Letters to the Editor

Indian Pediatrics 2002; 39:508-509

Reply

1. Arterial blood gas analysis is a difficult subject to understand, however, we had attempted to make the subject easy even to those who are fresh to the subject. pH is unitless, an acute 20 mm Hg increase in PaCO2 results in a pH fall of 0.1 and an acute decrease in the PaCO2 of 10 mm Hg results in a pH increase of 0.1 (it has been wrongly commented "0.1 rise for every 20 mm fall of PaCO2"). However, in chronic state the changes are not the same(1). An acute change in PaCO2 of 10 torr is associated with an increase or a decrease in pH of 0.08 units and this can also be practiced for simplicity.

2. The relationship of HCO3–/H2CO3 is usually expressed by the complicated Henderson-Hasselbalch equation which is highly mathematical and clinically difficult to use. For easy understanding, the oversimplified equation is depicted as pH = HCO3–/PaCO2 and it is not to be considered in purely mathematical sense. We want to stress the fact that the ratio of PaCO2 to HCO3– - really decides the pH (hydrogen ion concentration) rather than the absolute value.

3. Metabolic alkalosis is usually accompanied by hypokalemia, which if severe can cause cardiac arrhythmia, decreased oxygen delivery and neuro-muscular dysfunction. In metabolic alkalosis, extracellular volume depletion causes increase of HCO3– by either reduction in glomerular filtration rate or stimulation of proximal renal tubular absorption of sodium and HCO3– which are further complicated by low potassium level. Hence metabolic alkalosis has to be treated with volume expansion and maintenance of serum potassium to avoid complications.

While correcting metabolic acidosis of normochloremic variety (high anion gap), 0.6 formula is advised with half HCO3– correction immediately as an infusion and the rest only after 24 hours; thus there is hardly any difference between both the formulas. In general, the lower the pretreatment HCO3– , more the bicarbo-nate that is needed to produce a given increase in HCO3–. In patients with mild or moderate hypobicarbonatemia, about ¼ to ½ of infused bicarbonate remains unneutralized in the ECF. Thus, if 60 mmol bicarbonate is infused, 15-30mmol will remain, and ECF HCO3– will rise by about 1-2 mmol/L. In severe hypobicarbo-natemia, only 1/8 to 1/4 of infused bicarbonate remains unneutralized in the ECF. Thus, the same 1-2 mmol/L rise in HCO3– requires about 120 mmol of infused bicarbonate. The estimate of the dose of bicarbonate required is given as

(Cd – Ca) × K × Body wt. (in kg) = mEq required where ‘K’ for bicarbonate approximates 0.5 – 0.6,

These fractional guides are only approximate, it is extremely difficult to predict accurately how much bicarbonate will be needed. However correction should always be based on the severity of the condition(2,3).

4. Many drugs and toxins can cause lactic acidosis, of which common in clinical practice are salicylates, methyl and ethyl alcohol intoxication. Salicylate intoxica-tion causes metabolic acidosis and respiratory alkalosis. It is the salicylate (removal of acetyl group) that interferes with various enzymes leading to increase in production of organic acids especially keto and lactic acids. Protons liberated during the dissociation of acetylsalicylic acid do consume bicorbonate but the amount of aspirin typically ingested is too small to substantially lower HCO3–(2).

In clinical practice respiratory alkalosis may result from two causes - pulmonary and non pulmonary, among which pulmonary is common which involves intrinsic lung diseases, invariably presenting as hypoxemia. It has also been stressed that PaO2 of 60 mm of Hg saturates 90% of hemoglobin and majority of mild conditions may not require supple-mental oxygen therapy. However, when PaO2 falls below 60, hypoxemic complictions are drastic where administration of oxygen should be prompt.

All moderate and severe hypoxemic patients should be given supplemental oxygen to keep the SaO2 above 95%. Though pulmonary oxygen toxicity is the accepted complication, it is not a concern in the first few hours in a child with severe cardio-pulmonary problem where administration of high flow oxygen is a must to avoid hypox-emic complications of heart and brain(2).

5. Any two of the three measured laboratory values for the pH, PCO2 and HCO3 concentration can be used to calculate the third using the Henderson equation (Table I).

[H+] = 24 × (PCO2/[HCO3] and [HCO3]
  = 24 × (PCO2 / [H+])4

The above reference clearly explains the correct equation depicted in the article.

6. Examples were made for the beginners to understand easily stressing the three basic steps (pH, PaCO2 and HCO3–). 

We do agree that understanding the mixed disorder needs more accurately designed example or true arterial blood gas picture.

1. Subramanyan L, Vijayasekaran D, Somu N. Interpretation of blood gas analysis. In: Essentials of Pediatric Pulmonology, 2nd edn. Eds. Somu N, Subramanyam L. Madras, Siva and Co., 1996; pp 205-215.

2. Abelow B. Metabolic acidosis. In: Under-standing Acid Base. Ed. Abelow B. Baltimore Williams and Wilkins, 1998; pp 139-156.

3. Adelman RD, Soulhang MJ. Pathophysiology of Body fluids and fluid therapy. In: Nelson Textbook of Pediatrics, 16th edn. Eds. Behr-man RE, Kleigman RM, Jenson HB, Philadel-phia, W.B. Saunders Co., 2000; pp 220-223.

4. Brewer E D. Disorders of Acid Base Balance. Pediatr Clin N Am 1990; 37(2): 429-447.