Page 109 - Basic Monitoring in Canine and Feline Emergency Patients

using the alveolar gas equation (Box 5.6). # # The most common cause of hypoxemia is
The measured arterial oxygen content V/Q mismatch.
VetBooks.ir (PaO ) should not be less than 20 mmHg # # Assess ventilation as previously discussed.
2
below the calculated alveolar content (P O ).
Hypo ventilation will cause hypoxemia as
A
2
This is the ‘A–a gradient’ (Box 5.6).
A
2
A
2
alveolus (see Fig. 5.5).
3. If the patient is hypoxemic, consider which of P CO builds up and displaces P O from the
the five physiological causes of hypoxemia is most # # If the patient is hypoventilating, assessment of
likely (Table 5.6; see Figs 5.5 to 5.10). the A–a gradient (Box 5.6) can be helpful. A
# # As with most classification systems in medi- normal A–a gradient implies all hypoxemia
cine, there is often overlap with a single can be attributed to the hypoventilation; fix
patient having hypoxemia due to multiple the hypoventilation and the PaO should nor-
2
mechanisms simultaneously. malize. An abnormal A–a gradient implies
additional lung dysfunction on top of hypoven-
tilatory hypoxemia. See Case study 2.
Box 5.5. Calculation of the P/F ratio. # # A ‘quick and dirty’ version of the A–a gradient
is the ‘rule of 120.’ Breathing room air at sea
2 (
PaO / FiO asadecimal) level, the PaO and PaCO added together
2
2
2
Normal lungsonroomair:100 mmHg / .021 476 should be close to a sum of 120 or greater. If
=
Diseased lungson40% the sum is less than 120, there is lung dysfunc-
=
FiO:100 mmHg 040 250 tion that cannot be explained by hypoventila-
/.
tion alone, and one of the other four causes of
The normal P/F ratio is 400–500. The P/F ratio hypoxemia should also be considered.
a
is used in humans to classify severity of acute 4. Patients with a PaO <60 mmHg despite nor-
2
respiratory distress syndrome (ARDS), with <300 malized ventilation and traditional methods of
consistent with mild ARDS, <200 moderate
ARDS, and <100 severe ARDS. maximal oxygen supplementation (FiO 60%) are
2
candidates for mechanical ventilation.
a The P/F ratio does not take PCO into account, so hypo-
2
ventilation as a cause of hypoxemia will result in an abnor- While these methods of oxygen assessment may
mal P/F ratio even if there is no true lung pathology
provide useful clinical tools, studies in animals are
lacking. In a study by Briganti et al. (2015), there
Box 5.6. Calculation of the A–a gradient.
(
Alveolargasequation → P O = [ FiO P − P H2O )] −( PaCO /R)
B
2
2
2
A
Where FiO is presented as a decimal (0.21 on room air), P = barometric pressure in mmHg (760 at sea level),
2
B
P H2O represents water vapor pressure in the airways, (generally assumed to be 47 mmHg), PaCO is measured
2
from the arterial blood gas, and R represents the respiratory quotient, a constant for which 0.8 is commonly used
At sea level, breathing room air, the equation can be simplified to:
−
PO =150 ( PaCO /R)
2
2
A
PaO is measured from the arterial blood gas and subtracted from P O .
2 A 2
Normal PO − A 2 PaO 2 (Aagradient shouldbe<15 20mmHgFiO 21% or <<50 110mmHgFiO 100%)
−
−
−
)
(
(
)
2
2
What is a ‘normal’ A–a gradient becomes less clear as the FiO increases. Some references state that for every
2
10% increase in FiO , the A–a gradient may increase by 5–7, while others list gradients up to 110 as normal at
2
100% FiO . Therefore, many clinicians will choose to use the A-a gradient only when the animal is breathing
2
room air.
Venous and Arterial Blood Gas Analysis 101