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Letters to the Editor

Indian Pediatrics 2002; 39:506-508

Arterial Blood Gas Analysis in Clinical Practice

In the recent article on this subject(1), certain important aspects of arterial blood gases are confusing and need correction and clarification:

1. Regarding relationship of PCO2 and pH, the authors mention that pH falls by 0.1 U for every 20 mm rise of PCO2 and increase by 0.1 U for every 20 mm fall of PCO2. The relationship between pH and PCO2 is guided by an equation (Henderson-Hasselbalch equation) which remains same irrespective of rise or fall in PCO2. Thus the change in pH has to be equal for either rise or fall in PCO2. In this reference, American Academy of Pedia-trics guidelines on ‘Pediatric Advanced Life Support’(2) mention that a change in PCO2 of 10 mm Hg is associated with an increase or a decrease in pH of 0.08 units which seems more logical as per Henderson-Hasselbalch equation.

2. The Henderson-Hasselbalch equation given by the authors is not correct as pH is not simply the ratio of HCO3 to PCO2. The correct equation is

pH = 6.1 + log HCO3– / H2CO3

Because carbonic acid is in equilibrium with dissolved carbon dioxide, measure-ment of the partial pressure of carbon-dioxide (PCO2) can be used as a clinical estimate of carbonic acid (H2CO3) concentration. Thus, even if the ratio of HCO3 to PCO2 decides the pH, it can not be simply used to calculate pH as given in the equation by authors.

3. The authors have not used the word ‘judicious’ judiciously which is evident from following statements:

(i) Metabolic alkalosis is the most important to be managed judiciously as a rise of pH above 7.5 may cause arrhythmias (Page 1118, Acid-Base Disorders, last line).

(ii) If increased anion gap acidosis is evident, the etiology should be identified and corrected, where administration of bicarbonate may be hazardous. In children with normal anion gap acidosis bicarbonate correction should be done judiciously (page 1120, Para 3).

(iii) Oxygen is a drug, which should be used judiciously (Page 1121, Respiratory alkalosis, Para 2, Line 2).

Thus, different and inappropriate usage of this word in (i) and (ii) has caused plenty of confusion regarding management of these conditions.

4. Regarding management of these acid-base disorders:

I would like to differ with the author’s opinion that metabolic alkalosis is the most important condition to be managed in mixed disorders (Page 1118, Acid-Base Disorders, last line). Respiratory acidosis is the one that is to be managed on most urgent basis. Metabolic alkalosis on its own does not require any specific treat-ment most of the time except for the volume expansion.

The authors recommend use of 0.6 as the distribution of bicarbonate while recommending formula for correction of metabolic acidosis (Page 1120, Para 4). The distribution of bicarbonate detected by various studies is 0.2 to 0.5 and most of the available literature recommends 0.3 to be used in formula for correction of acidosis(2-3). Use of 0.6 for calculating amount of bicarbonate required seems to be too high.

Conditions like salicylate poisoing and organic acidemias are included by authors under the subheading ‘lactic acidosis’ in Table II. These conditions cause accumulation of other exogenous acids rather than lactic acid and their inclusion under lactic acidosis is not justified. Bicarbonate remains the preferred agent for correction of acidosis in conditions associated with an increased anion gap (e.g., lactic acidosis associated with hypoxia or shock)(3). This is in contra- diction to the authors’ statement that bicarbonate is not to be used in increased anion gap acidosis (Page 1120, Para 3).

The authors recommend prompt oxygen supplementation for respiratory alkalosis (Page 1121, respiratory alkalosis, Para 2, Line 1) which is not a correct thing to do unless the child has hypoxia. Most of the cases of respiratory alkalosis are neurogenic in origin and do not require any oxygen therapy.

The authors state that a critically ill child should be benefited with 100% oxygen irrespective of the oxygenation status. Such statements are unwarranted and unjustified in this era when oxygen free radical mediated damage has been implicated in so many serious conditions requiring intensive care(4).

5. Regarding detection of laboratory errors (Page 1125, last paragraph), the authors recommend use of Kasirer and Bleich equation (which is also given wrongly in the above article as the correct equation is H+ = 24 + PCO2/HCO3 and not H+ = 24 × PCO2/HCO3–). It is to be noted that all the blood gas measurement devices analyze PO2, PCO2 and pH. HCO3– is calculated by the machine from normograms based on Henderson-Hesselbalch equation. Thus any laboratory error in the measured values of pH and PCO2 will also be reflected in the bicarbonate value and simply calculating bicarbonate manually by these equations will not detect the error as bicarbonate is not an actually measured value by the machine. Only errors related to printing or writing of the report can be detected in this way and not the electrode or machine errors.

6. Examples are not designed carefully. In example (a) pH of 7.6 is too high for PCO2 of 30. A fall of 10 mm in PCO2 will decrease the pH by 0.1 in presence of nor-mal bicarbonate as indicated by authors themselves earlier. Thus the expected pH should be 7.5 rather than 7.6 or the PCO2 should have been 20 mmHg. Similarly, in example (e) the deficit in bicarbonate (around 16) will tend to decrease the pH by 0.24 (10 meq change in bicarbonate changes pH by 0.15 units)(2) while 10 mm fall in pH should increase the pH by 0.1. Thus the expected pH becomes 7.4 - 0.24 + 0.1 or 7.26. The pH given by authors is 7.12 which is much lower than expected for the values given by authors. Also, in this example, there is no respiratory acidosis as claimed by the authors as inadequate fall in PCO2 reflectes failure of compensatory mechanisms rather than respiratory acidosis.

Dheeraj Shah,

Lecturer,

Department of Pediatrics,

University College of Medical Sciences,

Dilshad Garden, Delhi 110 095, India.

 

References


1. Vijayasekaran D, Subramanyam L, Bala-chandran A. Arterial blood gas analysis in clinical practice. Indian Pediatr 2001; 38: 116-1128.

2. Emergency Cardiac Care Committee and Subcommittee, American Heart Association. Fluid therapy and medications. In: Pediatric Advanced Life Support. Eds. Chameides L, Hazinski MF. American Heart Association, 1994; pp 6.1-6.18.

3. Jospe N. Forbes G. Fluids and electrolytes: Clinical aspects. Pediatr Rev 1996; 17: 395-404.

4. Shah D, Garg K, Choudhury P. Oxidants and anti-oxidants in pediatric nutrition and disease. Nutrisearch 2001; 8:18.

 

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