Page 319 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

310 ACID-BASE DISORDERS

normal or only slightly abnormal. It is important to Gastric dilatation-volvulus complex has been
remember that dogs with long-standing respiratory associated with respiratory alkalosis and metabolic acido-
acidosis can have normal arterial pH. 22 When the mixed sis in dogs 37 in which the respiratory alkalosis may be the
2
disorder is confirmed, treatment should be directed at result of pain, 37 sepsis, or restriction of pulmonary
correcting the most life-threatening underlying disease expansion. Patients with liver disease may develop a wide
process first. No therapy is necessary to correct pH if variety of acid-base disturbances. Hyperventilation is
8
pH is normal or near normal. Patients with chronic common and appears to be multifactorial. Metabolic aci-
pulmonary disease that have hypoxemia and hypercapnia dosis also has been associated with liver disease in dogs. 13
are at greater risk from metabolic alkalosis than are others Human patients with cirrhosis demonstrate enhanced
because superimposition of metabolic alkalosis can proximal renal tubular sodium reabsorption that may
5
þ
further reduce ventilation and lead to worsening of limit distal H secretion and lead to hyperchloremic aci-
hypoxemia. 49 Therefore metabolic alkalosis should not dosis. Type B lactic acidosis also can develop in patients
be overlooked if the patient has a chronic lung disease. with liver failure because liver disease can decrease liver
uptake and metabolism of lactate. 40 Distal renal tubular
Respiratory Alkalosis and Metabolic acidosis has been associated with hepatic lipidosis in a
Acidosis cat with normal AG acidosis. 7
Many clinical situations can lead to this mixed disorder, Special considerations apply to cardiopulmonary
usually with high-AG metabolic acidosis. These patients resuscitation. Arterial blood gases may indicate respira-
have low PCO 2 and low [HCO 3 ], and their pH tends to tory alkalosis because gas exchange is occurring in blood

benearlynormal.Itisimportanttoremember thatchronic that traverses the pulmonary circulation. Mixed venous
respiratory alkalosis is a simple acid-base disturbance, and PCO 2 has been shown to be significantly higher than arte-
affected patients can have a normal pH. Thus in the pres- rial PCO 2 during cardiopulmonary resuscitation in dogs. 32
ence of a normal pH, low PCO 2 , and low [HCO 3 ], the cli- In this setting, arterial values reflect the adequacy of

nician must decide whether the patient has simple ventilatory support, whereas mixed venous values may
respiratory alkalosis or metabolic acidosis associated with correlate better with tissue pH. 58
respiratory alkalosis. In this situation, the history can pro- In patients with mixed metabolic acidosis and respira-
vide important clues. The presence of hypoxemia with tory alkalosis, pH tends to be normal, and specific treat-
increased hematocrit suggests chronic respiratory alkalo- ment to correct pH usually is not necessary. Treatment
sis. An increase in unmeasured strong anions is helpful should be directed at the underlying causes of the
because the majority of metabolic acidoses associated with metabolic acidosis and respiratory alkalosis.
respiratory alkalosis are normochloremic, whereas com-
pensation for chronic respiratory alkalosis is characterized Metabolic Acidosis and Metabolic Alkalosis
by corrected hyperchloremia. This mixed disorder usually is seen in patients with long-
Diseases associated with metabolic acidosis and standing, high-AG metabolic acidosis (e.g., chronic renal
respiratory alkalosis are shown in Box 12-6. In some failure, uncomplicated ketoacidosis) that begin vomiting
conditions such as sepsis, patients initially may have respi- and develop hypochloremic alkalosis. Because albumin is
ratory alkalosis; metabolic acidosis (usually caused by lac- a weak acid, a decrease in albumin concentration is
tic acidosis) only develops later. 23,26 Sepsis complicating associated with metabolic alkalosis. Superimposition of
any disease known to cause metabolic acidosis can result hypoalbuminemia on chronic metabolic acidosis also
in a metabolic acidosis superimposed on respiratory alka- can lead to this mixed acid-base disorder. Alternatively,
losis. Exercise also can cause a mixed disorder that begins this mixed metabolic disorder can begin as metabolic
with respiratory alkalosis. In mild exercise (35% of maxi- alkalosis with subsequent development of severe volume
38
mal O 2 consumption), mild respiratory alkalosis occurs. depletion resulting in hypoperfusion and lactic acidosis.
When dogs are maximally exercised, lactic acidosis is Depending on the relative severity of the two opposing
superimposed on the initial respiratory alkalosis. 27,38,45 disorders, pH and [HCO 3 ] can be increased, normal,

Dogs with heat stroke also initially have respiratory alka- or decreased. Recognition of both disturbances in this
losis and later develop mixed respiratory alkalosis and setting is very important because treatment of one with-
metabolic acidosis. 50 Salicylate toxicity in dogs and cats out attention to the other permits the unattended abnor-
causes hyperventilation initially, but metabolic acidosis mality to emerge unopposed. Information in Table 12-4
then develops. 42 The hyperventilation associated with can be used to help diagnose mixed metabolic acidosis
salicylate toxicity is caused by central stimulation, and and metabolic alkalosis. Mixed hyperchloremic metabolic
only a small portion of the hyperventilation can be acidosis and hypochloremic metabolic alkalosis can theo-
attributed to hyperthermia. 52 In human patients with retically coexist (e.g., patients with vomiting and diar-
salicylate intoxication, metabolic acidosis is caused by rhea), but because these disturbances have offsetting

accumulation of organic acids, including lactate and effects on [Cl ] and [HCO 3 ], only the prevailing disor-

ketoacids. 49 This also may be true in small animals. 42 der can be identified. Diseases associated with mixed