Blood Gas

Ok, so how are all of these gases and electrolytes measured?

Blood gas measurements are done using arterial blood (blood going to tissue) in a plastic syringe (most common), and they should be measured as soon as possible (within 15 minutes is best). The blood is transported in a syringe to limit exposure to air and it achieves this because the syringe is loaded directly into the blood gas instrument.

Why do blood gas samples need to be measured ASAP?
They change quickly due to the pressure differences of the atmosphere and the arterial blood.

Let’s quickly run through a few calculations to better understand gas pressure, don’t fall asleep! The atmospheric P0₂ and PCO₂ can be estimated using a few metrics.

1. Air is comprised of:
20.93% O₂
0.03% CO₂
78.1% N
1% other

2. Partial pressure is equal to barometric pressure multiplied by the % gas.

3. Vapor pressure of water at 37C is 47 mmHg.

4. Atmospheric pressure at sea level is 760 mmHg

Using this information:
PO₂ = (760-47) x 20.93 = 149 mm Hg

PCO₂ = (760-47) x 0.03 = 0.2 mm Hg

Why are these numbers important? Because gas flows from high concentration to low concentration. The normal range for PO₂ in arterial blood is 80-100 mmHg, so if the atmospheric O₂ is around 150 mmHg, sample PO₂ will increase if exposed to air. When not exposed to air, anaerobic PO₂ will decrease about 2 mmHg per hour at room temp due to cellular metabolism.

The reference range for PCO₂ in arterial blood is about 35-45 mmHg. Therefore if arterial blood is exposed to air, PCO₂ will decrease because atmospheric PCO₂ is close to zero.

The main takeaway is if you have a sample that has been sitting for more than 30 minutes (not on ice), the pH will decrease, PCO₂ will increase, and PO₂ will decrease.

If you have a sample that has been exposed to air or has an air bubble, the pH will increase, PCO₂ will decrease, and PO₂ will increase. Air bubbles in blood gas samples happen somewhat frequently. It’s important to be able to know what to do in these instances.

pH:
pH is measured using a H+ ion sensitive glass electrode.

CO₂:
PCO₂ is measured using a Severinghaus electrode. The Severinghaus electrode consists of a pH electrode separated from the sample by a gas permeable membrane and buffered solution. The CO₂ diffusing across the membrane lowers the pH. The relationship between CO₂ and pH gives you a CO₂ value in the sample.

O₂:
PO₂ is measured using a Clark electrode. Oxygen crosses the permeable membrane, is reduced, and the electrons produce a current proportional to the PO₂ in the sample.

To measure ions like Na+, K+, and Cl-, an ion-selective electrode (ISE) is used. An ISE is a potentiometric electrode consisting of a membrane selectively permeable to a single ionic species. (Think of it as a machine that can separate M&Ms by color). The potential produced at the membrane-sample solution interface is proportional to the log of the ionic concentration.

Sodium (Na+):
Sodium uses a glass membrane

Potassium (K+):
Potassium uses valinomycin in a plastic membrane

Chloride (Cl-):
Chloride uses quaternary ammonium salt anion exchangers

Direct ISE vs Indirect ISE:
An important concept to understand is the difference between direct and indirect ISE. In direct ISE the specimen is brought to the electrode surface without dilution. In indirect ISE the specimen is diluted with buffer prior to electrode contact.

Pseudohyponatremia: 
Sodium is only in the aqueous phase of plasma (it’s not in fat or protein) so diluting the specimen can cause an imbalance if low or high levels of protein and or lipids are present.