Base Excess 的臨床意義

ABG (Arterial Blood Gas)

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pH is a measurement of the acidity of the blood, reflecting the number of hydrogen ions present. 

Lower numbers mean more acidity; higher number mean more alkalinity.

pH is Elevated (more alkaline, higher pH) with:

·        Hyperventilation

·        Anxiety, pain

·        Anemia

·        Shock

·        Some degrees of Pulmonary disease

·        Some degrees of Congestive heart failure

·        Myocardial infarction

·        Hypokalemia (decreased potassium)

·        Gastric suctioning or vomiting

·        Antacid administration

·        Aspirin intoxication

·        Lasix

·        Steroid

pH is Decreased (more acid, lower pH) with:

• Strenuous physical exercise
• Obesity
• Starvation
• Diarrhea
• Ventilatory failure
• More severe degrees of Pulmonary Disease
• More severe degrees of Congestive Heart Failure
• Pulmonary edema
• Cardiac arrest
• Renal failure
• Lactic acidosis
• Ketoacidosis in diabetes

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pCO2 (Partial Pressure of Carbon Dioxide) reflects the the amount of carbon dioxide gas dissolved in the blood. 

Indirectly, the pCO2 reflects the exchange of this gas through the lungs to the outside air. Two factors each have a significant impact on the pCO2. The first is how rapidly and deeply the individual is breathing:

• Someone who is hyperventilating will "blow off" more CO2, leading to lower pCO2 levels
• Someone who is holding their breath will retain CO2, leading to increased pCO2 levels

The second is the lungs capacity for freely exchanging CO2 across the alveolar membrane:

• With pulmonary edema, there is an extra layer of fluid in the alveoli that interferes with the lungs' ability to get rid of CO2. This leads to a rise in pCO2.
• With an acute asthmatic attack, even though the alveoli are functioning normally, there may be enough upper and middle airway obstruction to block alveolar ventilation, leading to CO2 retention.

Increased pCO2 is caused by:

• Pulmonary edema
• Obstructive lung disease

Decreased pCO2 is caused by:

• Hyperventilation
• Hypoxia
• Anxiety
• Pregnancy
• Pulmonary Embolism (This leads to hyperventilation, a more important consideration than the embolized/infarcted areas of the lung that do not function properly. In cases of massive pulmonary embolism, the infarcted or non-functioning areas of the lung assume greater significance and the pCO2 may increase.)

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PO2 (Partial Pressure of Oxygen) reflects the amount of oxygen gas dissolved in the blood. It primarily measures the effectiveness of the lungs in pulling oxygen into the blood stream from the atmosphere.

Elevated pO2 levels are associated with:

• Increased oxygen levels in the inhaled air
• Polycythemia

Decreased PO2 levels are associated with:

• Decreased oxygen levels in the inhaled air
• Anemia
• Heart decompensation
• Chronic obstructive pulmonary disease
• Restrictive pulmonary disease
• Hypoventilation

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CO2 Content is a measurement of all the CO2 in the blood. 

Most of this is in the form of bicarbonate (HCO3), controlled by the kidney. A small amount (5%) of the CO2 is dissolved in the blood, and in the form of soluble carbonic acid (H2CO3).

For this reason, changes in CO2 content generally reflect such metabolic issues as renal function and unusual losses (diarrhea). Respiratory disease can ultimately effect CO2 content, but only slightly and only if prolonged.

Elevated CO2 levels are seen in:

• Severe vomiting
• Use of mercurial diuretics
• COPD
• Aldosteronism

Decreased CO2 levels are seen in:

• Renal failure or dysfunction
• Severe diarrhea
• Starvation
• Diabetic Acidosis
• Chlorthiazide diuretic use

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Base Excess or Base Deficit

Whenever there is an accumulation of metabolically-produced acids, the body attempts to neutralize those acids to maintain a constant acid-base balance. 

This neutralizing is achieved by using up various "buffering" compounds in the blood stream, to bind the acids, disallowing them from contributing to more acidity.

About half of these buffering compounds come from HCO3, and the other half from plasma and red blood cell proteins and phosphates.

The words "base deficit" and "base excess" are equivalent and are generally used interchangeably. The only difference is that base deficit is expressed as a positive number and base excess is expressed as a negative number.

A "Base Deficit" of 10 means that 10 mEqu/L of buffer has been used up to neutralize metabolic acids (like lactic acid). Another way to say the same thing would be the "Base Excess is minus 10."

More Negative Values of Base Excess may Indicate:

• Lactic Acidosis
• Ketoacidosis
• Ingestion of acids
• Cardiopulmonary collapse
• Shock

More Positive Values of Base Excess may Indicate:

• Loss of buffer base
• Hemorrhage
• Diarrhea
• Ingestion of alkali
  
  
  Source from： http://en.wikipedia.org/wiki/Base_excess
  
  

  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 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. 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 pCO₂ of 40 mmHg (5.3 kPa).^[2] 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 thehemoglobin is at 5 g/dl. The latter gives a better view of the base excess of the entire extracellular fluid.^[3]

  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 serum bicarbonate concentration ([HCO₃^-]) and pH by the equation:^[4]

  

  with units of mEq/L. The same can be alternatively expressed as

  

  
  

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Oxygen Saturation (SO2) measures the percent of hemoglobin which is fully combined with oxygen. 

While this measurement can be obtained from an arterial or venous blood sample, it's major attractive feature is that it can be obtained non-invasively and continuously through the use of a "pulseoximeter."

Normally, oxygen saturation on room air is in excess of 95%. With deep or rapid breathing, this can be increased to 98-99%. While breathing oxygen-enriched air (40% - 100%), the oxygen saturation can be pushed to 100%.

Oxygen Saturation will fall if:

• Inspired oxygen levels are diminished, such as at increased altitudes.
• Upper or middle airway obstruction exists (such as during an acute asthmatic attack)
• Significant alveolar lung disease exists, interfering with the free flow of oxygen across the alveolar membrane.

Oxygen Saturation will rise if:

• Deep or rapid breathing occurs
• Inspired oxygen levels are increased, such as breathing from a 100% oxygen source