Blood Gas  Components

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Clinical Pathology

Blood Gas Components

  • pH: pH is measured using a dedicated electrode and indicates the acidity or alkalinity of the sample. A low pH is compatible with acidemia and a high pH with alkalemia.

  • pO2: This is measured by a pO2 electrode. It is the partial pressure (tension) of oxygen in a gas phase in equilibrium with blood. High or low values indicate blood hyperoxia or hypoxia, respectively. pO2 in venous blood is lower than arterial blood due to oxygen extraction by peripheral tissues.

  • pCO2: This is measured using a pCO2 electrode. It is the partial pressure of pCO2 in a gas phase in equilibrium with the blood. The pCO2 gives an indication of the respiratory component of the blood gas results. A high and low value indicates hypercapnea (hypoventilation) and hypocapnea (hyperventilation), respectively. A high pCO2 is compatible with a respiratory acidosis and a low pCO2 with a respiratory alkalosis.

  • Bicarbonate: This is the concentration of bicarbonate in the plasma of the blood sample. It is actually a calculated value from the Henderson-Hasselbach equation as follows:
    Bicarbonate = 0.23 x pCO2 x antilog(pH - pKp), where pKp = 6.125 - log (1+ antilog(pH - 8.7))
    The bicarbonate on the blood gas report is usually lower than the bicarbonate from our chemistry panel (measured with the Hitachi). This is because the blood gas sample is kept strictly anaerobic and is a calculated value, whereas with the Hitachi, bicarbonate is measured directly. The bicarbonate, total CO2 and base excess are indicators of the metabolic component of the blood gas results. A low (and negative base excess) and high (and positive base excess) bicarbonate indicate metabolic acidosis and alkalosis, respectively.

  • Total CO2: This is another indicator of bicarbonate but is usually slightly higher as it also measures dissolved CO2. It is a calculated value from the pCO2 and bicarbonate as follows:
    total CO2 = 0.23 x pCO2 + bicarbonate.

  • Base excess: The base excess is the concentration of titratable base when the blood is titrated with a strong acid or base to a plasma pH of 7.40. The value provided is actually the standard base excess, which is an in vivo expression of the amount of base present. It is a calculated value using a very complicated formula. Together with the bicarbonate, the base excess gives you an indication of the metabolic component of the blood gas results. A positive base excess means excess base, i.e. a metabolic alkalosis, whereas a negative base excess means reduced base, i.e. a metabolic acidosis

  • Ionized calcium: Total calcium consists of free or ionized calcium (50%), calcium bound to protein (40-45%), principally albumin, and calcium complexed to anions (5-10%), e.g. citrate, lactate, bicarbonate. The total calcium concentration does not give an indication of what is available at the cellular level. Ionized calcium (iCa) is the form of calcium that is readily available to cells, and measurement of iCa is a more accurate reflection of the physiological calcium state. Unlike total calcium, ionized calcium is unaffected by albumin concentration, but it is affected by acid-base balance, with increases occurring when the sample becomes more acidetic (due to decreased protein-binding of calcium such as when there is delayed separation of plasma/serum from cells allowing increased production of acidic cellular metabolic byproducts.).