Skilled and accurate assessment of arterial blood-gas results leads to successful diagnosis, intervention, and treatment planning for patients.

Skilled and accurate assessment of arterial blood-gas results leads to successful diagnosis, intervention, and treatment planning for patients.

One of the most important skills that an RCP acquires in school is the ability to interpret arterial blood-gas (ABG) testing results. In clinical settings, the RCP is often the best educated, most experienced clinician available to interpret laboratory values associated with both normal blood-gas physiology and acid-base balance, as well as with the variations from these normal values that are associated with pathophysiologic processes. Skilled and accurate assessment of these parameters clearly leads to successful diagnosis, intervention, and treatment planning for the patient, but why would one want to try to predict ABG values?

In order to place an ABG interpretation into proper perspective, it is necessary to have a good idea of how ABG levels should look under various conditions (Table 1, page 60). These predicted values give the clinician a target against which to measure the observed results of analysis of the arterial blood sample. Before attempting prediction, RCPs should first avail themselves of a quick, easy method for interpreting ABG test results.

Included in the assessment are several steps aimed at providing accurate interpretation. Prior to interpreting the results of an ABG analysis, the RCP should be very familiar with Table 1, which provides a list of important information for the RCP, laboratory technician, and physician. Although current blood gas analyzers are highly automated and may provide tentative interpretation, they are not designed to function independently of clinician analysis, interpretation, and control. There are bits of information that are not integrated into the analysis algorithms of blood gas analyzers. This information can be divided into two categories.

The first category deals with what one must know in order to ensure accurate interpretation. Generally, these things involve the conditions under which the ABG sample was obtained; they include information concerning the patient’s fraction of inspired oxygen (Fio2), mechanical ventilation status, temperature, level of positive end-expiratory pressure (PEEP) or continuous positive airway pressure, age, and comorbidities.

The second type of information reflects a more in-depth evaluation of the patient and consists of items that the RCP should consider when interpreting a patient’s ABG results. These data include drug history, hemoglobin (Hgb) concentrations by species, chief complaint, tidal volume (V+), and respiratory rate.


There are many ways of interpreting ABG results. The method offered here is only one of those. Generally, the acid-base states are the most difficult to interpret. The more one practices this skill, the better an interpreter one will become.

To determine the type of acid-base imbalance, the clinician can use a simple (yet accurate) method of interpretation called four-step acid-base analysis. By following the four easy steps, one quickly masters the art of acid-base interpretation. It is important, however, to follow each step carefully, and to follow the steps in order. First, the clinician lists the three values responsible for acid-base balance: pH, Paco2, and bicarbonate. Second, one compares these with normal values and determines whether they are acidic, basic, or normal (writing A, B, or N beside each value). Third, one underlines any A, B, or N notations that are the same; these will usually be the pH and either the Paco2 or the bicarbonate. Fourth, one determines whether the nonunderlined value has moved in the direction opposite that of the circled values. If it has, compensation is occurring; if not, compensation is absent.

In addition, a clinician might wish to know whether the changes noted are acute or chronic in nature. There is a formula available to help one distinguish between the two. The anion gap, while often ignored, can be quite useful in assessing blood-gas changes. The anion gap is the difference between the anions (negatively charged electrolytes) and the cations (positively charged electrolytes) in the blood plasma. The cations are sodium, potassium, calcium, and magnesium. The anions are chlorine, bicarbonate, and phosphate. The anion gap represents the pool of volatile organic chemicals in the body. The organic compounds are created as the result of physiologic processes throughout the body. 

Because these chemicals are either acidic or alkaline in nature, their presence can and does affect the pH independently of the lung-kidney buffer system. These substances are difficult or impossible to analyze in a timely, cost-efficient manner. They do, however, act on the electrolytes, altering their plasma concentrations. Thus, the effect of the organic compounds can be indirectly inferred by studying alterations in electrolyte ion balance (the anion gap). To determine the anion gap, one simply subtracts the sum of chloride, bicarbonate, and phosphate from the sum of sodium, potassium, calcium, and magnesium. In normal subjects, the anion gap should be about 12±2 mEq/L.

Acid-base and blood-gas data, in conjunction with the anion gap, can be used in clinical situations to determine whether a change is acute or chronic. Acute situations are characterized by pH plus a change in one factor (Paco2, bicarbonate, or anion gap). Chronic alterations in blood-gas levels are indicated by pH and change in any two of the same three factors. In determining more precisely how the actual results of an ABG analysis correlate with the clinical picture, the ability to estimate and predict what the various blood-gas values should be is an important diagnostic and therapeutic tool. 


The ability to predict what the patient’s blood-gas levels should be, under given conditions, will be helpful in the interpretation of ABG results, but the making of a prediction often happens automatically, without the use of many specifics or guidelines. All RCPs have seen blood-gas results that do not look right, compared with their subconscious predictions of what the values ought to be. While such intuitive estimates may often be correct, other individuals may intuit other predictions.

Table 1. Things to know before ABG interpretation.
Things you MUST Know
• Fio2
• Patient’s temperature (some controversy here)
• Ventilator status
• Patient’s age
• Comorbidity

Things you SHOULD Know
• Hgb/Hct
• Drugs given
• Fraction type specific Hgb 
• Chief complaint
• Diagnosis
• Respiratory rate
• Vt or MV
• I and O (fluid balance)

Using formulas and guidelines, these predictions can be made more effectively and (perhaps more important) in a repeatable, documentation-friendly fashion. Tables 2 through 4 present several prediction formulae. Table 2 (page 61) examines the effect of changes in Paco2 on pH in both chronic and acute acidosis and alkalosis (respiratory or metabolic). Table 3 (page 61) illustrates how pH can be predicted if the Paco2 is known. Using the predicted pH, one can then employ the measured or observed pH to estimate the pH buffering taking place.

Table 2. Blood gas prediction formulae. 
Respiratory Driven pH Changes:
Acute Acidosis/Alkalosis
pH change = 0.008 units per 1 mm Hg Paco2 change 
Chronic Acidosis
pH change = 0.003 units per 1 mm Hg Paco2 change
Chronic Alkalosis
pH change = 0.0017 units per 1 mm Hg Paco2 change

Metabolic Driven Paco2 Changes:
Metabolic Acidosis 
Paco2 = 1.54 (Hco3) + 8 (+/-2)
Metabolic Alkalosis
Paco2 = 0.7 (Hco3) +20 (+/-1.5)

If the predicted Paco2 is greater than the observed, there is a superimposed respiratory alkalosis.
If the predicted Paco2 is less than the observed, there is a superimposed respiratory acidosis.


Table 3. Respiratory driven pH changes.
To predict pH or Paco2 change:
First, assume that pH = 7.40 and Paco2 = 40 mm Hg
For each acute 10 mm Hg rise of Paco2 the pH will decrease by 0.05 units. 
For each acute 10 mm Hg fall of Paco2 the pH will increase by 0.1 units.

Table 4 (page 62) takes the opposite tack, allowing the RCP to determine predicted pH using the measured Paco2. The method used to predict bicarbonate Hco3 change based on measured Paco2 is also shown in this table. Once more, variance between the real and predicted values tells the clinician that other forces are at work. Using the proper formulae will allow the RCP to hone interpretation skills and provide a more effective, efficient approach to the care of patients. It is known that factors other than physiologic parameters can affect acid-base and blood-gas status.

Table 4. Predicting respiratory pH and Hco3 changes.
To Predict Respiratory pH:
If the Paco2 > 40 mm Hg:
pH = 7.40 – {[(measured Pco2 – 40)/100]/ 2}
If Paco2 < 40 mm Hg:
pH = [(40-measured Paco2)/100] + 7.40

To Predict Hco3 Changes:
To predict Hco3 from Paco2 change:
Hco3 increases by 2 mmol/L for every 10 mm Hg decrease in Paco2
Hco3 decreases 1 mmol/L for every 10 mm Hg increase in Paco2

The last value to predict in the standard acid-base and blood-gas analysis is the Pao2, which depends primarily on barometric pressure (Pbar) and Fio2. One must also consider, however, the effects of water-vapor pressure (47 mm Hg) and of the Paco2. The formula:

predicted Pao2 in mm Hg=((Pbar-47)xFio2)-Paco2
predicts Pao2 with an accuracy that is within 10 mm Hg of the predicted value. 

One factor that can have a profound effect on ABG results, especially those dealing with oxygenation, is the patient’s chronological age. Through the normal aging process, both anatomic dead space and physiologic dead space occur. Emphysema of aging can be present, diffusion across the alveolar-capillary membrane becomes more difficult, and the physical movement of gas may be impaired. For all of these reasons, it is necessary to provide a correction factor relating age to Pao2. For each year of age beyond 60, there is a reduction of 1 mm Hg in Pao2 from a baseline of 90 mm Hg:

predicted Pao2 in mm Hg=90-(age in years-60).

For example, an 83-year-old male breathing room air would have a predicted Pao2 of 67 mm Hg. Of course, illness or exposure to cigarette smoke and other toxic materials can adversely affect the actual Pao2.


There are several equations that will allow practitioners to predict blood-gas and acid-base status based on the patient’s current physiologic status. The ability to determine what ABG values should be under a given set of circumstances (predicted values) is as important as knowing what those values actually are (observed values). Being able to compare the actual and predicted values allows more sensitive judgments about patient status to be made. Noting deviation from the predicted values can alert the clinician to possible conflicts between the clinical situation and the reported ABG values. This can lead to the detection of erroneous results. In addition, more effective strategies for the treatment of disorders revealed by the blood-gas analysis can be implemented. N 

Paul Mathews, PhD, RRT, FCCM, FCCP, is associate professor of respiratory care education, School of Allied Health, University of Kansas, Kansas City. Larry Conway, RRT, is director of respiratory, neurology, and sleep disorder services, North Mississippi Medical Center, Tupelo.