Blood Gases

Clinical Acid-Base Disturbances

Blood Gases Details

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Approach to the Abnormal Blood Gas

      Acid-base disorders can be approached with three questions:

           What is wrong?
           What caused it?
           What's being done about it?

      The answer to the first question, “What is wrong?” is obtained by simple inspection of the values on the pH, PaCO2, and HCO3-.
      If all three values are normal, the answer to “What is wrong?” is “Nothing,” and the other two questions can be ignored.
      If either pH, PaCO2, or HCO3- are abnormal, check the pH. If it’s below 7.4, the answer to “What is wrong?” is “Acidosis.” If above 7.4, “Alkalosis.”
      If the pH is within the normal range, but the PaCO2 or HCO3- (or both) are abnormal, an acid-base derangement exists, but the body has fully compensated for it. For example, with a pH of 7.35 (normal) and a decreased bicarbonate of 18, an acidosis exists.

           Check pH, PaCO2, HCO3-.
           Anything abnormal? If no, quit.
           pH greater than 7.4 = alkalosis
           pH less than 7.4 = acidosis

      The second question is “What caused it?” The answer is “Metabolic” if bicarbonate has caused the observed change in pH from 7.4. If carbon dioxide caused it, the answer is “Respiratory.” If both are guilty, the answer is “Mixed metabolic and respiratory.”
      First look at the bicarbonate. Is it guilty?
      Increased bicarbonate raises the pH. Low bicarbonate lowers the pH. If you see a pH above 7.4 and the bicarbonate is elevated above normal, it means bicarbonate is guilty of raising the pH. So a metabolic alkalosis exists.
      If the bicarbonate is above 7.4 and the bicarbonate is decreased or normal, bicarbonate is not guilty.
      Similarly, if the pH is below 7.4 and the bicarbonate is below normal, it means lack of bicarbonate is responsible for lowering the pH. Therefore a metabolic acidosis exists.
      Look at the CO2. Is it guilty?
      Carbon dioxide is acidic. A high CO2 will lower the pH, while a low CO2 will raise it. If the pH is above 7.4 and the PaCO2 is lower than normal, lack of CO2 is responsible. A respiratory alkalosis exists.
      If, however, the CO2 is normal or elevated while the pH is above 7.4, then CO2 can’t be contributing to the disturbance.
      If both PaCO2 and HCO3- are shifted in a direction that would contribute to the pH abnormality, both are guilty. A mixed metabolic and respiratory abnormality exists.

           Check pH, PaCO2, HCO3-.
           Anything abnormal? If no, quit
           pH greater than 7.4 = alkalosis
           pH less than 7.4 = acidosis
           Who’s responsible?
           HCO3- shifted in direction of pH = guilty
           CO2 shifted opposite of pH = guilty

      The final question is “What’s the body doing about it?” We’re checking for compensatory changes — changes the body has made to compensate for the abnormality. This is an inherantly inaccurate question, as I’ll discuss later. Consider your answer a “best guess.”
      Assume the body has only two mechanisms to affect pH: respiratory and metabolic. Respiratory is CO2 and metabolic is HCO3-.
      After you’ve identified the guilty party (CO2 or HCO3-), look at the other value. If that other value is abnormal, but in a direction that would move the pH back towards normal, then compensation is present.
      If you’ve found that both HCO3- and CO2 are guilty, then obviously compensation isn’t present. As an example, assume blood gases that show pH=7.33, HCO3=16.5, and PaCO2=32. The problem is acidosis (any abnormality plus pH >> 7.4). The guilty party is metabolic (HCO3- is low, shifted in a direction that causes acidosis). Respiratory compensation is present (CO2 is abnormal in a direction that would raise the pH back towards normal.
      Compensation by respiratory means is very fast, occuring within seconds or minutes. This compensation occurs via the body’s control of respiratory rate through the brain respiratory center. Thus respiratory compensation for metabolic abnormalities is seen almost immediately.
      Metabolic compensation, on the other hand, is slow. It occurs through elimination of acid or alkali by the kidney. Hours go by before significant compensation is seen. Metabolic compensation will occur for chronic respiratory disturbance, but also, metabolic correction through the kidney will be seen for metabolic disturbances.

           Check pH, PaCO2, HCO3-.
           Anything abnormal? If no, quit
           pH greater than 7.4 = alkalosis
           pH less than 7.4 = acidosis
           Who’s responsible?
           HCO3- shifted in direction of pH = guilty
           CO2 shifted opposite of pH = guilty
           If only one guilty, check “innocent” value.
           Shifted = compensation present.

      Is this accurate? No. Identifying the source of acidosis and presence of compensation assumes that the same process has been going on all along. If body’s state changes from one source of abnormality to another, or if two completely separate pathological processes are present, your “guess” will be wrong.
      For example, Phil Smith has a heart attack and goes into V-fib. He develops both respiratory and metabolic acidosis. Then he gets defibrillated and wakes up. As he realizes that he has to give up his favorite cholesterol-rich foods, he hyperventilates. Now blood gases are drawn.

           pH = 7.44       PaCO2 = 28       HCO3 = 18.6

      Looking at these gases, you diagnose “fully compensated (chronic) respiratory alkalosis.” Not true. Phil has an acute respiratory alkosis superimposed upon a “slightly less acute” metabolic acidosis. Remember that bicarbonate abnormalities cannot change quickly.
      Computer interpretation (such as that used in Mad Scientist Software's Blood Gases program) look for "zones" of blood gas values where clinical disturbances tend to fall. This gives a 95% level of certainty about mixed disorders and compensation. At the bedside however, you're on your own.
      Whenever you diagnose a respiratory cause for an acid-base abnormality, with metabolic compensation, consider whether the abnormal bicarbonate could be a “leftover” or separate metabolic abnormality of the opposite type.
      For example, in aspirin poisoning, both metabolic acidosis and respiratory alkalosis occur as a result of the aspirin. Depending on whether the pH happens to be above or below 7.4 at the moment, you might incorrectly call it a “compensated respiratory alkalosis” or a “compensated metabolic acidosis.” Always consider the clinical history. Then you can correctly decide whether a “compensation” is really compensation, or a separate abnormality.
      Like most everything else in medicine, blood gas interpretation requires a consideration of the patient history and your examination findings. Let's review our completed "bedside" algorithm.

           Acid-base Disturbance Diagnosis Algorithm
           Check pH, PaCO2, HCO3-.
           Anything abnormal? If no, quit
           pH greater than 7.4 = alkalosis
           pH less than 7.4 = acidosis
           Who’s responsible?
           HCO3- shifted in direction of pH = guilty
           CO2 shifted opposite of pH = guilty
           If only one guilty, check “innocent” value.
           Shifted = compensation present.
           History compatable with mixed disorder?
           Not true compensation.

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Metabolic Acidosis

Clinical causes of metabolic acidosis
      Tissue metabolism normally results in about 12,500 milliequivalents of acid production per day. This acid is in the form of CO2, and after circulation to the lungs, is removed from the body in expired air. If aerobic metabolism ceases (due to lack of oxygen or inability to use available oxygen due to metabolic poisoning), tissues are unable to completely oxidize sugar to CO2. Instead, sugar is only partly oxidized to lactic acid. As this acid cannot be expired through the lungs as CO2 is, it remains in circulation, causing metabolic acidosis.
      In untreated diabetes, normal sugar metabolism is deranged due to lack of insulin. In this case, acid buildup is due to acetoacetic and betahydroxybutyric acids.
      A certain amount of acid is formed when dietary proteins are metabolized. These proteins contain sulfate and phosphate groups that, after metabolism, form sulfuric and phosphoric acid. These acids amount to only about 150 mEq per day, however they must be excreted from the body through the slow process of kidney filtration. If the kidneys fail, acidosis results after several days. Ingestion of acidifying salts, and loss of bicarbonate through chronic diarrhea, are less common causes of metabolic acidosis.

           Example:
           pH = 7.21 PaCO2 = 40 HCO3 = 15.6

Compensation for metabolic acidosis
      As the blood becomes more acidic, the brain’s respiratory centers are stimulated to increase the rate and depth of breathing. This lowers the CO2 in the blood, decreasing its acidity.
      The kidney then begins to remove the excess acid. As the plasma is filtered, acid anions enter the urine. In the kidney tubules, hydrogen ion is secreted. For each hydrogen ion that enters the urine, a sodium ion and a bicarbonate ion are put back into the plasma. In this way, acid is eliminated from the body.

           Example of compensated metabolic acidosis:
           pH = 7.34 PaCO2 = 28 HCO3 = 14.7

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Metabolic Alkalosis

Causes of metabolic alkalosis
      Fruits are the normal source of alkali in the diet. They contain the potassium salts of weak organic acids. When the anions are metabolized to CO2 and removed from the body, alkaline potassium bicarbonate and sodium bicarbonate remain. Metabolic alkalosis may be found in vegetarians and fad dieters who are ingesting a low-protein, high fruit diet.
      A more efficient way to get alkali into the body is to consume sodium bicarbonate. This common heartburn remedy is probably the most common cause of symptomatic metabolic alkalosis.
      If acid is eliminated from the body, it has the same effect as adding alkali. Persons with protracted vomiting of acidic stomach juices will often develop metabolic alkalosis, as acid is secreted into the stomach then vomited out of the body.

           Example:
           pH = 7.51 PaCO2 = 39 HCO3 = 30.4

Compensation for metabolic alkalosis
      Short term, a decrease in respiratory rate leads to an increase in serum carbon dioxide levels. (The carbon dioxide is transported as hydrogen ion — buffered by hemoglobin — and bicarbonate.) This lowers the pH towards normal, partially compensating for the additional alkali present in the blood.
      The slow process of eliminating bicarbonate through the kidney then begins. Hydrogen ions are transported from the filtered urine back into plasma, with sodium ions and bicarbonate left behind. Alkaline sodium bicarbonate is thus eliminated.

           Example:
           pH = 7.45 PaCO2 = 46 HCO3 = 31.2

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Respiratory Acidosis

Causes of respiratory acidosis
      Buildup of carbon dioxide occurs when ventilations are inadequate. This is usually due to absense of adequate respiratory effort — such as when central control of respiration is depressed due to narcotics or barbiturates. When respiration ceases due to cardiac arrest, of course, respiratory acidosis is an immediate result.
      Respiratory acidosis can also result when obstruction of air motion leads to carbon dioxide buildup. Severe asthma and foreign body obstruction are examples.

           Example:
           pH = 7.21 PaCO2 = 55 HCO3 = 22

Compensation of respiratory acidosis
      Whereas respiratory changes can occur within seconds or minutes, metabolic changes take hours to days. Compensation for respiratory acidosis must occur through elimination of acid through the kidney, as discussed above under metabolic acidosis. Only in chronic respiratory problems, such as severe obstructive airway disease, will compensation be seen.

           Example:
           pH = 7.34 PaCO2 = 56 HCO3 = 29.5

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Respiratory Alkalosis

Causes of respiratory alkalosis
      Respiratory alkalosis occurs due to hyperventilation. The hyperventilation may be due to psychological causes — in fact, this is the most common cause.
      In other causes, the hyperventilation may be due to abnormal stimulation of ventilation due to disease. Changes in the lung due to pulmonary embolism, asthma, or pulmonary edema often trigger increased respiratory rate, resulting in respiratory alkalosis.
      Central stimulation of respiration occurs in aspirin poisoning. This respiratory alkalosis is a separate effect from the metabolic acidosis produced by aspirin.

           Example of respiratory alkalosis:
           pH = 7.57 PaCO2 = 24 HCO3 = 21.5

Compensation of Respiratory Alkalosis
      Respiratory alkalosis must exist for hours before metabolic compensation can be seen. Alkaline sodium hydroxide is eliminated by the kidney, returning the pH back towards normal, as discussed above under metabolic alkalosis.

           Example:
           pH = 7.46 PaCO2 = 22 HCO3 = 15.3

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Therapy of Respiratory Acidosis

      The treatment of respiratory acidosis isn’t difficult — in theory. All you have to do is increase the ventilation of the lungs. This removes carbon dioxide from the blood stream, raising the pH. The increase in ventilation may be easy in the intubated cardiac arrest or drug OD patient. Just turn up the ventilator, or tell the “bagger” to bag a little faster and deeper.
      In the conscious patient with severe asthma or pulmonary edema, a decision must be made whether to await results from conservative therapy, or to take control the airway through intubation and assisted ventilation. (This decision, in practice, is based more on “gestault” of the clinical picture rather than on the level of carbon dioxide.) You either improve air motion with drugs, or force better air motion with an artificial airway.
      In a patient with poor gas exchange due to intrapulmonary causes — that is, disease within the lung itself — increasing ventilatory rate and depth may be only marginally helpful. In this case, only improvement of the disease process will help.
      Some cases of carbon dioxide retention are better untreated. For example, consider this patient with CHF and emphysema:

           pH = 7.32 PaCO2 = 78 HCO3 = 39.3 PaO2 = 43

      Review of past hospital records consistantly shows a CO2 around 70 at discharge. This patient has a chronic (compensated) respiratory acidosis. Trying to “normalize” this patient’s blood gases would be dangerous. And even if you succeeded, once the patient was breathing on his own he would retain CO2 again acutely, resulting in a severe acute respiratory acidosis of pH =7.1! If the patient must be intubated, sufficient “dead space” must be provided within the ventilator tubing to keep the CO2 in the patient’s usual range. Back to Top


Therapy of Metabolic Acidosis

      Mild cases of metabolic acidosis are best left alone. Usually no treatment is needed if the pH is above 7.1, and rarely is it needed if the pH is above 7.2, although the patient’s level of discomfort and compensating hyperventilation must be considered.
      Metabolic acidosis is treated with sodium bicarbonate, given intravenously. There is considerable question, however, how beneficial acidosis treatment is for certain patients.
      For the semi-comatose diabetic in ketoacidosis, there’s no question that bicarbonate will raise the serum pH. But as the acid is neutralized in the blood, CO2 is formed (you remember the chemical reaction). The increase in pH decreases respiratory drive, which slows the elimination of this extra carbon dioxide. The CO2 diffuses into the cerebrospinal fluid, causing a paradoxical lowing of pH around the brain, with deepening of coma. The moral: give bicarb slowly and maintain the hyperventilatory state, even if bag-valve assist or intubation is required.
      For the patient in cardiac arrest, raising the pH hasn’t been shown to improve the ultimate outcome. And alkalosis caused by too much bicarbonate is positively deadly for the arrest victim. On the other hand, since the American Heart Association changed its standards to eliminate the routine use of bicarbonate, I’m seeing a lot of arrested patients from the field with pHs of 6.9 — which may lengthen the “code time” if there's pulseless electrical activity because the patient can’t be declared dead until he's both “warm and dead” and “acid-base normal and still dead.” For now, treat the cardiac arrest patient with bicarbonate only based on proven need by blood gases.
      Bicarbonate dosage recommendations vary widely — most sources recommend from 0.1 to 0.3 times the weight of the patient in kilograms times the (negative) base excess (BE) expressed in milliequivalents per liter. In my experience, 0.2 x weight x BE is about right for the typical patient. The calculated result of this formula will have units of milliequivalents — the number you calculate is the dose in milliequivalents.
      However, the recommendation I’ll give to you (and the formula given in both the Blood Gases disk and the ACLS training software) is based on the more conservative recommendation of the American College of Emergency Physician’s textbook. This formula is 0.1 x weight x BE. The minus sign on the base excess is ignored.

           Bicarb Dose = 1/10 of weight in kg times base excess

           Bicarb = 0.1 x wt x BE

      After giving bicarbonate, a repeat blood gas analysis should be performed (after a couple of minutes to “blow off” the CO2 that is formed). Often, an additional dose must be given. If you decide that use of bicarbonate is needed in a situation where blood gases are NOT available, for example with a tricyclic overdose or diabetic patient in coma far from a hospital, you need a reasonable way of calculating an empiric dosage.
      In this situation, give the patient one mEq for every kilogram of body weight:

           Emperic Bicarb = 1 mEq x weight in kg

      In the cardiac arrest victim, a continuing dosage may be necessary IF BLOOD GASES ARE NOT AVAILABLE. This dose is 1/2 mEq per kilogram every 10 minutes. However, you’ll probably never use this 1) because you should be getting blood gases, and 2) because if your CPR is so ineffective that acid continues to build up at that rate you’ll never save the patient anyway. The final words on bicarbonate therapy are: Have a good reason for using it, be aware of its problems and complications, and monitor your therapy with repeat blood gas analysis. Back to Top



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Copyright 1996 Mad Scientist Software
Citation:
Argyle, B., Blood Gases Computer Program Manual.
Mad Scientist Software, Alpine UT, 1996.

 

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