Eberhard KE, Sørensen EM, Thoft L, Uhrenholt S
© SATS Copenhagen 2017 - emss17.sats-kbh.dk
ARTERIAL BLOOD GAS ALGORITHM – GUIDE The arterial blood gas is primarily used to determine oxygenation and acid-­‐base status. pH: The acid-­‐base status is determined by the concentration of hydrogen ions. We use the logarithmic pH scale which express the concentration inversely, the normal value lie between 7.35 and 7.45. There are two key points to remember: 1) A pH value < 7.35 indicates an increase in hydrogen ions and the blood is referred to as acidic. Conversely a pH value > 7.45 indicates a decrease in hydrogen ions, the blood is alkaline. 2) Small changes in pH represent large change in hydrogen ion. It is important for the enzymatic processes in the body to keep pH within this narrow range. Different buffer systems as the intracellular proteins, phosphate and hemoglobin and the extracellular plasma proteins and bicarbonate try to prevent immediate changes but these are only temporary solutions. Excess of acids needs to be eliminated and buffers regenerated. This is done by the lungs and kidneys. PaO2: The ABG express the concentration of oxygen in partial pressure. At atmospheric pressure, the partial pressure of a gas is numerically the same as the percentage of gas by volume. E.g. if a patient breathes air with an oxygen percentage of 21 % the inspired partial pressure will be 21 kPa. The partial pressure is reduced passing down the respiratory tract because of addition of water vapour and carbon dioxide resulting in a pressure of about 13 kPa in the alveoli. As a rule of thumb the PaO2 should be about 10 less than the inspired oxygen concentration in a healthy individual, a larger gap indicates lung injury. NB! PaO2 decreases slightly with age. PaCO2: Carbon dioxide is a waste product of metabolism. The normal range is 4.5-­‐6 kPa. CO2 behaves like an acid when dissolving in plasma (the small increase in HCO3-­‐ has little effect on the overall concentration): CO2 + H2O ⇔ H+ HCO3-­‐ If the metabolic production of CO2 is constant a decrease in alveolar ventilation will reduce excretion of CO2 causing an increase in PaCO2 and therefore an increase in the concentration of H+. pH decreases and since the primary cause is the respiratory system we call the process a respiratory acidosis. Conversely if the alveolar ventilation increases more CO2 will be excreted and pH will increase causing a respiratory alkalosis. The respiratory center in the brain stem is very sensitive to the concentration of hydrogen ions in the blood and regulates ventilation within minutes. NB! Patients with COPD and a chronic increase of PaCO2 might have developed oxygen sensitivity instead making them less sensitive to PaCO2 increases. Base excess (BE) and bicarbonate: Base excess is calculated as the amount of strong acid or base needed to restore the normal pH of 7.4. We talk about base excess when > +3 mM indicating a metabolic alkalosis and base deficit when < -­‐3 mM indicating a metabolic acidosis. 1
Eberhard KE, Sørensen EM, Thoft L, Uhrenholt S
© SATS Copenhagen 2017 - emss17.sats-kbh.dk
Bicarbonate is the most important buffer. It neutralizes the effect of H+ and prevents decreases in pH. The normal range is 22-­‐28 mM. Bicarbonate is generated by the kidneys. If they fail to produce sufficient bicarbonate to meet the need of metabolic acids pH will decrease causing a metabolic acidosis. If there is an excess of bicarbonate to metabolic acids, pH will increase causing a metabolic alkalosis. Compensation: The respiratory and metabolic systems are linked: CO2 + H2O ⇔ H+ HCO3-­‐ and can therefore compensate for derangements in each other. The respiratory compensation of PaCO2 is relatively rapid while the metabolic adjustment of bicarbonate concentration takes several days. Examples of combined acid-­‐base disturbances with compensation are chronic obstructive pulmonary disease (COPD) and diabetic ketoacidosis. 2