Page 251 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

242 ACID-BASE DISORDERS

to establish a normal range because hyperventilation they were the only samples not collected under free-
related to fear or pain, increased muscular activity related flowing conditions. Data for the normal dogs in this
to struggling, and delays during sample transport and study are shown in Table 9-6.
analysis may have effects on the resulting normal range. Aging in humans has been associated with a decrease in
A review of previously published data provided the fol- Pao 2 and an increase in the alveolar-to-arterial PO 2 gradi-
lowing guidelines for normal arterial blood gas values ent, P(A a)O 2 . Mild or no changes in these values were
in dogs and cats: 25 observed in geriatric dogs, and no significant changes in
acid-base balance were found in geriatric dogs. 4,32
Dog Cat INTERPRETATION OF BLOOD
GAS DATA
pH 7.407 (7.351-7.463) 7.386 (7.310-7.462)
Correct identification of acid-base disturbances may pro-
PCO 2 (mm Hg) 36.8 (30.8-42.8) 31.0 (25.2-36.8)

HCO 3 (mEq/L) 22.2 (18.8-25.6) 18.0 (14.4-21.6) vide a clue to an underlying primary disease process and
PO 2 (mm Hg) 92.1 (80.9-103.3) 106.8 (95.4-118.2) aids in determining appropriate therapy for the patient.
A routine methodical approach to interpretation of blood
gas data facilitates the clinician’s approach to the patient.
Studies of normal unanesthetized dogs yielded venous The clinician should try to answer the following four
blood gas results as follows: pH 7.397 (7.351 to 7.443), questions:

PCO 2 37.4 (33.6 to 41.2) mm Hg, and HCO 3 22.5 1. Is an acid-base disturbance present?
(20.8 to 24.2) mEq/L. 52,76 In one of these studies, 2. What is the primary disturbance?
venous PO 2 values were reported to be 52.1 (47.9 to 3. Is the secondary, or adaptive, response as expected
56.3) mm Hg. 52 Studies of normal unanesthetized cats (i.e., is the disturbance simple or mixed)?
indicated venous blood gas values as follows: pH 7.343 4. What underlying disease process(es) is (are) responsi-
(7.277 to 7.409), PCO 2 38.7 (32.7 to 44.7) mm Hg, and ble for the acid-base disturbance(s)?
HCO 3 20.6 (18.0 to 23.2) mEq/L. 11,26,46 The possibility of an acid-base disturbance should be

When sampling sites were compared using unanesthe- considered when the history (e.g., vomiting, diarrhea)
tized normal dogs, blood gas data from three different or the pathophysiology of the patient’s disease (e.g., renal
venous sites (jugular vein, pulmonary artery, and cephalic failure, diabetes mellitus) is suggestive or when
þ
vein) were similar, but PCO 2 was higher and pH was lower abnormalities in total CO 2 or electrolytes (Na ,K ,
þ
when venous data were compared with results obtained and Cl ) are observed in the biochemical profile. Total

for the carotid artery. 29 The respiratory compensation CO 2 may be increased as a result of metabolic alkalosis
for metabolic acidosis in these dogs ranged from a 1.1- or renal adaptation to respiratory acidosis. Total CO 2
to 1.3-mm Hg decrement in PCO 2 for each 1-mEq/L may be decreased as a result of metabolic acidosis or renal

decrement in HCO 3 , whereas the respiratory compen- adaptation to respiratory alkalosis. Thus, the acid-base
sation for metabolic alkalosis ranged from a 0.4- to disturbance cannot be classified based on the total CO 2
0.6-mm Hg increment in PCO 2 for each 1-mEq/L incre- concentration alone. Objective physical findings sugges-
ment in HCO 3 for arterial, mixed venous, and jugular tive of an acid-base disturbance (e.g., hyperventilation)

venous samples. The increment was 1.3 mm Hg per 1- are unreliable as indicators of acid-base disturbances

mEq/L increment in HCO 3 for the cephalic samples, and are often not present. Blood gas analysis is required
which had the highest PCO 2 values, presumably because to identify and classify acid-base disorders conclusively.

TABLE 9-6 Blood Gas and Acid-Base Measurements (Mean Standard Deviation) in
Five Normal Unanesthetized Dogs

Value Arterial Mixed Venous Jugular Venous Cephalic Venous
pH (U) 7.395 0.028 7.361 0.021 7.352 0.023 7.360 0.022
PCO 2 (mm Hg) 36.8 2.7 43.1 3.6 42.1 4.4 43.0 3.2
PO 2 (mm Hg) 102.1 6.8 53.1 9.9 55.0 9.6 58.4 8.8

HCO 3 (mEq/L) 21.4 1.6 23.0 1.6 22.1 2.0 23.0 1.4
TCO 2 (mEq/L) 22.4 1.8 24.1 1.7 23.2 2.1 24.1 1.4
BE (mEq/L) 1.8 1.6 1.1 1.4 2.1 1.7 1.2 1.1

SHCO 3 (mEq/L) 22.8 1.3 23.0 1.2 22.2 1.3 23.2 1.1
From Ilkiw JE, Rose RJ, Martin ICA: A comparison of simultaneously collected arterial, mixed venous, jugular venous and cephalic venous blood samplesin
the assessment of blood gas and acid base status in dogs, J Vet Intern Med 5:294, 1991.
BE, Base excess; SHCO 3 , standard bicarbonate.