Partial preview of the text
Download ARTERIAL BLOOD GASES (ABG) and more Study notes Medicine in PDF only on Docsity!
Arterial blood gases Assessing respiratory function Arterial sampling helps in the assessment of a patient with low SpO, or pa- tients with known lung disease (especially if they are receiving supplemental O,). Document the FiO,,. Look specifically for: e Hypoxia (pO, <10.6kPa on air). e Hypercarbia (pco, >6.0kPa). e Bicarbonate retention (HCO, >28mmol/L). e Acidosis (pH <7.35). Differentiating between type | and type Il respiratory failure In type | failure, there is hypoxia with normal or + pCO., In type Il failure, there is hypoxia with tT pCO, and frequently t HCO,. In type II failure, the patient may develop life-threatening respiratory failure if administered high concentrations of O,. Aim to maintain SpO, at 88-92% in COPD, and recheck ABGs in 30min. Differentiating between acute and chronic type Il respiratory failure Patients who normally have a slightly t pCO, will also show t HCO, on ABG. The kidneys adapt over a period of days to retain HCO,, in an at- tempt to buffer the respiratory acidosis (see D nomogram inside front cover). Respiratory acidosis in a patient with chronic type II respiratory failure (t pCO,, t HCO,, and pH <7.35) indicates life-threatening impair- ment of lung function. In acute respiratory failure, the lungs are unable to eliminate CO, (caused by + GCS or hypoventilation from any cause), which results in t pCO, and respiratory acidosis. Patients may require ventilatory support. Metabolic acidosis The usual pattern of results in metabolic acidosis is pH <7.35, HCO,— <24mmol/L, and base excess (BE) <-2mmol/L. There may be compen- satory hypocarbia (pCO, <4.5kPa). Metabolic acidosis has many possible causes: e ft acid load (lactic acidosis, ketoacidosis, or ingestion of salicylates, methanol, ethylene glycol, or metformin). e + removal of acid (renal failure or renal tubular acidosis types 1 and 4). e Loss of HCO, from the body (diarrhoea, pancreatic or intestinal fistulae, acetazolamide, or renal tubular acidosis type 2). The anion gap The anion gap is the quantity of anions not balanced out by cations (a measurement of negatively charged plasma proteins). The normal value is 12—16mmol/L. It is measured by (all measured in mmol/L): (Na* +K*) - (cr +HCO,) Measuring the anion gap helps distinguish the cause of metabolic acidosis. A high anion gap indicates that there is excess H* in the body. The most common cause of a high anion gap metabolic acidosis is lactic acidosis. Most blood gas analysers measure lactate (normal <2.0mmol/L). 
