A guide to regulation of blood gases: part two

Arterial blood gases (ABGs) provide information about a patient’s respiratory and metabolic status. Equilibrium isachieved by a number of mechanisms including the respiratory and renal system. The degree to which blood gas results vary fromnormal limits helps the nurse to understand the effectiveness of organ function and the ability of the body to compensate foracute or chronic changes. To understand the significance of these changes it is necessary to look at the normal values andlimits. ABGs provide information on a series of measured and calculated values (Table 1). Interpretation of arterial blood gas resultsA systematic approach to ABG interpretation is essential (Resuscitation Council, 2004; Jevon and Ewens, 2002;Shoulders-Odom, 2000). The step-by-step approach to ABG analysis devised by Simpson (2004) is described below: Assess oxygenationExamine partial pressure (PaO₂) of oxygen and oxygen saturation (SaO₂). Assess whether the value is normal, high or low. Consider this in the light of any supplementary oxygen that thepatient is receiving (Simpson, 2004). Determine the status of the pHNote the pH value to determine the presence of acidosis or alkalosis. Normal pH is 7.35-7.45. Any value below 7.35indicates acidosis and any value above 7.45 indicates alkalosis. A normal pH indicates one of two scenarios. Either the bloodgas is normal in which case so will be the PaCO₂and HCO₃^-. Alternatively, full compensation is present in which case the PaCO₂and HCO₃^-will be deranged (outside normal values), as one parameter will be compensating for the deranged one (Tortora andDerrickson, 2005; Simpson, 2004). Assess the respiratory component (PaCO₂) PaCO₂values below 4.7kPa indicate a respiratory alkalosis (or respiratory compensation for a metabolic acidosis) andvalues above 6.0kPa indicate a respiratory acidosis (or respiratory compensation for a metabolic alkalosis) (ResuscitationCouncil UK, 2004). If the PaCO₂is deranged it is important to check whether it followsthe same direction (acidosis or alkalosis) as the pH. If so,this is the primary disorder (Simpson, 2004). Assess the metabolic component (HCO₃^-)Values below 22mmol/L indicate metabolic acidosis (or renal compensation for respiratory alkalosis) and values above26mmol/L indicate metabolic alkalosis (or renal compensation for a respiratory acidosis). If the HCO₃^-value is abnormal, check whether it follows the same direction (acidosis or alkalosis) as the pH. If so this is theprimary disorder (Simpson, 2004). Some clinicians prefer to use the base excess (or deficit) instead of the HCO₃^-values. A base excess is defined as the amount of acid required to restore 1L of blood to a normal pH of 7.4. Abase deficit is defined as the amount of alkali required to restore 1L of blood to a normal pH of 7.4 (Woodrow, 2004). Assess for a mixture of disordersA mixture of disorders may occur for two reasons: either a dual pathology is present or one disorder is compensating forthe other. For example, if the PaCO₂and HCO₃^-are both moving in the same direction (either indicating acidosis or alkalosis), a dual pathology must be presentsuch as metabolic and respiratory acidosis. If one parameter is acidic and the other is alkaline, then one parameter iscompensating for the other (Simpson, 2004; Heitz and Horne, 2001; Horne and Derrico, 1999). Assess for compensationThis is the body’s attempt to maintain a normal pH level. The respiratory system controls the carbon dioxide level and therenal system controls the bicarbonate level. The body uses these two systems to oppose each other in order to maintain a normalpH (Morton et al, 2005; Marieb, 2003). Three types of compensation are possible:

• In an uncompensated blood gas the pH is abnormal and either the CO ₂or HCO ₃^-is abnormal. There is no indication that the opposite system has tried to correct for the other (Morton et al, 2005; Pruitt and Jacobs, 2004).
• In a partially compensated blood gas, the pH is abnormal and both the CO ₂and HCO ₃^-are also abnormal. One parameter will be deranged and following the same direction as the pH. This is the primary problem. The third parameter will be moving in the opposite direction in order to compensate for the primary disorder (Morton et al, 2005; Tortora and Derrickson, 2005; Pruitt and Jacobs, 2004) but it will not have changed enough to bring the pH back to the normal limits.
• In a fully compensated state the pH is normal but is usually ‘tending towards’ alkalosis or acidosis (Morton et al, 2005). The two other parameters will be deranged and moving in opposite directions, one compensating for the other. The primary abnormality (metabolic or respiratory) is correlated with the abnormal pH (acidotic or alkalotic). Other clinical information will allow the clinician to interpret which parameter is compensating for which (Simpson, 2004). A typical example of a fully compensated status is a patient with COPD whose ABGs may show fully compensated respiratory acidosis (Pruitt and Jacobs, 2004; Bongard and Sue, 2003; Bourke, 2003).

The main causes of abnormal values are summarised in Tables 2 and 3. Anion gapAnother measurement that may be used when interpreting blood gas results, particularly when the cause of a metabolicacidosis is not clinically obvious, is calculation of the ‘anion gap’ (Heitz and Horne, 2001; Hinds and Watson, 1999). This is calculated as the difference between the sum of the bicarbonate and chloride concentrations and the sum of thesodium and potassium concentrations (Heitz and Horne, 2001; Hinds and Watson, 1999). The value normally ranges from 8-14mmol/Land represents the level of unmeasured anions in extracellular fluid (including sulphates, phosphates, plasma proteins, andorganic acids such as lactates and ketones). Table 1. Normal Values

If metabolic acidosis is due to bicarbonate loss the ‘anion gap’ will be normal. Conversely, a metabolic acidosis willcause an increase in the ‘anion gap’, for example, in ketoacidosis, renal failure, and lactic acidosis (Heitz and Horne, 2001;Hinds and Watson, 1999). Mixed respiratory and metabolic disturbances Examples of mixed acid-base disorders include:

• Diabetic ketoacidosis and vomiting;
• Metabolic acidosis and metabolic alkalosis;
• Pneumonia and COPD;
• Acute and chronic respiratory acidosis;
• Lactic acidosis and respiratory arrest - metabolic acidosis and respiratory acidosis;
• Renal failure and opiate overdose - metabolic acidosis and respiratory acidosis.

A mixed acid-base disturbance occurs when two or more simple acid-base disorders are present at the same time. Someclinical conditions may lead to complex acid-base disturbances and therefore an accurate assessment of the clinical details isvery important before making a diagnosis (Bourke, 2003). When two opposing disorders (alkalosis and acidosis) occursimultaneously, the pH is determined by the predominant disorder. In some cases the pH might be normal in the presence of amixed acid-base disorder (Heitz and Horne, 2001). This article has provided a systematic step-by-step approach in interpreting arterial blood gas results. In the nextarticle in this series, common causes will be discussed as well as treatment for acute acid-base disturbances. Arterial bloodgas interpretation is increasingly becoming an essential part of nursing practice. Therefore understanding the results can helpthe nurse to intervene safely, correctly and confidently and deliver better patient care.  Table 2. Causes of abnormal respiratory values

Table 3. Causes of abnormal metabolic values

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Jevon, P., Ewens, B. (2002) Monitoring the Critically Ill Patient. London: Blackwell Science.

Marieb, E. (2003)Human Anatomy and Physiology. London: Benjamin Cummings.

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Pruitt, W., Jacobs, M. (2004) Interpreting arterial blood gases. Easy as ABC Nursing; 34: 8, 50-53.

Resuscitation Council (2004) Acid-base balance: interpreting arterial blood gases. In: Advanced Life SupportCourse Appendices to the Provider Manual. London: Resuscitation Council.

Shoulders-Odom, B. (2000) Using an algorithm to interpret blood gases. Dimensions of Critical Care Nursing; 19: 1, 36-41.Simpson, H. (2004) Interpretation of arterial blood gases: a clinical guide for nurses. British Journal of Nursing; 13: 9,522-528.Tortora, G.J., Derrickson, B. (2005)Principles of Anatomy and Physiology. New York, NY: John Wiley & Sons.Woodrow, P. (2004) Arterial blood gas analysis. Nursing Standard;
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