#4 Lectures on arterial blood gas Analysis .pptx

Lectures on arterial blood gas Analysis fromharrisons

1. Arterial Blood Gas Analysis

2. C o n t e n t s Maintenance of Body pH Collecting ABG Sample Normal Values Assessment of Oxygenation Types of Acid Base Disorders Single Disorders Mixed Disorders Step wise analysis of Acid Base Disorders

3. Maintenance of Body pH Chemical Buffers Lungs Kidneys Act Immediately Acute- act within hours Slow -after 48 hrs HCO 3 - /H 2 CO 3 most imp. Proteins Hb PO 4 3- /H 3 PO 4 SO 4 2- /H 2 SO 4 pH decreases--> stimulation of resp. centres--> Hy pe r v e n t il a t i on- - > C O 2 washout-->PaCO 2 decrease HCO 3 - reabsorption by proximal tubules H + excretion by distal tubules NH 3 synthesis & excretion by proximal tubules-->binds with diffusible H + in lumen and make it non- diffusible

4. Collecting ABG Sample Take an arterial sample at room air and start O2 supplementation immediately if required. Preference--Radial>Brachial>Femoral 22 Gauge needle flushed with Heparin Cool the sample immediately Avoid air bubbles Venous sample- absence of flash of blood, pulsations & autofilling of syringe. Metabolism- blood cells consume O2, produce CO2 and decrease pH.

5. Normal Values PARAMETER ARTERIAL BLOOD VENOUS BLOOD pH 7.35- 7.45 7.31- 7.41 pCO2 (mm of Hg) 35-45 41-51 pO2 (mm of Hg) >80 30-40 HCO3 (mmol/l) 22-26 23-29 SaO2 100 75

6. Assesment of Oxygenation Look at PaO 2 and SaO 2 Look at PaO 2 /FiO 2 ratio - Normal- 1:400- 1:500 <1:400- VQ mismatch/diffusion defect/intracardiac shunt -<1:300- ARDS A-a gradient= PAO 2 - PaO 2 = [FiO 2 x (Patm - PH 2 0)- (PaCO 2 /0.8)] - PaO 2 -For Room Air, = 150- PaCO 2 /0.8 Normal A-a gradient for 20 yr person is 5 mmHg , which increases to 10 mmHg in a 35 year old person A-a gradient >= 20 mmHg at any age is abnormal

7. Types of Acid Base Disorders S. No. Primary Disorder Main Mechanism Compensatory Mech. C o m p e n s a t o r y Response 1. Metabolic Acidosis ↓ s. HCO 3 - Hyperventilation ↓ PaCO 2 2. Metabolic Alkalosis ↑ s. HCO 3 - Hypoventilation ↑ PaCO 2 3. Respiratory Acidosis ↑ PaCO 2 ↑ HCO 3 reabsorption - by kidneys ↑ s. HCO 3 - 4. Respiratory Alkalosis ↓ PaCO 2 ↓ HCO 3 - reabsorption by kidneys ↓ s. HCO 3 - PaCO2 and HCO3 change in the same direction known as SAME DIRECTION RULE

8. Metabolic Acidosis High Anion Gap Metabolic Acidosis Ketoacidosis Lactic Acidosis Salicylic Acid Overdose Methanol Poisoning Ethylene Glycol Poisoning Renal Failure (eGFR <20 ml/min)

9. Metabolic Acidosis High Anion Gap Metabolic Acidosis Non Anion Gap Metabolic Acidosis aka Hyper Chloridic Acidosis Ketoacidosis Lactic Acidosis Salicylic Acid Overdose Methanol Poisoning Ethylene Glycol Poisoning Renal Failure (eGFR <20 ml/min) Diarrhea Pancreatic Fistula Antacids containing Mg, Al(eg Digene) Renal Tubular Acidosis Renal Tubular dysfunction (eGFR 20-50 ml/min) Drugs causing ↑ s K + = ACE -, ARB, K sparing diuretics, Pentamidine, Trimethoprim

10. Metabolic Alkalosis Chloride Responsive(gain of HCO3-) Chloride Unresponsive(loss of H+ due to increase in Aldosterone Activity) Vomiting Cl - losing diarrheas Diuretics- Thiazides, Loop Sweat Loss- Cystic Fibrosis Primary increase in Aldosterone Activity : High BP Primary Hy pe r al d o s t er o n i s m- Conn’s Syndrome, Cushing’s Syndrome Renin secreting tumor Liddle’s Syndrome Raectionary increase in Aldosterone Activity: Normal or Low BP Bartter’s Syndrome Gitelman’s Syndrome Other Causes: Milk Alkali Syndrome Penicillin Overdose

11. Respiratory Acidosis aka Type 2 Respiratory Failure Mechanism-> Hypoventilation--> ↑ PaCO 2 C a u s e s : Site Affected Cause Resp. Center Damage Drugs- Morphine, BZD Stroke- dec. blood supply to resp. centre Spinal Cord Damage (@C3 or above) #, Polio, ALS Phrenic N. Damage Neuropathy eg GBS NMJ Disorder -presynaptic- Lambert Eaton Synd -postsynaptic- Myasthenia Gravis Muscle Damage (Diaphragm) -Dystrophy (Duchenne’s) -Myopathies Chest Wall Stiffness AS, Obesity, Kyphoscoliosis Chronic Damage of Small Airways Chronic Bronchitis

12. Respiratory Alkalosis Mechanism- Primary stimulation of Resp. Centers Causes: CENTRAL PERIPHERAL 1. Drugs- Progesterone( eg Pregnancy) Salicylate Infections -Meningitis -Encephalitis Anxiety Lungs Compensatory in cases of Pneumothorax, Alveolar edema, Pneumonia, ARDS Systemic Cytokines IL 1, IL6--> stimulate resp. center eg Fever, Sepsis

13. STEP WISE ANALYSIS OF ABG

14. Clinical Information Likely Disorder Type 1 Diabetes with poor compliance to Insulin DKA (High AG Metabolic Acidosis) Chronic Bronchitis Respiratory Acidosis Severe Vomiting Metabolic Alkalosis (Cl - responsive) Diarrhea Metabolic Acidosis (Non AG)

15. For analysis purpose take normal pH as 7.4 pH<7.4 = Acidosis pH>7.4 = Alkalosis For eg 7.36= 7.42= 7.40= Either no disorder or mixed disorder

16. 1. Compare pHand HCO 3 If going in same direction= Metabolic If going in opposite direction= Respiratory eg pH 7.2 , HCO3 28 2. Compare pHand PaCO 2 If going in same direction= Metabolic If going in opposite direction= Respiratory eg eg pH 7.5 , PaCO2 60

17. If from Step 3, primary disorder is Metabolic, calculate compensatory PaCO2 Compensatory or Expected PaCO2= 15 + HCO3 - Eg Metabolic Acidosis= pH -7.3 , HCO3-20 , Expected PaCO2- Metabolic Alkalosis= pH -7.49 , HCO3-30 , Expected PaCO2-

18. If from Step 3, primary disorder is Respiratory, calculate compensatory HCO3- For Respiratory Acidosis: Eg Ph- 7.26 PaCO2- 70 Expected HCO3- (acute)- Expected HCO3- (chronic)- Resp. Acidosis Expected HCO3- Acute For every 1↑ of PaCO2 from 40 HCO3- will ↑ by 0.1 from normal value of 24 Chronic For every 1↑ of PaCO2 from 40 HCO3- will ↑ by 0.4 from normal value of 24

19. For Respiratory Alkalosis: Eg Ph- 7.54 PaCO2- 20 Expected HCO3- (acute)- Expected HCO3- (chronic)- Resp. Alkalosis Expected HCO3- Acute For every 1↓ of PaCO2 from 40 HCO3- will ↓ by 0.2 from normal value of 24 Chronic For every 1↓ of PaCO2 from 40 HCO3- will ↓ by 0.4 from normal value of 24

20. Given value= Expected Value Diag.= Single Disorder If the primary diagnosis - Metabolic Expected PaCO2 > Given PaCO2= Additional Resp. Acidosis Expected PaCO2 < Given PaCO2= Additional Resp. Alkalosis If the primary diagnosis - Respiratory Expected HCO3 > Given HCO3= Additional Metabolic Acidosis Expected HCO3 < Given HCO3= Additional Metabolic Alkalosis Given value= Expected Value Diag.= Mixed Disorder

21. Examples pH- 7.30 Hco3- 20 paCO2- 35 pH- 7.28 HCO3- 20 PaCO2- 15 pH- 7.50 Hco3- 30 paCO2- 60

22. If pH is in extremes, it is likely a acute disorder (as kidneys work slowly to compensate) Change in pH > 0.2 If pH is near normal, it is likely a chronic disorder (as kidneys have compensated) Change in pH <0.1 Resp. Disorder (Acidosis & Alkalosis) Expected pH ACUTE For every 10 change of PaCO2 from 40 pH changes by 0.08 from 7.40 CHRONIC For every 10 change of PaCO2 from 40 pH changes by 0.03 from 7.40