Page 126 - Basic Monitoring in Canine and Feline Emergency Patients

of the ETCO which allows for detection of sudden tilation (high PaCO ) can cause cerebral vasodilation
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changes in the carbon dioxide values. It is impor- which in turn can increase intracranial pressure.
VetBooks.ir tant to remember that this relationship between intubated patients with severe neurologic decom-
Manual hyperventilation may be considered for
ETCO and PaCO will only be valid if there are no
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major respiratory or hemodynamic changes in the
tion. The recommended ETCO target for this
patient. It is not recommended to use ETCO solely pensation or those at high risk of cerebral hernia-
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to assess PaCO without at least periodic compari- sub-category of patients is 30-35 mmHg. If manual
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sons to the arterial carbon dioxide value. hyperventilation is performed it should be limited
Measuring paired PaCO and ETCO values also to a duration of 4-6 hours (such as during advanced
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allows the clinician to determine the difference imaging like magnetic resonance imaging; MRI) to
between the arterial and end-tidal CO . This differ- prevent excessive cerebral vasoconstriction and
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ence is referred to as the P (a-ET) CO gradient. The possible ischemic damage to the brain. It is not
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arterial to ETCO gradient should be ≤ 5 mmHg in recommended to provide prophylactic hyperventi-
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anesthetized normal dogs. The P (a-ET) CO gradient lation during the initial resuscitation of patients
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can provide the clinician with valuable information with traumatic brain injury so as to avoid vasocon-
regarding the clinical progress of a critically ill striction that could impede oxygen delivery to
patient; trends of this value may be used to assess damaged brain cells.
improvement. Table 6.1 summarizes some of the
causes of altered PaCO –ETCO gradient. 6.4 Interpretation of End-Tidal
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Carbon Dioxide
Traumatic brain injury patients
Clinical information can be obtained from three
Patients with traumatic brain injury require careful sources in CO analysis: (i) numerical values of
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monitoring of their ventilatory status. Hyperventilation ETCO (capnometry); (ii) shape of the waveform
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(low PaCO ) leads to cerebral vasoconstriction which (capnogram); or (iii) the difference between ETCO
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helps to preserve intracranial pressure while hypoven- and PaCO (P (a-ET) CO gradient).
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Table 6.1. The most common causes of altered P CO gradient.
(a-ET) 2
Increased P (a-ET) CO gradient Examples
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Ventilation perfusion mismatch Acute respiratory distress syndrome
Pulmonary infiltrates (edema, hemorrhage, pneumonia)
Diffusion barrier preventing diffusion of carbon dioxide into the alveoli (ETCO to
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be significantly lower than PaCO )
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Pulmonary thromboembolism
Decreased lung perfusion and increased alveolar dead space causing ETCO to
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be significantly lower than PaCO 2
Increased alveolar dead space Chronic obstructive pulmonary-like disease
Pulmonary thromboembolism
These conditions tend to cause an incomplete alveolar emptying (i.e. chronic
airway obstruction causes gas to be trapped in the alveoli not allowing complete
gas exchange therefore the ETCO reads low)
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Decreased cardiac output Heart disease
Less delivery of carbon dioxide from the tissues to the alveoli for gas exchange
resulting in low ETCO
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Low patient tidal volume to Increase in dead space ventilation
equipment dead space Patient exhaling into dead space; CO not measured by capnograph
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Leak in the sampling system or Exhaled gases lost from system
around endotracheal tube False decrease in ETCO value measured by capnograph
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PaCO , arterial concentration of carbon dioxide (measured in mmHg); ETCO , end-tidal carbon dioxide concentration (measured in
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mmHg); P CO , difference between arterial and end-tidal carbon dioxide concentrations.
(a-ET) 2
118 L.A.M. Ilie