Blood gas machine

A small amount of heparinised blood is either injected from a syringe or aspirated from a capillary tube into the machine. The blood comes into contact with three electrodes which measure pH, PO2 and PCO2.

• pH — measured by the potential across a pH-sensitive glass membrane separating a sample of known pH and the test sample.

• PO2 — the partial oxygen pressure, is measured by applying a polarising voltage between a platinum cathode and a silver anode (Clark electrode). O2 is reduced, generating a current proportional to the PO2.

• PCO2 — the partial pressure of carbon dioxide, utilises a pH electrode with a Teflon membrane (Severinghaus electrode) which allows through uncharged molecules (CO2) but not charged ions (H + ). CO2 alone thus changes the pH of a bicarbonate electrolyte solution, the change being linearly related to the PCO2.

• Hb — estimated photometrically; this is not as accurate as co-oximetry (see below).

• Bicarbonate — calculated by the Henderson-Hasselbach equation

Actual HCO3- includes bicarbonate, carbonate and carbamate.

• Actual base excess (deficit) — the difference in concentration of strong base (acid) in whole blood and that titrated to pH 7.4, at PCO2 5.33kPa and 37°C.

• Standard base excess (deficit) — a calculated in vivo base excess (deficit).

• Standard bicarbonate — the plasma concentration of hydrogen carbonate equilibrated at PCO2 5.33kPa, PO2 13.33kPa and temperature 37°C.

Blood gas values can be given either as 'pHstat' or 'alphastat', the former correcting for body temperature by shifting the calculated Bohr oxyhaemoglobin dissociation curve (hyperthermia to the right, hypothermia to the left). Alphastat measures true blood gas levels in the sample.

0 0

Post a comment