Alan W. Grogono, M.D.
Chairman and Merryl and Sam Israel Professor
Department of Anesthesiology
Tulane University School of Medicine, New Orleans
The following diagrams represent the behavior of the whole body during acid-base disturbances and the body's responses to therapeutic intervention.
Interactive Hypercard Stack: If you are using a Macintosh, you can download an Interactive Hypercard Stack which allows you to move the mouse around in several different Acid-Base Diagrams. In each diagram you can obtain a numerical analysis and a text interpretation of any point you choose. These results can also be "pasted" into any other document. There are two versions:
Acid Base Stack (AB.stk). Downloads fast (46 k); requires Hypercard or Hypercard Player.
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This diagram is derived from the Siggaard-Andersen In-Vivo Nomogram .
The respiratory and metabolic components are used for the axes. This
has the effect of showing how the two components determine the pH.
The respiratory acidosis (PCO2) is along the x-axis and
the Base Excess is along the y-axis. The diagrams which follow use
Metabolic Acidosis for the Y-axis to emphasize the relationship
between Metabolic Acidosis and Negative Base Excess.
The metabolic acid scale is equivalent to negative base excess, i.e.,
a metabolic acid level of 10 is equivalent to base excess of -10.
Each pH line shows all of the different combinations of respiratory
and metabolic abnormalities capable of producing that pH. To use the
diagram enter the PCO2 along the x-axis; the intersection
with the measured pH allows the level of metabolic acidosis to be
read from the left hand scale horizontally opposite the intersection.
When ventilation is altered then the position on the graph moves
horizontally at the same level of metabolic disturbance. When the
patient receives bicarbonate, then the position is lower in
proportion to the dose. Thus metabolic and respiratory changes each
move the patient along their respective axes without affecting the
other axis.
The diagram has three principal lines which facilitate the
recognition of characteristic syndromes:
1) no metabolic compensation (zero on the metabolic acid scale)
2) no respiratory compensation (PCO2 = 40 mmHg)
3) complete compensation (pH = 7.4).
The first lines represents a zone in which patients are characteristically found who have acute, uncompensated respiratory disturbances. However, all three lines are important because they help anticipate the location of the chronic conditions.
Chronic conditions lie approximately halfway between no compensation
and complete compensation.
Metabolic Disturbances. A patient with a metabolic disturbance neither remains on the PCO2 = 40 line ("No Compensation"), nor achieves the pH 7.4 line ("Complete Compensation"), but lies about halfway between no compensation and complete compensation.
Respiratory Disturbance. A patient with a chronically abnormal PCO2 will not remain on the Zero line ("No Compensation"), nor will achieve the pH 7.4 line ("Complete Compensation"), but will slowly achieve partial compensation about halfway between no compensation and complete compensation. There is only one area where this is a slight over-simplification. In patients with chronic hyperventilation, compensation often produces a pH close to 7.4. The zone representing chronic hyperventilation extends, therefore, up towards pH 7.4.
A few examples employing the diagram may make these concepts easier to understand.
PCO2 = 53, pH = 7.3. The pH = 7.3 line crosses PCO2 = 53 at a metabolic acidosis of about 0 mEq/1 (BE = 0), i.e., no metabolic compensation (zero line), which is characteristic of pure (acute) respiratory acidosis. This occurs with acute respiratory depression or by setting an anesthetic ventilator to deliver a minute ventilation smaller than the patient's normal. It takes a day or two for a patient's kidney to respond and produce the typical partial correction.
PCO2 = 28, pH = 7.5. The pH = 7.5 line crosses PCO2 = 28 at a metabolic acidosis of about 0 mEq/1 (BE = 0), i.e., no metabolic compensation (zero line), which is characteristic of pure (acute) respiratory alkalosis. This occurs in acute hyperventilation or by setting an anesthetic ventilator to deliver a minute ventilation greater than the patient's normal. It takes a day or two for a patient's kidney to respond and produce the typical partial correction.
PCO2 = 64, pH = 7.3. The pH = 7.3 line crosses PCO2 = 64 at a metabolic alkalosis of about 5 mEq/1 (BE = +5). This location is, vertically, about halfway between no compensation (zero line) and full compensation (pH 7.4), characteristic of chronic hypoventilation. The compensation has corrected the pH about halfway towards normal - a typical compensation which might be seen in chronic obstructive pulmonary disease.
PCO2 = 28, pH = 7.45. The pH = 7.45 line crosses PCO2 = 28 at a metabolic acidosis of about 4 mEq/1 (BE = -4). This location is, vertically, about halfway between no compensation (zero line) and full compensation (pH 7.4), characteristic of chronic hyperventilation. The compensation has corrected the pH about halfway towards normal - a typical compensation which occurs at high altitude in response to hypoxia.
PCO2 = 31, pH = 7.3. The pH = 7.3 line crosses PCO2 = 31 at a metabolic acidosis of 10 mEq/1 (BE = -10). Horizontally, the location is equidistant from PCO2 = 40 and pH = 7.4, i.e., midway between no compensation and complete compensation. This is characteristic of metabolic acidosis and might be found in lactic acidosis following tissue ischemia.
PCO2 = 49, pH = 7.45. The pH = 7.45 line crosses PCO2 = 49 at a metabolic alkalosis of 10 mEq/1 (BE = +10). Horizontally, the location is equidistant from PCO2 = 40 and pH = 7.4, i.e., midway between no compensation and complete compensation. This is characteristic of metabolic alkalosis and might be found following vomiting or gastric suction, both of which remove acid from the body.
PCO2 = 60, pH = 7.1. The pH = 7.1 line crosses PCO2 = 60 at metabolic acidosis of 9 mEq/1 (BE = -9). This represents a combination of metabolic and respiratory acidosis. It might occur following trauma with tissue ischemia and respiratory depression.
PCO2 = 31, pH = 7.6. The pH = 7.6 line crosses PCO2 = 30 at metabolic alkalosis of 10 mEq/1 (BE = 10). This represents a combination of metabolic and respiratory alkalosis. It might occur with gastric aspiration and mechanical hyperventilation.
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