Page 250 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Introduction to Acid-Base Disorders 241

increases with metabolic alkalosis, regardless of changes in The PCO 2 of dry room air is extremely low, and the
PCO 2 in the sample. PCO 2 of the blood sample decreases, and its pH increases
The calculations of standard bicarbonate and whole if it is exposed to air. 51 The PO 2 of room air is higher than
blood buffer base were introduced before the concept that of arterial or venous blood, and the PO 2 of the sample
of whole body titration was developed 22 and represented increases if it is exposed to air. The increase is much
an attempt to use in vitro titration of whole blood samples greater for venous than arterial blood samples. Air
to separate the respiratory and metabolic components of bubbles may also cause an increase in PO 2 and a decrease
acid-base disturbances. These methods do not account in PCO 2 if they occup. 10% or more of the sample volume.
for other buffering effects in the body (e.g., intracellular Analysis of the sample within 15 to 30 minutes of collec-
proteins other than hemoglobin, intracellular organic tion is desirable. The PCO 2 of a blood sample increases and
phosphates, bone carbonate). the pH decreases as the sample is allowed to stand before
analysis. The rate of change is much greater at 25 Cthan

SAMPLE COLLECTION AND at 4 C. These changes in PCO 2 and pH are accompanied

HANDLING by decreased glucose and increased lactate concentrations
Proper collection and handling of samples for blood gas and are attributed to glycolysis by white cells, red cells,
analysis are as important as accurate measurement of pH and platelets. Aerobic metabolism by white cells also
and PCO 2 by the blood gas analyzer. 23 In small animals, decreases PO 2 . By cooling the blood sample, these changes
arterial samples usually are taken from the femoral artery. are minimized. Therefore, if the sample cannot be analyzed
This procedure can be performed in unanesthetized dogs soon after collection, the syringe should be immersed in
with minimal discomfort and restraint but is difficult in a mixture of ice and water. Samples are stable for up to

unanesthetized cats. Samples for venous blood gas analy- 2 hours at 4 C, but PCO 2 begins to increase and pH
sis usually are taken from the jugular vein. However, to decrease after 20 to 30 minutes at 25 C. 40

venous stasis and muscular activity can result in accumu- Arterial samples are preferred to venous ones because
lation of acid metabolites. Thus, an attempt should be oxygenation of blood can be evaluated, and the sample is
made to obtain a free-flowing venous sample by releasing not affected by stasis of blood flow and local tissue metab-
digital pressure on the vein after venipuncture has been olism. The most conspicuous difference between arterial
achieved. and venous samples is the difference in PO 2 , which reflects
For femoral artery samples, the hair over the medial oxygenation of blood in the lungs and use in the tissues.
thigh is clipped, and the puncture site is disinfected. A Conversely, arterial samples may not reflect the acid-base
3-mL syringe with a 25-gauge needle is coated with a status in peripheral tissues. This may present a problem
small amount of heparin (1000 U/mL). Enough heparin during cardiopulmonary resuscitation (see Chapter 10).
is drawn into the syringe to coat the interior of the entire The PCO 2 is slightly higher and the pH is slightly lower
barrel, and air is expelled, leaving the dead space of the in venous samples because of local tissue metabolism.
syringe filled with heparin. The dead space of 1- to 5- Free-flowing capillary blood that has been “arterialized”
mL syringes is 0.1 to 0.2 mL, and this volume provides by warming the skin puncture site is used as an alternative
more than enough heparin for anticoagulation. 51 Dilu- to arterial samples in human medicine.
tion of the sample with heparin should be avoided Capillary blood obtained from the caudal medial
because it can cause erroneously low values for pH, ear margin of unanesthetized dogs had blood gas values
24,27,51
PCO 2 , and HCO 3 . similar to those of arterial samples and did not require
An assistant restrains the dog in lateral recumbency, induction of arteriolar vasodilatation by warming
and the rear limb closest to the table is extended. The (“arterialization”). 52 In a study of cats, arterialized capil-
artery is located by palpating the femoral pulse and is lary blood was obtained from the cut claw after previously
62
immobilized beneath the first and second fingers of the warming the paw. In this study, mean PO 2 and PCO 2 did
operator’s free hand. The artery is punctured with the not differ from those of arterial blood, but mean pH was
needle directed at an angle approximately perpendicular significantly higher (7.432 versus 7.419). Capillary blood
to the course of the vessel. At least 1.5 mL of blood is is collected directly into a heparinized capillary tube; a
withdrawn, and the site of puncture is manually com- small metal “flea” is added for mixing; and the ends of
pressed for 3 to 5 minutes after needle withdrawal to pre- the tube are sealed with clay. During states of peripheral
vent hematoma formation. If necessary, air bubbles are vascular collapse (e.g., hypovolemic shock), capillary
dislodged by flicking the barrel of the syringe with the blood does not provide meaningful blood gas values for
index finger and expelling any air from the hub of the comparison with those of arterial samples. 52,70
syringe. Usually, the needle is inserted into a rubber stop-
per to prevent exposure of the sample to room air. NORMAL VALUES
A tightly fitting cap placed over the hub of the syringe Normal blood gas values for dogs and cats should be
may be superior. 51 The syringe is rolled between the established by the laboratory performing the analysis.
palms of the hands to mix the sample. Extreme care must be taken in obtaining blood samples