In
physiology
Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...
, base excess and base deficit refer to an excess or deficit, respectively, in the amount of
base present 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 typical
reference range
In medicine and health-related fields, a reference range or reference interval is the range or the interval of values that is deemed normal for a physiological measurement in healthy persons (for example, the amount of creatinine in the blood, o ...
for base excess is −2 to +2 mEq/L.
Comparison of the base excess with the reference range assists in determining whether an
acid/base disturbance is caused by a respiratory, metabolic, or mixed metabolic/respiratory problem. While
carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
defines 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, by
titrating back to a standardized blood
pH of 7.40.
The predominant base contributing to base excess is
bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochem ...
. 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
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 pCO
2 of .
A base deficit (i.e., a negative base excess) can be correspondingly defined in terms of the amount of strong base that must be added.
A further distinction can be made between actual and standard base excess: actual base excess is that present in the blood, while standard base excess is the value when the
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
is at 5 g/dl. The latter gives a better view of the base excess of the entire
extracellular fluid
In cell biology, extracellular fluid (ECF) denotes all body fluid outside the cells of any multicellular organism. Total body water in healthy adults is about 60% (range 45 to 75%) of total body weight; women and the obese typically have a low ...
.
Acid-Base Tutorial — Terminology
/ref>
Base excess (or deficit) is one of several values typically reported with arterial blood gas analysis that is derived from other measured data.
The term and concept of base excess were first introduced by Poul Astrup and Ole Siggaard-Andersen in 1958.
Estimation
Base excess can be estimated from the bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochem ...
concentration ( 3−">CO3− and pH by the equation:[Medical Calculators > Calculated Bicarbonate & Base Excess](_blank)
Steven Pon, MD, Weill Medical College of Cornell University
with units of mEq/L. The same can be alternatively expressed as
----
Calculations are based on the Henderson-Hasselbalch equation:
:
Ultimately the end result is:
:
Interpretation
Base excess beyond the reference range indicates
* metabolic alkalosis
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35–7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct res ...
or respiratory acidosis
Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis).
Carbon dioxide is produced continu ...
with renal compensation if too high (more than +2 mEq/L)
* metabolic acidosis
Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys ...
or respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
with 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 of carbon dioxide
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...
). Often a disturbance in one triggers a partial compensation in the other. A secondary (compensatory) process can be readily identified because it ''opposes'' the 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, thus metabolic alkalosis
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35–7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct res ...
, usually involves an excess of bicarbonate
In inorganic chemistry, bicarbonate (IUPAC-recommended nomenclature: hydrogencarbonate) is an intermediate form in the deprotonation of carbonic acid. It is a polyatomic anion with the chemical formula .
Bicarbonate serves a crucial biochem ...
. It can be caused by
* Compensation for primary respiratory acidosis
Respiratory acidosis is a state in which decreased ventilation (hypoventilation) increases the concentration of carbon dioxide in the blood and decreases the blood's pH (a condition generally called acidosis).
Carbon dioxide is produced continu ...
* Excessive loss of HCl in gastric acid by vomiting
* Renal overproduction of bicarbonate, in either contraction alkalosis or Cushing's disease
Cushing's disease is one cause of Cushing's syndrome characterised by increased secretion of adrenocorticotropic hormone (ACTH) from the anterior pituitary (secondary hypercortisolism). This is most often as a result of a pituitary adenoma (spe ...
A base deficit (a below-normal base excess), thus metabolic acidosis
Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys ...
, usually involves either excretion of bicarbonate or neutralization of bicarbonate by excess organic acids. Common causes include
* Compensation for primary respiratory alkalosis
Respiratory alkalosis is a medical condition in which increased respiration elevates the blood pH beyond the normal range (7.35–7.45) with a concurrent reduction in arterial levels of carbon dioxide. This condition is one of the four primary dis ...
* Diabetic ketoacidosis
Diabetic ketoacidosis (DKA) is a potentially life-threatening complication of diabetes mellitus. Signs and symptoms may include vomiting, abdominal pain, deep gasping breathing, increased urination, weakness, confusion and occasionally loss of ...
, in which high levels of acidic ketone bodies are produced
* Lactic acidosis
Lactic acidosis is a medical condition characterized by a build-up of lactate (especially -lactate) in the body, with formation of an excessively low pH in the bloodstream. It is a form of metabolic acidosis, in which excessive acid accumulates d ...
, due to anaerobic metabolism
Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O2). Although oxygen is not the final electron acceptor, the process still uses a respiratory electron transport chain.
In aerobic organisms undergoing r ...
during heavy exercise or hypoxia
* Chronic kidney failure
Chronic kidney disease (CKD) is a type of kidney disease in which a gradual loss of kidney function occurs over a period of months to years. Initially generally no symptoms are seen, but later symptoms may include leg swelling, feeling tired, vo ...
, preventing excretion of acid and resorption and production of bicarbonate
* Diarrhea
Diarrhea, also spelled diarrhoea, is the condition of having at least three loose, liquid, or watery bowel movements each day. It often lasts for a few days and can result in dehydration due to fluid loss. Signs of dehydration often begin w ...
, in which large amounts of bicarbonate are excreted
* Ingestion of poisons such as methanol, ethylene glycol
Ethylene glycol (IUPAC name: ethane-1,2-diol) is an organic compound (a vicinal diol) with the formula . It is mainly used for two purposes, as a raw material in the manufacture of polyester fibers and for antifreeze formulations. It is an odo ...
, or excessive aspirin
Aspirin, also known as acetylsalicylic acid (ASA), is a nonsteroidal anti-inflammatory drug (NSAID) used to reduce pain, fever, and/or inflammation, and as an antithrombotic. Specific inflammatory conditions which aspirin is used to treat inc ...
The serum anion gap
The anion gap (AG or AGAP) is a value calculated from the results of multiple individual medical lab tests. It may be reported with the results of an electrolyte panel, which is often performed as part of a comprehensive metabolic panel.
The an ...
is 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 for chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
during excretion.
See
* Acid–base homeostasis
Acid–base homeostasis is the homeostatic regulation of the pH of the body's extracellular fluid (ECF). The proper balance between the acids and bases (i.e. the pH) in the ECF is crucial for the normal physiology of the body—and for cellu ...
* Metabolic acidosis
Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys ...
/ Metabolic alkalosis
Metabolic alkalosis is a metabolic condition in which the pH of tissue is elevated beyond the normal range (7.35–7.45). This is the result of decreased hydrogen ion concentration, leading to increased bicarbonate, or alternatively a direct res ...
* Arterial blood gas
An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe an ...
References
External links
acid-base.com
Emedicine: Lactic Acidosis
{{Blood tests
Chemical pathology
Diagnostic intensive care medicine