- ABG INTERPRETATION - Flashcards
What do ABGs measure?
• Partial pressure of O2 • pH • Partial pressure of CO2 (PaCO2) • Bicarbonate (HCO3) concentration • Base excess • Anion gap
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
What is the estimated PaO2, relative to FiO2
FiO2 40% - PaO2 200mmHg
FiO2 60% - PaO2 300mmHg
FiO2 80% - PaO2 400mmHg
Discuss the role of SaO2 in oxygenation
- the percentage of Hb molecules saturated with O2
- 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
Discuss 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 the causes of respiratory acidosis
- hypoventilation
- CNS/respiratory centre depression
- airway obstruction
- lung disease
- neuromuscular interference
List the 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 the causes of respiratory alkalosis
- hyperventilation/CNS irritation
- hypoxia/hypoxaemia
- anxiety
- mechanical ventilation
- fever/sepsis
- hyperthyroidism
- lung disorders
- liver failure
- drugs - salicylates
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)
Discuss the role of the Anion Gap
The anion gap is the difference between the plasma concentration of the major measured cation (Na+ & K+ ) and the sum of the measured anions (Cl- & HCO3 - )
It is useful in identifying the cause of metabolic acidosis
Normal anion gap < 18 mEq/L
AG = (Na+ + K+ ) – (HCO3 - + Cl)
A high AG means there is an accumulation of acid in body fluids that is not being buffered (loss of HCO3 - without a subsequent increase in Cl)
Discuss the normal ranges on an ABG
pH 7.35 – 7.45 PaO2 80 – 100 mmHg SaO2 95 - 100% PaCO2 35 – 45 mmHg HCO3 - 22 – 26 mEq/L Base Excess -2 - +2 Anion Gap <18
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) - Look at anion gap
Na+ + K+ – Cl- + HCO3 <18 accumulation of acid in body fluid (lactic, ketones, uric, phosphoric etc.)