Page 93 - Basic Monitoring in Canine and Feline Emergency Patients

VetBooks.ir 5 Venous and Arterial Blood

Gas Analysis

Aimee C. Brooks*

Purdue University College of Veterinary Medicine, West Lafayette, Indiana, USA

Venous blood gas analysis is often used as first-line abnormalities can cause significant dysfunction of
‘stat’ blood work to assess acid–base status, electro- the cardiovascular and neurological systems, and
lytes, and (depending on the monitor) other param- can result in organ failure and death.
eters such as renal values and lactate. Arterial blood When considering the pH, even small deviations
gas analysis is generally performed when assessment from the normal range can be important. Since pH
of the oxygenation/ventilation status of the patient is is a logarithmic scale, a change in pH from 7.4 to
+
desired. As other chapters of this book (see Chapters 7.1 indicates the amount of H in the system has
1 and 8) discuss point-of-care values such as lactate doubled. Small changes can therefore have large
and electrolyte abnormalities, this chapter will focus effects; cellular enzymatic functions can be affected
on acid–base assessment and assessment of oxygena- by pH deviations as small as 0.1 from the patient’s
tion/ventilation status. Indirect monitors of oxygen- normal baseline.
ation and ventilation are discussed in Chapters 4 Because maintenance of normal pH within a tightly
and 6, respectively. controlled range is so critical, the body has developed
multiple buffering systems to help maintain pH
within a normal range and mitigate the changes in
5.1 Basic Physiology and Anatomy
pH that may occur with disease. The most impor-
Blood gas analysis: Acid–base tant of these systems is the bicarbonate– carbonic
acid system, commonly represented by the carbonic
Acid–base: Traditional approach
anhydrase equation (Box 5.1).
The blood pH is inversely proportional to the con-
centration of hydrogen ions ([H ]) in the blood-
+
+
stream; as [H ] goes up, pH goes down. Therefore,
a decreasing pH is consistent with acidemia (more Box 5.1. Carbonic anhydrase equation.
acidic blood) and an increasing pH with alkalemia + −
(more basic or alkaline blood). The underlying CO + HO↔ HCO ↔ H + HCO 3
2
2
2
3
process that causes the pH to go up or down is the CO , carbon dioxide; H O, water; H CO , carbonic
2
2
3
2
−
+
–osis, e.g. an uncomplicated metabolic acidosis will acid; H , hydrogen ion; HCO , bicarbonate.
3
result in acidemia.
The normal pH of most mammals is around 7.4.
For dogs, the normal range of pH is 7.35–7.46, and
for cats, 7.31–7.46, although these values will vary The equation illustrates that carbon dioxide
slightly by analyzer and readers should use the (CO ), when combined with water, will form car-
2
normal ranges provided for their machine. Values bonic acid. This reaction is catalyzed by the enzyme
outside of these ranges should be considered abnor- carbonic anhydrase, allowing it to happen rapidly in
malities, with the more extreme deviations leading the body. Carbonic acid quickly dissociates into
to more significant side effects. Severe acid–base hydrogen ion and bicarbonate. Because the body can
* Corresponding author: brook108@purdue.edu
© CAB International, 2020. Basic Monitoring in Canine and Feline Emergency Patients 85
(eds E.J. Thomovsky, P.A. Johnson and A.C. Brooks)