by "Grog" (Alan W. Grogono), Professor Emeritus, Tulane University Department of Anesthesiology

Clinical Considerations

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Clinical Aspects

Page Index.

Treating Respiratory Disturbances
Treating Metabolic Disturbances
Rapidity of Compensation
Anion Gap
Effects of temperature.
Kilopascals for PCO2
Converting pressures in different units
Arterial vs. End-exhaled PCO2 difference
Summation of PO2 and PCO2
Therapeutic implications
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Introduction

This page covers selected clinical topics. Separate pages deal with the treatment of Respiratory and Metabolic disturbances.

In addition, the Rapidity of Compensation is covered in the page on the production of acid.


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Anion Gap.

Gap = Na+ + K+ - Cl- - HCO3-

Some causes of metabolic acidosis, e.g., lactic acidosis, release anions into the extracellular fluid which are not normally measured. When this occurs there will be an unexpected discrepancy between the sums of the principal cations and anions. The usual sum is:

Gap   =   Na+   +   K+   -   Cl-   -   HCO3-

  15    =   140   +   5     -  105  -     25     mMol/L

In addition to Cl- + HCO3- there are extra unmeasured anions, e.g., lactate, phosphate, sulphate, which increase the "gap". A gap greater than 30 indicates a significant concentration of unmeasured anions. Because this calculation relies upon the accuracy of the other measurements, small errors in these large numbers cause a disproportionately great error in the "gap". If information is required about the unmeasured anions, it is probably more appropriate to measure their concentration, i.e., lactate in tissue hypoxia, 3-hydroxybutyrate in diabetic ketosis, and phosphate or sulfate in renal failure.


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No "Normal" Cold Humans
Cold Blood

Effects of temperature.

(Click on Picture on Left)

We normally assess blood gases at 37oC. When blood is cooled, carbon dioxide becomes more soluble; this reduces the PCO2 by about 4.5% per oC. The pH rises about 0.015 per oC because hemoglobin, the principal buffer in the blood, accepts more hydrogen ions on its alpha imidazole group when cooled. However, as it is rewarmed the blood returns to normal again. Such changes characteristically occur when blood reaches a cold extremity (fishing in a cold stream) but is normal again when it returns to the heart. Analogous changes occur when a poikilothermic ("cold-blooded") animal is cooled and then rewarmed. For these reasons it is best to judge the normality of the blood at 37oC. Our familiarity with the PCO2 and pH at this temperature makes recognition easy. Then, without knowing the actual values at the low temperature, normal values at 37o can be assumed to meet the needs of the cold tissues (Reeves and Rahn, 1979).

When judging gas exchange between air and blood, however, it is necessary to correct the value of the arterial PCO2 for the patient's actual temperature. Only then can the arterial blood gas values be meaningfully compared to exhaled gas partial pressures, or to the gases in a pump-oxygenator. Because they unfamiliar, however, they are useless when judging "normality"; there is no such thing as "normal" hypothermic human being for us to use as a standard.


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40 = 5.3 !
One Pascal

Kilopascals for PCO2.

(Click on Picture on Left)

One pascal: Many texts and papers express PCO2 in kilopascals (kPa). It is useful to remember that this value is almost the same as the percentage of atmospheric pressure. For example, the normal arterial PCO2 of 40 mmHg is 5.33 kPa or 5.61 % of atmospheric pressure. One pascal is one newton per square meter. One newton is about 102 grams weight or about 3.6 ounces of fluid.

Spill a "newton" on a Square Meter: When one newton (a small cup of coffee) is spilled on to a table one meter square, the resulting layer of water is 0.102 mm deep. This is, by definition, the pressure exerted by one pascal.


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One Kilopascal

Kilopascals

The kiloPascal: A pressure of one thousand pascals (1 kPa) is about 10.2 cm H2O or about 7.75 mmHg.

Atmospheric pressure is about 1034 cm H2O or 101.9 kPa. The useful approximations are 1000 cm H2O or 100 kPa.

mmHg to kPa: To convert pressure in mmHg to kPa, divide the value in mmHg by 7.5.


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1 : 760 : 1000 : 100
Pressure in mmHg

Pressure Conversion (approximate).

(Click on Picture on Left)

We should all be able to quickly convert between the various units used for the measurement of pressure. It is helpful to recognize that, for practical purposes:

1 atmos = 760 mmHg  = 1000 cmH2O = 100 kPa 

or, more useful to clinicians:

10 cmH2O = 7.6 mmHg  = 1 kPa = 1% atmos


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Difference between arterial PCO2 and end-exhaled PCO2.

PACO2 - PaCO2 = 5 (!)

Discrepancy

In awake healthy people, there is little difference between an end-tidal PCO2 and an arterial PCO2. During an anesthetic, end-tidal PCO2 is commonly lower than a simultaneous arterial PCO2 . Several factors may be contributing to such a discrepancy:


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Summation of PO2 and PCO2.

PaO2 + PaCO2 = 140 (?)

Nitrogen and Water Vapor:

The body neither consumes nor manufactures nitrogen. Accordingly, the partial pressure of nitrogen is approximately constant throughout the trachea, bronchi, and alveoli; it occupies most of the atmospheric pressure. The alveolar partial pressure of water vapor is also stable; it depends on the body temperature. With nitrogen and water vapor "fixed", oxygen and carbon dioxide share the remaining space. With normal lung function and accurate analysis, the arterial blood gas values reflect this.

Practically, breathing air: PaO2 + PaCO2 = 140 mmHg

If the sum of PO2 + PCO2 is greater than expected, then the analysis contains an error. A sum which is smaller suggests that the lungs are failing to adequately transfer oxygen. For greater accuracy, this sum incorporates the respiratory quotient (RQ). Then, at atmospheric pressure breathing room air, the corrected sum of the respiratory gases is 150 mmHg:

Accurately, breathing air: PaO2 + PaCO2 / RQ = 150 mmHg

This degree of accuracy is not required for most purposes. Moreover, the RQ is usually unknown. Accordingly it is simpler and reasonable to use the first equation and add the PaCO2 and PaO2 directly:


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Therapeutic implications.

Not the Disease

The pH and PCO2 normally vary very little. Customarily stable, they demonstrate predictable changes with many disease states. Understandably, the concentration of the hydrogen ion used to be perceived, itself, as an etiologic factor in many syndromes. This concept has been refuted by demonstrations by Xu et al that the body and brain tolerate pH values as low as 6.2 without damage provided circulation and oxygenation are maintained. In addition, rats in hyperbaric conditions can survive undamaged when subjected to a PCO2 exceeding one atmosphere.

An Indicator

The implication of these results is that an abnormal acid-base value is best regarded as an indicator of trouble, not as pathology in its own right.

Pharmacology

Variation in the pH alters the degree of ionization of proteins and many drugs. As most ionized substances do not cross cell membranes readily, alterations in pH affect both cellular function and the potency of many pharmaceutical agents. Relative acidity of tissues, for example in the vicinity of an abscess, is recognized to reduce the efficacy of local anesthetic solutions. Conversely, relative alkalinity enhances the uptake of local anesthetic solutions. Alkalinity also potentiates drugs such as meperidine and morphine by increasing the availability of lipophilic, uncharged base, to cross the blood-brain barrier (Shulman et al 1984)


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Acid-Base Tutorial
Alan W. Grogono
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All Rights Reserved
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