Blood gas interpretation

Measures:

• pH: normally 7.35-7.45
• PaO2: arterial partial pressure of oxygen - a reflection of the amount of oxygen dissolved in the blood. Normal 10-14kPa.
• PaCO2: Normal 4.5-6kPa - reflects the absolute ventilatory state of a patient and possible respiratory compensatory mechanisms
• HCO3-: normal 24-28mmol/L. Bicarbonate is the main plasma buffer. A low value suggests consumption, often due to increased acid load e.g. lactic acid. A high value suggests retention of base to compensate for hypoventilation causing an acidaemia.
• Base deficit/base excess: normal +2 to -2. This describes whether the body's buffers are being consumed (deficit) or retained (excess).
  • A measure of the amount of fixed base added to an aliquot (1L) of blood to bring pH to 7.4
  • Essentially goes lower in anaerobic metabolism and acidosis
  • Might actually be a better predictor of poor outcomes in trauma patients than lactate - in one study a moderate BD on admission meant a mortality of 15%, while severe BD meant 35% mortality
  • Normal 2 to -2
  • Mild -3 to -5
  • Moderate -6 to -9
  • Severe <-9
• Serum lactate - normal <1.2mmol/L. A reflection of the extent of anaerobic metabolism occurring, and secondarily a measure of the liver's ability to metabolise lactate and regenerate bicarbonate ions.
• Anion gap - normal range 10-15mmol/L.

Relative hypoxaemia is a more useful measure than absolute PaO2, which is essentially comparing it to FiO2

• As FiO2 increases towards 1, PaO2 should also increase
• Use PaO2/FiO2 (should be >40kPa, or patient is hypoxic)

Metabolic acidosis

• Impaired tissue perfusion - deal with cause
• Renal failure - deal with cause, bicarbonate, RRT
• Hepatic failure - deal with cause
  • Or KULT (ketones urea lactate toxins)
• NAGMA - usually hyperchloraemic acidaemia - most often seen following vigorous resus with 0.9% saline, also bladder surgery and ileal conduit formation.

Resp acidosis

• Inadequate CO2 elimination by lungs relative to rate of CO2 produced by cellular metabolism
  • Head or spinal injury - ventilation
  • Drug overdose - ventilate, antidote
  • Chest wall injury - ventilate
  • Myopathy/peripheral neuropathy - ventilate
  • Pulmonary disease - treat disease, resp support +/- ventilate
  • Massive PE - re-establish perfusion of ventilated lung

Metabolic alkalosis

• Typically occurs when there is both an increase in alkali and impaired excretion of bicarb
• In surgical patients, frequently as a result of H+ loss (vomiting or NGT drainage, especially in gastric outlet obstruction), or from excess bicarb regeneration as a result of diuretic therapy
• Can also occur with mineralocorticoid excess

Resp alkalosis

• Occurs when alveolar ventilation exceeds the rate necessary to eliminate CO2 produced by cellular metabolism