Page 105 - Basic Monitoring in Canine and Feline Emergency Patients

Table 5.7. Common pre-analytical factors that can affect blood gas analysis.

VetBooks.ir Confounder Parameter affected, direction of Comment
change
Temperature  =
Temperature
more gas to dissolve into liquid and decreasing the
 PO 2 Gas solubility increases at lower temperatures, allowing
 PCO 2 partial pressure. The subsequently decreased PCO will
2
 pH inversely affect pH
(Temperature  will have
the opposite effects)
Delay in analysis For every hour delay at Metabolism of O and production of CO by cells will
2
2
room temp: continue within the blood, changing the levels in the
 PO by 2 mmHg/h sample (assuming an anaerobic environment)
2
 PCO by 1 mmHg/h
2
 pH 0.02–0.03/h
All changes will be faster
with increased WBC counts
WBC  PO over time Increased cellular mass consumes oxygen more rapidly
2
Excessive heparin  pH Dilution with excessive heparin will cause decreases
 HCO − 3 in most variables; the PO of liquid heparin itself is
2
 PCO 2 150 mmHg (same as atmospheric PO ) and so will raise
2
 BE or lower the patient value toward 150 mmHg depending
PO – variable on the patient’s starting PaO (e.g. whether patient
2
2
 iCa receiving supplemental O or not)
2
Pigmented/opaque SaO – variable effects Methylene blue, cobalamin, very pronounced lipemia (e.g.
2
substances in plasma TPN therapy), isosulfant blue, and fetal Hb can interfere
with spectrophotometric measurements; endogenous
pigments (bilirubin) should not
Anesthetic gas  PO In older blood gas machines, anesthetic gas can be reduced
2
by the Clarke electrode and spuriously detected as O
2
Air contamination PaO – variable Values will equilibrate toward room air which should have
2
(bubbles)  PCO 2 PCO of 0 and PO of 150 mmHg at sea level. PaO
2
2
2
 pH may be spuriously elevated or decreased depending on
the FiO breathed by the patient
2
Hemolysis  iCa The exact reason hemolysis induces these changes in PO and
2
 PO 2 PCO is unclear; there are suspected influences of cell-free
2
 PCO 2 hemoglobin and other intracellular molecules independent of
influence of pH; this bias may be analyzer dependent
Cl, chloride; iCa, ionized calcium; K, potassium; TPN, total parenteral nutrition; WBC, white blood cells.
typically self-calibrate and bring the cartridge and than portable units. Some benchtop analyzers can
sample up to temp (37°C) for analysis. They there- measure bodily fluids in addition to blood and usu-
fore need less maintenance and manual calibration ally have panels that can be customized for each
of the machine. The cartridges contain set variables patient. These units are larger and are generally
and are limited in what they will analyze. In addi- housed in a central area where samples are brought
tion, cartridge-based systems are generally less to them, such as a clinical pathology lab, or inten-
cost-effective if large volumes of samples are being sive care unit. While they are more cost-effective for
run. However, they are generally small, portable, high-throughput clinics, they generally require more
and battery-operated, making it easier to take them manual calibration and maintenance.
to the patient in the field. Measurement of electrolytes is discussed in
In contrast, benchtop analyzers contain multi-use Chapter 8. For blood gas analysis, pH, PO , and
2
sensors and can measure a greater range of analytes PCO are directly measured, and other variables
2
Venous and Arterial Blood Gas Analysis 97