Who Is Balancing: Is It RBC or Acid-Base Status?

T. Rajini Samuel

### Abstract

Hemoglobin is an important intracellular protein buffer present inside the red blood cells (RBC). When the partial pressure of carbon dioxide (pCO2) is increased, it freely diffuses into the RBC where it reacts with water molecules to form carbonic acid which dissociates to form bicarbonate and hydrogen ions by the enzyme carbonic anhydrase. Hydrogen ions liberated in this reaction are buffered by hemoglobin. Oxyhemoglobin is a stronger acid than deoxyhemoglobin. Oxygenation of hemoglobin causes an increase in net titratable hydrogen ion due to the Haldane effect. As the oxygen saturation of hemoglobin (sO2) increases, the base excess is changed in the acidic direction, or as the sO2 decreases, the base excess is changed in alkaline direction. The changes in the level of the enzyme carbonic anhydrase in RBC are related to the changes in pH, pCO2, and bicarbonate levels in the blood. The understanding of the acid-base balance is a challenging task, but at the same time, it has immense clinical value. The relationship of carbonic anhydrase enzyme present inside the RBC in maintaining the acid-base balance to the commonly employed arterial blood gas (ABG) parameters like pH, pCO2 bicarbonate, and base excess may help us for better understanding.

Keywords: acid-base balance, carbonic anhydrase enzyme, oxygen saturation, hemoglobin

#### 1. Introduction

Arterial blood gas (ABG) analysis plays a vital role in the management of intensive care unit patients, especially for critically ill patients, but the interpretation is sometimes a challenging task especially if the acid-base disturbances are complex [1–5]. In ABG analysis, the pH and pCO2 are measured parameters, but bicarbonate concentration is a calculated parameter derived from the modified Henderson equation [2]. Davenport or bicarbonate-pH diagram is a graphical tool representing the relationship between pH, pCO2, and bicarbonate to depict the respiratory and metabolic acid-base disturbances. This Davenport diagram is rarely used in clinical setting [1].

Simple acid-base disorders are very easy to diagnose, but combined acid-base disorders due to either compensatory mechanisms or mixed disorders are often difficult and sometimes confusing. The four acid-base disorders are metabolic acidosis, metabolic alkalosis, respiratory acidosis, and respiratory alkalosis. Simple acid-base disorder is the presence of any of the four disorders with appropriate compensations. Mixed acid-base disorder denotes the presence of more than one primary disturbances which can be suspected from a lesser or greater than expected

compensations. Respiratory disorders are associated with appropriate renal compensatory mechanisms, and similarly metabolic disorders are compensated by respiratory mechanisms [6, 7].

or extracellular base excess is the base excess at hemoglobin concentration of 5 g/dl

cBase B; oxygenated <sup>¼</sup> cBase Bð Þ� ; actual <sup>0</sup>:<sup>2</sup> � ctHb � ð Þ <sup>1</sup> � sO2

cBase Bð Þ¼ ; actual cBase Bð ; oxygenatedÞ þ 0:2 � ctHb � ð Þ 1 � sO2

As the sO2 increases, the term 0.2 � ctHb � (1 � sO2) decreases, so the base excess is changed in the acidotic direction because it is slightly decreased, or as the sO2 decreases, the term 0.2 � ctHb � (1 � sO2) increases, so the base excess is

The correlation between pCO2 and (HCO3 � standard HCO3)/H2CO3 and pCO2 and ratio of (HCO3/standard HCO3) is clearly shown in Figures 1 and 2, respec-

changed in alkaline direction because it is slightly increased [8–10].

tively. From that, it is very clear that as the pCO2 decreases, the ratio of

(HCO3 � standard HCO3)/H2CO3 also decreases and, as the pCO2 increases, the ratio of (HCO3 � standard HCO3)/H2CO3 also increases and, thereafter, the curve flattens. At pCO2 of 40 mmHg, the ratio of (HCO3 � standard HCO3)/H2CO3 is zero because the difference between bicarbonate and standard bicarbonate value is zero (HCO3 � standard HCO3 is zero). In respiratory acidosis (due to hypoventilation), pCO2 retention occurs, and in respiratory alkalosis (due to hyperventilation), the pCO2 value is decreased. The ratio of (HCO3 � standard HCO3)/H2CO3 changes in respiratory disorders and also in metabolic acid-base disturbances associated with respiratory compensations. The ratio of (HCO3 � standard HCO3)/H2CO3 is greater positive for respiratory acidosis and greater negative for respiratory alkalosis [1]. The normal range for standard base excess is �2 mmol/L. If the value is >2 mmol/L, then it denotes metabolic alkalosis, and if the value is <�2 mmol/L, then it denotes metabolic acidosis (base deficit). Using this concept a four-quadrant graphical tool can be constructed for ABG interpretation using standard base excess

Oxyhemoglobin is a stronger acid than deoxyhemoglobin. Oxygenation of hemoglobin causes an increase in net titratable hydrogen ion because hydrogen ions are liberated from the oxygen-linked buffer groups due to the Haldane effect. So, the variation of oxygen saturation of hemoglobin (sO2) influences the base excess

[8–12].

or

Figure 1.

35

Relation between pCO2 and (HCO3 � standard HCO3)/H2CO3.

result. The formula for calculating this is

Who Is Balancing: Is It RBC or Acid-Base Status? DOI: http://dx.doi.org/10.5772/intechopen.84768

Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg. The normal level for base excess is 2 to +2 mEq/L. A negative base excess indicates the presence of base deficit. Actual base excess is the base excess of the blood, while standard base excess is the base excess of the extracellular fluid (ECF) at hemoglobin concentration of 5 gm/dL [8–10].

Under normal ventilation, bicarbonate parameter is useful, but in patients with abnormal ventilation (respiration), it may not reflect the true status because bicarbonate is a dependent variable and it changes with the concentration of pCO2. As pCO2 increases, it reacts with water molecules to form carbonic acid which dissociates into hydrogen and bicarbonate ions. The hydrogen ions are buffered by nonbicarbonate buffers like albumin, hemoglobin, and phosphate buffer system. So, the concentration of bicarbonate increases as pCO2 also increases. This problem is solved by measuring standard bicarbonate [11, 12].

Standard bicarbonate is the concentration of bicarbonate in the plasma from blood which is equilibrated with a normal pCO2 (40 mmHg) and a normal pO2 (over 100 mmHg) at a normal temperature (37°C). The actual bicarbonate and the standard bicarbonate concentrations are approximately equal under normal ventilation, but in abnormal respiration (either hypoventilation or hyperventilation), the two values alter and deviate from each other depending on the changes in the concentration of pCO2 [1].

The bicarbonate value is increased in respiratory acidosis and decreased in respiratory alkalosis. So, the difference between bicarbonate and standard bicarbonate value is positive for respiratory acidosis and negative for respiratory alkalosis. If the acid-base disorder is purely metabolic without respiratory compensation, then the bicarbonate and standard bicarbonate values are more or less closer. If the metabolic disorder is compensated by respiratory mechanisms, then the two values alter and deviate from each other.

The most commonly used approach for arterial blood gas (ABG) analysis interpretation is a physiological approach based on the bicarbonate-carbon dioxide buffer system. The major buffer system in the ECF is the carbon dioxidebicarbonate buffer system, and other buffer systems that play a role in buffering are protein and phosphate buffer systems. The buffers are substances that resist changes in pH. All buffers in a common solution are in equilibrium with the same hydrogen ion concentration. Therefore, whenever there is a change in hydrogen ion concentration in the extracellular fluid, the balance of all the buffer systems changes at the same time. This phenomenon is called the isohydric principle. Henderson-Hasselbalch equation concentrating on the bicarbonate-pCO2 buffer is based on this principle. This approach is very simple and easier, but a major drawback of this is it is unable to quantify the metabolic (non-respiratory) component and does not explain the causative mechanism of metabolic acid-base disturbances [8].

#### 2. Base excess

Base excess approach was developed to quantify the metabolic component, but it was criticized because it represents the whole blood and did not accurately represent the whole body behavior. Blood volume diluted with interstitial fluid represents the effective extracellular fluid hemoglobin concentration of 5 g/dl. Standard base excess Who Is Balancing: Is It RBC or Acid-Base Status? DOI: http://dx.doi.org/10.5772/intechopen.84768

or extracellular base excess is the base excess at hemoglobin concentration of 5 g/dl [8–12].

Oxyhemoglobin is a stronger acid than deoxyhemoglobin. Oxygenation of hemoglobin causes an increase in net titratable hydrogen ion because hydrogen ions are liberated from the oxygen-linked buffer groups due to the Haldane effect. So, the variation of oxygen saturation of hemoglobin (sO2) influences the base excess result. The formula for calculating this is

$$\mathbf{c} \mathbf{B} \mathbf{s} \mathbf{e} \left( \mathbf{B}, \alpha \mathbf{y} \mathbf{y} \mathbf{e} \mathbf{a} \mathbf{t} \mathbf{e} \right) = \mathbf{c} \mathbf{B} \mathbf{s} \mathbf{e} \left( \mathbf{B}, \mathbf{a} \mathbf{t} \mathbf{u} \mathbf{a} \right) - \mathbf{0} \, \mathbf{2} \times \mathbf{c} \mathbf{t} \mathbf{H} \mathbf{b} \times \left( \mathbf{1} - \mathbf{s} \mathbf{O}\_2 \right)$$

or

compensations. Respiratory disorders are associated with appropriate renal compensatory mechanisms, and similarly metabolic disorders are compensated by respira-

Base excess is defined as the amount of strong acid that must be added to each liter of fully oxygenated blood to return the pH to 7.40 at a temperature of 37°C and a pCO2 of 40 mmHg. The normal level for base excess is 2 to +2 mEq/L. A negative base excess indicates the presence of base deficit. Actual base excess is the base excess of the blood, while standard base excess is the base excess of the extracellular

Under normal ventilation, bicarbonate parameter is useful, but in patients with abnormal ventilation (respiration), it may not reflect the true status because bicarbonate is a dependent variable and it changes with the concentration of pCO2. As pCO2 increases, it reacts with water molecules to form carbonic acid which dissociates

bicarbonate buffers like albumin, hemoglobin, and phosphate buffer system. So, the concentration of bicarbonate increases as pCO2 also increases. This problem is solved

Standard bicarbonate is the concentration of bicarbonate in the plasma from blood which is equilibrated with a normal pCO2 (40 mmHg) and a normal pO2 (over 100 mmHg) at a normal temperature (37°C). The actual bicarbonate and the standard bicarbonate concentrations are approximately equal under normal ventilation, but in abnormal respiration (either hypoventilation or hyperventilation), the two values alter and deviate from each other depending on the changes in the

The bicarbonate value is increased in respiratory acidosis and decreased in respiratory alkalosis. So, the difference between bicarbonate and standard bicarbonate value is positive for respiratory acidosis and negative for respiratory alkalosis. If the acid-base disorder is purely metabolic without respiratory compensation, then the bicarbonate and standard bicarbonate values are more or less closer. If the metabolic disorder is compensated by respiratory mechanisms, then the two values

The most commonly used approach for arterial blood gas (ABG) analysis interpretation is a physiological approach based on the bicarbonate-carbon dioxide

Base excess approach was developed to quantify the metabolic component, but it was criticized because it represents the whole blood and did not accurately represent the whole body behavior. Blood volume diluted with interstitial fluid represents the effective extracellular fluid hemoglobin concentration of 5 g/dl. Standard base excess

buffer system. The major buffer system in the ECF is the carbon dioxidebicarbonate buffer system, and other buffer systems that play a role in buffering are protein and phosphate buffer systems. The buffers are substances that resist changes in pH. All buffers in a common solution are in equilibrium with the same hydrogen ion concentration. Therefore, whenever there is a change in hydrogen ion concentration in the extracellular fluid, the balance of all the buffer systems changes at the same time. This phenomenon is called the isohydric principle. Henderson-Hasselbalch equation concentrating on the bicarbonate-pCO2 buffer is based on this principle. This approach is very simple and easier, but a major drawback of this is it is unable to quantify the metabolic (non-respiratory) component and does not explain the causative mechanism of metabolic acid-base disturbances [8].

into hydrogen and bicarbonate ions. The hydrogen ions are buffered by non-

fluid (ECF) at hemoglobin concentration of 5 gm/dL [8–10].

by measuring standard bicarbonate [11, 12].

concentration of pCO2 [1].

alter and deviate from each other.

2. Base excess

34

tory mechanisms [6, 7].

Erythrocyte

cBase Bð Þ¼ ; actual cBase Bð ; oxygenatedÞ þ 0:2 � ctHb � ð Þ 1 � sO2

As the sO2 increases, the term 0.2 � ctHb � (1 � sO2) decreases, so the base excess is changed in the acidotic direction because it is slightly decreased, or as the sO2 decreases, the term 0.2 � ctHb � (1 � sO2) increases, so the base excess is changed in alkaline direction because it is slightly increased [8–10].

The correlation between pCO2 and (HCO3 � standard HCO3)/H2CO3 and pCO2 and ratio of (HCO3/standard HCO3) is clearly shown in Figures 1 and 2, respectively. From that, it is very clear that as the pCO2 decreases, the ratio of (HCO3 � standard HCO3)/H2CO3 also decreases and, as the pCO2 increases, the ratio of (HCO3 � standard HCO3)/H2CO3 also increases and, thereafter, the curve flattens. At pCO2 of 40 mmHg, the ratio of (HCO3 � standard HCO3)/H2CO3 is zero because the difference between bicarbonate and standard bicarbonate value is zero (HCO3 � standard HCO3 is zero). In respiratory acidosis (due to hypoventilation), pCO2 retention occurs, and in respiratory alkalosis (due to hyperventilation), the pCO2 value is decreased. The ratio of (HCO3 � standard HCO3)/H2CO3 changes in respiratory disorders and also in metabolic acid-base disturbances associated with respiratory compensations. The ratio of (HCO3 � standard HCO3)/H2CO3 is greater positive for respiratory acidosis and greater negative for respiratory alkalosis [1].

The normal range for standard base excess is �2 mmol/L. If the value is >2 mmol/L, then it denotes metabolic alkalosis, and if the value is <�2 mmol/L, then it denotes metabolic acidosis (base deficit). Using this concept a four-quadrant graphical tool can be constructed for ABG interpretation using standard base excess

Figure 1. Relation between pCO2 and (HCO3 � standard HCO3)/H2CO3.

clinical value. The relationship of the formation of bicarbonate from pCO2 with the help of carbonic anhydrase enzyme present inside the RBC plays a significant role in maintaining the acid-base balance. The application of standard bicarbonate in the calculation of non-respiratory hydrogen ion concentration and development of a novel four quadrant graphical method for arterial blood gas interpretation may help

About 188 arterial blood gas sample data were utilized. Strict precautions were taken to avoid pre-analytical errors, and the consistency of the ABG report was

] {nanomoles/L = 10<sup>9</sup> moles/L}

The main parameters like measured pH, pCO2, HCO3, standard HCO3, and standard base excess values were noted. Carbonic acid concentration was calculated from pCO2. The difference between bicarbonate and standard bicarbonate was calculated. The ratio of (HCO3 standard HCO3)/H2CO3 was calculated [1].

us for better understanding.

Who Is Balancing: Is It RBC or Acid-Base Status? DOI: http://dx.doi.org/10.5772/intechopen.84768

3. Materials and methods

Calculation of H<sup>+</sup>

<sup>=</sup> log [H<sup>+</sup>

pH <sup>=</sup> <sup>9</sup> log [H<sup>+</sup>

H+

NRH<sup>+</sup>

Figure 4.

37

Relation between NRH<sup>+</sup> and Std HCO3.

:

<sup>=</sup> log [H<sup>+</sup> <sup>10</sup><sup>9</sup> moles/L]

]

] log [10<sup>9</sup>

(At pCO2 of 40 mmHg)

the relationship is clearly shown in Figure 4:

pH = log[H<sup>+</sup> nanomoles/L]

checked by using the modified Henderson equation [2].

—hydrogen ion concentration at actual pH

(Calculated using modified Henderson equation) H+ (hydrogen ion concentration) = {24 pCO2}/HCO3

Calculation of NRH<sup>+</sup> (non-respiratory hydrogen ion concentration):

—hydrogen ion concentration at non-respiratory pH

This calculated hydrogen ion concentration equivalent of standard bicarbonate has thus been called the "non-respiratory" hydrogen ion concentration or NRH<sup>+</sup> [13, 14]. It has a unique value for a given standard bicarbonate concentration, and

Figure 2. Relation between pCO2 and HCO3/Std HCO3.

and the ratio of (HCO3 standard HCO3)/H2CO3 in the two axes that demarcate the various acid-base disturbances which are shown in Figure 3 [1].

The aim of the manuscript is to increase in depth the understanding of the acidbase balance which is a challenging and at times an arduous task, yet it has immense

#### Figure 3.

Analysis of various acid-base disturbances using standard base excess (x-axis) and the ratio of (HCO3 standard HCO3)/H2CO3 (y-axis) in the four-quadrant graph.

Who Is Balancing: Is It RBC or Acid-Base Status? DOI: http://dx.doi.org/10.5772/intechopen.84768

clinical value. The relationship of the formation of bicarbonate from pCO2 with the help of carbonic anhydrase enzyme present inside the RBC plays a significant role in maintaining the acid-base balance. The application of standard bicarbonate in the calculation of non-respiratory hydrogen ion concentration and development of a novel four quadrant graphical method for arterial blood gas interpretation may help us for better understanding.
