Page 108 - Basic Monitoring in Canine and Feline Emergency Patients

# # Case studies 1 and 2 in Section 5.4 illustrate
Box 5.4. Traditional acid–base analysis the traditional and non-traditional approaches
VetBooks.ir Patient values:
to acid–base analysis.
normal=74.)
pH=73.(
Blood gas analysis: Ventilation/oxygenation
PCO =25 mmHg normal=40 mmHg)
(
2
Ventilation
−
=
HCO = 12 mmolL normal 22 mmolL/)
/(
3
1. Assess venous or arterial CO levels. PaCO val-
2
2
A pH of 7.3 is low = acidemia. The PCO (acid) ues <35 mmHg are indicative of hyperventilation,
2
is low at 25 mmHg; the acidemia cannot be the 35-45 mmHg are considered normal, and values
‘fault’ of the PCO . Therefore, a low HCO (base) >45 mmHg are consistent with hypoventilation. Venous
−
2
3
of 12, indicating a metabolic acidosis, would be values will average about 5 mmHg higher than arterial
the culprit. values, assuming circulatory flow is adequate.
Compensation: The body is breathing off PCO
2
(acid) to compensate for the metabolic acidosis # # For example, if the PvCO is 70 mmHg, one
2
−
−
created by the low HCO . The HCO is 10 can reasonably infer that the PaCO is likely
2
3
3
points below normal. (22 − 12 = 10). To compen- to be around 65 mmHg and the patient is
sate, the PCO should change 0.7 for every 1 severely hypoventilating.
2
−
point the HCO changes (see Table 5.1). 10 × 0.7 2. Patients who have a PaCO >60 mmHg are
3
2
= 7. A normal PCO of 40 − 7 = 33 ± 3 mmHg severely hypoventilating and mechanical ventila-
2
would be the expected compensation. This tion should be considered if the underlying cause
patient’s PCO of 25 is in excess of expected of hypoventilation cannot be rapidly reversed (e.g.
2
compensation, therefore it has a mixed distur- reversible sedatives).
bance of both metabolic acidosis and respiratory # # Patients with excessive work of breathing
alkalosis.
(unable to rest) are in danger of exhaustion
and respiratory arrest, and should also be
considered candidates for ventilation. A PCO
2
level that is “normal” or rising in these patients
Non-traditional (semi-quantitative) analysis is concerning for exhaustion and imminent
1. Evaluate the pH. An elevated pH is an alkalemia, respiratory arrest. See Case study 3.
and a low pH an acidemia. 3. Be aware that assessment of ventilation and
2. Determine the respiratory aspect by evaluat- assessment of the respiratory side of acid–base analy-
ing the PCO . This is the same as in traditional sis are the same thing (assessment of PCO ).
2
2
analysis; elevations in PCO will cause a respira-
2
tory acidosis and decreases in PCO will cause a Oxygenation
2
respiratory alkalosis. 1. Determine the fraction of inspired oxygen
3. Evaluate the metabolic aspect of the patient (FiO ) and the barometric pressure (Pb). If the
2
by mathematically calculating the contribution patient is breathing room air at sea level, FiO =
2
of electrolytes, protein, lactate, and water to the 21% and Pb = 760 mmHg.
pH. Use the equations in Table 5.5, being sure to 2. Look at the PaO . The PaO should be approxi-
2
2
keep track of positive results (alkalinizing effect) or mately four or five times the FiO . This relation-
2
negative results (acidifying effect). Sum the output ship is the basis of the ‘P/F ratio’ (see Box 5.5).
of each equation and subtract it from the BE calcu- # # Breathing room air at sea level, healthy lungs
lated by the blood gas analyzer. should create a PaO of 80–100 mmHg in
2
# # If the sum of the equations subtracted from arterial blood. A PaO of <80 mmHg is
2
the BE approaches zero, there are no ‘unmeas- hypoxemia, with <60 mmHg considered
ured’ acid–base influences. References for severe hypoxemia.
‘normal’ unmeasured anions range up to ± 5. # # A patient breathing 100% oxygen should
# # If the difference between the equations and the have a PaO of 400–500 mmHg.
2
BE is greater than 5, there are ‘unmeasured # # At altitude, PaO will drop proportionally
2
influences’ on the acid–base. These are gener- as Pb decreases. The expected alveolar
ally toxins/drugs or errors of metabolism. concentration of oxygen can be calculated
100 A.C. Brooks