Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (623 page)

   Coexistence of Metabolic Acidosis of Hyperchloremic Type and Increased Anion Gap

   
⁻
May be suspected by plasma HCO
₃
^‒
that is lower than is explained by the increase in anions (e.g., AG = 16 mmol/L and HCO
₃
^‒
= 5 mmol/L)

   Examples: Uremia and proximal RTA, lactic acidosis with diarrhea, excessive administration of NaCl to a patient with organic acidosis

   Coexistence of Metabolic Alkalosis and Metabolic Acidosis

   May be suspected by acid–base values that are too normal for clinical picture

   Examples: Vomiting causing alkalosis plus bicarbonate-losing diarrhea causing acidosis

   
PEARLS

   
Pulmonary embolus
: Mild to moderate respiratory alkalosis is present unless sudden death occurs. The degree of hypoxia often correlates with the size and extent of the pulmonary embolus. pO
₂
>90 mm Hg when breathing room air virtually excludes a lung problem.

   
Acute pulmonary edema
: Hypoxemia is usual. CO
₂
is not increased unless the situation is grave.

   
Asthma
: Hypoxia occurs even during a mild episode and increases as the attack becomes worse. As hyperventilation occurs, the pCO
₂
falls (usually <35 mm Hg); a normal pCO
₂
(>40 mm Hg) implies impending respiratory failure; increased pCO
₂
in a true asthmatic (not bronchitis or emphysema) indicates impending disaster and the need to consider intubation and ventilation assistance.

   
Chronic obstructive pulmonary disease
(bronchitis and emphysema) may show two patterns—“pink puffers,” with mild hypoxia and normal pH and pCO
₂
and “blue bloaters,” with hypoxia and increased pCO
₂
; normal pH suggests compensation and decreased pH suggests decompensation.

   
Neurologic and neuromuscular disorders
(e.g., drug overdose, Guillain-Barré syndrome, myasthenia gravis, trauma, succinylcholine): Acute alveolar hypoventilation causes uncompensated respiratory acidosis with high pCO
₂
, low pH, and normal HCO
₃
^‒
. Acidosis appears before significant hypoxemia, and rising CO
₂
indicates rapid deterioration and need for mechanical assistance.