Serum [H+] is maintained within a narrow range through a series of reversible chemical buffers and physiologic pulmonary and renal responses. Although the pH of the extracellular fluid (ECF) is maintained between 7.38-7.42 in the elderly, there is evidence to suggest this occurs at the expense of a reduced serum НСОз reserve. A recent review noted a significant increase in the steady-state blood [H+] and a reduction in steady-state serum НСОз from subjects aged 20-100 years, suggesting a progressive age-related, low-level metabolic acidosis. The homeostatic responses to chronic metabolic acidosis in aging may engender pathologic consequences, such as nephrolithiasis, bone demineralization, and muscle protein breakdown. These maladaptive changes, which may reflect subtle degrees of acidosis or the intermittent nature of the acidosis, suggest a “eubicarbonatemic” metabolic acidosis may exist in older patients and emphasizes the importance of recognition and treatment of even mild acid loads to prevent these maladaptive homeostatic responses.

This eubicarbonatemic metabolic acidosis with aging appears to be most directly related to a decline of renal function and possibly an age-related, low-grade, diet-dependent metabolic acidosis. In addition to the normal physiologic changes that occur with aging, the increased frequency of comorbid conditions and/or medications which may further impact upon both pulmonary and renal function increase the susceptibility towards the development of acid-base disturbances.

Table 2. Simple Guide to Estimating Physiologic Response to a Primary Acid-Base Disturbance

Primary Disturbance

Actual Response #

Estimate

Metabolic acidosis                  (10 meq/l fall НСОз)

10-12 mmHg fall PCO2

(10)

Metabolic alkalosis                 (10 meq/l rise HCO3)

6-7 mmHg rise PCO2

(5)

Respiratory acidosis (chronic)*   (10 mmHg rise рСОг)

3.5 meq/L rise HCO3

(5)

Respiratory alkalosis (chronic)*   (10 mmHg fall PCO2)

5 meq/l fall HCO3

(5)

* If acute then 10 mmHg change pCO* 1-2 change HCO3; # The actual response can be rounded off to 10
for metabolic acidosis and five for each other disturbance for easy recall with only a slight loss of accuracy

Approach to Acid-Base Disorders

Acidosis is a process that, if left unopposed, results in acidemia (pH <7.38). Likewise, alkalosis is a process that, if left unopposed, results in alka-lemia (pH >7.42). The physiologic response to changes in pH involves changes in alveolar ventilation (рСОг), renal acid excretion and/or НСОз reclamation. The full respiratory response to metabolic acidosis requires hours, while the maximal renal response to respiratory disturbances usually requires three-to-four days. A rough guide to determining the appropriate physiologic response to a given primary disturbance is shown in Table 2.
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