What type of clinical test is this and what does it measure?

What is the importance of temperature in the reported result?

What is the pH-stat approach?

What is the α-stat approach?

Which is better?

This is an arterial blood gas (ABG) analysis; it gives information about the adequacy of a patient’s gas exchange and acid–base status. It is used perioperatively, during CPB and also in severe lung disease (severe asthma in the ER), cardiac and kidney failure, uncontrolled diabetes, severe infections, drug overdose, and also in the ICU. An abnormal pH value as in acidosis or alkalosis can occur in disease states. ABG helps us to determine if the acid–base derangement is respiratory or metabolic in origin. The result is always reported taking into consideration the temperature of the patient at the time of collection.

The arterial blood sample is preheated to 37°C prior to measurement. If the actual patient temperature is keyed in, modern blood gas machines will report the pH value for that temperature as well. This is calculated mathematically from the pH measured at 37°C. For clinical use, the Rosenthal correction factor is recommended and is done as follows:

According to Henry’s law, the solubility of a gas increases with decrease in temperature. PO₂ is 5 mmHg lower and PCO₂ is 2 mmHg lower for each degree below 37°. Hypothermia causes a decrease in the PCO₂ (hypocarbia) and a concomitant increase in the pH (alkalemia), yet the total body CO₂ content remains the same. There are two blood gas management strategies in hypothermia—temperature correction (pH stat) or not (α stat). These have different effects on cerebral blood flow, oxygen dissociation curve, and intracellular enzyme and protein activity.