Understanding ABGs Flashcards
Normal Ranges
pH: 7.35 – 7.45 PaCO2: 4.7 – 6.0 kPa || 35.2 – 45 mmHg PaO2: 11 – 13 kPa || 82.5 – 97.5 mmHg HCO3–: 22 – 26 mEq/L Base excess (BE): -2 to +2 mmol/L
PaO2 of Patient on O2 Therapy
PaO2 should be approximately 10kPa less than the % inspired concentration FiO2
Hypoxaemia
Hypoxaemia: <10kPa PaO2 on air
Severe hypoxaemia/resp failure: <8kPa on air
Type 1 Resp Failure
Hypoxaemia and normocapnia
Result of ventilation/perfusion (V/Q) mismatch
Examples of V/Q Mismatch
Reduced ventilation w normal perfusion (pulm oedema, bronchoconstriction)
Reduced perfusion w normal ventilation (PE)
Type 2 Resp Failure
Hypoxaemia and hypercapnia
Result of alveolar hypoventilation
Causes of hypoventilation
Incr airway resistance due to obstruction (COPD)
Reduced lung tissue/chest wall compliance (pneumonia, rib fracture, obesity)
Reduced strength of resp muscles (MNR)
Drugs acting on resp centre reducing overall ventilation (opiates)
Acidotic/Normal/Alkalotic
Acidotic: pH <7.35
Normal: pH 7.35 – 7.45
Alkalotic: pH >7.45
Respiratory or Metabolic Change
Changes in pH are caused by an imbalance in the CO2 (respiratory) or HCO3– (metabolic)
Compensation
The body can try and adjust other buffers (CO2/HCO3) to keep the pH within the normal range.
Compensation in Respiratory
Adjust HCO3
Compensation in Metabolic
Adjust CO2
HCO3
Base which helps mop up acids (H+ ions).
When HCO3– is raised –> pH is increased as there are less free H+ ions (alkalosis).
When HCO3– is low –> pH is decreased as there are more free H+ ions (acidosis).
Resp Acidosis
Decr pH
Incr CO2
Normal HCO3
Resp Alkalosis
Incr pH
Decr CO2
Normal HCO3
Resp acidosis w metabolic compensation
Decr/normal pH
Incr CO2
Incr HCO3
Resp alkalosis w metabolic compensation
Incr/normal pH
Decr CO2
Decr HCO3
Metabolic Acidosis
Decr pH
Decr HCO3
Normal CO2
Metabolic Alkalosis
Incr pH
Incr HCO3
Normal CO2
Metabolic acidosis w resp compensation
Decr pH
Decr HCO3
Decr CO2
Metabolic alkalosis w resp compensation
Incr pH
Incr HCO3
Incr CO2
Base excess
High BE: >+2mmol/L, primary metabolic alkalosis/comp resp acidosis
Low BE:
Rate of Compensation
Resp compensation for metabolic disorder: quickly (by adjusting alveolar vent)
Metabolic compensation for resp disorder: few days (kidneys must adjust HCO3 production)
Mixed Acidosis and Alkalosis
CO2 and HCO3 moving in opposite directions
Treatment is directed towards correcting each primary acid/base disturbance
Causes of Resp Acidosis
Respiratory depression (e.g. opiates)
Guillain-Barre: paralysis leads to an inability to adequately ventilate
Asthma
COPD
Iatrogenic (incorrect mechanical ventilation settings)
Causes of Resp Alkalosis
Anxiety (i.e. panic attack)
Pain: causing an increased respiratory rate.
Hypoxia: resulting in increased alveolar ventilation in an attempt to compensate.
Pulmonary embolism
Pneumothorax
Iatrogenic (e.g. excessive mechanical ventilation)
Causes of Metabolic Acidosis
Increased acid production or acid ingestion.
Decreased acid excretion or rate of gastrointestinal and renal HCO3– loss.
High Anion Gap Metabolic Acidosis
Diabetic ketoacidosis
Lactic acidosis
Aspirin overdose
Renal failure
Normal Anion Gap Metabolic Acidosis
Gastrointestinal loss of HCO3– (e.g. diarrhoea, ileostomy, proximal colostomy)
Renal tubular disease
Addison’s disease
Causes of Metabolic Alkalosis
Gastrointestinal loss of H+ ions (e.g. vomiting, diarrhoea)
Renal loss of H+ ions (e.g. loop and thiazide diuretics, heart failure, nephrotic syndrome, cirrhosis, Conn’s syndrome)
Iatrogenic (e.g. addition of excess alkali such as milk-alkali syndrome)
Causes of Mixed Resp/Metabolic Acidosis
Cardiac arrest
Multi-organ failure
Causes of Mixed Resp/Metabolic Alkalosis
Liver cirrhosis in addition to diuretic use
Hyperemesis gravidarum
Excessive ventilation in COPD
