Arterial Blood Gases

1. Arterial Blood Gases ABGs

2. Samples and Management • Samples are obtained from a needle puncture into the peripheral artery or from an arterial line. • ABG samples must be mixed with heparin to prevent coagulation. • Pressure must be applied for 5 minutes after the needle is withdrawn, to prevent haemorrhage or haematoma. • The blood sample should be labelled correctly, including the time, date and level of oxygen therapy the patient is receiving. • Non-iced samples should be analysed within 10-15 minutes. • Iced samples should be analysed within 1 hour.

3. ABG’s are taken to Evaluate the Patient’s…. Oxygenation Ventilation Acid base balance This can help indicate respiratory, cardiac, renal and metabolic disorders.

4. PaO2 SaO2 PaCO2 pH HCO3 Base excess 10– 13 kPa 95 – 100% 4.5 – 6.0 kPa 7.35 – 7.45 units 22 – 26 Eq/L or 24 - 27mmol/L -2 - +2 Normal ABGs Values

5. PaO2 - arterial partial pressure of Oxygen • Measures the amount of dissolved oxygen in the blood. • Indicates potential tissue oxygenation. • Pa02 <10kPa indicates hypoxaemia, this is when the cells are deprived of oxygen, this can lead to irreversible damage. • Causes – hypoventilation, airway obstruction, COPD.

6. SaO2 – Oxygen Saturation • This measures the percentage of saturated haemoglobin carrying oxygen. • When measured using a pulse oximetry it is recorded as SpO2. • SaO2 measures the saturation of arterial blood • SpO2 measures the saturation of peripheral capillary blood.

7. PaCO2- arterial partial pressure of carbon dioxide • Measures the concentration of carbon dioxide in the blood. • Provides information on ventilation and if the respiratory system is working effectively. • Carbon dioxide is transported in the plasma in a solution form called carbonic acid. This is an acidic solution and if the patient has too little or too much CO2 it can effect the blood pH. i.e too little CO2 = alkaline too much CO2 = acidic

8. pH - of the blood • Indicates the acidity or alkalinity of the blood. • Small pH changes can be life-threatening. • A blood pH of <7.0 or >8.0, a patient would be unlikely to survive. • A pH of <7.35 is acidic, decreases cardiac contractions. • A pH of >7.45 is alkaline, interferes with tissue oxygenation.

9. HCO3 – Bicarbonate ion concentration • Bicarbonate is the main chemical buffer in plasma and prevents large changes in pH. • It is of an alkali. • It indicates how well the metabolic system is working, this is closely linked to the kidneys.

10. BE – Base Excess • Measures the amount of buffer requires to maintain normal pH to a litre of blood. • BE is negative the blood is alkali • BE is positive the blood is acidic

11. Respiratory acidosis • This is when the level of CO2 builds up as the lungs aren’t expelling it properly. This forces the level of carbonic acid to increase causing the blood pH to drop. • Causes hypoventilation, airway obstruction, bronchopneumonia. • Respiratory acidosis is identified when the blood pH is less then 7.35 and the PaCO2 greater than 6kPa. • The increase in carbonic acid causes the blood pH to drop. Bicarbonate cannot directly buffer carbonic acid but it can prevent hydrogen ions being released into the blood maintaining pH. This is known as the compensated respiratory acidosis.

12. Respiratory alkalosis • Is when the lungs are eliminating too much CO2 and the level of carbonic acid in the blood can drop and the blood can become alkali. • This is caused by hyperventilation, fall in oxygen levels, pulmonary disease and high altitude. • Respiratory alkalosis is identified when the blood pH is greater than 7.45 and the PaCO2 less than 4.5kPa. • The kidneys can compensate this by reducing the level of bicarbonate in the blood (as bicarbonate is an alkyl) therefore the pH is decreased.

13. Metabolic acidosis • Bicarbonate levels are low because the buffer has already been used or been lost, possibly turning the blood acidic. • Is caused by metabolic problems such as cardiac arrest, diabetic ketoacidosis, diarrhoea, kidney failure. • Metabolic acidosis is identified by a blood pH less than 7.35 & a HC03 level of less than 24mmol/L • The lungs can compensate by breathing off more CO2 reducing the levels of CO2 which therefore increases alkalinity returning blood pH to normal levels

14. Metabolic alkalosis • Happens when bicarbonate levels increase or there is lose of acid. • Causes are vomiting, diuretic therapy, dehydration and electrolyte imbalance. • Metabolic alkalosis is identified by a pH greater than 7.45 and HCO3 greater than 27mmol/L. • The lungs compensate by retraining CO2 which increase carbonic acid levels and therefore lowers pH.

15. Four steps to analysing ABGs • Examine the pO2 values. Is the patient hypoxamia? • Note the pH value, does it present acidosis or alkalosis • Study the pCO2 and HCO3 values. If the pCO2 is abnormal and the HCO3 normal this indicates respiratory irregularity. If the HCO3 is abnormal and pCO2 normal this indicates metabolic irregularity. 4. If both pC02 and HCO3 values are abnormal then you need to investigate which is the primary disorder.

16. References Allen, K. (2005) Four-step method of interpreting arterial blood gas analysis. Nursing Times. 101 (1) p. Claydell, H. and Derrico, D. (1999) Mastering ABGs. American Journal of Nursing. 99 (8) p.26-32. Coombs, M. (2001) Making sense of arterial blood gases. Nursing Times. 97 (27) p. Lynes, D. (2003) An introduction to blood gas analysis. Nursing Times. 99 (11) p. Woodrow, P. (2004) Arterial blood gas analysis. Nursing Standard. 18 (21) p.45-52.