Chap. 54 traditional acid-base Flashcards
Traditional acid-base analysis uses:
Blood pH is a measure of:_____ and is dependent on the ratio of _______:_______
PCO2 behaves as an acid in the body and represents
metabolic contribution to acid-base balance
AG is a diagnostic tool that may help identify the cause of
The treatment of most acid-base disorders is focused on resolution of the underlying disease.
Sodium bicarbonate therapy is primarily indicated for:
Henderson-Hasselbalch equation to evaluate:
pH as a direct consequence of the PCO2, HCO3, BE
hydrogen ion concentration
ratio of HCO3:PCO2
respiratory contribution to acid-base balance.
Bicarbonate, base excess, and TCO2
metabolic acidosis
tx. of metabolic acidoses w.:
kidney disease
diarrhea-associated loss of bicarbonate
HH equation:
carbonic acid equation:
pH = 6.1 (pka) + log ([HCO3]/[0.03xPCO2])
HH equation, pH is consequence ratio HCO3-:PCO2
H2O + CO2 - H2CO3 - H+ + HCO3-
why does CO2 act as an acid in body:
bc of ability to react with H2O to produce carbonic acid
H2CO3
major criticisms of using bicarbonate as the measure of the metabolic component
criticisms to trad approach as a whole:
not independent of changes in PCO2
=important that changes in bicarbonate concentration are always evaluated in terms of the pH and PCO2
failure to identify individual disease processes that are contributing to the acid-base abnormality
Base excess (BE) is the titratable acidity (or base) of the blood sample. It is defined as:
advantage
amount of acid or base that must be added to a sample of oxygenated whole blood to restore the pH to 7.4 at 37° C and at a PCO2 of 40 mmHg
independent of changes in the respiratory system
BE is a parameter calculated by an algorithm programmed into blood gas machines
Herbivores more positive BE
carnivores more negative BE than people
BE can be estimated by
estimated by the measured HCO3 - normal HCO3 (22)
in the face of substantial abnormalities in PCO2, the BE is a more reliable measure of metabolic component
TCO2 misleading b/c:
majority of carbon dioxide carried in blood is in what form:
TCO2 will be _____ mmol/L higher than ____
represents metabolic acid-base component, not the respiratory
TCO2 is a measure of all the carbon dioxide in a blood sample, and the majority of carbon dioxide is carried as bicarbonate in the blood
1 to 2 mmol/L higher than bicarbonate
Anion Gap why we calc: what is it: calc.: normal AG composed of: false decrease:
better define the cause of a metabolic acidosis
electroneutrality requires there to be equal # anions:cations in physiologic systems
no actual AG; the apparent AG exists because more cations in the system are readily measured than anions
AG is a reflection of unmeasured cations and unmeasured anions and is calculated:
[Na] + [k] - [HCO3] - [Cl]
negatively charged plasma proteins, mostly albumin
two common mechanisms of metabolic acidosis
loss of bicarbonate: GI or kidneys
involves the exchange of bicarbonate and chloride
=hyperCl metabolic acidosis
gain of acid: DUEL, hydrogen ions will titrate (combine) with bicarbonate, leading to a fall in bicarbonate concentration
- anion that accompanied the hydrogen ion (the conjugate base) will accumulate, maintaining electroneutrality
= increasing the AG
Common acids associated with an increased AG
low AG:
lactate, ketone bodies, sulfate, phosphate, and toxins such as ethylene glycol
DUEL
(sulfates, phosphates)
Renal bicarbonate loss
Gastrointestinal bicarbonate loss
Dilutional acidosis
Hypoalbuminemia
metabolic compensatory response to a primary respiratory disorder takes hours to begin and 2 to 5 days
espiratory response to a primary metabolic abnormality is rapid in onset and complete within hours
metabolic compensatory response to a primary respiratory disorder takes hours to begin and 2 to 5 days
“overcompensation” assumption is
mixed acid-base disorder
changes in the secondary system are not within a range
disturbance of the secondary system preventing appropriate compensation or causing appearance of “overcompensation” (which does not occur)
Metabolic acidosis Metabolic alkalosis Respiratory acidosis—acute Respiratory acidosis—chronic Respiratory alkalosis—acute Respiratory alkalosis—chronic
↓ PCO2 of 0.7 mm Hg per 1 mEq/L decrease in [HCO3−] ±3
↑ PCO2 of 0.7 mm Hg per 1 mEq/L decrease in [HCO3−] ±3
↑ [HCO3−] of 0.15 mEq/L per 1 mm Hg ↑ PCO2 ±2
↑ [HCO−3] of 0.35 mEq/L per 1 mm Hg ↑ PCO2 ±2
↓ [HCO−3] of 0.25 mEq/L per 1 mm Hg ↓ PCO2 ±2
↓ [HCO−3] of 0.55 mEq/L per 1 mm Hg ↓ PCO2 ±2
compensation in cats:
cats do not develop respiratory compensation
extrapolation of the canine calculations of expected metabolic compensation to respiratory disorders should be performed with caution in cats
extrapolation of the canine calculations of expected respiratory compensation to metabolic disorders cannot be recommended in cats
Traditional acid-base analysis identify 4 simple disorders
Metabolic acidosis Metabolic alkalosis Respiratory acidosis Respiratory alkalosis mixed
approach:
- pH - acidemia or alkalemia
- CO2 and HCO3:
influencing pH in same direction is the primary disorder - Calc. compensation
- AG if metabolic acidosis (assuming normal albumin)
Bicarbonate Therapy
adverse effects of metabolic acidosis:
indications for HCO3-:
controversies:
strict contraindication:
Bicarbonate Therapy
- decreased myocardial contractility
- arterial vasodilation
- impaired coagulation
- decreased renal and hepatic blood flow
- insulin resistance
controversial, although some consensus has been reached in recent time. There are several concerns with bicarbonate therapy (Box 54-5).
-first is that its use is based on the premise that acidemia has substantial negative consequences to the patient - human studies have demonstrated that a low pH is well tolerated; this includes patients subjected to permissive hypercapnia and patients with DKA
one of the most commonly cited adverse effects of acidemia is decreased myocardial contractility and vascular tone. Investigations have not been able to consistently demonstrate these negative hemodynamic effects
- studies have failed to demonstrate that bicarbonate administration will improve hemodynamic performance in the face of acidemia (in some studies hemodynamic performance actually deteriorates after bicarbonate administration).
- another concern is that sodium bicarbonate therapy does not reliably increase pH.
- after administration, the HCO3- ions rapidly dissociates to CO2 and water
- ventilation does not increase appropriately, an elevated PCO2 will cause a decrease in pH
=sodium bicarbonate therapy is strictly contraindicated in patients with evidence of hypoventilation
HCO3
strict contraindication:
adverse effects CNS acidosis explained:
-after administration, the HCO3- ions rapidly dissociates to CO2 and water
-ventilation does not increase appropriately, an elevated PCO2 will cause a decrease in pH
=sodium bicarbonate therapy is strictly contraindicated in patients with evidence of hypoventilation
- **Of greater concern is paradoxical intracellular acidosis
- bicarbonate cannot freely cross cell membranes, but the CO2 produced freely enter cells
- once intracellular, CO2 combines with water, leading H+ release and causing intracellular acidosis
-associated with increases in blood lactate concentration in studies of lactic acidosis, hemorrhagic shock, and DKA - exact mechanism for this response is not known, but left shifting of the oxygen-hemoglobin dissociation curve because of increases in blood pH may play a role
HCO3 adverse effects:
Increased hemoglobin affinity for oxygen
Increased blood lactate concentration
Paradoxical intracellular acidosis
Hypercapnia
Hypervolemia
Hyperosmolality
Hypernatremia
Hypocalcemia (ionized)
Hypokalemia
Phlebitis
WHen to give HCO3:
- HCO3 is not indicated with:
- HCO3 is indicated when:
If a specific therapy exists for the underlying cause of a metabolic acidosis, this in combination with appropriate IV fluid therapy should be the focus
ventilation inadequate
lactic acidosis
DKA
where bicarbonate therapy has been associated with no improvement in outcome or clinical deterioration despite severe acidemia
diseases causing bicarbonate loss, such as chronic kidney disease and diarrhea (an uncommon cause of metabolic acidosis in small animal patients)
management of patients with acute kidney injury (AKI) is less well defined
management of metabolic acidosis and hyperkalemia is a reasonable option, although caution must be used to avoid volume overload in the oliguric or anuric patient.
Hypertonic sodium bicarbonate should never be administered rapidly bc:
diluted to osmolality of less than _____:
or give it by____:
Commercially available 8.4% sodium bicarbonate solution has an osmolality of approximately ____:
so a dilution of 1 part sodium bicarbonate to 3 parts diluent (e.g., sterile water for injection) =
can cause vasodilation and increases in ICP, which can be fatal
slow (over 30 minutes or longer) or diluting it with sterile water to make it an isotonic solution
dilution usually results in a significant volume for administration; the rate of infusion should then be governed by the perceived fluid tolerance
less than 600 mOsm/L
central catheter to avoid phlebitis
2000 mOsm/L /3 = 666.6mOsml/L
