ABG and Blood Gas Analysis Lecture PDF

Acid Base Balance & Arterial Blood gases A Clinical approach Part 1: Respiratory Dr Kathryn Steven Objectives • Very brief overview of acid base physiology • Consider the respiratory causes of, and responses to, acid base abnormalities • To learn a simple stepwise approach to the clinical assessm...

Acid Base Balance & Arterial Blood gases A Clinical approach Part 1: Respiratory Dr Kathryn Steven Objectives • Very brief overview of acid base physiology • Consider the respiratory causes of, and responses to, acid base abnormalities • To learn a simple stepwise approach to the clinical assessment of arterial blood gas results • Consider some common clinical scenarios from a respiratory perspective pH scale Physiology • Human body is continually producing acid • pH of the body must be maintained • Buffers – proteins haemoglobin carbonic acid / bicarbonate • Excretion lungs kidneys We measure pH/ acid base with an ABG Normal Values • pH • pO2 • pCO2 • Bicarbonate 7.35 – 7.45 12 -13 kPa 4.5 – 5.6 kPa 22 – 26 mmol/l Remember H2O + CO2 H2CO3 H+ + HCO3- CO2+ H2O = H2CO3 = H+ + HCO3- 07/11/2023 How do we take an ABG? CO2+ H2O = H2CO3 = H+ + HCO3- 7 07/11/2023 CO2+ H2O = H2CO3 = H+ + HCO3- Acid – base disturbances occur when… • There is a problem with ventilation • There is a problem with renal function • Overwhelming acid or base load the body can’t handle Interpretation of ABG Results Step 1: Assess the oxygenation  look at the pO2. Is the patient hypoxic? We have different ways to measure oxygenation…. What if the pO2 is low? There may be a problem with - getting oxygen into the lungs - oxygen getting into the blood - oxygenation of tissues CO2+ H2O = H2CO3 = H+ + HCO3- 07/11/2023 Alveolar – arterial oxygen gradient • Remember to think - what is the inspired oxygen concentration? • Use your clinical history & exam to help deduce what might be the cause of low oxygen levels CO2+ H2O = H2CO3 = H+ + HCO3- 07/11/2023 pO2- 14kPa CO2+ H2O = H2CO3 = H+ + HCO3- 07/11/2023 Is the pO2 too high? Adverse effects of high oxygen levels • • • • Increases risk of hypercapnic respiratory failure in acute exacerbations of COPD Increased mortality survivors of cardiac arrest Increased mortality intensive care patients Increased mortality in acute severe asthma Why?? • Generates free radicals • Collapse of alveoli due to atelectasis • Irritating to mucous membranes Leads to ocular toxicity, myocardial damage, neuro… 17 British Thoracic Society guidelines • Oxygen is a treatment for hypoxia not dyspnoea alone. • In an unstable medical emergency give high conc of oxygen then titrate to target once stable • Targets 94-96% (normally) 88-92% (type 2 resp failure) 18 Step 2: Assess the pH • pH <7.35 acidaemia • pH >7.45 alkalaemia • pH between 7.35 and 7.45  2 options – normal mixed acid base abnormality Step 3: What appears to be the cause for the acidosis / alkalosis? If the PH is low - what would a RESPIRATORY cause for this acidosis look like in the ABG? - what might a METABOLIC cause for this acidosis look like in the ABG? Step 4: Is there compensation seeming to occur? Remember we said that the body will always try to maintain pH between 7.35 – 7.45. Compensation is altering of function of the respiratory or renal system in an attempt to correct an acid – base imbalance If the PH is low because of a respiratory cause – how might the body compensate? If the pH is high because of a respiratory cause – how might the body compensate? If the pH is low because of a metabolic cause – how might the body compensate? Time for an example! Remember, today we are thinking about respiratory causes Example 1 Arterial blood gas results • pH • pO2 • pCO2 • HCO3- 7.20 28.7kPa 11kPa 25mmol/l What does this blood gas show? Is compensation occurring? • Look at the pCO2 • Look at the bicarbonate • If pCO2 and HCO3- move in the same direction compensation is possibly occurring • If both values move in opposite directions more than 1 pathology must be present Arterial blood gas results • pH • pO2 • pCO2 • HCO3-(standard) 7.20 28.7kPa 11kPa ↑ 25mmol/l Example – acute respiratory acidaemia Example 2 A patient with a history of COPD and currently smoking 20 cigarettes per day attends the respiratory clinic. Results • pH • pO2 • pCO2 • HCO3(standard) 7.32 6kPa 10.6kPa 37mmol/l What does this blood gas show? Chronic Respiratory Alkalosis • In chronic respiratory acidosis the kidneys compensate by retaining bicarbonate. This takes a few days to reach its maximal value. Summary - Respiratory Acidosis • pH • pO2 • pCO2 • HCO3-(std) 07/11/2023 Acute Chronic 7.20 7.32 28.7kPa 6.0kPa 11kPa 10.6kPa 25mmol/l 37mmol/l CO2+ H2O = H2CO3 = H+ + HCO3- Example 3 A 25 year old man with a history of asthma is brought to A&E by ambulance with acute shortness of breath and wheezing for the last two hours. His pulse is 120, RR 36 and auscultation reveals widespread wheeze throughout the chest Arterial Blood Gas Results pH pO2 pCO2 HCO3(std) 7.55 17.0kPa 3.3kPa 22mmol/l What does this blood gas show? Causes of hyperventilation • Acute severe asthma • Pulmonary embolism • Pulmonary oedema • Anxiety attack Abnormal level of central respiratory drive Hypoxia Direct stimulation of respiratory centre Psychogenic Chronic respiratory alkalosis Don’t worry about it unless considering high altitude when… Hypoxaemia induced hyperventilation is prominent Compensation occurs by renal excretion of bicarbonate For interest Caudwell expedition http://www.xtreme-everest.co.uk/ Example 4 A 17 year old patient with Type 1 Diabetes Mellitus (T1DM) is rushed to A&E. A finger prick capillary blood glucose is 28.6 Her pulse is 112, RR 28 with sighing deep breaths Arterial blood gas results • pH • pO2 • pCO2 • HCO3-(standard) 7.28 12.7kPa 3.0 kPa 16 mmol/l What does this ABG show? Summary • Use a structured approach to interpretation of ABG • Always consider the clinical context • Treat the patient not the numbers!! Recommended Reading: • Arterial Blood Gases Made Easy Hennessey & Japp ISBN 978-0-443-10414-5