Acid Base Disturbances, Physiological Compensation Flashcards
acid base disorders; what causes them conceptually
Problems with normal organ physiology can lead to acid base
disturbances. Conceptually, this might result from:
1) Excessive production of acids (diabetic ketoacidosis, lactic acidosis etc.).
2) Loss of substances (eg. acids) that would normally make the blood more acidic (H + - vomiting) or that would buffer against pH changes (HCO3- - diarrhea, renal failure).
3) Changes in ventilation that affect the exchange of CO2 (CO2 accumulation = acidosis, CO2 loss = alkalosis)
approach to acid base problems; 3 steps
1) History and physical examination findings
– History of vomiting or diarrhea
– Evidence of altered respiratory function
– How long has the problem existed?
2) Analysis of blood gas/pH data – This is called “Blood Gas Analysis”
– Is the pH altered?
– What is the pCO2 ?
– What is the HCO3- ?
– What is the Base Excess?
3) Identify underlying problem and treat
– ± fluid therapy to “correct” A/B disorder
blood gas analysis; what is measured w a blood gas analyzer
When the blood gas analyzer is used:
* pH, pCO2 and pO2 are measured.
* HCO3- is calculated by the instrument
– From H-H equation
- A value called the “Actual Base Excess” (ABE or BE) is calculated (by the instrument)
actual base excess; defined as what, positive vs negative values, defines what, takes into account what, what is corrected
-Actual Base excess is defined as the amount of strong acid that must be added to each litre of blood to yield a pH to 7.40 at a temperature of 37oC and a pCO2 of 40 mmHg.
-A positive value indicates that acid must be added to correct the pH (animal is alkalotic – typically this is already recognized as pH is above normal range).
-A negative value indicates that base must be added to correct the pH (animal is acidotic – typically this is already recognized as pH is below normal range).
-Base excess defines the metabolic component of an animal’s acid base status.
-Takes into account multiple factors (eg. Hb concentration, Phosphates) – not just HCO3-
-Essentially measures all buffering capacity rather than just HCO3-
-Standard Base Excess – is “corrected” to a standard level of hemoglobin (this is important in anemia for example)
systematic approach to blood gas analysis; outside normal range, steps to determine origin
If blood pH is outside the normal range, then an acid/base imbalance exists
* If the pH is low - Acidosis
* If the pH is high – Alkalosis
The next step is to determine the origin
1) Examine the pCO2 – if it is abnormal (high in animal with
acidosis, low in animal with alkalosis), then the problem is considered to be respiratory in origin. If pCO2 is normal – it is considered to be metabolic in origin (compensation modifies this statement – see later slides)
2) Examine the Base Excess – if this is outside the normal range, then this helps further suggest that it is metabolic in origin (note: you can only do this once you have ensured that the problem is not respiratory in origin because sometimes these patients can also experience a base excess or deficit)
classification of acid base disturbances, simple vs mixed
- Classification: Based on these approaches we describe….
– Simple acid base disturbances – only one type of A/B disorder is present
* Respiratory acidosis
* Respiratory alkalosis
* Metabolic acidosis
* Metabolic alkalosis
– Mixed – more than one cause for the disturbance
* e.g. metabolic acidosis and respiratory acidosis
resp acidosis and alkalosis; result from what, most important indicator and levels, occur when
- Result from failure of the respiratory system to balance pH
- PCO2 is the single most important indicator of respiratory inadequacy
- PCO2 levels
– Normal PCO2 fluctuates between 40 and 50 mm Hg
– Values above 50 mm Hg suggest respiratory acidosis (when the animal is known to be acidotic)
– Values below 40 mm Hg indicate respiratory alkalosis (when the animal is known to be alkalotic). - Respiratory Acidosis
– Occurs when an animal breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, tracheal obstruction etc. - Respiratory Alkalosis
– Occurs when an animal hyperventilates
metabolic acidosis; incliudes what, what happens, common type, causes
-Includes all acidosis cases except those caused by high blood CO2 levels (high pCO2)
-Essentially any acidosis that isn’t respiratory
* Metabolic acid-base imbalance – ABE levels are less than ~-2mEq/L
* Often bicarbonate ion levels are below normal range (22-26 mEq/L)
- Metabolic acidosis: The most common acid-base imbalance
– Causes include accumulation of lactic acid, shock, ketosis in diabetic
crisis, starvation, and kidney failure
metabolic alkalosis; how to confirm case, typical causes
- Any alkalosis that isn’t respiratory in origin
- High BE/bicarbonate levels support or confirm metabolic origin in most cases
- Typical causes are:
– Vomiting of the acid contents of the stomach
– Intake of excess base (e.g., from antacids)
– Constipation, in which excessive bicarbonate can be reabsorbed
resp and renal compensations for AB disturbances; body tries to what and details
The body tries to correct acid-base imbalances due to the inadequacy of one body system (lung, gut, kidney etc.) by compensating with “other” system(s).
For example:
– The respiratory system will attempt to correct metabolic acid-base imbalances
– The kidneys will work to correct imbalances caused by respiratory disease (or non-renal metabolic problems)
– Usually, the presence of renal compensation suggests that the A-B problem has been present for at least some time.
Compensations are often reflected in the values observed for pCO2, HCO3- and ABE
resp compensation for metabolic acidosis
- To compensate for metabolic acidosis:
– The rate and depth of breathing become elevated (although you may not observe this clinically).
– Blood pH is below 7.35 and bicarbonate level is low
– As carbon dioxide is eliminated by the respiratory system, pCO2 falls below normal - Contrast this with respiratory acidosis where pCO2 is high - this is the actual cause of the acidosis in the first place.
-To “counteract” the low blood pH, breathing rate and depth increase, blood CO2 level falls (even though this wasn’t the initial cause of the acidosis), blood pH rises closer to normal values.
resp compensation for metabolic alkalosis; how is compensation seen
- In metabolic alkalosis:
– Compensation is seen as slow, shallow breathing, allowing carbon dioxide to accumulate in the blood
(again, the change in breathing pattern may be difficult to discern clinically) - Compensation is revealed by observing :
– High pCO2 and high blood pH. (Remember, if the primary problem was respiratory alkalosis, the pCO2
would be low)
-To “counteract” the high blood pH, breathing rate and depth decrease, blood CO2 level rises, blood pH decreases closer to normal values
renal compensation for resp acidosis
- To correct respiratory acid-base imbalance, renal mechanisms that control pH are induced or enhanced.
- Compensated Respiratory Acidosis has high pCO2 and high bicarbonate/BE levels
– The high pCO2 is the initial cause of acidosis
– The high bicarbonate levels indicate the kidneys are retaining bicarbonate to offset the acidosis
To “counteract” the low blood pH (initially caused by high CO2), kidneys
conserve bicarbonate, increase acid secretion. Blood bicarbonate level rises, blood pH rises closer to normal values.
renal compensation for resp alkalosis
- Compensated Respiratory Alkalosis has low pCO2 and high pH with a low (or at least falling) HCO3-
– The low CO2 caused the alkalosis. The kidneys respond by eliminating bicarbonate from the body by
reabsorbing less in the tubules - so over time, bicarbonate levels drop and pH of the blood decreases closer to normal
To “counteract” the high blood pH, body increases elimination of HCO3
through kidneys and retains more H+ by decreasing glutamine metabolism
and decreasing “titration” against SO4- and HPO4- .
what is an anion gap
-Unmeasured anions are often derived from acids, so if their numbers increase, acidosis is
develops
-Total measured cations exceeds measured anions Þ anion gap
