Arterial Blood Gases Dr Mark Young Mater Health Services
Why do them? Quick results Bedside test Range of important information Oxygenation Effectiveness of gas exchange Control of ventilation Acid base status Haemoglobin Electrolytes
ph acid base Normal range 7.35-7.45 Low ph = acidaemia High ph = alkalaemia Acidosis/alkalosis refers to the process that changes ph
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-].
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
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 3 - + H +
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
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
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
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
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
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
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
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