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Four-step method of interpreting arterial blood gas analysis

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Patients being cared for in general clinical areas are becoming sicker (McGloin et al, 1999) and sicker patients require more complex care and treatment (Department of Health, 2002; DoH, 2000). As a consequence the accurate and timely interpretation of test results has become an essential skill for nurses to function effectively (Muir et al, 2002). However, many nurses find acid-base balance confusing and view interpreting arterial blood gas (ABG) results as outside of the scope of their practice.

VOL: 101, ISSUE: 01, PAGE NO: 42

Kate Allen, MSc, PGCE, RGN, is lecturer, critical care nursing, School of Nursing and Midwifery, University of East Anglia, Norwich

Due to the increase in patient dependency it is no longer unusual for nurses to be caring for patients who need frequent ABG analysis. Nurses are often the first members of the health care team to see ABG results and an understanding of their significance and the ability to decide when medical staff need to be informed is important (Woodrow, 2004). However, acid-base balance and ABG analysis are complex concepts, requiring a great deal of study to master. For most health care professionals, nurses included, such in-depth knowledge is not necessary to enable the level of interpretation required. By following a simplified, systematic four-step approach it is possible for all nurses to correctly interpret ABGs and so gain vital information about their patients’ condition.

Why monitor arterial blood gas?

ABG analysis is a diagnostic tool that allows the objective evaluation of a patient’s oxygenation, ventilation and acid-base balance (Coombs, 2001). The results from an ABG will indicate how well a patient’s respiratory system is working. However, ABGs can offer more than just information on the respiratory system - they also indicate how well a patient’s kidneys and other internal organs (the metabolic system) are functioning. Although all of the data in an ABG analysis can be useful, it is possible to accurately interpret the results without considering all of the values. It is only essential to consider a maximum of six values:

- Oxygen concentration (pO2);

- Oxygen saturation (SaO2);

- Bicarbonate ion concentration (HCO3);

- Base excess;

- Carbon dioxide concentration (pCO2);

- Hydrogen ion concentration (pH).

Oxygen concentration and saturation

In the UK pO2 is a pressure measured in kilopascals (kPa) and this indicates whether the patient has enough oxygen in their blood. The normally accepted range for pO2 is 10-13kPa (Muir et al, 2002), but the exact value will depend upon age and history. For example a 20 year old may have a normal pO2 of 12.5-13.0kPa while a 65 year old may have a normal pO2 of 10.8kPa (Resuscitation Council UK, 2001).

It is also necessary to consider this value in respect of any previous history, as a patient with chronic lung disease may be perfectly able to function with a low pO2 level (Parsons and Heffner, 2002). A pO2 of less than 8kPa is frequently the level required for a low oxygen level (hypoxia) diagnosis (British Thoracic Society, 2002). In clinical practice, however, unless there is evidence to suggest otherwise, 10kPa should be the lowest pO2 level accepted before others are alerted.

Saturation (SaO2) measures how well the haemoglobin in the blood is saturated. The SaO2 value derived from a blood gas is very similar to the SpO2 value gained from pulse oximetry. The only difference is that in a blood gas we are measuring the saturation of arterial blood (SaO2) and in pulse oximetry we are measuring the saturation of peripheral capillary blood (SpO2). In practice the differences between these are negligible (Woodrow, 2004) with the normally accepted range being 95-100 per cent (Esmond, 2001). This value may need to be reconsidered in view of a patient’s history and age. As these values essentially measure the same thing it is not necessary to consider both. Accurate ABG analysis can be achieved by considering the pO2 level alone.

Bicarbonate ion level and base excess

In simple terms the bicarbonate ion (HCO3) concentration and base excess (BE) provide information about how much alkali there is in the blood. This will indicate how well the patient’s metabolic system is working, which relates closely to kidney function (Hutto Faria, 1997).

HCO3 and BE are referred to as the metabolic parameter. The normal range for HCO3 is 22-26mEq/L and for base excess (BE) it is -2 to +2 (Woodrow, 2004). As both of these values essentially measure the same thing it is not necessary to consider both.

Accurate ABG analysis can be achieved by considering the HCO3 level alone. If pO2 alone is used to tell us about oxygenation and HCO3 to tell us about the functioning of the metabolic system, we only need to consider four values in order to accurately interpret any ABG.

Carbon dioxide concentration

The carbon dioxide concentration (pCO2) provides information concerning ventilation and reveals whether the respiratory system is working effectively. Because of this the pCO2 is often called the respiratory parameter. The pCO2 is also measured in kPa and indicates how much carbon dioxide a patient has in their blood. The normal range is 4.5-6.0kPa (Cornock, 1996).

Carbon dioxide results from all metabolic processes in the body. For example, when you wiggle your toes carbon dioxide will be produced. In order to be removed from the body it is transported to the lungs and then exhaled. It is transported to the lungs in a plasma solution in the form of a chemical called carbonic acid (H2CO3). This is an acidic solution, so if the patient has too much or too little carbon dioxide it will become difficult for them to maintain the correct blood pH.

Hydrogen ion concentration

The hydrogen ion concentration (pH) provides information on acid-base balance. This relates to how much acid or alkali a patient has in their blood. The pH scale uses values from one to 14 as a way of indicating whether a solution is acidic, neutral or alkaline (Fig 1). The pH scale indicates the concentration of hydrogen ions. Lots of hydrogen ions will make a solution acidic and the solution will have a low pH value. A solution that has few hydrogen ions will be alkaline and will have a high pH value.

For the body to work effectively it needs the pH of arterial blood to be 7.4. Only small variations of 0.05 can be tolerated without causing ill effects. So within an arterial blood gas the normal range for pH should be 7.35-7.45 (Guyton and Hall, 2000). In the simplest terms, if the pH is within the range of 7.35-7.45 the acids and the alkalis within the blood are correctly balanced. Thus, the acid-alkali balance is being measured, but this is not referred to as a patient’s acid-alkali balance. The term ‘base’ is used to describe an alkali and the term acid-base balance is used.

Acid-base balance

As small changes in pH levels are life-threatening, mechanisms for maintaining acid-base balance are vital (Box 1, p43). Although the pH scale measures 1.0-14.0, a blood pH of <7.0 or >8.0 makes a patient’s survival unlikely (Woodrow, 2004). Therefore, the body relies on a range of chemicals to counteract dramatic changes in blood pH - buffers. These act like chemical sponges, clearing up the problems of too much or too little acid or alkali.

Buffers are commonly pairs of chemicals, for example, H2CO3 and sodium bicarbonate (NaHCO3). The role of H2CO3 in carbon dioxide transport and its link to the respiratory system have already been discussed. The other chemical in the pair is NaHCO3, which is linked to the metabolic system. These two chemicals are normally combined in a 1:20 ratio, (H2CO3 to NaHCO3), keeping the pH within the normal range. To maintain this 1:20 ratio, when H2CO3 levels increase, so do NaHCO3 levels and when H2CO3 levels decrease, so do NaHCO3 levels.

As the body always attempts to maintain a normal state by homeostasis (Clancy and McVicar, 2002), if the pH level becomes abnormal, the body will attempt to return to normal by compensation. In compensation, the system not experiencing problems will attempt to correct the 1:20 ratio, thus returning the pH to normal.

Respiratory imbalances

H2CO3 is unique in its ability to be controlled by the respiratory system. If there is too much H2CO3 it can be broken down and the lungs can expel the extra carbon dioxide. If there is not enough H2CO3 the respiratory rate decreases and the lungs retain carbon dioxide. This then combines with water to form more H2CO3.

As long as a patient has healthy lungs, these processes will maintain the normal acid-base balance. However, if the lungs are unable to exhale carbon dioxide adequately, such as in emphysema or respiratory arrest, the level of carbon dioxide and H2CO3 will increase. This causes the blood pH to decrease and the patient develops respiratory acidosis - an acid-base imbalance. The ABG result would indicate a respiratory problem as the carbon dioxide level would have changed but the sodium bicarbonate level would have remained normal. Due to the increase in carbon dioxide and therefore H2CO3 the 1:20 ratio no longer exists. So NaHCO3, the metabolic buffer, needs to increase in order to rebalance the scales.

By this process the metabolic system is trying to compensate for the problem in the respiratory system. As long as the patient’s kidneys are healthy they should be able to adjust the level of NaHCO3 and the patient’s pH will return to the normal range. However, if too much carbon dioxide was expelled, for example if the patient was taking lots of rapid, shallow breaths, the level of carbon dioxide in the blood would decrease. This would result in the amount of H2CO3 falling and the pH of the blood rising. The patient would have respiratory alkalosis, another acid-base imbalance. The ABG result would show that it was a respiratory problem as the carbon dioxide level would have changed but the NaHCO3 level would have remained normal. However, as long as the patient’s kidneys were healthy, they would try to compensate by excreting NaHCO3. The 1:20 ratio would be regained and the patient’s pH would have returned to the normal range.

Metabolic imbalances

As previously described the kidneys control the metabolic buffer sodium bicarbonate (NaHCO3). Metabolic control of excess acid and alkali is more complicated than respiratory control and relies upon a variety of mechanisms, including:

- Loss of acid in urine;

- Production and reabsorption of buffers by the liver, kidneys and gut;

- Reabsorption of alkali in the kidneys;

- Production of acid in cells/stomach (Woodrow, 2004);

- Adequate nutrition and hydration.

If there is a problem in the metabolic system causing an acid-base imbalance the respiratory system will try to compensate. For example, if a patient has diabetic ketoacidosis they will have lost bicarbonate. This would cause an imbalance as they would not have enough sodium bicarbonate and their blood pH would decrease. The patient would have a metabolic acidosis. The results would detect that it was a metabolic problem because the bicarbonate level would have changed but the carbon dioxide level would remain normal.

To regain balance the respiratory system would compensate by increasing the respiratory rate and removing more carbon dioxide. This would reduce the amount of carbonic acid and resume the 1:20 ratio. If the metabolic system gained bicarbonate, this would result in metabolic alkalosis.

For example, if a patient took too many of some types of indigestion tablets (essentially bicarbonate) the bicarbonate level would rise. ABG results would detect it was a metabolic problem because the bicarbonate level would have changed but the carbon dioxide level would still be normal. In order to regain balance the respiratory system would compensate by decreasing the respiratory rate - holding on to more carbon dioxide. This would increase the amount of carbonic acid and reinstate the 1:20 ratio.

If the lungs compensate for a metabolic abnormality, compensation occurs within hours, but the kidneys take 2-4 days to compensate for respiratory abnormality. Any acute respiratory problem will always be uncompensated, as the kidneys would not have had time to compensate.

The four-step ABG analysis

As long as the normal values (Box 2) are remembered and the four-step system is carefully followed, it will be possible for ABGs to be accurately interpreted:

- Examine pO2 and SaO2 to determine oxygen status. If pO2 and SaO2 values are decreased, the patient has hypoxaemia. Normal or slightly elevated pO2 and SaO2 levels indicate that the patient is well oxygenated;

- Note the pH value to determine the presence of acidosis or alkalosis. Acidosis is indicated by a pH of <7.35 and alkalosis by a pH >7.45;

- Study the pCO2 and HCO3 values. A respiratory irregularity exists if the pCO2 value is abnormal and the HCO3/BE value is normal. Conversely, an abnormal HCO3/BE value and a normal pCO2 indicate a metabolic irregularity.

For many blood gases the fourth step is not needed, but if the pCO2 and HCO3/BE are both abnormal it is necessary to continue;

- Is compensation of any type taking place? In all cases of compensation the respiratory and metabolic parameters will be moving in differing directions. Is compensation partial? Are the pH, and the HCO3 and pCO2 parameters all abnormal? If so, whichever parameter would produce the pH is the primary disorder.

So, if pH is acidic (low), HCO3 is acidic (low) and pCO2 is alkalotic (low), the result would be partially compensated metabolic acidosis. This would be partially compensated as the pH had not been returned to the normal range. Is compensation complete? Is the pH in normal range, but both the HCO3 and pCO2 parameters are abnormal? Reconsider the pH, using 7.4 as the only normal value. Is the pH on the acid (low) or alkali (high) side? Once again, whichever parameter would produce the pH is the primary disorder.

Conclusion

Arterial blood gas analysis is valuable as a diagnostic tool as it enables objective evaluation of a patient’s oxygenation, ventilation and acid-base balance. Such information demonstrates how well a patient’s respiratory and metabolic systems are working. It is clear that such information has the potential to be invaluable in the treatment of a wide range of patients.

Within the current health care climate patients being nursed in all clinical areas are becoming sicker. It is not unusual for a nurse to care for patients who need frequent arterial blood gas tests, so it has become necessary for nurses to interpret the results of these tests. This will enable medical staff to be rapidly alerted to any potential problems and care to be tailored to the exact needs of the patient. The simple four-step approach described in this article has been specifically designed to enable all nurses to accurately and quickly interpret results of blood gas analysis, a skill that will improve patient care.

LEARNING OBJECTIVES

Each week Nursing Times publishes a guided learning article with reflection points to help you with your CPD. After reading the article you should be able to: - Understand why arterial blood gas needs to be monitored; - Be familiar with the different gases involved; - Identify the six values needed to accurately interpret the results; - Understand the different imbalances that the analysis can identify; - Be familiar with four-step arterial blood gas analysis.

Guided reflection

Use the following points to write a reflection for your PREP portfolio: - Describe why you read this article and how it is relevant to your work; - Summarise the main points the article makes about arterial blood gas analysis; - Identify a new piece of knowledge about arterial blood gas analysis that you have learnt from this article; - Consider how you will use this information in your practice; - How will you follow up what you have learnt?

KEY POINTS

- Lots of pCO2 makes a patient acidic - Not enough pCO2 makes a patient alkalotic - Lots of HCO3 makes a patient alkalotic - Not enough HCO3 makes a patient acidic - If the level of pCO2 is causing the problem - it is respiratory in nature - If the level of HCO3 is causing the problem - it is metabolic in nature

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