ABG's acid/base balance

  1. Can someone who is great at interpreting ABGs please help me? I continue to struggle with Respiratory vs. Metabolic Acidosis especially when compensation is thrown into the mix. How can you tell resp/vs. metabolic and how can you tell partial compensation ( all values are out of whack still, but how can one tell if there's compensation or not.... I guess?)

    Thanks in advance!
  2. 16 Comments

  3. by   Katie71275
    Do you know all of your normal ABG's?
    Try this link, see if this clears things up for you some.
    how to read an arterial blood gas.
  4. by   Tinknurse2B
    Hope this helps....this is what I used to learn the differnce and now I am a pro at it. Let me know if it helps

    Normal pH pH (7.35-7.45)
    < 7.35 Acidosis
    > 7.45 Alkalosis
    7.40 is the exact normal
    7.35-7.45 Compensated
    < 7.35 or > 7.45 Uncompensated
    CO2 (35-45) normal CO2 in artery
    HCO3 (22-26) normal Bicarbonate in artery
    Respiratory Acidosis CO2 > 45
    Respiratory Alkalosis CO2 < 35
    Metabolic Acidosis HCO3 <22
    Metabolic Alkalosis HCO3 > 26
    pH and HCO3 are cousins they go in the same direction for Acidosis and Alkalosis
    But CO2 is an outsider it goes the opposite direction for Acidosis and Alkalosis
    Abnormal pH and CO2 in opposite directions respiratory problem
    Abnormal pH and HCO3 in the same direction metabolic problem
    HCO3 and CO2 in the same direction trying to compensate for abnormal pH
    HCO3 and CO2 in opposite directions both imbalances (mixed)
    What are some causes of low PaO2? Hypoxic Hypoxia--there’s just not enough of a supply of O2 ( COPD, pneumonia, ARDS, suffocation)
    What are some causes of low PaO2? Anemic Hypoxia There’s plenty of O2—but not enough HGB to carry it to the tissues
    What are some causes of low PaO2? Stagnant Hypoxia There may be enough O2 coming in and enough HGB to carry it--but the circulation is stagnant due to a decreased Cardiac Output. The O2 is not being adequately carried to the tissues.
    What are some causes of low PaO2? Histotoxic Hypoxia Poisoning like Carbon Monoxide or Cyanide. Either the blood can’t carry the O2 or the cells can’t receive the O2 from the blood.
    What is saturation? SaO2 (oxygen saturation) measures the percent of oxygen bound to hemoglobin. This tells weather the patient has HYPOXIA (decreased O2 in the tissues).
    Normal SaO2 ? Greater that 95%
    In Carbon Monoxide Poisoning the HGB is saturated with Carbon Monoxide. Although the patient is hypoxemic because there is no room on the HGB for O2 to be carried—the Saturation looks good because it can’t distinguish between the two.
    What does the PaO2 mean? The O2 tells us if the patient has HYPOXEMIA (decreased oxygen in the blood).
    Normal PaO2 = 80-100. (Hypoxemia = PaO2<80)
    What does it do? PaO2 assesses Perfusion (gas exchange).
    What does it do? PaCo2 asseses the adequacy of Ventilation (breathing pattern).
    What does it do? The PaO2 is very important in determining your patient’s oxygen status and needs
    Decreased pH with Decreased HCO3 ACIDOSIS.
    Increased pH with Increased HCO3 ALKALOSIS.
    Decreased pH with Increased CO2 ACIDOSIS.
    Increased pH with Decreased CO2 ALKALOSIS.
    If it is 7.35-7.45 (normal) is COMPENSATED
    If the CO2 is <35 or >45-- RESPIRATORY.
    If the HCO3 is <22 or >26-- METABOLIC.
    If the pH is <7.35 or >7.45 is UNCOMPENSATED.
    Carbon Dioxide is an Acid
    Increasing your respiratory rate(hyperventilation) you "blow off" CO2 decreasing your acid. Giving you ALKALOSIS
    Decreasing your respiratory rate (hypoventilation) you retain CO2(acid) therefore increasing your CO2 giving you ACIDOSIS
    Hydrogen or HCO3 is Bicarbonate or Basic or a base
    If you have excess H+ and decreased HCO3(base):decrease in pH Acidosis; the kidneys will try to adjust to this by excreting H+ and retaining HCO3
    When H+(acid) decreases and you hace increased HCO3(base) Alkalosis;Kidneys excrete HCO3(base), retains H+
    Respiratory AcidosisH<7.35;CO2>45 Causes HypoventilationDepression of respiratory center (sedatives, narcotics,drug overdose, mi,Spinal cord injuryChest wall disorders(pnuemo)Disorders of lung(COPD, CHF, pneum, aspiration
    S/S of Respiratory Acidosis Muscle twichingTachycardiadysrythmiasdiaphorisispalpitati onsserum electrolyte abnomalitiesCNS depression
    Treatment of respiratory acidosis physically stimulate pt to breatheVigorous chest PTC & DB, Spirometerrespiratory treatmentsreversal of sedativesantibiotics for infectionsdiuretics for oveload
    Respiratory Alkalosis: pH > 7.45 CO2 < 35 Alveolar HyperventilationPsychogenic(fear,pain,anxiety)CNS stimulation(brain injury, ETOH, brain tumorHypoxiaventilator rate too fast
    S/S of respiratory alkalosis HeadacheVertigoTinnitusElectrolyte abnormalitiesParesthesias
    Treatment for Alkalosis Sedatives or analgesicsCorrection of HypoxiaAntipyretics for fevertreat for hyperthyroidismbreathe into a paper bag

    a pH of less than 7.35 on an ABG would be considered acidosis
    a PH of greater than 7.45 on an ABG would be considered alkalosis
    alkalosis along with a PaCO2 of less than 35 on an ABG would be considered respiratory alkalosis
    acidosis along with a PaCO2 of greater than 45 on an ABG would be considered respiratory acidosis
    acidosis with an HCO3 level of less than 22 would be considered metabolic acidosis
    alkalosis with an HCO3 level of greater than 26 would be considered metabolic alkalosis
    normal PaCO2 range 35-45
    normal HCO3 range 22-26
    normal SpO2 range 95-100
  5. by   RLtinker
    When looking at acid/base balance you need to look at the normal values,
    Blood pH 7.35-745
    HCO3 22-26
    PaCO2 38-42
    First the pH will tell you if this is acidosis or alkalosis, Low ph ( <7.35) indicates acidosis and a high pH (>7.45) indicates alkalosis. To figure out if its metabolic or Respiratory, you have to look at HCO3 (metabolic Higher= alkalosis, lower= acidosis) and PaCO2 (respiratory Lower= alkalosis, higher= acidosis). The acronym ROME may help, Respiratory Opposite -Metabolic equal.
    Now this is a bit tricky,
    To answer the question of compensation you need to ask does either the PaCO2 or
    HCO3 go in the opposite direction of the pH? If so, then that system is compensating. For an example acidosis in the ph and respiratory with alkalosis in the metabolic is a sign of metabolic compensation. Technically this is called partial compensation. If the PH is in the normal range (but usually closer to one end then the other) and the metabolic and respiratory are opposites, your looking at Complete compensation.
    Last edit by RLtinker on May 9, '12
  6. by   Esme12
  7. by   TnMtnRose
    This may help too. My instructor told me to visualize the "poster child" for each : metabolic acidosis ~ diabetic ketoacidosis, metabolic alkalosis ~ PT with NG tube/suction , respiratory acidosis ~ PT with COPD, respiratory alkalosis ~ PT who is hyperventilating. Putting that "face" with the "numbers" helped me
  8. by   mindlor
    Why dont you post some values and we can take a squiz at them.....
  9. by   Juniper248
    I have a question related to this. I'm not in nursing school yet, but I've taken chemistry. Sorry if this is a stupid question.
    Say someone has respiratory alkalosis. According to Le Chatelier's Principle the equation would look like:
    HCO3 + H+ --> H2CO3 --> CO2 + H2O
    The equation would move towards the right so there would be less HCO3 + H3 in the blood, which makes it basic.
    Why do the values show, for example a PaCO2 that is on the lower side of normal, if the equation shifts to produce more CO2 (in respiratory alkalosis)?

  10. by   pghnurse527
    First, memorize normal values.
    pH= 7.35-7.45
    pCO2 = 35-45 (I remember that pH and CO2 have 35 and 45)
    HCO3 = 22-26

    Step 1) Look at the pH. The lower the number the more acidic, the higher the number the more basic/alkalotic. So if the pH is 7.34 it is acidosis, if the pH is 7.46 it is alkalosis. Now you have the second part of your answer.

    Step 2) Does the pCO2 deviate further from normal values or does HCO3 deviate further from normal values? For instance, if the pCO2 given was 58 and the HCO3 was 28, you can see that 58 is a lot further from the normal range of 35-45 than 28 is from 22-26. So, the pCO2 is altered. CO2 = respiratory and HCO3= metabolic. If CO2 deviates further, your answer is respiratory. If HCO3 deviates further, your answer is metabolic.

    Step 3) Compensated-- Only needed if the pH is WNL. The acidosis/alkalosis comes from whichever end of the normal pH range (7.35-7.45) your number is at. So if your number is 7.37 it is closer to acidosis. If your number is 7.42 it is closer to alkalosis. Next, choose which number deviates the furthest between the pCO2 and hCO3.
  11. by   mindlor
    your equation goes both directions..........if you start with CO2 you have carbonic acid.....

    The more CO2, the more carbonic acid, the more carbonic acid, the lower the blood pH thus acidosis, not alkalosis.......
  12. by   daisyfleur70
    Thanks so much for all of your replies... this has been very helpful!
  13. by   tariq_ze
    Can someone tell me what is the good book for biochemistry for nurses. Is there any written specially for nurse students.

    Thanks in advance!
  14. by   mindlor
    Quote from tariq_ze
    Can someone tell me what is the good book for biochemistry for nurses. Is there any written specially for nurse students.

    Thanks in advance!
    You need to spend much less time on biochem and much more time knowing what side to position a client in order to give them a suppository...

    I assure you there will be no biochem questions on the nclex