Page 94 - Basic Monitoring in Canine and Feline Emergency Patients

rapidly change CO levels by changes in breathing, acidosis because the body cannot excrete metabolic
2
this ‘open’ buffering system is extremely effective in acids appropriately. This will cause a decrease in
VetBooks.ir keeping pH levels in the body near normal. pH, which will trigger pH receptors in the carotid
body and aortic arch that will stimulate ventilation.
The classic Henderson–Hasselbalch equation
(Box 5.2) describes how the relationship between
the main components of this system (CO and Increasing ventilation will drop the amount of
PCO (acid) in the blood (i.e. a respiratory alkalo-
2
2
−
HCO ) influence pH. sis), bringing the pH closer to normal. Compensation
3
can never bring the pH completely back to normal.
Respiratory compensation for metabolic disorders
happens quickly (within minutes) as changes in
Box 5.2. Henderson–Hasselbalch ventilation can occur rapidly. Metabolic compensa-
equation. tion for primary respiratory disorders will take
longer (2–5 days) as the kidneys must adjust the
+
−
.
pH=61 log[ HCO /( 003 × PCO )] OR amount of acid retained or excreted in order to
.
2
3
  H  =24 × PCO / HCO 3 − compensate.
+

2
−
HCO , bicarbonate; PCO , partial pressure of Expected compensatory responses derived from
2
3
carbon dioxide dissolved in plasma. healthy experimental dogs are shown in Table 5.4.
To the author’s knowledge, while these values are
commonly extrapolated to cats, there are no pub-
lished guidelines for expected compensatory
The Henderson–Hasselbalch equation demon- responses in cats.
strates that the pH of the body is determined by the In traditional acid–base analysis as described
−
−
−
ratio between HCO and PCO . That is, as the HCO above, both PCO and HCO are viewed as inde-
2
3
3
3
2
content increases, pH will increase (alkalemia), and pendent variables and the ratio between the two
−
as the HCO content decreases, pH will also decrease determines the pH. On the respiratory side, this is
3
(acidemia). Therefore, we think of HCO as acting fairly straightforward, as differentials for abnor-
−
3
as the main base in the body. Conversely, as PCO malities in PCO are synonymous with those for
2
2
content increases, pH will decrease (acidemia), and hypoventilation or hyperventilation (see Table 5.2).
as PCO content decreases, pH will increase The weakness in simple traditional acid–base anal-
2
(alkalemia). Hence, we think of PCO as the main ysis comes with interpretation of the metabolic side
2
acid in the body. of the equation. A strictly traditional approach
The PCO level is determined by alveolar ventila- does not allow differentiation between the many
2
tion (see below), and therefore it is representative metabolic acids – both endogenous and exogenous
−
of the respiratory side of acid–base assessment. – that HCO may be buffering. Therefore, addi-
3
−
Similarly, the HCO is regulated primarily by renal tional analyses, including the non-traditional/semi-
3
function, and therefore is representative of the quantitative approach as described in the next
metabolic side of acid–base assessment. Any pro- section, have been developed in an attempt to bet-
cess that changes the ratio between PCO and ter understand the underlying variables affecting
2
−
HCO will alter the pH of the system (Box 5.2). the metabolic side of the equation.
3
For example, increasing PCO or decreasing HCO Two modifications to traditional acid–base anal-
−
2
3
will shift the blood towards acidemia, whereas ysis involve using base excess (BE) and anion gap
decreasing PCO or increasing HCO will shift the (AG) calculations to parse out the various causes of
−
2
3
blood towards alkalemia. Common differentials metabolic acidoses.
for respiratory acidosis/alkalosis (aka hypoventila-
tion and hyperventilation) are shown in Table 5.1.
Common differentials for metabolic acidosis and Acid–base: Modifications of traditional
alkalosis are shown in Tables 5.2 and 5.3, interpretation
respectively. base excess One of the downsides of using HCO
−
3
When an acid–base abnormality occurs, the as the sole variable to assess the metabolic side of
body attempts to compensate and bring the pH acid–base problems is that it can be influenced by
back toward normal by using the opposing system. PCO . As seen in Box 5.1, addition of PCO to the
2
2
For example, renal failure causes a primary metabolic system will shift the carbonic anhydrase equation
86 A.C. Brooks