Base excess
Base excess | |
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LOINC | 11555-0 |
Inphysiology,base excessandbase deficitrefer to an excess or deficit, respectively, in the amount ofbasepresent in the blood. The value is usually reported as a concentration in units of mEq/L (mmol/L), with positive numbers indicating an excess of base and negative a deficit. A typicalreference rangefor base excess is −2 to +2 mEq/L.[1]
Comparison of the base excess with the reference range assists in determining whether anacid/base disturbanceis caused by a respiratory, metabolic, or mixed metabolic/respiratory problem. Whilecarbon dioxidedefines the respiratory component of acid–base balance, base excess defines the metabolic component. Accordingly, measurement of base excess is defined, under a standardized pressure of carbon dioxide, bytitratingback to a standardized bloodpHof 7.40.
The predominant base contributing to base excess isbicarbonate.Thus, a deviation of serum bicarbonate from the reference range is ordinarily mirrored by a deviation in base excess. However, base excess is a more comprehensive measurement, encompassing all metabolic contributions.
Definition
[edit]BMP/ELECTROLYTES: | |||
Na+= 140 | Cl−= 100 | BUN= 20 | / Glu= 150 \ |
K+= 4 | CO2= 22 | PCr= 1.0 | |
ARTERIAL BLOOD GAS: | |||
HCO3−= 24 | paCO2= 40 | paO2= 95 | pH= 7.40 |
ALVEOLAR GAS: | |||
pACO2= 36 | pAO2= 105 | A-a g= 10 | |
OTHER: | |||
Ca= 9.5 | Mg2+= 2.0 | PO4= 1 | |
CK= 55 | BE= −0.36 | AG= 16 | |
SERUM OSMOLARITY/RENAL: | |||
PMO= 300 | PCO= 295 | POG= 5 | BUN:Cr= 20 |
URINALYSIS: | |||
UNa+= 80 | UCl−= 100 | UAG= 5 | FENa= 0.95 |
UK+= 25 | USG= 1.01 | UCr= 60 | UO= 800 |
PROTEIN/GI/LIVER FUNCTION TESTS: | |||
LDH= 100 | TP= 7.6 | AST= 25 | TBIL= 0.7 |
ALP= 71 | Alb= 4.0 | ALT= 40 | BC= 0.5 |
AST/ALT= 0.6 | BU= 0.2 | ||
AF alb= 3.0 | SAAG= 1.0 | SOG= 60 | |
CSF: | |||
CSF alb= 30 | CSF glu= 60 | CSF/S alb= 7.5 | CSF/S glu= 0.6 |
Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2of 40 mmHg (5.3 kPa).[2]A base deficit (i.e., a negative base excess) can be correspondingly defined by the amount of strong base that must be added.
A further distinction can be made between actual and standard base excess:actualbase excess is that present in the blood, whilestandardbase excess is the value when thehemoglobinis at 5 g/dl. The latter gives a better view of the base excess of the entireextracellular fluid.[3]
Base excess (or deficit) is one of several values typically reported with arterial blood gas analysis that is derived from other measured data.[2]
The term and concept of base excess were first introduced byPoul AstrupandOle Siggaard-Andersenin 1958.
Estimation
[edit]Base excess can be estimated from thebicarbonateconcentration ([HCO3−]) andpHby the equation:[4]
with units of mEq/L. The same can be alternatively expressed as
Calculations are based on theHenderson-Hasselbalchequation:
Ultimately the end result is:
Interpretation
[edit]Base excess beyond the reference range indicates
- metabolic alkalosis,orrespiratory acidosiswith renal compensation if too high (more than +2 mEq/L)
- metabolic acidosis,orrespiratory alkalosiswith renal compensation if too low (less than −2 mEq/L)
Blood pH is determined by both a metabolic component, measured by base excess, and a respiratory component, measured by PaCO2(partial pressure ofcarbon dioxide). Often a disturbance in one triggers a partial compensation in the other. A secondary (compensatory) process can be readily identified because itopposesthe observed deviation in blood pH.
For example, inadequate ventilation, a respiratory problem, causes a buildup of CO2,hence respiratory acidosis; the kidneys then attempt to compensate for the low pH by raising blood bicarbonate. The kidneys only partially compensate, so the patient may still have a low blood pH, i.e. acidemia. In summary, the kidneys partially compensate for respiratory acidosis by raising blood bicarbonate.
A high base excess, thusmetabolic alkalosis,usually involves an excess ofbicarbonate.It can be caused by
- Compensation for primaryrespiratory acidosis
- Excessive loss of HCl in gastric acid by vomiting
- Renal overproduction of bicarbonate, in eithercontraction alkalosisorCushing's disease
A base deficit (a below-normal base excess), thusmetabolic acidosis,usually involves either excretion of bicarbonate or neutralization of bicarbonate by excess organic acids. Common causes include
- Compensation for primaryrespiratory alkalosis
- Diabetic ketoacidosis,in which high levels of acidicketone bodiesare produced
- Lactic acidosis,due toanaerobic metabolismduring heavy exercise orhypoxia
- Chronic kidney failure,preventing excretion of acid and resorption and production of bicarbonate
- Diarrhea,in which large amounts of bicarbonate are excreted
- Ingestion of poisons such asmethanol,ethylene glycol,or excessiveaspirin
The serumanion gapis useful for determining whether a base deficit is caused by addition of acid or loss of bicarbonate.
- Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis).
- Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea). The anion gap is maintained because bicarbonate is exchanged forchlorideduring excretion.
See
[edit]References
[edit]- ^Frances Talaska Fischbach; Marshall Barnett Dunning (2008),A Manual of Laboratory and Diagnostic Tests(8th ed.), p. 973,ISBN978-0-7817-7194-8.
- ^abJonathan D. Kibble; Colby R. Halsey (2009),Medical Physiology: The Big Picture,p. 249,ISBN978-0-07-164302-3.
- ^Acid-Base Tutorial — Terminology
- ^Medical Calculators > Calculated Bicarbonate & Base ExcessSteven Pon, MD, Weill Medical College of Cornell University