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Metabolic Acidosis Pco2

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Acid-base (anesthesia Text)

There are four native buffer systems – bicarbonate, hemoglobin, protein, and phosphate systems. Bicarbonate has a pKa of 6.1, which is not ideal. Hemoglobin has histidine residues with a pKa of 6.8. Chemoreceptors in the carotid bodies, aortic arch, and ventral medulla respond to changes in pH/pCO2 in a matter of minutes. The renal response takes much longer. Arterial vs. Venous Gases Venous blood from the dorsum of the hand is moderately arterialized by general anesthesia, and can be used as a substitute for an ABG. pCO2 will only be off by ~ 5 mm Hg, and pH by 0.03 or 0.04 units [Williamson et. al. Anesth Analg 61: 950, 1982]. Confounding variables include air bubbles, heparin (which is acidic), and leukocytes (aka “leukocyte larceny”). VGB/ABG samples should be cooled to minimize leukocyte activity, however when blood is cooled, CO2 solubility increases (less volatile), and thus pCO2 drops. As an example – a sample taken at 37°C and at 7.4 will actually read as a pH of 7.6 if measured at 25°C. Most VBG/ABGs are actually measured at 37°C. A-aDO2 increases with age, as well as with increased FiO2 and vasodilators (which impair hypoxic pulmonary vasoconstriction). In the Continue reading >>

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Popular Questions

  1. sonias

    3 This is my topic for this week in nursing school, respiratory & metabolic acidosis/ alkalosis. I am having trouble breaking it down. Can someone please help me understand this please? Any and all help is greatly appreciated.

  2. Esme12

    Normal values:
    PH = 7.35 - 7.45
    C02 = 35 - 45
    HC03 = 21-26
    Respiratory acidosis = low ph and high C02
    hypoventilation (eg: COPD, narcs or sedatives, atelectasis)
    *Compensated by metabolic alkalosis (increased HC03)
    For example:
    ph 7.20 C02 60 HC03 24 (uncompensated respiratory acidosis)
    ph 7.33 C02 55 HC03 29 (partially compensated respiratory acidosis)
    ph 7.37 C02 60 HC03 37 (compensated respiratory acidosis)
    Respiratory alkalosis : high ph and low C02
    hyperventilation (eg: anxiety, PE, pain, sepsis, brain injury)
    *Compensated by metabolic acidosis (decreased HC03)
    examples:
    ph 7.51 C02 26 HC03 25 (uncompensated respiratory alkalosis)
    ph 7.47 C02 32 HC03 20 (partially compensated respiratory alkalosis)
    ph 7.43 C02 30 HC03 19 (compensated respiratory alkalosis)
    Metabolic acidosis : low ph and low HC03
    diabetic ketoacidosis, starvation, severe diarrhea
    *Compensated by respiratory alkalosis (decreased C02)
    examples:
    ph 7.23 C02 36 HC03 14 (uncompensated metabolic acidosis)
    ph 7.31 C02 30 HC03 17 (partially compensated metabolic acidosis)
    ph 7.38 C02 26 HC03 20 (compensated metabolic acidosis)
    Metabloic alkalosis = high ph and high HC03
    severe vomiting, potassium deficit, diuretics
    *Compensated by respiratory acidosis (increased C02)
    example:
    ph 7.54 C02 44 HC03 29 (uncompensated metabolic alkalosis)
    ph 7.50 C02 49 HC03 32 (partially compensated metabolic alkalosis)
    ph 7.44 C02 52 HC02 35 (compensated metabolic alkalosis)
    *Remember that compensation corrects the ph.
    Now a simple way to remember this......
    CO2 = acid, makes things acidic
    HCO3 = base, makes things alkalotic
    Remember ROME
    R-Respiratory
    O-Opposite
    M-Metabolic
    E-Equal
    Ok always look at the pH first...
    pH<7.35 = acidosis
    pH>7.45 = alkalosis
    Then, if the CO2 is high or low, then it is respiratory...If the HCO3 is high or low then it is metabolic. How you remember that is that the respiratory system is involved with CO2 (blowing air off or slowing RR), and the kidneys (metabolic) are involved with HCO3 (excreting or not excreting).
    Here is how you think thru it: pH = 7.25 CO2 = 40 HCO3 = 17
    Ok, first, the pH is low so think acidosis. CO2 is WNL. HCO3 is low. Draw arrows if it helps. The abnormal values are both low (think Equal). Metabolic imbalances are equal. So, this must be metabolic acidosis!
    Now, for compensation...If you have a metabolic imbalance, the respiratory system is going to try to compensate. Respiratory = CO2. If the CO2 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will decrease the CO2 because you want to get rid of the acid (CO2). In alkalosis, it will increase because you want to add more acid (CO2)
    If you have a respiratory imbalance, the kidneys will try to compensate. Kidneys = HCO3. If the HCO3 is normal in the ABG, then there is no compensation going on. Compensation in acidosis will increase HCO3 because you want to hold on to the base to make it more alkalotic. In alkalosis, it will decrease because you want to excrete the base to make it more acidic.

  3. Esme12

    Check out this tutorial
    Interactive Online ABG's acid base

  4. -> Continue reading
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