Arterial Blood Gases. Dr Mark Young Mater Health Services

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1 Arterial Blood Gases Dr Mark Young Mater Health Services

2 Why do them? Quick results Bedside test Range of important information Oxygenation Effectiveness of gas exchange Control of ventilation Acid base status Haemoglobin Electrolytes

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4 ph acid base Normal range Low ph = acidaemia High ph = alkalaemia Acidosis/alkalosis refers to the process that changes ph

5 The 4 simple acid base disorders : Respiratory acidosis Respiratory alkalosis Metabolic acidosis Metabolic alkalosis. Respiratory disorders are caused by abnormal processes which tend to alter ph because of a primary change in pco2 levels. Metabolic disorders are caused by abnormal processes which tend to alter ph because of a primary change in [HCO3-].

6 PaCO 2 Reflects alveolar ventilation High PaCO 2 = under ventilation Low PaCO 2 = over ventilation Acute changes in PaCO 2 will alter ph CO 2 is an respiratory acid Underventilation leading to high PaCO 2 respiratory acidosis Over ventilation leading to low PaCO 2 respitatory alkalosis PaCO2 is normally around 2-5mmHg greater than ETCO 2

7 HCO 3 - Bicarbonate is a weak base Regulated by kidneys Is the metabolic component Together CO 2 and HCO 3 act as a buffer for each other H 2 O + CO 2 <-> H 2 CO 3 <-> HCO H +

8 Buffer changes Body aims to maintain status quo (homeostasis) CO2 - HCO 3 system acts as a buffer Example underventilation Increasing PaCO 2 Buffer leads to increased HCO 3 - and H + production More acid is produced = acidaemia ph goes down = acidosis Cause is ventilation/co 2 = respiratory acidosis

9 Compensatory changes Longer term Control of bicarbonate through kidney Eg Patient being underventilated long term in ICU Initial increase in PaCO 2 and decrease in ph respiratory acidosis Body responds to increased acid by retaining more bicarbonate in the kidneys Leading to increase in HCO 3 - metabolic compensation Respiratory acidosis Increase PaCO 2, decreased ph If normal HCO 3 acute respiratory acidosis If high HCO 3 compensated/chronic respiratory acidosis

10 Metabolic acidosis Eg diabetic ketoacidosis Body is producing too much acid acidosis H + combines with HCO 3 Decreased HCO 3 Increased CO 2 production Patient increases respiratory rate to keep CO 2 normal = Metabolic acidosis Decreased HCO 3 Decreased ph CO 2 if normal = acute metabolic acidosis If low then chronic metabolic acidosis

11 PaO 2 Reflects gas exchange Normally PaO 2 decreases with age Expected PaO 2 = 100 (age x ¼ ) Low PaO 2 = hypoxaemia Hypoventilation Mismatch of ventilation/perfusion V/Q mismatch Therefore if PaCO 2 is normal, then hypoxaemia is almost certainly caused by ventilation-perfusion mismatch

12 The alveolar arterial oxygen gradient P(A-a)O 2 = PAO 2 PaO 2 PaO 2 = arterial oxygen tension PAO 2 = alveolar oxygen tension PAO 2 = FiO2(P B - P H2O ) - 1.2(PaCO 2 ) FiO 2 = oxygen fraction in inspired air P B = barometric pressure (760 mmhg at sea level) P H2O = water vapour tension (47 mmhg at 37 C) Normal value <15 mmhg If >15mmHg suggests a VQ mismatch related to the lung

13 Base excess The metabolic component of the acid-base balance Calculated Amount of acid required to restore a litre of blood to it s normal ph at a PaCO 2 of 40mmHg Increases in metabolic alkalosis Decreases/becomes more negative in metabolic acidosis Can become quite confusing when Mixed pattern Partial compensation

14 Factors influencing ABG results Delay in sampling Falsely low PaO 2 due to oxygen consumption Transport on ice if a delay Air bubbles Falsely high PaO 2 Falsely low PaCO 2 Gently remove air bubbles immediately after collection without agitating Body temperature Complex arrangement affecting PCO 2 and ph Venous blood gas Hb and electrolytes the same ph, CO 2 and HCO 3 are similar Oxygen is less than half that of arterial

15 ABG is just a test It doesn t diagnose Eg a patient with severe asthma could have the same ABG as a patient with pneumonia The degree of abnormality on an ABG doesn t reflect the degree of abnormality for the patient Eg low PaO 2 doesn t indicate tissue hypoxia a normal PaO 2 doesn t mean tissue oxygenation in normal Influenced by Hb, CO, regional blood flow Therefore always need to interpret ABG results in the clinical context to facilitate diagnosis and managment