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Acid
Base Balance – A Primer for the Rest of Us
by Mike Kennamer, EMT-P, M.P.A, Northeast Alabama Community
College
The concept of acid base balance is simple, for our purposes.
The body needs acid to perform certain essential functions.
However, buildup of these acids is harmful, therefore a balance
must be maintained.
Before
we embark in the specifics of acid base we should first address
some specialized terminology. PH, or parts of hydrogen, refers
to the hydrogen ion concentration. The PH scale is a logarithmic
scale that goes from one (1) to fourteen (14). It is inversely
proportional in that 1 on the PH scale means that all that
is present is hydrogen ions. 14 means that there are no hydrogen
ions present. 7 on the PH scale is neutral.
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Acidosis occurs
then the PH is low or there are too many hydrogen ions present.
Alkalosis occurs when the PH is high indicating that there are too
few hydrogen ions present. Normal PH in human blood ranges between
7.35 and 7.45. This must stay closely balanced as even a small shift
in PH can be fatal.
The body has
several mechanisms for maintaining acid base balance. The buffer
system, in concert with the respiratory and renal systems, maintains
a tight reign on PH level. This is how it works.
When there are
too many hydrogen (H+) ions in the bloodstream, the buffer system
releases bicarbonate (HCO3), the chief buffer in the body. When
these two mix, carbonic acid (H2CO3) forms. Carbonic acid is a weak
but highly volatile acid. This means that it is just as dangerous
as hydrogen, but easier to remove. Carbonic acid, in time, will
reduce itself to water (H2O). Unfortunately, when dealing with acid
base time can mean the difference between life and death. That is
why the body releases an enzyme produced in the erythrocytes called
carbonic anhydrase. Carbonic anhydrase speeds the natural process
that would have occurred anyway and allows the carbonic acid (H2CO3)
to break down, almost immediately, into water (H2O) and carbon dioxide
(CO2)
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The
water is eliminated through the renal system (urination) and
the carbon dioxide is eliminated through the respiratory system
(hyperventilation).
Even with the body’s best efforts, imbalances sometimes
occur. That is when we, as EMS personnel, must intervene to
correct an acid base derangement. Before we begin figuring
acid base derangements, we should know the normal ranges for
arterial blood gas figures (ABGs). Although ABG machines vary,
the accepted norms are listed in table 1. |
Follow these
steps to determine the type of acid base problem that your patient
is experiencing. I call this the tic-tac-toe method because of the
resemblance to a tic-tac-toe board. Each step will be listed and
a box below it will illustrate the process using this method.
Our sample ABG
readings are as follows:
PH
7.58; PO2 108; PCO2 39; HCO3 32.
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Begin by drawing the tic-tac-toe board.
We know
that alkalosis means high PH and acidosis refers to low PH.
We also know that the respiratory component in acid base is
related to PCO2 and the metabolic component in acid base is
related to HCO3. Label the tic-tac-toe board as indicated. |
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2.
Determine the presence of acidosis or alkalosis by looking at
the PH.
If the PH is higher than 7.45 alkalosis is present; if the PH
is lower than 7.35 acidosis is present.
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the PH of 7.58 is higher than 7.45, alkalosis is present.
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| 3.
Look at the determining component for both the respiratory (PCO2)
and metabolic (HCO3) sides. Determine if they are higher
or lower than the established norms. Indicate this by drawing
arrows (up for high and down for low) in the appropriate spaces.
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A
PCO2 of 39 is normal so we write “normal”
in this space. A HCO3 of 32 is high so we put a
up arrow in this space. |
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| 4.
Now look at the PH and each of the determining components (PCO2
and HCO3) to see if you are going the same way (both
up or both down) or if they are opposing (one upand one down).
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Compare:
• PH and PCO2 – no comparison since PCO2
is normal.
• PH and HCO3 – both arrows point up |
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| 5.
If the PH and the determining component go in opposite directions,
the problem is respiratory in nature. If the PH and
the determining component go in the same direction, the problem
is metabolic in nature. Use the acronym ROME (Respiratory opposite,
Metabolic equal) to help you remember. |
| Since
the PH and HCO3 arrows point in the same direction, we know
that this is a metabolic problem. Since we have already determined
that alkalosis is present, we know that this derangement is
metabolic alkalosis. |
Build
upon this foundation to further your knowledge and understanding
of human physiology.
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