Page 118 - Basic Monitoring in Canine and Feline Emergency Patients

6

VetBooks.ir Capnography

Laura a.M. ILIe*

Veterinary Specialty Center, Buffalo Grove, Illinois, USA

Capnography is the measurement of the partial pres- The gold standard measurement of the PaCO is
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sure of carbon dioxide (CO ) in exhaled respiratory performed by analyzing an arterial blood sample (see
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gases at the end of expiration. The exhaled CO in turn Chapter 5). Unfortunately, collection of arterial blood
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is an indirect assessment of the partial pressure of CO samples is not always easily achieved in small animals
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in the arterial blood (PaCO ) and as such provides and can be associated with pain, thrombosis, and pos-
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insight into important life-sustaining systems includ- sible infection. Additionally, an arterial blood gas
ing the respiratory, circulatory, and metabolic systems. sample only provides a snapshot of the patient’s CO
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The measurement of the exhaled CO has been value at one point in time and requires repeat blood
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used for many decades in human and veterinary medi- sampling to gage the animal’s condition over time.
cine to assess the adequacy of ventilation under gen- A noninvasive and indirect measurement of the
eral anesthesia, appropriateness of intubation and, PaCO can be obtained by measuring the CO level
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more recently, the efficacy of chest compressions of the patient’s expired breaths (i.e. end-tidal CO
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during cardiopulmonary resuscitation (CPR). (ETCO )) using a capnograph. In order to measure
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ETCO the patient has to be intubated or to have a
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tight-fitting mask. If ventilation and perfusion are
6.1 Basic Physiology
well matched, the ETCO should be nearly equal to
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Carbon dioxide is a metabolic byproduct produced that of the PaCO (i.e. 35–45 mmHg). Usually the
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when carbon is combined with oxygen as part of the ETCO value is 2 to 5 mmHg less than that of arte-
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body’s energy-making processes. As it is formed, CO rial CO because alveolar gases containing CO
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diffuses from the tissues into the bloodstream and will mix with dead-space gases containing no CO
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then is transported to the lungs where it diffuses into in the anesthetic tubing, slightly reducing the
the alveoli. At the alveolar level, the red blood cells ETCO measurement versus the concentration of
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are ‘unloading’ CO . This carbon dioxide is then CO in the alveolus. Conditions that may interrupt
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expelled through the breathing process (Fig. 6.1). the transfer of CO from the blood to the alveoli
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The PaCO is carefully maintained by the body at an (i.e. pulmonary edema, hemorrhage, pneumonia,
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approximate constant pressure of 35–45 mmHg in pulmonary embolism, emphysema) can lower the
non-sedated/anesthetized dogs and cats. This is because ETCO . In these situations, the difference between
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CO plays an important role in homeostasis. Specifically, PaCO and ETCO levels can be increased.
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it acts as a buffer to maintain normal blood pH, plays In addition, ETCO provides insight regarding the
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an important role in the regulation of cerebral perfu- function of the circulatory system. For the CO to
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sion by regulating vascular tone, stimulates breathing travel from the cells to the alveolus, there must be
in the brain’s ventilator centers, and influences the appropriate cardiac output (CO) and at least relatively
affinity hemoglobin has for oxygen (O ). normal flow of blood through the heart and pulmo-
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Measurement of the PaCO concentration pro- nary system. If the CO is decreased in conditions like
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vides the clinician with useful information regard- hypovolemia or poor cardiac output or the blood does
ing the ventilation status of a patient. High PaCO not appropriately travel from the cells back to the
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(hypercapnia) suggests hypoventilation while low alveolus (as occurs with vasodilatory shock or extreme
PaCO (hypocapnia) suggests hyperventilation. vasoconstriction), the ETCO will be low.
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* Corresponding author: LIlie@vetspecialty.com
110 © CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)