B4-038 Interpreting Arterial Blood Gases Flashcards
a change in the ratio of PCO2/HCO3 predicts the change in
pH
normal acid base values
pH
PCO2
HCO3
pH: 7.4
PCO2: 40
HCO3: 24
decrease in blood pH below normal range
acidemia
elevation in the blood pH above normal range
alkalemia
process that increases [H+] by increasing PCO2 or by reducing [HCO3-]
acidosis
process that reduces [H+] by reducing PCO2 or by increasing [HCO3-]
alkalosis
in a simple disorder, both PCO2 and HCO3 should move
in the same direction
will compensation return the pH to normal?
no
just trend it the right way
primary disturbance of blood [HCO3-] bicarbonate concentration
metabolic
2 types of metabolic acidosis
- anion gap
- non anion gap (hyperchloremic)
changes in ventilation are mediated by chemoreceptors in the […] and […]
carotid
lower brainstem
- [HCO3-] retention
- [HCO3-] is produced more than it is excreted
metabolic alkalosis
- nasogastric suction, vomiting
- diuretic therapy
- hyperaldosteronism (Addison’s)
- Cushing
- Exogenous steroids
- Licorice injestion
- Alkali injestion
possible causes of…
metabolic alkalosis
[HCO3-] depletion
metabolic acidosis
[HCO3-] used up as buffer for acid
anion gap acidosis
[HCO3-] loss through urine or bowel
non-anion gap acidosis
- methanol
- uremia
- DKA
- paraaldehyde
- INH/iron
- lactic adiosis-sepsis
- ethylene glycol
- salicylates/starvation
can cause…
anion gap metabolic acidosis
- hyperailmentation
- acetazolamide
- renal tubular acidosis
- diarrhea
- ureteroenteric fistula
- pancreticoduodenal fistula
can cause…
non-anion gap metabolic acidosis
equation used for metabolic acidosis when trying to assess if respiratory compensation is adequate
winter’s formula
expected pCO2= (1.5x[HCO3]) +8 +/-2
- CO2 retention
- decreased alveolar ventilation
respiratory acidosis
- CNS depression, sedatives
- neuromuscular disorders
- thoracic cage limitations
- acute obstruction
- chronic obstruction
- ventilator malfunction
possible causes of…
respiratory acidosis
- CO2 depletion
- increased alveolar ventilation
respiratory alkalosis
- anxiety
- CNS disorders
- salicylates, analeptics
- fever, sepsis
- pregnancy -FRC decreases
- liver insufficiency
- hyperthyroidism
possible causes of…
respiratory alkalosis
pH < 7.35
acidemic
pH > 7.45
alkalemic
primary respiratory disturbances will change the […] primarily
PCO2
small changes in serum HCO3 in compensation
a primarily high PCO2 defines
respiratory acidosis
a primarily low PCO2 defines
primary respiratory alkalosis
primary metabolic disturbances change the […]
HCO3
a primarily low HCO3 defines
primary metabolic acidosis
a primarily high HCO3 defines a
primary metabolic alkalosis
determining acute vs chronic respiratory acidosis
Expected pH for acute: pH=7.4 – 0.008 (pCO2 – 40)
Expected pH for chronic: pH= 7.4 – 0.003 (pCO2 – 40)
if there is a metabolic acidosis, use Winter’s formula to
assess if respiratory system is compensating adequately
the physiologic response to metabolic acidosis is
hyperventilation
if the actual measured PCO2 is much greater than the expected PCO2 from Winter’s formula…
the respiratory system is not fully compensating
respiratory acidosis is concurrently present
if there is metabolic acidosis, check the
anion gap
[Na+] - ([Cl-] + [HCO3])
normal anion gap
12
if there is increased anion gap metabolic acidosis, are there other [….] present
metabolic issues
determine corrected bicarb
electroneutrality principle
for every molecule of unmeasured anion present, one molecule of bicarb is lost
if the corrected bicarbonate is < 22 then
there is additional metabolic acidosis present
if the corrected bicarbonate is > 26 then
there is an additional metabolic alkalosis present
considered present when there is an abnormality in HCO3 or PCO2 or pH
acid-base disorder
key organs to maintaining acid/base balance
3
- kidney
- respiratory system
- CNS
required lab values to assess acid/base disorders
- ABG: pH, pCO2, bicarb
- electrolytes: Na, K, Cl, HCO3
- BUN, glucose, creatinine
- good history
an imbalance between Na, Cl, and total CO2 is measured as
anion gap
MUDPILES for high anion gap acidosis
Methanol
Uremia
DKA
Propylene glycol
Infection, Iron, Isoniazid
Lactic acidosis
Ethylene glycol
Salicylates
HARDUPS for normal anion gap acidosis
Hyperailmentation
Acetozolamide
RTA
Diarrhea
Ureteroenteric fistula
Pancreaticoduodenal fistula
Spironolactone
the compensatory response of the pulmonary system for a metabolic alkalosis is limited a PCO2 between
40 and 50
