Blood Gas, pH

pH is the negative logarithm of the hydrogen ion concentration and is an index of acidity and alkalinity of the blood. It changes nonlinearly masking magnitude of acid " “base disorders. The hydrogen ion concentration is dictated by the ratio of two quantities: the HCO3 ¢ ˆ ’ concentration, which is regulated by the kidneys, and the PCO2, which is controlled by the lungs.

Normal range:

Arterial: 7.35 " “7.45

Venous: 7.31 " “7.41

Use

To evaluate acid " “base disorders

Interpretation

Increased In

Metabolic alkalosis (plasma bicarbonate excess)

Excessive alkali administration

Potassium depletion (GI loss, lack of potassium intake, diuresis)

Stimulation of respiratory center by increased intracranial pressure

Hypoxia with normal overall alveolar diffusion of CO2

Fever

Salicylate poisoning (early)

Excessive artificial ventilation

Decreased In

Metabolic acidosis (bicarbonate deficit)

Increased formation of acids

Ketosis (DM, starvation, hyperthyroidism, high-fat low-carbohydrate diet, after trauma)

Cellular hypoxia including lactic acidosis

Decreased excretion of H+

Renal failure (prerenal, renal, and postrenal)

Renal tubular acidosis

Fanconi syndrome

Acquired (drugs, hypercalcemia)

Inherited (cystinosis, Wilson disease)

Addison disease

Respiratory acidosis

Emphysema, pneumonia, and pulmonary edema

Bronchoconstriction, plugs, and drugs depressing the respiratory center

Obstructive or restrictive pulmonary disease

Limitations

The pH of freshly drawn blood decreases on standing at a rate of 0.04 " “0.08 pH U/hour at 37 ‚ °C, by approximately 0.03 U/hour at 25 ‚ °C, but only 0.008 U/hour at 4 ‚ °C.