PaO2PaO2

- partial pressure of oxygen dissolved in blood plasma. PaO2

reflects only 3% of total oxygen in blood.

PaO2 : 80 – 100mm of hg.

SaO2 - represents oxygen combined with available

haemoglobin – approximately 97% of total oxygen in blood

Hence the oxygenation status of the patient can be properly assessed based on 3 elementsPaO2, SaO2 and Haemoglobin

pHpH is the hydrogen ion concentration in plasma. Ideal

value for pH is 7.40Condition were pH lower than 7.35 is called acidosis and

above 7.45 is called alkalosisYou don’t have to remember the equation given below as the pH is already calculated for you by the blood gas machine.

Henderson – Hasselbalch equation for blood pH

pH = pK + logHCO3

CO2

PaCO2Partial pressure of carbon dioxide dissolved in

blood plasma. PaCO2 value indicates whether the patient can ventilate well enough to rid the body of carbon dioxide produced as a consequence of metabolism.

Normal range : 35 – 45 mm of HgPaCO2 less than 35 defines respiratory

alkalosisPaCO2 greater than 45 defines respiratory

acidosis

HCO3-

HCO3- bicarbonate level is the acid base component

that reflects kidney function. Normal range: 22 – 26mEq/LBicarbonate is increased or decreased in the blood

plasma by renal mechanism.Bicarbonate level less than 22 defines metabolic

acidosis and greater than 26 defines metabolic alkalosis.

Base Excess and Base deficitBase excess and base deficit reflect the non respiratory

contribution to acid-base balanceNormal range: -2mmol/L to +2mmol/L.Negative base level is reported as base deficit and is

associated with metabolic acidosisPositive base level is reported as base excess and is

associated with metabolic alkalosis.Bicarbonate level and base level goes in the same

direction

Anion GapAnion gap is computed by subtracting the major plasma

anions (chloride and bicarbonate) from the major plasma cations (sodium).

Normal range: 8 to 16 mEq/L. This value represents the amount of unmeasured anions such as ketones or lactate in plasma.

Plasma anion gap = (Na+ + K+) – (Cl- + HCO3-)A non-anion gap metabolic acidosis can occur through the loss of bicarbonate and the retention of chloride ion (hyperchloremic metabolic acidosis) . Clinically a non-anion gap acidosis is associated with diarrhoea, renal failure, hyperalimentation etc.

Oxyhaemoglobin dissociation curve

The relationship between the two axes of this curve assumes normal values for haemoglobin, pH, temperature and PCO2 and In the tissues, the oxygen-haemoglobin curve dissociation curve shifts to the right. As pH decreases, PCO2 increases, or temperature rises, the curve shifts to the right, resulting in an increased release of oxygen.In the lungs, the oxygen-haemoglobin curve dissociation curve shifts to the left. As pH increases, PCO2 decreases, or temperature falls, the curve shifts to the left, resulting in an increased ability of haemoglobin to pick up oxygen.