Accuracy Check and Metabolic Acid-Base Indices

Of the three variables in the H-H (Henderson-Hasselbalch) equation, any one obviously can be calculated if the other two are known.

Charles B. Spearman ⁵⁹¹

The problem in acid-base balance has been quantitation of the metabolic, or nonrespiratory, component, as HCO₃⁻ ion concentration is strongly dependent on the PCO₂.

John W. Severinghaus ⁵⁸⁸

ACCURACY CHECK

The novice in blood gas application must master Chapter 2 before addressing the content in this chapter. Indeed, Chapter 5 deals with the less frequent and more subtle technical nuances of blood gas classification and analysis and may lead to confusion if routine blood gas classification is not fully understood. This chapter is intended for use when the clinician is faced with some real or perceived inconsistency in the reported information.

From an educational perspective, some instructors or clinicians may prefer to skip this chapter until a solid foundation of clinical blood gas interpretation is assured. Again, this chapter addresses primarily the recognition of “exceptions” to normal blood gas classification and interpretation and may be more suitable as supplementary reading, reference, or advanced study.

Fortunately, the types of technical errors discussed herein are less common than in the past. In most state-of-the-art blood gas laboratories, blood gas results are directly printed to a report from the blood gas measurement device. These direct printouts preclude the possibility of human errors in transcription or oral reporting. The data on direct printouts can generally be assumed to be accurate assuming controls and safeguards discussed in previous chapters are adhered to. Notwithstanding, occasional errors still may occur and result in inappropriate diagnosis or treatment if they are not recognized.

The present chapter is included in this text for completeness, comprehensiveness, and critical analysis. Literal life-and-death decisions are made based on blood gas data. Errors in the diagnosis or management of acid-base or oxygenation status may have dire consequences. Therefore, individuals who are responsible for these decisions must have a thorough understanding of each index and the normal inter-relationships between them. One must be able to detect inaccurate data, or explain and interpret seemingly inconsistent information.

On Call Case 5-1 ABGs and Critical Thinking

You are the only person available to care for this patient. You must assess the patient/situation and act accordingly.

A 55-year-old man in the burn unit suffers a cardiac arrest. Blood gases drawn during the arrest are called to the floor and reported as shown. Due to the negative base excess reported, the patient is given two ampules of sodium bicarbonate.

ARTERIAL BLOOD GASES

SaO₂	91%
pH	7.52
PaCO₂	47 mm Hg
PaO₂	63 mm Hg
[BE]	− 11 mEq/L

ASSESSMENT

ABNORMALITIES: List abnormal data and other noteworthy information. Classify ABG.

Explanation: List possible diseases, pathology, or other situations which may have led to this patient’s condition.

Evaluation: Suggest additional data which would be useful in helping understand the situation or in making diagnosis.

INTERVENTION

Importance: Prioritize concern(s) of treatment in order of urgency and/or seriousness as you see the overall situation.

Thus, at some point during the assessment of blood gas data, the clinician should briefly pause and consider the plausibility, consistency, and harmony of the various recorded data. Assessment of the consistency of the reported measurements is recommended in comprehensive and systematic acid-base assessment by most experts.¹⁷⁶ To ensure accuracy, blood gas values should be evaluated for internal consistency and external congruity.

Internal Consistency

Techniques for Evaluating Internal Consistency

Most often, gross inspection of blood gases facilitates the detection of internal inconsistencies. Conceptually, acidemia (i.e., pH < 7.35) cannot be present in the absence of a causative acidosis (i.e., PaCO₂ > 45 mm Hg and/or [HCO₃] < 22 mEq/L). Likewise, alkalemia cannot occur without alkalosis. Another type of gross inconsistency might be the presence of a normal pH with concurrent respiratory and metabolic acidosis. Here again, this combination of circumstances is impossible. Once the basic relationship between acidosis and acidemia is understood, most gross errors in blood gas data are easily and readily recognized.

Occasionally, technical error is less obvious. In these cases, the blood gas values generally make sense but something doesn’t seem exactly right. Example 5-1 may serve as an example.

Example 5-1

pH	7.60
PaCO₂	30 mm Hg
[HCO₃]	23 mEq/L

Example 5-1 is not internally consistent. The pH is too high given a PaCO₂ of 30 mm Hg and essentially normal metabolic status. Actually, the pH must be approximately 7.50. Failure to detect this inconsistency could have undesirable diagnostic or therapeutic consequences. A pH of 7.60 indicates severe alkalemia and should be cause for serious concern. A pH of 7.50 is common in the intensive care unit and most often requires no intervention.

The following discussion explores four methods that can be used to assess internal consistency when errors may be more subtle. These methods are indirect metabolic assessment, the rule of eights, the modified Henderson equation, and an acid-base map. All of these methods make use of the principle that the three acid-base components (i.e., pH, PaCO₂, and [HCO₃] or alternatively [BE]) are interrelated such that if two components are known, the third component can be deduced.

Indirect Metabolic Assessment

The metabolic tendency (e.g., presence of metabolic acidosis or metabolic alkalosis) can actually be determined without ever looking at a metabolic index. Indirect metabolic assessment is based on the principle that any change in pH must be a result of either the metabolic or the respiratory component because these components are the two sole factors that determine pH. If pH has changed and it is not of respiratory origin, it must be due to a metabolic acid-base change.

Acute PaCO₂–pH Relationship

To simplify these relationships for a moment, assume that all metabolic factors are normal and constant. Under these circumstances, any deviation of pH from normal must be a result of a change in respiratory status. If it was known exactly how much a given acute change in PaCO₂ would alter pH, then the precise pH that would be present with a given PaCO₂ could be predicted.

Assuming normal, constant metabolic status, the amount that pH will change in response to a given PaCO₂ change is constant and is termed the acute PaCO₂–pH relationship. The acute PaCO₂–pH relationship for both increases or decreases in PaCO₂ is shown in Table 5-1. A different pH change factor is necessary for a decrease in PaCO₂ than for an increase in PaCO₂ because of the logarithmic nature of these relationships.

Table 5-1

Acute PaCO₂–pH Relationship

PaCO₂ Change	pH Change
DECREASE	INCREASE
1 mm Hg	0.01
10 mm Hg	0.10
INCREASE	DECREASE
1 mm Hg	0.006
10 mm Hg	0.06

Expected pH

Using a pH of 7.40 and a PaCO₂ of 40 mm Hg as our baseline, the expected pH that would result from a specified change in PaCO₂ could thus be calculated. For example, if PaCO₂ decreases 10 mm Hg (i.e., from 40 to 30 mmHg), pH increases 0.10 or from 7.40 to 7.50. Conversely, if PaCO₂ increased acutely to 50 mm Hg, the expected pH (assuming normal metabolic tendency) would be 7.34.

Important landmarks in the relationship between PaCO₂ and pH are shown in Table 5-2. It may be useful for the clinician to memorize these relationships. When a more precise calculation is desired, specific calculation of the expected pH for any PaCO₂ can also be accomplished by application of the formulas shown in Box 5-1.

Box 5-1 Calculation of Expected pH

IN HYPOCARBIA

Expected pH = 7.40 + (40 mm Hg -PaCO₂) 0.01

IN HYPERCARBIA

Expected pH = 7.40 -(PaCO₂ -40 mm Hg) 0.006

Table 5-2

Important Landmarks of Acute PaCO₂–pH Relationship

PaCO₂	pH
20	7.60
25	7.55
30	7.50
35	7.45
40	7.40
50	7.34
60	7.28
70	7.22
80	7.16
90	7.10

Indirect Metabolic Status

After the expected pH has been calculated for a given PaCO₂, it can be compared with the actual pH on the blood gas report as a means of “indirect metabolic assessment.” Because the acute PaCO₂–pH relationship holds true only when metabolic status is normal, if actual pH is equal to the expected pH it can be concluded that metabolic status is, indeed, unchanged and normal.

On the other hand, it is likewise true that if the actual pH is not equal to the expected pH, the metabolic status cannot be normal. This is intuitively correct because any alteration in pH that is not of respiratory origin must be metabolic. The clinician should understand that indirect metabolic assessment is only an approximation (albeit a very good one) and that very small differences between actual and expected pH (e.g., ±0.02) can be attributed to slight measurement error. Blood gas electrode error alone may result in discrepancies of at least ±0.01.

For this reason, a ±0.03 comparison factor is recommended. Indirect metabolic assessment using this factor correlates very well with metabolic assessment using the base excess of extracellular fluid [BE]ecf that is described later in this chapter.

The possible outcomes of indirect metabolic assessment based on comparison of actual and expected pH are defined in Table 5-3. When the actual pH is equal to the expected pH ±0.03, the metabolic status must be normal. When the actual pH is significantly more acidic than expected (i.e., more than 0.03 pH units lower), a metabolic acidosis must be present. Conversely, when actual pH is significantly more alkaline (more than 0.03 pH units) than expected, a metabolic alkalosis (nonrespiratory condition tending to cause alkalemia) must be present. These conclusions closely parallel metabolic diagnosis that is made directly with the [BE]ecf to be discussed later in this chapter.

Table 5-3

Indirect Metabolic Assessment

Actual pH–Expected pH Relationship	Indirect Metabolic Status
Actual pH = expected pH ± 0.03	Normal metabolic status
Actual pH > expected pH + 0.03	Metabolic alkalosis
Actual pH < expected pH − 0.03	Metabolic acidosis

The values in Example 5-1 presented earlier are used to show the application of indirect metabolic assessment to evaluate internal consistency. Based on the acute PaCO₂–pH relationship, the expected pH for a PaCO₂ of 30 mm Hg is 7.50. Therefore, in a patient with this PaCO₂ level and normal metabolic status, one would expect to find an actual pH within ±0.03 of 7.50.

The patient’s actual pH in Example 5-1 is 7.60, which is much higher than expected. Therefore, the patient must also have a concomitant metabolic alkalosis based on the acute PaCO₂–pH relationship. The finding of a normal metabolic index (i.e., [HCO₃] 23 mEq/L) is not consistent with the indirect finding. The problem in this case could have been a transcription error. The patient’s actual pH should have been 7.50. A pH of 7.50 would make these data internally consistent.

In summary, metabolic status may be accurately assessed indirectly without ever seeing a metabolic index. In fact, using indirect metabolic assessment, a complete blood gas acid-base classification can also be made based on the PaCO₂ and pH alone in the event that a metabolic index is, for some reason, unavailable. This technique is likewise a useful tool in attempting to validate or invalidate the internal consistency of a questionable blood gas report.

A lack of internal consistency does not indicate where an error has occurred. Nevertheless, it is clear evidence that an error is present, and the clinician or laboratory diagnostician should be alerted with regard to the need for further investigation and clarification.

Rule of Eights

Table 5-4

Rule of Eights (Factor × PaCO₂) = Predicted bicarbonate

pH	Factor
7.60	8/8
7.50	6/8
7.40	5/8
7.30	4/8
7.20	2.5/8
7.10	2/8

Modified Henderson Equation

The Henderson equation to be described later in Chapter 8 can be modified to relate [H⁺] in nanequivalents per liter (instead of pH) to PaCO₂ and [HCO₃] as shown in Equation 5-1. Thus, if any two of these three variables are known the third variable can be calculated.

[H+]=24×PaCO2[HCO3] Equation 5-1

Equation 5-1

To apply this formula, however, one must be able to convert pH units to hydrogen ion concentration in nanequivalents per liter (nEq/L). To get an idea of how minute this unit is, a nanequivalent is one millionth of an equivalent. A milliequivalent is equal to 10⁻³ equivalents. A microequivalent is equal to 10⁻⁶ equivalents, and a nanequivalent is equal to 10⁻⁹ equivalents.

There is a near-linear relationship between [H⁺] in nEq/L and pH over the pH range (7.20 to 7.50) shown in Table 5-5. It can also be seen that this linear relationship begins to deteriorate quickly beyond this range, especially with acidemia.

Table 5-5

Relationship Between pH and [H⁺]

pH	[H⁺] nEq/L
7.80	16
7.70	20
7.60	25
7.55	28
7.50	32
7.45	35
7.40	40
7.35	45
7.30	50
7.25	56
7.20	63
7.15	71
7.10	79
7.00	100
6.90	126
6.80	159

A pH of 7.40 is equivalent to a [H⁺] of 40 nEq/L. As a general rule, for each 0.01 change in pH within the range of 7.20 to 7.50, there is a 1 nEq/L inverse change in [H⁺]. Thus, a pH of 7.50 is equal to a [H⁺] of 30 nEq/L, and a pH of 7.30 is equal to a [H⁺] of 50 nEq/L.

By converting [H⁺] to pH, Equation 5-1 can be used to check internal consistency of questionable blood gases. This equation is especially useful because it can be used to determine any of the three variables (i.e., pH, PaCO₂, or [HCO₃]) if the other two variables are known.

With regard to internal consistency, it should also be pointed out that acid-base values can be internally consistent, yet they can still be wrong. For example, if the pH is measured incorrectly, the [HCO₃] is also wrong (although it is internally consistent) because it is calculated in the blood gas machine based on the pH and the PCO₂.

Acid-Base Map

The acid-base map is discussed in detail in Chapter 14 (see Fig. 14-1), which addresses the identification of mixed acid-base disturbances. Using an acid-base map, one can easily plot two acid-base variables on the map and determine the third. When readily available, the acid-base map is probably the easiest and most expedient way to assess internal consistency.

External Congruity

In addition to assessing internal consistency, the clinician should also evaluate reported data from other laboratories and the patient’s general appearance to ensure external congruity. External congruity in this context means ensuring that all laboratory tests and observations are in concert and are harmonious with the blood gas results. A sign of incongruity is when blood gas numbers are not in harmony with the patient’s appearance or other laboratory values. Incongruity is often the first clue regarding incorrect and/or misleading laboratory measurements.

Laboratory to Laboratory Congruity

Calculated Bicarbonate

The plasma bicarbonate can be used to evaluate external congruency because it is most often reported by two different, unrelated laboratories. Plasma bicarbonate is reported routinely on the arterial blood gas report. However, the bicarbonate reported on the blood gas report is not directly measured. Rather, it is a calculated value based on the Henderson-Hasselbalch equation.

Total CO₂

The plasma bicarbonate is also usually reported from the chemistry laboratory with standard electrolytes as total CO₂ ([total CO₂]). Because plasma [HCO₃] comprises approximately 95% of total CO₂, these two measurement may essentially be viewed as being interchangeable.¹⁷⁶ ¹⁷⁷ One caveat is that total CO₂is measured in the chemistry laboratory using venous blood, which is typically 2 to 3 mEq/L higher in bicarbonate. Nevertheless, total CO₂ is, for all practical purposes, an index of plasma bicarbonate. Therefore, the electrolyte report may be used as a crosscheck regarding the accuracy of the [HCO₃] reported on the blood gas report.

Historically, total CO₂ was introduced as a clinical metabolic acid-base index before the routine availability of blood gases. Today, total CO₂ is considered to have only minimal value as an isolated index because it must be interpreted in the context of pH and PaCO₂. Nevertheless, the [HCO₃] can easily be approximated from the total CO₂, and this value can be compared with the blood gas bicarbonate as a gross index of external congruity. As previously stated, total CO₂ should be expected to be slightly higher than bicarbonate from the blood gas report because it is measured from venous blood.

Total CO₂ Components

As discussed in Chapter 8, CO₂ is transported in the blood as bicarbonate, dissolved CO₂, and carbamino-compounds. Bicarbonate and dissolved CO₂ are responsible for almost all of the CO₂ present in the blood plasma. Therefore, total CO₂, usually reported in mEq/L, is presumed to be equal to the sum of CO₂ dissolved in the plasma and plasma bicarbonate, which is shown in Equation 5-2. PaCO₂ may be multiplied by the conversion factor (0.03 mEq/L/mm Hg) to determine the dissolved CO₂ concentration in mEq/L.

[Total CO2]=[dissolvedCO2]+[bicarbonate][Total CO2]=(PaCO2×0.03)+2425.2=1.2+24(mEq/L) Equation 5-2

Equation 5-2

Bicarbonate Calculation from Total CO₂

To calculate the specific [HCO₃], dissolved CO₂ in mEq/L is subtracted from the reported total CO₂ (see Equation 5-3). The difference represents plasma bicarbonate in mEq/L. Because the concentration of dissolved CO₂ in mEq/L is so small (typically 1 to 2 mEq/L) and bicarbonate represents 95% of the total CO₂ value, gross inspection of the total CO₂ provides a reliable estimation of plasma bicarbonate even without this calculation.

[Total CO2]–[dissolvedCO2]=[bicarbonate]25.2–(PaCO2×0.03)+24=[bicarbonate]25.2–1.2=24(mEq/L) Equation 5-3