ABG INTERPRETATION Flashcards
What do ABGs measure?
- pH
- Partial pressure of O2 (PaO2)
- Partial pressure of CO2 (PaCO2)
- Bicarbonate (HCO3) concentration
- Base excess
Discuss the role of PaO2 in oxygenation
- partial pressure of oxygen in arterial blood
- guide to efficiency of gas exchange from the lungs into the blood
- normal 80-100mmHg on 21% of FiO2
- should be assessed in conjunction with FiO2
Discuss the role of SaO2 in oxygenation
- the percentage of Hb molecules saturated with O2 in arterial blood • measure of their ability to carry O2
- important for oxygen content and delivery to the tissues
- normal 95-100% on 21% of FiO2
- should be assessed in conjunction with FiO2
Define full, partial and uncompensated metabolic and respiratory abnormalities
- Full compensation – pH within normal limits with values of respiratory and metabolic components outside normal range and in opposite directions
- Partial compensation – compensation goes in the same direction as the cause but unable to normalise pH
- Uncompensated – one system has made no correction
List some causes of respiratory acidosis
- hypoventilation
- CNS/respiratory centre depression
- airway obstruction
- lung disease
- neuromuscular interference
List some causes of metabolic acidosis
increased production H+ (ketoacidosis)
- altered cellular metabolism (lactic acidosis)
- ingestion toxins increase lactate production
- renal failure (decreased excretion H+)
- excessive loss HCO3 (diarrhoea, renal tubular acidosis)
List some causes of respiratory alkalosis
- hyperventilation/CNS irritation
- hypoxia/hypoxaemia
- anxiety
- mechanical ventilation
- fever/sepsis
- hyperthyroidism
- lung disorders
- liver failure
- drugs
List the causes of metabolic alkalosis
• excess loss H+ (vomiting, NG suctioning, Cl- wasting diarrhoea) • excess HCO3 - (diuretic therapy, overuse antacids, citrate in blood transfusion)
State the normal ranges on an ABG
- pH 7.35 – 7.45
- PaO2 80 – 100 mmHg
- PaCO2 35 – 45 mmHg
- HCO3 - 22 – 26 mEq/L
- Base Excess -2 - +2
- SaO2 95 - 100%
Discuss a clinical approach to ABG analysis
- Look at oxygenation (PaO2) Hypoxia?
- Look at pH Is the pH normal?
- Is your patient acidotic (pH < 7.35) Is your patient alkolitic (pH > 7.45)
- Look at PaCO2 to determine if respiratory or metabolic Is pCO2 normal?
- Is pCO2 increased? (acidosis or compensation)
- Is pCO2 decreased? (alkolosis or compensation)
- Look at HCO3 - Is the HCO3 normal?
- Is the HCO3 increased (alkolosis or compensated)
- Is the HCO3 decreased? (acidosis or compensated)
Interpret the following:
- 10L of oxygen via a mask
PaO2: 7.0 kPa (11-13 kPa)
pH: 7.29 (7.35 – 7.45)
PaCO2: 9.1 kPa (4.7-6.0 kPa)
HCO3–: 26 (22-26 mEg/L)
Base excess: +1 (-2 to +2)
Respiratory acidosis, caused by type 2 respiratory failure (a failure of ventilation) leading to increased levels of CO2 (hypercapnia).
- The PaO2 is low, so we know the patient is in respiratory failure, however, we don’t yet know what type.
- pH reveals an acidosis and look at the CO2 to see if it is contributing to the acidosis (↑CO2).
- In this case, the PaCO2 is raised significantly and this is likely the cause of the acidosis. In the context of low PaO2, a raised PaCO2 suggests the patient type 2 respiratory failure.
- The HCO3– is normal, so the metabolic system is not contributing to the acidosis and also isn’t compensating for the respiratory acidosis, suggesting that this is an acute derangement.
- The base excess is within normal limits as there has been no significant change in the amount of HCO3–. If this respiratory acidosis was chronic we would expect that the kidneys would have generated more HCO3– to compensate, which would have resulted in an increased BE.
Interpret the following:
- On room air
PaO2: 14 kPa (11-13 kPa)
pH: 7.49 (7.35 – 7.45)
PaCO2: 3.2 kPa (4.7-6.0 kPa)
HCO3: 22 (22-26 mEg/L)
BE: +2 (-2 to +2)
Respiratory alkalosis occuring as a result of increased ventilation
- A PaO2 of 14 kPa on air is at the upper limit of normal, so the patient is not hypoxic.
- A pH of 7.49 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).
- The CO2 is low, which would be in keeping with an alkalosis, so we now know the respiratory system is contributing to the alkalosis and is likely the entire cause of it.
- HCO3- is normal, ruling out a mixed respiratory and metabolic alkalosis, leaving us with an isolated respiratory alkalosis.
- Base excess is on the low end of normal, suggesting there has been no addition of bicarbonate to cause the alkalosis, ruling out the metabolic system as the cause
Compensation would involve a much more significant reduction in HCO3***
Interpret the following:
- On room air
PaO2: 12.7 kPa (11-13 kPa)
pH: 7.50 (7.35 – 7.45)
PaCO2: 5.5 kPa (4.7-6.0 kPa)
HCO3-: 29 (22-26 mEg/L)
BE: +3 (-2 to +2)
Metabolic alkalosis
- A PaO2 of 12.7 kPa on air is normal, so the patient is not hypoxic.
- A pH of 7.50 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).
- The CO2 is normal, which is not in keeping with an alkalosis, so we now know the respiratory system is not the cause of this derangement. The next step is to look at the HCO3- and see if it explains the alkalosis.
- HCO3– is high, which is in keeping with a metabolic alkalosis.
- Base excess is increased, in keeping with an excess of HCO3–.
The respiratory system can attempt to compensate for a metabolic alkalosis by increasing PaCO2 (decreasing ventilation), but in the short term, the respiratory system will likely maintain PaCO2 within the normal range.
If the metabolic alkalosis persists, however, you would expect the PaCO2 to rise and compensate for the metabolic alkalosis, as the respiratory centre becomes progressively desensitized to the increasing levels of PaCO2.
Interpret the following:
- On 3l oxygen via NC
PaO2: 9.1 kPa (11-13 kPa)
pH: 7.30 (7.35 – 7.45)
PaCO2: 8.4 kPa (4.7-6.0 kPa)
HCO3-: 29 (22-26 mEg/L)
BE: +4 (-2 to +2)
Respiratory acidosis with metabolic compensation
- A PaO2 of 9.1 kPa is low, confirming that the patient is hypoxic. It is important to recognise that this PaO2 is much lower than you would expect for a patient on 3L of oxygen.
- A pH of 7.30 is lower than normal and therefore the patient is acidotic. The next step is to figure out whether the respiratory system is contributing to the acidosis (e.g. ↑ CO2).
- The CO2 is raised significantly, which is in keeping with an acidosis (and also type 2 respiratory failure), so we now know the respiratory system is likely the cause of this derangement (or at least a contributor).
- HCO3- is high, which is not in keeping with an acidosis, so the metabolic system is not contributing to the acidosis. In fact, the raised HCO3– is compensating for the low pH.
- Base excess is increased, in keeping with an excess of HCO3–.
An oxygen flow rate of 3L via nasal cannulae would be expected to deliver an inspired concentration (FiO2) of around 32%, therefore you would expect that the PaO2 would be approximately 10 kPa less than this (e.g. 22 kPa). A PaO2 of 9.1 kPa is therefore grossly abnormal and indicates significant hypoxia.
Interpret the following:
- On room air
PaO2: 12.4 kPa (11-13 kPa)
pH: 7.29 (7.35 – 7.45)
PaCO2: 5.5 kPa (4.7-6.0 kPa)
HCO3-: 15 (22-26 mEg/L)
BE: – 4 (-2 to +2)
Metabolic Acidosis
- A PaO2 of 12.4 kPa is normal, ruling out hypoxia
- A pH of 7.29 is abnormally low and therefore the patient is severely acidotic. The next step is to figure out whether the respiratory system is contributing to the acidosis (e.g. ↑ CO2).
- The CO2 is normal and therefore the respiratory system doesn’t appear to be contributing to the acidosis.
- HCO3– is low, which is in keeping with an acidosis, so the metabolic system is the cause of this patient’s acidosis.
- Base excess is low, in keeping with a metabolic acidosis.
- There is no evidence of respiratory compensation for this metabolic acidosis (e.g. ↓CO2).
