Arterial Blood Gas (ABG) Analysis Flashcards
ABG analysis
Sample of arterial blood that is used to measure pH, partial pressure of oxygen, partial pressure of carbon dioxide, bicarbonate level, base excess
Indication of - ventilation, gas exchange, acid-base status
Readings should be related to previous values, clinical presentation of the patient, fraction of inspired oxygen (FiO2)
Normal values for ABGs: pH
7.35 to 7.45
Normal values for ABGs: PaO2
10.7 to 13.3 kPA
Normal values for ABGs: PaCO2
4.7 to 6 kPA
Normal values for ABGs: HCO3-
22 to 26 mmol/L
Normal values for ABGs: BE
-2 to +2
Normal values for ABGs: oxygen saturation (SaO2)
95 to 100%
Steps to analysing an ABG
Check the PaO2
Check the pH
Check the PaCO2
Check the HCO3- and BE
Is it a respiratory or metabolic problem?
Is there any compensation
Abnormalities in PaO2
Check the PaO2 first as hypoxia will kill someone long before anything else
Hypoxia
Hypoxaemia
Abnormalities in PaO2: hypoxia
Reduced oxygen at tissue level
Causes - hypoxaemia, reduced cardiac output, reduced oxygen carrying capacity of the blood, reduced/ restricted blood flow
Abnormalities in PaO2: hypoxaemia
Reduced oxygen in arterial blood
Causes - V/Q mismatch due to wasted perfusion; ventilation isn’t good due to atelectasis or consolidation/ sputum plug, V/Q mismatch due to wasted ventilation; e.g. pulmonary embolism, hypoventilation, diffusion abnormality, available oxygen
pH
Acidotic - pH < 7.35
Alkalotic - pH > 7.45
Responds to respiratory and metabolic changes but cannot differentiate between them
PaCO2 indicates respiratory changes
HCO3- and BE indicates metabolic changes
The renal and respiratory systems work jointly to maintain the blood pH within normal limits
Abnormalities in PaCO2
In the presence of altered pH
Respiratory alkalosis - PaCO2 < 4.7kPA
Respiratory acidosis - PaCO2 > 6.0kPA
Causes of hypocapnia; low CO2 - hyperventilation
Causes of hypercapnia; high CO2 - hypoventilation
Type 2 respiratory failure - high CO2 in the presence of low oxygen
Abnormalities in HCO3- and BE
In the presence of altered pH
Metabolic acidosis - HCO3- < 22mmol/L and BE < -2
Metabolic alkalosis - HCO3- > 26mmol/L and BE > +2
Indicate the extent of renal compensation and thus the metabolic contribution to an acid-base disturbance
Causes of metabolic acidosis - diabetic ketoacidosis, lactic acidosis, diarrhoea/ high GI output, renal failure
Causes of metabolic alkalosis - vomiting, over diuresis
Compensation
Where one system adjusts for a disturbance in the other to try and keep the pH within normal parameters
Partial - trying to balance things out but cannot, respiratory acidosis, as the pH is low, pH - 7.32; acidotic. PaO2 - 10.8 kPa; normal. PaCO2 - 7.2 kPa; acidosis, type 2 respiratory failure. HCO3- - 27.0 mmol/L; alkalosis, trying to balance out the acidotic. BE - +2.7 mmol/L; alkalosis, trying to balance out the acidotic.
Full - they have balanced things out, pH - 7.45; normal. PaO2 - 10.8 kPa; normal. PaCO2 - 7.2 kPa; acidosis. HCO3- - 28.3 mmol/L; alkalosis, trying to balance out the acidosis. BE - +3.2 mmol/L; alkalosis, trying to balance out the acidosis.
Type 1 respiratory failure
Type 1 respiratory failure - PaO2 < 8kPa
Respiratory impairment - PaO2 <10.7-8kPa
The PaCO2 will be normal or low
Does not affect pH
Causes - pneumonia, pulmonary embolus (PE), pneumothorax, pulmonary oedema, acute asthma, acute respiratory distress syndrome (ARDS), fibrosis, chronic obstructive pulmonary disease (COPD), covid
Type 2 respiratory failure
Indicated by - PaCO2 > 6.0kPa and low PaO2
In acute type 2 failure the pH with be acidotic
In chronic type 2 failure the pH may be normal due to compensation.
Occurs due to inadequate alveolar ventilation
Causes - chronic obstructive pulmonary disease (COPD), fatigue, kyphoscoliosis, drugs e.g. opiates, neuromuscular disorders
Respiratory drive
Brain
Central and peripheral chemoreceptors
Mechanoreceptors in the lungs/ chest wall
Other factors - pain, fear etc.
affect rate and depth
Ventilation
Breathing - mechanical flow of air into (inhalation) and out (exhalation) of the lungs
Process by which gases flow between the atmosphere and the alveoli
Air flow is generated because of alternating pressure differences created by contraction and relaxation of respiratory muscles
Factors affecting ventilation - airway latency, air flow, respiratory drive, skeletal structure, muscle function, BMI
Ventilation: inhalation
Pressure inside the alveoli must become lower than atmospheric pressure
Achieved by increasing the volume of the lungs
Boyle’s law
Ventilation: exhalation
Occurs due to elastic recoil of the chest wall and lungs
Passive process
As muscles relax the size of the thoracic cavity reduces and the volume decreases so the alveolar pressure increases
Pressure inside the lungs is greater than the atmospheric pressure gases are exhaled
Pulmonary gas exchange
Cells use oxygen to generate energy and produce carbon dioxide as waste
Depends on the lungs being able to move oxygen in and carbon dioxide out of the blood
Gas exchange - transfer of oxygen from the alveolar air to the blood (oxygenation) and carbon dioxide from the blood to the alveolar air (carbon dioxide elimination)
Diffusion
Partial pressure and diffusion
Partial pressure - pressure exerted by each individual gas in a mixture of gases
Partial pressure of oxygen and carbon dioxide
When a gas is dissolved in a liquid the amount dissolved depends on the partial pressure
Ficks’ law of diffusion - rate of diffusion is proportional to the surface area and concentration difference and is inversely proportional to the thickness of the membrane
Factors affecting diffusion - thickened membrane e.g. fibrosis, Bullae reduce surface area
Oxygen-haemoglobin dissociation curve
Shows relationship between amount of oxygen bound to haemoglobin and the partial pressure of oxygen in the blood
Haemoglobin affinity for oxygen is affected by 4 physiological factors - carbon dioxide, acid-base balance, pH, hypoxic drive
Sifts the curve to the left or right
