Page 99 - Basic Monitoring in Canine and Feline Emergency Patients

The ventilatory rate is driven primarily by PaCO respiration (e.g. intercostals, abdominal muscula-
2
levels sensed by receptors in the brain. Peripherally, ture) will also be activated, increased the work of
VetBooks.ir PaO and pH levels sensed within the carotid bodies breathing.
2
and aortic arch also influence the ventilatory drive.
Any pathology affecting these normal pathways
Other factors such as mental/emotional status, tem-
perature (primarily in dogs), lung irritant receptors, needed for ventilation can result in hypoventilation
and elevated PaCO (see Table 5.1). In contrast,
2
etc. can also influence breathing (see Table 5.1, hyperventilation means PaCO levels will be low,
2
hyperventilation). These signals will be summated and therefore CO is not stimulating the increase in
2
within the respiratory centers in the brainstem, breathing. Instead, other stimuli such as those out-
which will send signals down the cervical spinal cord lined in Table 5.1 must be driving the hyperventila-
to the paired phrenic nerves, which exit the spinal tion in the face of an already low PaCO .
2
cord around segments C5–C7 (up to C4 in cats). The
paired phrenic nerves innervate each crus of the
diaphragm, the main muscle of breathing. Contraction Oxygenation
of the diaphragm results in expansion of the thoracic With each breath, the alveolus is refreshed with
cavity and a drop in intrathoracic pressure (it gas. In a patient breathing room air, this gas is com-
becomes more negative relative to atmosphere). This posed of 21% oxygen, 78% nitrogen, and 1%
results in a concurrent drop in alveolar pressure as other miscellaneous gases. Dalton’s law of partial
the lungs expand, drawing air down the airways and pressures states that the total pressure (barometric
into the alveolus. In health, expiration happens as a pressure (Pb) = 760 mmHg at sea level) must equal
result of relaxation of the diaphragm and elastic the sum of the partial pressure of each gas making up
recoil of the lungs passively expelling air. With the mixture. Therefore, room air is approximately
increased respiratory drive, accessory muscles of 160 mmHg O , 593 mmHg N , and 7 mmHg other
2
2
Air is humidified, taking up 47 mmHg as
water vapor pressure

(760–47) 0.21 = 150
mmHg O entering
2
Pb sea level = 760 mmHg:
the alveolus
21%O 2
78%N 2
1% other gasses
P A O = 100
2
P CO = 40 Natural
A
2
P N = 573 venous
2
A
admixture
Venous blood: Arterial blood:
PvO = 30–40 mmHg PaO = 100 PaO = 80–100 mmHg
2
2
2
PvCO = 41–51 mmHg PaCO = 35–45 mmHg
2
2
Fig. 5.3. Alveolar gas exchange breathing room air (21%) at sea level (barometric pressure (Pb) = 760 mmHg). As
gas enters the airways, it is humidified, adding 47 mmHg of water vapor pressure to the total gas mixture. Once gas
enters the alveolus, CO diffuses into the alveolus from the bloodstream, further ‘diluting’ the alveolar oxygen content
2
(P O ). Oxygen diffuses from the alveolus into the bloodstream; increasing oxygen content of the capillary blood
A
2
(PaO ) is denoted as a blue to red gradient. Oxygen is taken up by hemoglobin (Hb), saturating the Hb in red blood
2
cells. Even in health, a small amount of natural venous admixture occurs, mixing deoxygenated venous blood with the
maximally oxygenated blood leaving the lungs (see text). This causes a slight drop in the oxygen level between the
Alveolus and arterial blood (the A–a gradient).
Venous and Arterial Blood Gas Analysis 91