ABG: Blood Gas Analysis Respiratory and Metabolic acidosis and Alkalosis

 Blood gas analysis 

Blood gas analysis is used to direct ventilation and oxygenation strategies and act as a guide to the acid-base balance of the patient. Normal arterial and venous blood gas values listed below

Normal arterial and venous blood gas values

Normal arterial value.                         Normal venous values

 pH             7.35 - 7.45                           pH.         7.35 - 7.45

paCO2       4.6 - 6.0 kPa                         PvCO2    4.5 - 6.0 kPa

HCO3-       22 - 26 mmol/L                   HCO3-    22 - 28 mmol/L

Base excess +2 to -2 mmol/L             Base excess +2 to 2 mmol/L

Lactate       0.5 - 1.5 mmol/L              Lactate      0.5 - 2.2 mmol/L

PaO2          10.0 - 13.3 kPa                    PvO2        11 - 13 kPa

SaO2          >95%                                  SvO2         65 - 75%

pH- this is measure of how acid or alkaline the blood is in the presence of H+ ions.  A low pH is acidotic (more H+ ions), whereas a high pH is alkalotic (fewer H+ ions)

PaCO2- partial pressure of arterial carbon dioxide (CO2), which is the primary metabolite of cellular respiration. CO2 dissolves in water to form carbonic acid, which disassociates into H+ and HCO3 ions. This process is speeded up in the presence of the enzyme carbonic anhydrase.

HCO3- bicarbonate is a buffer found in the plasma, which resists changes in pH by retaining or releasing H+ ions. If there are more HCO3- ions than H+ ions, the blood is more alkalotic. Bicarbonate is produced in the liver and kidneys.

Buffering is the first line of correction of pH, but the HCO3- supply will fall,  so H+ ion elimination is also required. Excess H+ ions are removed from the circulation by the kidneys, which excrete H+ ions in the urine and absorb HCO3 ions in the proximal tubule of the nephron, or by the lungs, which excrete H+ ions in H2O. Removal from the lungs occurs within minutes or hours, whereas removal from the kidneys occurs over a period of days.

Base excess (BE) refers to the amount of acid required to return 1L of blood to pH 7.4 at a pCO2 of 5.3kPa. It reflects the metabolic component of a pH imbalance. 

• Negative BE indicates the amount of acid that needs to be removed in order to return the pH to normal (i.e. as a result of metabolic acidosis )
• Positive BE indicates the amount of acid that needs to be retained or added in order to return the pH to normal (i.e. as a result of a metabolic alkalosis)

Lactate (lactic acid) -this chemical is produced in anaerobic metabolism, and it is in an indicator that increased metabolic demand is exceeding the supply of oxygen available. A raised lactate concentration is associated with acidosis. 

PaO2 - partial pressure of arterial oxygen. Oxygen is the primary substrate for cellular respiration. It binds to haemoglobin to form oxyhaemoglobin. Different physical conditions will affect the affinity of haemoglobin for oxygen. Theses include pH, pCO, temperature and 2, 3- diphosphoglycerate (2, 3-DPG)

SaO2-oxygen saturation is measure of how much oxygen is bound to haemoglobin. The higher the partial pressure of oxygen (PO2), the higher the oxygen saturation will be.

Values are corrected for the patient's specific body temperature, and fractionated inspired oxygen concentration (FiO2) is entered. The standard bicarbonate (stdHCO3-) and the standard base excess values are given with the respiratory components of the acid-base disturbance removed. Therefore if the stdHCO3- is lower than HCO3- the acidosis is metabolic origin.

Arterial blood gas analysis 

Check whether the pH is within the normal range

• If the pH is below the normal range this is acidosis 
• If the pH is above the normal level this is alkalosis
• If the pH is within the normal level, check whether the pCO2 and the and the HCO3- are also within the normal range

Check the cause of the acidosis (low pH)

• If the PCO2 is above the normal range this is respiratory acidosis 
• If the HCO3- is below the normal range this is metabolic acidosis 
• If the PCO2 is above the normal range and the HCO3 is below  the normal range this is mixed acidosis 

Check the cause of alkalosis 

• If the PCO2 is below the normal range this is respiratory alkalosis 
• If the HCO3 is above the normal range this is metabolic alkalosis 
• If the PCO2 is below the normal range and HCO3- is above the normal range this is mixed alkalosis 

Check for compensation 

• If the pH is not within the normal range this is not compensated 
• If the pH is within the normal level and PCO2 and HCO3- are outside the normal range but in opposite directions (i.e. either high PCO2 and low HCO3- or low PCO2 and high HCO3-) this is compensated 

Check whether the pO2 is within the normal range 

• If the PO2 is below  the normal range this is hypoxaemia 
• If the pO2 is above the normal range this is hyperoxia

                              Acidosis and Alkalosis  values 

                                   

                                                     pH              PCO2             HCO3-

Respiratory acidosis                      ↓                   ↑              Normal

Respiratory alkalosis                     ↑                   ↓             Normal

Metabolic acidosis                         ↓              Normal            ↓

Metabolic alkalosis                        ↑              Normal           ↑

Respiratory and metabolic           ↓                   ↑                 ↓

acidosis                                       

 

Respiratory and Metabolic           ↑                  ↓                  ↑

alkalosis

Acid-base disturbances 

Respiratory acidosis - increased production and/or decreased excretion of CO2 causes include 

• Obstructive lung disease 
• Respiratory depression ( over sedation, brain injury, neuromuscular disorder )
• Hypoventilation ( error in non-invasive or mechanical ventilation, chest deformities, or pain restricting ventilation )

Respiratory alkalosis - increased excretion of CO2; causes include :

• Restrictive lung disease 
• Pulmonary q
• Hyperventilation ( error in non-invasive or mechanical ventilation, hypoxia, anxiety )

Metabolic acidosis -increased production or retention of acid and/or increased excretion of base/alkali, causes include 

• Uraemia
• Acid ingestion ( aspirin, ethylene glycol )
• Acid from abnormal metabolism (ketoacidosis)
• Hyperchloraemia ( due to excess intravenous infusion )
• Lactic acidosis ( due to exercise, shock, hypoxia, liver failure, or trauma )
• Loss of alkali ( due to diarrhoea, bowel fistula, or renal tubular necrosis )

Metabolic alkalosis - increased production or retention of base/ alkali and increased excretion of acid. Causes include 

• Alkali ingestion 
• Excess bicarbonate or buffer infusions
• Renal disorders 
• Hypokalaemia
• Drugs ( diuretics, ingestion of alkali)
• Loss of acid (due to gastric aspirates or vomiting ).