Arterial blood gas

The respiratory and metabolic systems work together to keep the body’s acid-base balance within normal limits. Indications for ABG analysis include: • To diagnose and establish the severity of respiratory failure • To evaluate the need for long-term oxygen therapy. • Manage patients in a critical care setting • Monitor the patients receiving non-invasive ventilation • Assess patients’ condition following cardiac/respiratory arrest • Establish a baseline prior to surgery • Monitor patients during cardiorespiratory exercise tests and sleep studies. The parameters measured are: • pH—an indication of hydrogen ion (H+) concentration in the blood, pH shows the blood’s acidity or alkalinity • Partial pressure of arterial oxygen (PaO2)—PaO2 reflects the body’s ability to pick up oxygen from the lungs • Partial pressure of arterial carbon dioxide (PaCO2)—PaCO2 reflects the adequacy of lung ventilation and carbon dioxide elimination • Bicarbonate (HCO3)—known as the metabolic parameter it reflects the ability of the kidneys to retain and excrete HCO3 • Base excess—a mathematically calculated figure that reflects the amount of acid or base needed to change one litre of blood to a pH of 7.4. A systematic approach should be followed when interpreting ABG tests, and the results should be considered in the context of the patient’s history. The results can be misleading if there are mixed respiratory and non-respiratory factors.

Compensation

This is the body’s attempt to maintain a normal pH level. The body uses the respiratory and metabolic systems to oppose each other to maintain a normal pH. Three types of compensation are possible: Uncompensated, partially compensated, and fully compensated. • Uncompensated The pH is abnormal and either the PaCO2 or HCO3 is abnormal. There is nothing in the results to indicate that the opposite system has tried to correct for the other. This is often seen when a patient is acutely unwell. An example of uncompensated respiratory acidosis would be: ✅ pH 7.17 This is low (acid) ✅ PaO2 6.6kPa This is low (hypoxic respiratory failure) ✅ PaCO2 9.4kPa This is high which explains why the pH is acid ✅ HCO3 25 This is normal so no evidence of compensation • Partially compensated The pH is again abnormal and both the PaCO2 and the HCO3 are also abnormal. One of the parameters will follow the same direction as the pH. This will indicate the primary problem (respiratory or metabolic). The third parameter will be moving in the opposite direction in order to compensate for the primary disorder but will not have changed enough to normalize the pH. An example of partially compensated respiratory acidosis would be: ✅ pH 7.3 This is low (acid) ✅ PaO2 6.8kPa........ This is low (hypoxic respiratory failure) ✅ PaCO2 8.6kPa..... This is high which explains why the pH is acid ✅ HCO3 30................ This is high, so evidence that compensation is starting to take place. • Fully compensated The pH is normal but may be at the upper or lower range of normal. The PaCO2 and HCO3 will be abnormal and moving in opposite directions, one compensating for the other ✅ pH 7.37 This is just within normal limits but slightly acid ✅ PaO2 6.9kPa This is low (hypoxic respiratory failure) ✅ PaCO2 7.3kPa This is high which explains why the pH is slightly acid, so the problem is respiratory in origin ✅ HCO3 29.8 This is high, the kidneys have produced bicarbonate which explains why pH is in normal limits despite a high CO2

Causes, signs, and symptoms of deranged ABGs

1. Respiratory acidosis Causes: Mechanisms that reduce ventilation such as: COPD/asthma, blocked airway by tumour or foreign body, spontaneous lung collapse, chest wall injury, Central nervous system depression from: drugs (morphine, barbiturates), Hypoventilation from: pulmonary, cardiac, musculoskeletal or neuromuscular diseases Signs and symptoms: • Headache • Tachycardia • Confusion • Restlessness • Sweating • Fatigue 2. Respiratory alkalosis Hyperventilation from: anxiety, pain, infection causing fever, brainstem damage drugs (aspirin) Increased ventilation–caused by hypoxic drive: pneumonia, diffuse interstitial lung disease, high altitude Signs and symptoms: • Rapid respirations • Numbness • Light-headedness • Twitching • Anxiety • Fear 3. Metabolic acidosis Causes: HCO3 depletion from diarrhoea Liver, renal and endocrine disease Drug intoxication Signs & Symptoms: • Deep respirations • Fatigue • Headache • Drowsiness • Nausea/vomiting • Abdominal pain • Coma (if severe) 4. Metabolic alkalosis Loss of hydrochloric acid from prolonged vomiting Loss of potassium from diuretics or steroids Excessive alkali ingestion Signs& Symptoms: • Slow, shallow respirations • Restlessness • Twitching • Confusion • Irritability • Coma (if severe)