1、ABG INTERPRETATION Debbie Sander PAS-II Objectives Whats an ABG?Understanding Acid/Base Relationship General approach to ABG Interpretation Clinical causes Abnormal ABGs Case studies Take home What is an ABG Arterial Blood Gas Drawn from artery-radial,brachial,femoral It is an invasive procedure.Cau
2、tion must be taken with patient on anticoagulants.Helps differentiate oxygen deficiencies from primary ventilatory deficiencies from primary metabolic acid-base abnormalities What Is An ABG?pH H+PCO2 Partial pressure CO2 PO2 Partial pressure O2 HCO3 Bicarbonate BE Base excess SaO2 Oxygen Saturation
3、Acid/Base Relationship This relationship is critical for homeostasis Significant deviations from normal pH ranges are poorly tolerated and may be life threatening Achieved by Respiratory and Renal systems Case Study No.1 60 y/o male comes ER c/o SOB.Tachypneic,tachycardic,diaphoretic and Cyanotic.Dx
4、 acute resp.failure and ABGs Show PaCO2 well below nl,pH above nl,PaO2 is very low.The blood gas document Resp.failure due to primary O2 problem.Case Study No.2 60 y/o male comes ER c/o SOB.Tachypneic,tachycardic,diaphoretic and Cyanotic.Dx acute resp.failure and ABGs Show PaCO2 very high,low pH and
5、 PaO2 is moderately low.The blood gas document Resp.failure due to primarily ventilatory insufficiency.There are two buffers that work in pairs H2CO3 NaHCO3 Carbonic acid base bicarbonate These buffers are linked to the respiratory and renal compensatory system Buffers Respiratory Component function
6、 of the lungs Carbonic acid H2CO3 Approximately 98%normal metabolites are in the form of CO2 CO2 +H2O H2CO3 excess CO2 exhaled by the lungs Metabolic Component Function of the kidneys base bicarbonate Na HCO3 Process of kidneys excreting H+into the urine and reabsorbing HCO3-into the blood from the
7、renal tubules 1)active exchange Na+for H+between the tubular cells and glomerular filtrate 2)carbonic anhydrase is an enzyme that accelerates hydration/dehydration CO2 in renal epithelial cells H2O +CO2 H2CO3 HCO3 +H+Acid/Base Relationship Normal ABG values pH 7.35 7.45 PCO2 35 45 mmHg PO2 80 100 mm
8、Hg HCO3 22 26 mmol/L BE -2-+2 SaO2 95%Acidosis Alkalosis pH 45 HCO3 7.45 PCO2 26 Respiratory Acidosis Think of CO2 as an acid failure of the lungs to exhale adequate CO2 pH 45 CO2+H2CO3 pH Causes of Respiratory Acidosis emphysema drug overdose narcosis respiratory arrest airway obstruction Metabolic
9、 Acidosis failure of kidney function blood HCO3 which results in availability of renal tubular HCO3 for H+excretion pH 7.35 HCO3 7.45 PCO2 7.45 HCO3 26 Causes of Metabolic Alkalosis loss acid from stomach or kidney hypokalemia excessive alkali intake How to Analyze an ABG 1.PO2 NL =80 100 mmHg 2.pH
10、NL =7.35 7.45 Acidotic 7.45 3.PCO2 NL =35 45 mmHg Acidotic 45 Alkalotic 35 4.HCO3 NL =22 26 mmol/L Acidotic 26 Four-step ABG Interpretation Step 1:Examine PaO2&SaO2 Determine oxygen status Low PaO2(80 mmHg)&SaO2 means hypoxia NL/elevated oxygen means adequate oxygenation Step 2:pH acidosis 7.45 Four
11、-step ABG Interpretation Step 3:study PaCO2&HCO 3 respiratory irregularity if PaCO2 abnl&HCO3 NL metabolic irregularity if HCO3 abnl&PaCO2 NL Four-step ABG Interpretation Step 4:Determine if there is a compensatory mechanism working to try to correct the pH.ie:if have primary respiratory acidosis wi
12、ll have increased PaCO2 and decreased pH.Compensation occurs when the kidneys retain HCO3.Four-step ABG Interpretation PaCO2 pH Relationship 80 7.20 60 7.30 40 7.40 30 7.50 20 7.60 Compensated Respiratory Acidosis CO2 More Abnormal Respiratory Acidosis CO2 Expected Mixed Respiratory Metabolic Acidos
13、is CO2 Less Abnormal CO2 Change c/w Abnormality Metabolic Metabolic Acidosis CO2 Normal Compensated Metabolic Acidosis CO2 Change opposes Abnormality Acidosis ABG Interpretation Compensated Respiratory Alkalosis CO2 More Abnormal Respiratory Alkalosis CO2 Expected Mixed Respiratory Metabolic Alkalos
14、is CO2 Less Abnormal CO2 Change c/w Abnormality Metabolic Alkalosis CO2 Normal Compensated Metabolic Alkalosis CO2 Change opposes Abnormality Alkalosis ABG Interpretation Respiratory Acidosis pH 7.30 PaCO2 60 HCO3 26 Respiratory Alkalosis pH 7.50 PaCO2 30 HCO3 22 Metabolic Acidosis pH 7.30 PaCO2 40
15、HCO3 15 Metabolic Alkalosis pH 7.50 PCO2 40 HCO3 30 What are the compensations?Respiratory acidosis metabolic alkalosis Respiratory alkalosis metabolic acidosis In respiratory conditions,therefore,the kidneys will attempt to compensate and visa versa.In chronic respiratory acidosis(COPD)the kidneys
16、increase the elimination of H+and absorb more HCO3.The ABG will Show NL pH,CO2 and HCO3.Buffers kick in within minutes.Respiratory compensation is rapid and starts within minutes and complete within 24 hours.Kidney compensation takes hours and up to 5 days.Mixed Acid-Base Abnormalities Case Study No.3:56 yo neurologic dz required ventilator support for several weeks.She seemed most comfortable when hyperventilated to PaCO2 28-30 mmHg.She required daily doses of lasix to assure adequate urine out
