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ABG & Spirometry Sufian Naseer ILOs Understand that clinical history and current clinical status is important to give context to ABG results Recall and apply the steps of ABG interpretation to clinical data, including: Interpretation of PaO2 in the context of supplemental oxygen Interpretation of pH Interpretation of PaCO2 Interpretation of HCO3-, base excess Understand the differences between type I and type II respiratory failure and recall common associated conditions Understand the meaning of partial and complete respiratory or metabolic compensation Understand basic physiology behind acid-base balance in the context of ABG Interpret clinical data and link to a clinical history to generate a list of differential diagnosesUse code CBOSCECREW24 at checkout on geekyquiz.com for 10% off OSCE flashcards, OSCE stations and knowledge bundles.Arterial Blood Gases (ABGs) Know your patient ABG is a useful tool Provides information about the physiology of the patient But needs to be understood in the clinical context - Are they on Oxygen? If yes, then how much? - Do the patient’s symptoms match up with the ABG result? Have a structured approach 1. Oxygenation (PaO2) 2. pH 3. PaCO2 4. Bicarbonate 5. Base Excess Oxygenation (PaO2) - On Room Air, PaO2 should be greater than 10 kPa - If on Oxygen - PaO2 should be 10 kPa less than % inspired concentration FiO2 - 40% oxygen -> ? - Different oxygen delivery devices will offer different flow rates - Nasal cannula, Simple face mask, Non-rebreathers, Venturi masks Golden Rule: if PaO2< 8 kPa (on air) - patient is in respiratory failure Respiratory Failures Type I respiratory failure - Hypoxaemia (<8 kPa) - Normocapnia (PaCO2 < 6 kPa) - Cause - V/Q mismatch - Pulmonary oedema, pneumonia, pulmonary embolism Type II respiratory failure - Hypoxaemia (<8 kPa) - Hypercapnia (PaCO2 > 6 kPa) - Cause - hypoventilation - COPD, obesity, Opiates, GBS pH Main question to ask - is the pH normal, acidotic or alkalotic? - Normal = pH - 7.35 - 7.45 - Acidotic = pH < 7.35 - Alkalotic = ph > 7.45 Now we need to find the cause? Either respiratory or metabolic pH has changed due to imbalance in CO2 and HCO3- PaCO2 This helps in identifying the cause - Carbonic acid - will decrease pH - If CO2 is retained - more carbonic acid -> decrease in pH - If CO2 is being blown off - less carbonic acid -> increase in pH PaCO2 - compensation - Body can adjust to bring pH back to normal - If too much CO2 (respiratory acidosis) -> body will try to increase HCO3- (metabolic compensation) - If too little CO2 (respiratory alkalosis) -> body will try to decrease HCO3- (metabolic compensation) - Metabolic compensation takes a few days PaCO2 - What question should we ask? - Is the CO2 normal or abnormal? - Abnormal - Does it fit with the change in pH? - CO2 increases - we expect pH to decrease due to respiratory acidosis - But if pH was to increase then it wouldn’t correlate - If it doesn’t match the change in pH -> think metabolic HCO3- - Now we have a clue whether the abnormality is respiratory or metabolic - HCO3- is a base and - If HCO3- increases - binds with H+ - pH increases (alkalosis) - If HCO3- decreases - more free H+ ions - pH decreases (acidosis) HCO3-- compensation - Body can adjust to bring pH back to normal - If too much HCO3- (metabolic alkalosis) -> body will try to increase CO2 retention (respiratory compensation) - If too little HCO3- (metabolic acidosis) -> body will try to blow off CO2 (respiratory compensation) - Respiratory compensation is quick HCO3- - Is the HCO3- normal or abnormal? - Abnormal - Does it fit with pH change? - Low HCO3- should result in acidosis - If it doesn’t - cause is respiratory Base Excess - Tells you if there is more or less base in the body - High - > +2 mmol/L - Low - < -2 mmol/L - Normal - in between the 2 valuesExampleExample pH CO2 HCO3- Cause Respiratory Low High Normal Opiates Acidosis GBS Asthma COPD Iatrogenic Respiratory High Low Normal Anxiety Alkalosis Pain Hypoxia PE / Pneumothorax Iatrogenic Metabolic Low Normal Low High anion gap Acidosis vs Normal anion gap Metabolic High Normal High GI loss Alkalosis Renal loss Anion Gap Anion Gap EXPLAINED - YouTube - [Na+] - [(Cl-) + HCO3-] - Normal - 4 to 12 mmol/L - Tells you how many unmeasured anions are present (albumin) - High Anion Gap Metabolic Acidosis [increased acids or impaired H+ excretion] - CAT MUDPILES - DKA - Lactic acidosis - Renal failure - Normal Anion Gap Metabolic Acidosis [loss of bicarbonate] - GI loss (diarrhoea) - Renal tubular disease - Addison’s disease - Diuretics (Acetazolamide) Mixed acidosis and alkalosis - Easy way to spot is that CO2 and HCO3- will move in opposite directions - Increased CO2 and Decreased HCO3- -> mixed respiratory and metabolic acidosisSpirometry ILOs Spirometry Understand the differences between obstructive and restrictive lung diseases Recall common obstructive and restrictive lung diseases, such as: Asthma COPD Interstitial lung disease Obesity Neuromuscular disease Interpret the values of FEV1, FVC and FEV1:FVC to differentiate obstructive and restrictive lung disease Recognise obstructive and restrictive lung disease from a flow-volume curve and volume-time graph Identify lung volumes on a spirometry graph, including: Total lung capacity (TLC) Vital capacity (VC) Functional residual capacity (FRC) Tidal volume (TV) Residual volume Interpret clinical data and link to a clinical history to generate a list of differential diagnoses What is it? - A method of assessing lung function - Differentiates obstructive disorders from restrictive disorders - Helps monitor disease severity Important terminology - Forced expiratory volume in 1 second (FEV1): the volume exhaled in the first second after deep inspiration and forced expiration, similar to PEFR. - Forced vital capacity (FVC): the total volume of air that the patient can forcibly exhale in one breath. - FEV1/FVC: the ratio of FEV1 to FVC expressed as a percentage. Obstructive disorders - Reduced FEV1 - <80% of predicted - Reduced FVC (to a lesser amount than FEV1) or may be normal - FEV1/FVC ratio <0.7 Obstructive disorders - Causes - COPD - Asthma - reversible obstruction after bronchodilator - Bronchiectasis - Cystic Fibrosis Restrictive disorders - Reduced FEV1 - <80% of predicted - Reduced FVC - <80% of predicted - FEV1/FVC ratio > 0.7 Restrictive disorders - Causes - Pulmonary fibrosis - Skeletal abnormalities (kyphoscoliosis) - NMDs (MND, Myasthenia Gravis, GBS) - Obesity - Pregnancy Resources ABG Interpretation | A guide to understanding ABGs | Geeky Medics Arterial blood gas (ABG) • LITFL • CCC Spirometry Interpretation | Obstructive vs Restrictive | Geeky Medics Spirometry • LITFL• CCC Lung Function Tests – Zero To Finals