gas to be expired
around the tube rather than going through the analyzer Inappropriate placement of the ETT
(i.e. into the esophagus)
● ● If a non-rebreathing system is used (patient smaller than 7 kg), the fresh gas flow may be
too high, causing dilution of the expired gas and lowering the ETCO 2
● ● Increased dead space (especially in small-sized patients) causing carbon dioxide to stay in
the system rather than pass through the capnograph
● ● Accidental extubation
Technical malfunction ● Expiratory valve malfunction (accumulation of expired gases including CO ) Slow fresh gas
●
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of the anesthesia flow (with non-rebreathing systems) leading to build-up of carbon dioxide in the system
machine ● ● Exhausted CO absorbent (rebreathing carbon dioxide)
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ETCO , end-tidal carbon dioxide; ETT, endotracheal tube; CO , carbon dioxide.
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● ● Phase I (respiratory baseline): this phase repre- ● ● Phase 0 (inspiratory downstroke): the CO con-
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sents inspiration which means the CO concen- centration rapidly declines to zero during inha-
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tration should be zero. The expiration phase lation as CO -free gas is drawn into the lungs,
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starts right before the end of this phase, but past the carbon dioxide sensor.
because it does not contain any expired CO ● ● Alpha angle: the angle between phase II and III
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(due to the presence of dead space) it is displayed is the α angle. This angle is typically considered
as a flat line. to be an indirect representation of the ventila-
● ● Phase II (expiratory upstroke): as the name tion: perfusion status of the lung. The slope and
implies, in this phase the CO concentration the height of phase III can be influenced by air-
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rises rapidly, and an upswing of the baseline will way resistance, cardiac output, and CO pro-
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be seen. As the expiration progresses, the CO duction, resulting in changes in the degree of the
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from the alveoli will replace the ‘CO -free gas’ alpha angle. Normally this angle is between
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previously present in the trachea and endotra- 100° and 110°. For example, an increase in
cheal tube and be sensed by the capnometer. alpha angle may suggest increased airway resist-
● ● Phase III (alveolar plateau): as expiration con- ance (i.e. an obstruction from things such as
tinues, the alveoli become empty and the CO asthma, bronchospasm, kinked endotracheal
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level reaches a plateau level. Because not all the tube, etc.). The angle increases since it takes
alveoli empty at the same exact time, the alveolar longer to expel the carbon dioxide-rich gas from
plateau phase is slightly sloped. This happens the alveoli when there is airway spasm or other
because the alveoli with a low ventilation:perfusion airway obstruction.
ratio (i.e. well perfused with blood but not venti- ● ● Beta angle: the angle between phase III and the
lated as effectively) have a higher CO concen- inspiratory downstroke is the β angle. This angle
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tration and usually empty late in the exhalation is usually about 90 degrees. This angle is used to
phase. The highest point of phase III corresponds assess the degree of rebreathing; it will increase
to the actual ETCO . as the rebreathing increases since it will take
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120 L.A.M. Ilie