Q J Med 2002; 95: 125
© 2002 Association of Physicians
Correspondence |
Assessing motor neuron disease
Havant
Sir,
In their paper (Q J Med 2001; 94:491–5), Hadjikoutis and Wiles draw attention to a number of circumstances in which the serum chloride and bicarbonate can be misleading. They do not mention that they will also be misleading if they are measured when hypercapnia is increasing or decreasing.
An exacerbation of hypercapnia will take 4–6 days to raise the serum bicarbonate and lower the serum chloride to new steady levels.1 How long it will take them to recover in a remission (or in a period of assisted respiration) is more unpredictable, since it is strongly influenced by the patient's chloride intake, which in turn depends mainly on the salt intake.2 If the intake is low, the serum chloride may remain low, and the bicarbonate high, for a long time. A prognosis based on those levels would be unduly pessimistic.
References
1. Polak A, Haynie GD, Hays RM, Schwartz WB. Effects of chronic hypercapnia on electrolyte and acid-base equilibrium: I. Adaptation. J Clin Invest1961; 40:1223–7.
2. Schwartz WB, Hays RM, Polak A, Haynie GD. Effects of chronic hypercapnia on electrolyte and acid-base equilibrium: II. Recovery, with special reference to the influence of chloride intake. J Clin Invest1961; 40:1248–9.
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