content, oxygen delivery, the oxygen-hemoglobin dissociation curve, ventilation perfusion mismatch, shunt, and the alveolar-arterial gradient [P(A-a)O2] are.
How to Interpret Arterial Blood Gas Data? 145
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How to Interpret Arterial Blood Gas Data? ALLADI MOHAN, SK SHARMA Table 1: Relationship between pH and [H+]
INTRODUCTION Arterial blood gas (ABG) analysis, considered to be one of the most precise measurements in medicine, is a crucial investigation that is frequently employed in the critical care setting1,2. A structured approach to rational interpretation of the ABG report requires a clear understanding of the basic concepts and physiological principles underlying the disorders of acid-base homeostasis, oxygenation and ventilation. In this review we have provided a brief overview regarding the essential fundamental physiological concepts and have attempted to provide a practical approach for bed-side interpretation of ABG results. BASIC PHYSIOLOGICAL AND CHEMICAL CONCEPTS Acid-base Chemistry
pH The concept of pH was put forward by the Danish chemist, Soren Peter Sorensen in 1909 to refer to the negative logarithm of hydrogen ion (H+) concentration3. pH = – log [H+] The pH is a ‘dimensionless representation of the H+, has no units and is just a number. There are several advantages of the expression pH compared to H +. Measured by the pH electrode, pH, is related to the logarithm of H+ “activity” rather than “concentration” of H+ and this seems physiologically more appropriate. The relationship between pH and H + is shown in Table 1. For each 0.1 pH unit increment, H+ falls by 20%; by remembering this relationship, the intermediate values can be derived accurately4-7.
pH
[H+] (nmol/l)
6.8
158
6.9
125
7.0
100
7.1
79
7.2
63
7.3
50
7.4
40
7.5
31
7.6
25
7.7
20
7.8
15
Overview of Acid-base Balance in the Human Body The normal extracellular fluid (ECF) H + concentration is 40 nmol/l. Under normal physiological conditions, the H+ concentration varies little from its normal value due to the processes of acid base regulation, as maintenance of H+ concentration at this level is considered to be essential for normal cellular processes. In order to achieve this, close interaction between three physiological systems of the body, namely, the chemical buffers of the body, kidneys and the lungs is considered essential4-7. Dissolved carbon dioxide is in equilibrium with carbonic acid Carbonic Anhydrase CO2 + H2O H2CO3 Carbonic acid ionizes according to the equation Carbonic Anhydrase + H2CO3 H + HCO3–
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By the law of mass action: [H+] [HCO3–] _________________ [H2CO3]
= K’
Taking logarithm on both sides we have: log
[H+] [HCO3–] _________________ [H2CO3]
= log K’
Therefore, log [H+] + log
[HCO3–] ___________ [H2CO3]
= log K’
and log [H+] = log K’ – log
[HCO3–] ___________
[H2CO3]
By changing signs on both sides of the equation, we have log – [HCO 3] ___________
— log [H+] = – log K’ + log [H2CO3]
Since –log [H+] is called pH and –log K’ is called pK, the equation can be rewritten as pH = pK + log
[HCO3–] ___________
[HCO3–] _____________________ 0.0301 × PaCO2
This equation may also be rewritten as H+ = 24 ×
Alveolar Ventilation Alveolar ventilation is best defined in terms of ventilation of carbon dioxide. The PaCO2 directly reflects the adequacy of alveolar ventilation. Elevation of PaCO2 > 50 mmHg indicates alveolar hypoventilation and low PaCO 2 7.44 Simple acid-base disorders are those in which there is a single primary aetiological acid-base disorder. Mixed acid-Base Disorders are those in which two or more primary etiological disorders are present simultaneously Respiratory disorders are caused by abnormal processes which tend to alter pH because of a primary change in PaCO2 levels Metabolic disorders are caused by abnormal processes which tend to alter pH because of a primary change in [HCO3–]
Table 2b: Nomenclature for clinical interpretation of ABG measurements: ventilatory status
PaO2 < 80 mmHg = mild hypoxemia PaO2 < 40 mmHg = severe hypoxemia Effect of oxygen therapy
Uncorrected hypoxemia = PaO2 < room air acceptable limit* Corrected hypoxemia = PaO2 > room air acceptable limit; < 100 mm Hg Excessively corrected hypoxemia = PaO2 > 100 mmHg; 7.50 = Partially compensated metabolic alkalosis
pH 7.48, PaCO2 = 20
pH 7.30 – 7.50 = Chronic ventilatory failure
40 - 20 = 20
pH 10 mOsm/l is considered abnormal when calculated using this formula. Conditions causing high AG metabolic acidosis include ethanol, ethylene
How to Interpret Arterial Blood Gas Data? 151 glycol, methanol, acetone, isopropyl ethanol and propylene glycol poisoning. Assessment of Ventilatory and Oxygenation Status The approach shown in Table 2b can be used to interpret the ABG report and classify the ventilatory status in conjunction with the pH. Finally, looking at the PaO2 in the context of the FIO2 (Table 2c) can aid in the evaluation of hypoxemia. CONCLUSIONS
Vol. IX. A publication of Indian Association of Clinical Medicine. New Delhi: Jaypee Brothers Medical Publishers 2006;73-81. 2. Driscoll P, Brown T, Gwinnutt C, Wardle T. A simple guide to blood gas analysis. New Delhi: Jaypee Brothers Medical Publishers; 2002. 3. Sörenson SPL. Enzyme studies II. The measurement and meaning of hydrogen ion concentration in enzymatic processes. Biochemische Zeitschrift 1909;21:131-200. Excerpts from pages 131-134 and 159-160 of the paper available at http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/ Sorenson-article.html. Accessed on 21 September 2006. 4. Shapiro BA, Harrison RA, Cane RD, Kozlowski-Templin R. Clinical application of blood gases. 4th ed. Chicago; Year Book Medical Publishers, Inc; 1989.
Detailed history taking and thorough physical examination are vital for obtaining clues regarding the underlying etiological conditions. Assessment of pH, PaCO2, and HCO3– facilitates determination of whether a primary metabolic or respiratory disorder is present. Estimation of the predicted compensatory response for simple acid-base disorders might suggest the presence of an additional disease process if compensation is not appropriate. Calculation of the various gaps (AG, UAG, osmolar gap) can be helpful in differential diagnosis. With a clear understanding of the pathophysiological processes and by following the stepwise approach described, majority of the ABG reports can be interpreted with clarity.
10. Wrenn K. The delta (delta) gap: An approach to mixed acidbase disorders. Ann Emerg Med 1990;19:1310-3.
REFERENCES
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5. Fall PJ. A stepwise approach to acid-base disorders. Practical patient evaluation for metabolic acidosis and other conditions. Postgrad Med 2000;107:249-50, 253-4, 257-8 passim. 6. Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore) 1980;59:161-87. 7. Williams AJ. ABC of oxygen: assessing and interpreting arterial blood gases and acid-base balance. BMJ 1998;317:1213-6. 8. Beasley R, McNaughton A, Robinson G. New look at the oxyhaemoglobin dissociation curve. Lancet 2006;367:1124-6. 9. Ishihara K, Szerlip HM. Anion gap acidosis. Semin Nephrol 1998;18:83-97.
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