Page 98 - Basic Monitoring in Canine and Feline Emergency Patients

acid–base analysis, and Fig. 5.2 shows an example Blood gas analysis
of how counterbalancing abnormalities not seen Ventilation
VetBooks.ir with traditional analysis would be seen with non- The amount of air cycled through the lungs per
traditional analysis.
minute is called the minute ventilation, and is the
product of respiratory rate and tidal volume of
Table 5.5. Calculations for non-traditional (semi-quanti- each breath. Ventilation refreshes the gas in the
tative) acid–base analysis.
alveolus, replenishing oxygen for diffusion into the
Effect Formula a blood and removing accumulated CO .
2
CO is produced during aerobic metabolism in
2
Free water effect Dogs: 0.25[(Na ) – (Na )] cells and is carried via the venous circulation back
r
p
Cats: 0.22[(Na ) – (Na )] to the lungs for excretion. It is a very soluble gas
p
r
Corrected chloride Cl × (Na /Na ) that equilibrates rapidly between body compart-
p
r
p
Chloride effect Cl − Cl corrected ments, blood, and the alveolar space. Because it
r
Phosphate effect 0.58 (Phos – Phos ) equilibrates so rapidly, as long as circulatory flow
p
r
Albumin effect 3.7 (Alb –Alb ) is adequate, the level of CO in the blood and the
p
r
Lactate effect −1 × lactate p 2
Sum of effects Free water effect + Cl effect level in the alveolus can be assumed to be equal.
+ Phos effect + Alb effect + Therefore, the level of CO in the bloodstream is
2
lactate effect directly proportional to alveolar ventilation. This is
Unmeasured Base excess – sum of effects why we use partial pressure of carbon dioxide
anion effect (PCO ) levels in the blood as the gold standard
2
measurement of effective alveolar ventilation. A
a Subscript p = patient value; subscript r = median value of
analyzer reference range for that analyte. Calculated values normal arterial PCO (PaCO ) level is approxi-
2
2
that result in positive numbers will have alkalinizing effects, mately 40 mm Hg, with venous PCO (PvCO )
2
2
while those resulting in negative numbers will have acidifying usually only slightly higher at ~45 mmHg.
effects. Significant differences between arterial and venous
Alb, albumin; Cl, chloride; Na, sodium; Phos, phosphorus. CO imply issues with circulation in the tissue bed
Adapted from In: Silverstein, D.C., Hopper, K. (eds), Small 2
Animal Critical Care Medicine, 2nd edn. Elsevier, St. Louis, sampled, issues with generalized circulation (e.g.
Missouri, USA. recent cardiopulmonary arrest), or sampling error.
+
+
UC (Ca, Mg) AG Albumin UA UC (Ca, Mg) AG Albumin UA
Lactate
K + K +
HCO – Lactate (near HCO –
3 zero in health) 3
Increased
lactate
Na + Cl – “hidden” by Na + Cl –
a concurrent
decrease in
albumin
Fig. 5.2. Gamblegram of elevated lactate/high anion gap masked by concurrent hypoalbuminemia. As shown in
Fig. 5.1, increasing lactate should normally increase the anion gap. However, as shown in the boxes to the right, a
concurrent decrease in albumin will mask the expected increase in AG by maintaining the original ‘size’ of the UA box.
In traditional acid–base analysis, no abnormalities would be noted even with calculation of the anion gap because
these counterbalancing effects resulted in an unchanged bicarbonate level. The non-traditional approach measures
the impact of albumin and lactate independently, revealing even the ‘hidden’ effects of counterbalancing abnormalities
to the clinician. Cl, chloride; Ca, calcium; K, potassium; Mg, magnesium; Na, sodium; UA, unmeasured anions; UC,
unmeasured cations.
90 A.C. Brooks