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    《ISPN学习》总第572期
    日期:2020-01-14 20:23:00    
     
    《ISPN学习》2019年12月16日总第572期
       
    Acid-Base Balance and Blood Gas Interpretation
    酸碱平衡和动脉血气解读

    内容提要:
    • 酸释放氢离子;碱捕获氢离子。体液的酸性或碱性用氢离子浓度即pH表示。
    • 氢离子与pH呈反相关性,即氢离子多,pH低,为酸性;氢离子少,pH高,为碱性。
    • 血液中碱积累或酸丢失,造成碱毒性。
    • 影响酸碱体内平衡的因素,即酸碱平衡调节器,包括肺、肾和缓冲系统。
    • 酸碱平衡的正常范围为:    pH = 7.35 – 7.45;
      • HCO3 : H2CO3 = 20:1
      • PaCO2 = 35 – 45 mm Hg
      • HCO3 = 22 -- 26 mEq/L
      • SaO2 = 95 -- 100%
      • PaO2 = 80 – 100 mm Hg
    • 酸性指标是CO2;碱性指标是HCO3.
     
    FIGURE: Acid-base balance. In the healthy state, a ratio of 1 part carbonic acid to 20 parts bicarbonate provides a normal serum pH between 7.35 and 7.45. Any deviation to the left of 7.35 results in an acidotic state. Any deviation to the right of 7.45 results in an alkalotic state.
    : 酸碱平衡。在健康状态下,20:1的碳酸与碳酸氢盐比使血清正常pH值维持在7.35-7.45之间。偏向7.35左侧导致酸毒性,偏向7.45右侧导致碱毒性。
     
     
    Welcome to the video tutorial on acid-base balance and blood gas interpretation. Acid-base balance is critical, maintaining homeostasis in the body. Acids are substance that can give up a hydrogen ion and basis are substances can accept the hydrogen ion. Acidity or alkalinity as body fluid is expressed in terms of the concentration of hydrogen ion or pH. Hydrogen ions have an inverse relationship to the pH. More hydrogen ions cause a lower pH, which is acidic. Fewer hydrogen ions cause a higher pH, which is alkaline. The accumulation of acid in the blood or loss of base causes acidosis. The accumulation of base in the blood or loss of acid causes alkalosis.
    The primary factors in the homeostatic process include the lungs, kidneys, and the buffer systems.
    The lungs control certain amounts of carbon dioxide in the blood. Therefore if the lungs are not functioning properly, CO2 builds up, causing increased carbonic gases, which affects blood pH and leads to acidosis. The main function of the kidneys is retaining or excreting bicarbonate, or HCO3, which is to neutralize the excess acid in the blood. If both of the lungs and kidneys are working properly, the natural build-up of acids will be neutralized effectively by the buffer system. The buffer system works very quickly to maintain proper pH of the blood in body tissue. The bicarbonate will neutralize the correct amount of carbonic acid molecules to maintain the correct ratio of 20:1 acid molecules. This ratio of 20 bicarbonate (HCO3) to 1 carbonic acid (H2CO3) will keep the blood pH in the range of 7.35 to 7.45. The arterial blood gas or ABG provide the valuable information about the acid-base balance of the blood and the oxygenation status of the blood. It measures the arterial pH, PaCO2, bicarbonate, PaO2, oxygen saturation, and base excess. Arterial blood is used because venous blood is not suitable for the assessment of oxygen tension and pH. The nurse draws blood from either the radial, brachial, or femoral artery; or from an arterial line. The radial artery is most commonly used because it is accessible, easily positioned, and more comfortable for the patient. The sample should be drawn with a heparinized syringe without exposure to air. The sample is then placed on an ice, which slows the clotting of blood, and rushed to the lab for analysis.
    When analyzing ABGs, there are several normal values that need to be memorized.
    When assessing acid-base status, the most important component to look at first is the pH. 7.4 is the perfect pH with a normal range of 7.35 to 7.45. PaCO2, the partial pressure of carbon dioxide, reflect alveolar function and ranges from 35 to 45 millimeters of mercury. Bicarbonate, or HCO3, the major base component, has a normal value at 22 to 26 mEq/L. The total concentration of bicarbonate in the blood may be referred to as serum carbon dioxide, serum bicarbonate, and total carbon dioxide content.
    When you need to interpret ABGs, it is helpful to lay out a chart, with pH at the top of the first column; then lungs or PaCO2, and kidneys, HCO3. Start labeling in the second column, acid, base, acid, continuing on to the third column, base acid base. And if you memorize the normal value, you can put them right in. pH is 7.35 to 7.45; PaCO2 is 35 to 45; and HCO3 is 22 to 26. With the chart written out, you can easily figure out ABG problems.
    Let's go over a few examples.
    If the pH is 7.29, it's less than 7.37, it is acidic; the PaCO2 is 50, greater than 45, so it is acidic. The HCO3 is 24, which is normal. Since the pH is acidotic and CO2 is acidotic, we know the acid-base disturbance is being caused by the respiratory system and it's called respiratory acidosis. Since PaCO2 is high and represents lung function, it tells us there is excess CO2 retention from hypoventilation or CNS depression.
    In our next example, the pH is 7.5. It is basic, circle base. The PaCO2 is 33; it is also basic. The HCO3 is 25, which is within normal range. In this case there is excess CO2 being excreted, possibly caused by hyperventilation or fever. The acid-base imbalance is again caused by the respiratory system, but with an alkalotic pH, meaning it will be respiratory alkalosis.
    OK now we will look at metabolic situations.
    Metabolic acidosis and metabolic alkalosis are caused by the levels of bicarbonate ion in the blood. The kidneys excrete these ions into the urine, so they can be eliminated when not needed. If the pH is low, in this example, 7.28, circle acid; the PaCO2 is within the normal range of 40, and the HCO3 is acidic at 20, with low bicarb levels and a low pH, you have metabolic acidosis.
    In our next example, the pH is 7.51; so it is basic, the PaCO2 is 39, within the normal range; the bicarb level is above 26, it is high, also basic; this means we have metabolic alkalosis. Metabolic alkalosis can result from inadequate excretion of acids due do the renal disease, loss of potassium from diuretic therapy, and other causes.
    OK we’ve looked at how the levels of CO2 and HCO3 can alter the blood pH. However the body does try to bring the pH back to normal by compensating for the imbalances. For example, the pH is acidotic, the CO2 is alkalotic, and the HCO3 is acidotic. Both the HCO3 and pH are acidotic, making the primary acid-base disorder metabolic acidosis. The CO2 is opposite of the pH and will be evidence of compensation from the respiratory system.
    Another way to remember acid-base imbalances is with the mnemonic ROME. Respiratory opposite, metabolic equal. We know respiratory involves CO2, and CO2 will be going in the opposite direction of the pH. The pH is low for acidotic, the CO2 will be high, also acidotic, resulting in respiratory acidosis. When the pH is high, or alkalotic, and the CO2 is low, also alkalotic, we have respiratory alkalosis. We know metabolic involves the bicarbonate ion, HCO3. When the pH is low, or acidotic, and the HCO3 is low, also acidotic, the result is metabolic acidosis. If the pH is high, alkalotic, and the HCO3 is also high, also alkalotic, we have metabolic alkalosis.
    Let's go over a review. Look at the pH first. If it is low, it indicates acidosis; if it is high, it indicates alkalosis. Next, look at the CO2. The respiratory system controls CO2. So determine if it's higher than 45 or lower than 35. Next, look at the HCO3, which is controlled by the kidney. Is it less than 22 or greater than 26? Then determine if there is any compensation present. It is also important to know the oxygenation status of a patient. The partial pressure of arterial oxygen is the amount of oxygen that is dissolved in the blood. It is referred to as PaO2 and is normally 80 to 100 millimeters of mercury. The PaO2 does not play alone in acid-base regulation. However, low PaO2 indicates an alteration in respiratory function, or breathing air with the low level of oxygen. Oxygen is also combined with hemoglobin. The SaO2 for the oxygen saturation of hemoglobin, is expressed as a percentage. At full saturation, the normal SaO2 is 95 to 100%. The SaO2 and PaO2 are directly related to each other. As one increases, the other usually increases also. So oxygen is present in the blood in 2 forms: oxygen dissolved in the blood, and oxygen combined with hemoglobin. If they are below normal, there is evidence of hypoxemia.
    Thank you for watching this video on acid-base balance and blood gas interpretation.

     
    Questions
    1. A client arrives in the physician’s office complaining of insomnia, decreased appetite, and weight loss due to a recent job loss. The physician diagnoses the client with generalized anxiety disorder and prescribes two common antianxiety medications. The client expresses concern to the nurse about taking medication for the symptoms and asks about alternatives for treating the anxiety. The nurse’s best action is to:
    A. Instruct the client to begin yoga and provide the times and locations of local classes in the area.
    B. Instruct the client in basic rhythmic breathing techniques as well as a simple modified autogenic relaxation exercise.
    C. Educate the client in various relaxation techniques while determining which seems most appealing to the client.
    D. Refer the client to a local financial advisor.
    2. A client is admitted to the hospital with a blood pressure of 85/53 mm Hg and is complaining of light-headedness and near syncope. The client’s current medications include carvedilol (Coreg), amiodarone (Cordarone), warfarin (Coumadin), Lanoxin (Digoxin), furosemide (Lasix), potassium (K-Dur), and a daily vitamin. The client also reports using both St. John’s wort and cat’s claw. Based upon this information, what is the nurse’s best action?
    A. Notify the physician immediately reporting the client is critically ill and needs transfer to the ICU.
    B. Notify the physician that the client’s use of cat’s claw may interact with some of the other heart medications that the client is taking, resulting in hypotension.
    C. Notify the physician that the client’s use of St. John’s wort may interact with the diuretic the client is taking, resulting in light-headedness.
    D. Notify the physician that the client’s use of cat’s claw may interact with the anticoagulant the client is taking, resulting in near syncope.
     
    Answers
    1. C. Educating the client in various relaxation techniques, while determining which seems most appealing to the client, helps to alleviate the client’s anxiety while allowing the client to maintain control by making choices.
    2. B. Notify the physician that the client’s use of cat’s claw may interact with some of the other heart medications the client is taking, resulting in hypotension.





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