Chapter VIII Arterial Blood Gas Interpretation(ABG) Arterial Blood Gas Analysis2 is used to measure the partial pressures of oxygen (PaO2), carbon dioxide (PaCO2), and the pH of an arterial blood sample. Oxygen content (O2CT), oxygen saturation (Sa)2), and bicarbonate (HCO3) values are also measured. A blood sample for ABG analysis may be drawn by percutaneous arterial puncture from an arterial line. The ABG analysis is mainly used to evaluate gas exchange in the lungs. I is also used to assess integrity of the ventilator control system and to determine the acid-base level of the blood. The ABG analysis is also used for monitoring respiratory therapy (again by evaluating the gas exchange in the lungs). Nursing Considerations: Your first look at an ABG result might prove to be confusing. Any patient who is critically ill might be given this test at regular intervals. Arterial blood gas determinations will indicate two basic bodily functions:
2 The term “analysis” is often used when the LABORATORY talks about the ABG. In the lab, the specimen is “analyzed”. However, when the doctor or the nurse talk about the ABG, they often use the term “interpretation”. The nurse and the doctor are more concerned with the interpretation of the specimen results. Therefore, you will often see “ABG Interpretation” used in the clinical area.ABG’s will also indicate other important facts about a patient’s status. However, the two functions above are the most important. In a clinical situation, most nurses need only to understand these two basic concepts. When the results of an ABG are abnormal, most hospitals today will have a lab procedure for notification of the MD or to the ICU staff. But if you should be one of those “lucky” nurses who is floated to a critical care area or a respiratory care area, you may have to interpret the results by yourself. If you are able to do this, and fast, it may mean that the patient will get help fast. Hypoxemia, academia, and alkalemia are important concepts which should be understood before beginning. Hypoxemia is a term which refers to a lowered blood oxygen content. This term and the term hypoxia are probably quite familiar to most nurses. They both will be used as meaning exactly the same. Hypoxia is the basis of one part of interpretation process. From above, we know that oxygenation status of the patient can be critical during certain disease states. Acidemia or acidosis is a term which refers to excessive amounts of acid in the blood. Acids are produced naturally in the body as a product of metabolism and other specific body processes. If our blood acid levels rise too high, it will interfere with the health of the individual. This will be in addition to the disease which is already present causing the acidosis. Alkalosis, alkalemia is the term which refers to the condition of excessive bicarbonate ions (bases) in the blood. As we mentioned above, this imbalance in the blood pH will then cause further problems as the normal body recovery mechanism may also be interrupted. On the next pages you will find an explanation of what the ABG test is all about. We will also present the nursing considerations surrounding their interpretation. Read each section of the following text in order. The text builds up from the simpler concepts to the more complex concepts so each nurse will be able to easily follow the interpretation process. When you fully understand one section, then go on to the next section until you finally are able to interpret the ABG with the fullest understanding. Since this course is very clinically oriented, we will concentrate on the aspects of ABG interpretation that apply to direct patient care. The clinical uses of ABG studies will be listed on the following pages. ABG studies may be helpful to diagnose and treat the following (Brunner, 1994):
Collecting the ABG specimen The ABG is performed on a sample of arterial blood. The specimen is then obtained in a syringe prepared with heparin so as to prevent coagulation from occurring. The sample is then placed in crushed ice and rushed to the lab for analysis. Each institution will have a slight variation in the method of the collection and in which department the sample will be handled. The reason for rushing the specimen and for using the ice is to prevent coagulation of the specimen, and specifically, ice slows the clotting of the blood. Be sure you are familiar with that procedure in your facility: Terms used in connection with ABG’s:
Acid/Base Balance pH is the measurement used to determine acidity or alkalinity of arterial blood. pH is a measure of an acid or base solution and the relative strength of that solution. Below is the pH scale, 7 being the arbitrary center point indicating a neutral solution. An example of an acid is carbonic acid. Carbonic Acid is formed when carbon dioxide (CO2) chemically combines with water (H20) to form carbonic acid (HxCO3). The “H” at the beginning of a chemical formula usually designates an acid.
The further away from 7 in either direction indicates the strength of the acid or base. An acid can donate the hydrogen ion (H+) and the base is a substance which can accept the ion. The pH then is the concentration of the ion in solution. Normal blood pH ranges from 7.35 to 7.45 this is slightly to the alkaline side of the scale. If the pH is at the low end of the scale or if it is actually below 7.35, the condition is academia. Thus if it is above 7.45 it is described as alkalemia. The body is in a state of constant change. Thus, the pH is constantly changing within this range of values. This of course is called the homeostatic process. Body waste products are constantly being produced, and affecting the pH of the blood. As food is metabolized, these wastes are dumped into the blood and affect the pH. There are also concurrent processes which act to balance these actions. They are known as buffers. If the body pH should start to become too acid, the buffers work to neutralize them and balance the pH at normal levels. The exact opposite occurs in an alkaline pH situation. This buffer pair of acid-base work to maintain pH at an optimum 7.40. Carbonic acid and the ion bicarbonate is the buffer pair we refer to. The buffer systems The lungs, kidneys, and the buffer system are the primary considerations in the homeostatic process. The lungs can control certain small amounts of carbon dioxide in the blood. Carbon dioxide in the blood chemically produces carbonic acid. Thus, in cases where the lungs do not function properly, CO2 builds up, causing increased carbonic acid. This increase in acid can affect the blood pH, leading to acidosis. The main function of kidneys is retaining or excreting of the bicarbonate ion (HCO3). This is the ion which neutralizes the excess acid in the blood. If both organs are working properly, the natural build-up of acids can be neutralized effectively by the buffer system. The buffer system in the body is able to work very quickly to maintain proper pH of the blood and the body tissues. The prime buffer system is the system of carbonic acid and bicarbonate. Bicarbonate will neutralize the correct numbers of carbonic acid molecules to maintain the correct ratio of 20:1 acid molecules. This 20:1 ratio will preserve the blood pH at the normal range of 7.35 to 7.45. Bicarbonate ions and carbonic acid are constantly being produced and combined in order to keep the optimal pH. The respiratory system also works to maintain the proper blood pH. When the bicarbonate/carbonic acid buffer system cannot work fast enough to compensate for pH disturbances, the respiratory system has a mechanism for buffering the blood. Hyperventilation and hypoventilation can be used by the body to control the amount of carbonic acid in the blood. The respiratory center in the brain responds to changing levels of carbonic acid in the blood. When the acid level of blood increases, and is not controlled by the first buffer system, the respiratory system responds. Hyperventilation causes the body to exhale and “get rid of” CO2 from the blood, through the lungs. This reduction of CO2 causes the blood pH to become less acid. Reduce the CO2 and the acid level of the blood is reduced. This is how the body responds to excess acid in the blood. The opposite mechanism occurs with hypoventilation. Hypoventilation will cause the retention of CO2 in the blood. As we discussed earlier, this CO2 becomes carbonic acid when it remains in the blood and mixes with water. If you retain CO2, the acid level of the blood will go up. This increased acid could “buffer” any excess base that is present in the blood. If the blood becomes alkaline, then hypoventilation may be another way to neutralize it and get the blood pH back to normal. These respiratory conditions will be discussed in more detail later in this text. In the lab, pH is measured directly using an electrode placed in the blood sample. The “p” of pH is actually defined as “percent Hydrion” or called the negative logarithm of the hydrogen concentration. The concentrations can be expressed as 10 -7, for example, this means: 0.0000001. This negative logarithm can also be expressed as the inverse ratio (Cooper 1987). The more hydrogen ions there are, the lower the pH, or acid. On the other hand, as the hydrogen ion concentration decreases in the blood, the pH increases (alkalinity). The third buffer system exists that will react if the first two methods fail to correct an abnormal blood pH. This third and powerful buffer system is the kidney. The kidneys will react to sustained and/or high levels or acid and/or alkalinity. The kidney buffer system responds to these dangerous level, called “metabolic” conditions. These conditions are metabolic acidosis and alkalosis, and will be discussed later.
Normal HCO3- is: 24 to 28 mEq/L NORMAL vs. ABNORMAL ABG VALUES To continue the discussion from the previous section, we must now look at the value of the carbon dioxide in the blood. CO2 levels are reported on the ABG test as the partial pressure of carbon dioxide. PCO2 levels will directly affect the levels of acid in the blood. PCO2 normal: 35 to 45 mm Hg Increases above the levels indicated, could possibly mean that the CO2 is building due to hypoventilation or respiratory failure of some kind. Decreased levels of CO2 can indicate the opposite type of problem, hyperventilation, as discussed earlier. Analysis of respiratory status First: Examine pH value; if HIGH (above 7.45), ALKALOSIS is present As you see, the conditions of respiratory acidosis or respiratory alkalosis can be determined by examining just the pH and the carbon dioxide levels in the blood. In fact, there are two ways that the pH values can be affected. Earlier we demonstrated that the respiratory system will increase or decrease breathing when the acid levels are too high or too low. The reverse condition can also occur. If some other factor (s) directly causes either hyperventilation or hypoventilation, then the acid content of the blood will be forced to go up or down. Examples of these conditions are described below. So remember that respirations can be considered a buffer to help the body; or, if there is a primary respiratory problem, it can adversely affect the blood pH. In most cases, the respiratory conditions of acidosis or alkalosis can be corrected quite simply, by merely improving the patient’s respiratory status. Respiratory alkalosis can be reversed in most cases by merely stopping the hyperventilation. Nursing Considerations: As we look at the medical conditions which can produce pH imbalances, we will first concentrate on respirations. Any diagnosis which has decreased breathing as a symptom, can lead to either previously mentioned condition. Respiratory Acid-Base Disorders Respiratory Acidosis Findings:
Possible Causes:
Signs and Symptoms:
Respiratory Alkalosis Findings:
Possible Causes:
Signs and Symptoms:
Recognition of these conditions can be the key to prevention. When administering pain meds, remember possible respiratory problems which can occur. With fever, remember hyperventilation can happen, quite subtly. METABOLIC CONDITIONS: Now we will discuss metabolic situations. Metabolic acidosis and metabolic alkalosis conditions are determined by the levels of bicarbonate ion in blood. The kidneys excrete these ions into the urine and out of the body when not needed. As the body demands the bicarbonates to neutralize acids, the kidneys conserve bicarb ions to keep the body in balance. Bicarb ions, are also metabolic by-products (normal by-products of metabolism). To detect metabolic conditions: FIRST: Examine pH values –High pH 9above 7.45) SECOND: Examine CO2 levels (assumed to be normal) THIRD: Examine bicarb levels – high bicarbonate (above 22 to 26 mEq/L) Condition: METABOLIC ALKALOSIS *opposite conditions indicate METABOLIC ACIDOSIS FIRST: Low pH (below 7.35) SECOND: Normal CO2 levels THIRD: Low bicarb levels Nursing Considerations in Metabolic Conditions: Metabolic Acidosis can be caused by many conditions:
The above conditions can all lead to metabolic acidosis. Patients who have had pancreatic drainage and have had ureterosigmoidostomy are also prone to develop a metabolic acidosis. The nurse should observe for any of the signs or symptoms of dehydration, shock or diabetic acidosis. Mental confusion, disorientation, and other neurological signs should not be overlooked, especially if the patient is an unstable diabetic. Remember, the kidneys will work to relieve the acidosis, but it may not be enough to fully compensate such as in the case of aspirin overdose. With salicylate poisoning, initially there is a respiratory alkalosis due to the stimulant effect of aspirin on the respiratory system. However, later ABG’s will show the true danger of salicylate poisoning, in the fact that metabolic acidosis will shortly follow. Metabolic Alkalosis can be caused by many disease conditions as well as by iatrogenic causes. The following are the most frequently seen causes of metabolic alkalosis:
Every nurse should be aware of the great imbalances which might be brought on by suctioning of any kind. Especially long-term nasogastric suctioning can induce fluid and electrolyte imbalances and can lead to alkalosis. A common cause of alkalosis is hyperventilation. This respiratory condition can lead to metabolic alkalosis especially if another of the above disorders is present. One of the first symptoms seen in these cases is dizziness. Other symptoms of increased alkalemia include numbness and tingling in extremities, weakness, twitching of the muscles, and some arrhythmias may be seen. Metabolic Acid-Base Disorders Metabolic Acidosis: Findings:
Possible Causes:
Signs and Symptoms:
Metabolic Alkalosis: Findings:
Possible Causes:
Signs and Symptoms:
Oxygenation Status In the previous section, acid-base balance concepts were presented. Those simple respiratory and metabolic disease conditions can be determined by analysis of the results of the ABG. We also discussed the many clinical applications of this knowledge. Next, we will present the oxygenation concepts involved with interpretation of the ABG. Oxygen as a gas in our atmosphere is in the concentration of about 21%. It is important to know that the patient was breathing room air when the ABG sample was obtained. As with all gases, oxygen is also measure in its partial pressure. Partial pressure of a gas refers to the pressure a gas exerts as a result of its molecular activity in a mixture of gases. The lab results of the ABS’s are reported as percentages and partial pressures of these gases. For our purposes as nurses, these percentages and partial pressures should only be used as a comparison figure to the norm when interpreting the results. As an example, the normal PO2 (partial pressure of oxygen) is 80 – 100 mmHg. All this should really mean to us is that in arterial blood, 80 to 100 mmHg represents the “amount” of oxygen that is dissolved in each 100 ml of the arterial blood. If a patient’s PO2 results are 70, then we know there is an insufficient amount of dissolved oxygen present. Clinically, there can be many different reasons for this. The patient may be anemic, or may have decreased respirations, or may have pneumonia. All or any of these conditions may lead to low PO2. Oxygen Content of the Blood: Another term with which nurses should be familiar is FIO2. This term refers to fractional inspired oxygen (FIO2). If a patient is breathing other than 21% room air, the FIO2 is said to be higher or at a greater percentage. In some cases, ABS’s are analyzed simply for the results of the oxygen content. Perhaps it might already be known that the patient does not have an acid-base imbalance, but the physician is more interested in the amount of oxygen in the blood. Remember that many COPD patients will almost always have a slight imbalance in the pH of the blood due to a chronically high CO2 level. In these cases the PO2 is critically important for diagnosis. Oxygen Saturation of the Blood: Next we will present saturation of hemoglobin in determining ABG results. The SO2 value is defined as the extent to which oxygen saturates the hemoglobin molecules in the RBC’s. It is expressed in a percentage, compared to the full potential of the blood to be saturated. Therefore, at full saturation the normal SO2 is 95% to 100%. As you can then see, the SO2 and PO2 are directly related to each other. As one increases, so does the other, usually. This holds true in the upper level numbers. However, when the relationship between those two numbers changes, it also indicates that saturation is affected by other factors not just the amount of oxygen present. Remember that oxygen is present in the blood in two forms. Oxygen is dissolved in the blood and oxygen is combined with hemoglobin. The solubility of oxygen depends upon the pressure of oxygen and its solubility as a gas. Oxygen dissolved in the blood represents only a very small part of the total oxygen in the blood. Most oxygen is carried on the hemoglobin. Arterial oxygen pressure values (PaO2) are used to calculate the hemoglobin saturation. These values are also used to estimate the availability of oxygen for the vital organs of the body. The PaO2 is also used with the PaCO2, arterial carbon dioxide pressure, can be used to estimate the alveolar-arterial oxygen gradient (Aagradient). Calculation of the Aagradient serves as an index of lung effectiveness in gas exchange. The wider the difference, the greater the severity of the lung dysfunction. As an example, even if the PO2 reaches as low as 50 to 60 mmHg, the oxygen saturation can remain at 85% - 90%. That is an indication that even though the oxygen levels are quite low, the saturation will be nearly normal. Clinically, this means that the patient has very little oxygen in reserve. The patient may seem quite normal while at rest, but even a slight exertion will be too much to handle and will probably cause a crisis. Again, this is due to the ability of the hemoglobin to remain saturated at relatively high levels, even though there is actually a reduced amount of oxygen in the blood. (For instance, in anemia, where there is a reduced number of cells and hemoglobin, but the cells that are present, are fully saturated.) By this time, the clinical ramifications should be much clearer. A person who has a respiratory disease has the double danger of low oxygen levels, but also high CO2 levels. Now we see how these two problems can lead not only to an oxygen problem, but also an acid-base problem. We will discuss this further in the next section. Pick an ABG result which indicates hypoxia:
Answer: A & B are hypoxic Compensation We have seen how imbalance in the levels of CO2 and H2O3 can disturb the blood pH. However, the body has mechanisms to counteract these imbalances. Compensation is the process of the body’s response to these imbalances, and tries to bring the pH back to normal. If there is a hypo- or hyperventilation causing a rise or fall in the CO2 levels, the pH will also change. The response of the kidneys would be to conserve or excrete bicarbonate, in order to get the pH back to normal. As an example: a patient is hyperventilating, CO2 is “blown off” thus causing lowered acid levels and alkalosis. The kidneys respond by excret8ng HCO3, to try to restore the normal pH. The ABG’s might be: pH 7.45 CO2 36 HCO3 22 As you see, the pH is high normal, indicating that the patient is borderline alkalotic. The low normal is trying to compensate. Another ABG will be needed soon to see if the patient has stabilized or if they are now in full blown alkalosis. If it was recognized that the patient was in compensation, the patient would be watched very carefully and probably have frequent ABG determinations to see if they were able to handle the mild hyperventilation which lead to the alkalosis. As another example, if we are dealing with a serious metabolic problem, the condition can be much more unstable. For example, with renal failure, the kidneys will not be able to excrete even normal amounts of HCO3. This renal failure will cause alkalosis as bicarbonate builds up in the blood. The body’s initial response will be hypoventilation, in an effort to build up CO2 and thus neutralize the bicarb with acid. The ABG’s might be: pH 7.45 CO2 45 HCO3 25 You can see that the patient is in compensation now, but if the kidneys continue to fail, the situation will become worse, rapidly. Compensation is a delicate situation. The patient can easily go into acidosis or alkalosis with little or no reserve power to fight the situation. Also, compensatory situations can last for only a short time. When the lungs or the kidneys respond to a pH change, they have limits to what they can do to correct the situation. If the person is already sick, and then they also develop a pH disturbance, they are probably in serious trouble. The lungs and the kidneys will only be able to compensate for a short time, due to low body reserves. In completing our discussion on compensation, we also have to remember the patient. He/she will need to be treated as soon as possible. Since the body’s own defense mechanism will last just a short time, the nurse must look for and accurately report symptoms. The susceptible patient must be identified and observed for life-threatening complications in the acid-base balance. However, do not forget the patient’s oxygenation status. Up to this point, we have primarily been concerned with Ph of the blood. We should also remember that changes in the acid-base balance may also effect the oxygen content. In cases of compensation, the patient’s respiratory status may be severely compromised. For example, if the patient begins to hyperventilate, it may be due to the primary cause of reduced CO2 in the blood. However, hypoventilation may still occur in a person who is going into metabolic alkalosis. In that case, the patient may be severely hypoxic and needs to hyperventilate, but the overpowering effect of alkalosis still causes the patient to slow respirations instead of increase them. Therefore, the patient may show signs of hypoxia (cyanosis, lethargy, etc.), but they may still be unable to breathe on their own due to the pH problem which effects the respiratory center in the brain. Clinically, the patient looks terrible, and cannot breathe well. In fact, the breathing may become erratic. First there may be hyperventilation which changes rapidly to hypoventilation, the patient may experience long periods of eupnoea, even though they may actually be hypoxic and in alkalosis. This is why nurses must also be aware of the delicate situation the compensation creates. The nurse should:
Analyzing the ABG This section is a guide to analysis of the ABG. Follow the steps as indicated in order to best interpret the results. Step 1. Examine pH
Step 2. Examine CO2
Step 3 Examine HCO3
Step 4 Check PO2 Levels
Step 5 Check Signs/Symptoms of Patient
Patient 1 pH 7.45 CO2 32 HCO3 23 Identify:
Answers: a. respiratory alkalosis; b. yes because HCO3 is less than 24; c. possible hyperventilation Possible causes: hyperventilation, respiratory stimulation, gram-negative bacteremia Part B Use this guide to analyze ABG’s if the patient’s primary diagnosis is hypoxia or any condition where o2 may be compromised. Step 1 Examine PO2
*may require immediate intervention, as in obstructed airway, COPD, or if on a ventilator Step 2 Examine pH
Step 3 Examine Patient Symptoms If you have checked all of the above steps and they are within normal limits, then your patient is either in compensation or is adequately ventilated. If ABG’s are normal, but the patient still has symptoms of hypoxia, then repeat ABG’s in a short time. Then the problem should be apparent. |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||