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.
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