Arterial blood gases

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Slide 1 :
Slide 2 : Dr. Vishram Buche Om Child Trust Hospital NAGPUR Arterial Blood Gases …A systematic Approach
Slide 3 : The Goal : To provide simple bedside approach to interpretation of ABG …. Not to: To teach physiology . To teach theories on acid-base regulation To look for alternative approaches to interpretation In detail
Slide 4 : Acid-Base status… Oxygenation…. Ventilation….
Slide 5 : …A respiratory component …A respiratory acid …Moves opposite to the direction of pH. …A metabolic component …It is a base (Metabolic) …Moves in the same direction of pH. …Moves in same direction ...Primary disorder …Moves in opposite direction …Mixed Disorder CO2 HCO3 CO2 HCO3 Facts about Acid-Base balance……
Slide 6 : H a 1 pH H+ nmoles /L. pH 20 7.60 30 7.50 40 7.40 50 7.30 60 7.20 0 14 H+ = 80- last two digits of pH OH ion H+ ion Alkaline Acidic
Slide 7 : Getting a Feel Of Blood Gases
Slide 8 : Primary lesion compensation pH HCO3 CO2 METABOLIC ACIDOSIS HYPER VENTILATION BICARB CHANGES pH in same direction LOW HCO3 LOW pH LOW pCO2 Low Alkali
Slide 9 : Primary lesion compensation pH HCO3 CO2 METABOLIC ALKALOSIS HYPO VENTILATION BICARB CHANGES pH in same direction HIGH HCO3 HIGH pH HIGH CO2 High Alkali
Slide 10 : CO 2 CHANGES pH in opposite direction Primary lesion compensation pH CO 2 BICARB Respiratory acidosis HIGH pCO2 LOW pH HIGH HCO3 High CO2
Slide 11 : CO 2 CHANGES pH in opposite direction Primary lesion compensation pH CO 2 BICARB Respiratory alkalosis LOW pCO2 HIGH pH LOW HCO3 Low CO2
Slide 12 : Body’s physiologic response to Primary disorder in order to bring pH towards NORMAL limit Full compensation Partial compensation No compensation…. (uncompensated) BUT never overshoots, If overcompensation is there, Take it granted it is a MIXED disorder Compensation….
Slide 13 : Remember……. Respiratory compensation is always FAST …12-24 hrs Metabolic compensation is always SLOW ...5 -7 days
Slide 14 : How to identify the type of compensation…..? pH HCO3 CO2 7.20 15 40 7.20 15 30 7.37 15 20 Un-Compensated Partially Compensated Fully Compensated
Slide 15 : Now that I have this data, what does it mean? ----- XXXX Diagnostics ------ Blood Gas Report 248 05:36 Jul 22 2000 Pt ID 2570 / 00 Measured 37.0o C pH 7.463 pCO2 44.4 mm Hg pO2 113.2 mm Hg Corrected 38.6o C pH 7.439 pCO2 47.6 mm Hg pO2 123.5 mm Hg Calculated Data HCO3 act 31.1 mmol / L HCO3 std 30.5 mmol / L BE 6.6 mmol / L O2 CT 14.7 mL / dl O2 Sat 98.3 % ct CO2 32.4 mmol / L pO2 (A - a) 32.2 mm Hg pO2 (a / A) 0.79 Entered Data Temp 38.6 oC ct Hb 10.5 g/dl FiO2 30.0 % output
Slide 16 : It is very simple to make things difficult It is very difficult to make things simple
Slide 17 : The Anatomy of a Blood Gas Report -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0C pH 7.436 pCO2 47.6 mm Hg pO2 122.4 mm Hg Calculated Data HCO3 act 31.2 mmol / L HCO3 std 30.5 mmol / L B E 6.6 mmol / L O2 ct 15.8 mL / dl O2 Sat 98.4 % ct CO2 32.5 mmol / L pO2 (A -a) 30.2 mm Hg ? pO2 (a/A) 0.78 Entered Data Temp 38.6 0C FiO2 30.0 % ct Hb 10.5 gm/dl Measured values… most important Temperature Correction : Is there any value to it ? Calculated Data : Which are useful one? Entered Data : Important
Slide 18 : Uncorrected pH & pCO2 are reliable reflections of in-vivo acid base status Temperature correction of pH & pCO2 do not affect calculated bicarbonate “ There is no scientific basis ... for applying temperature corrections to blood gas measurements…” Shapiro BA, OTCC, 1999. pCO2 reference points at 37o C are well established as a reliable reflectors of alveolar ventilation Reliable data on DO2 and oxygen demand are unavailable at temperatures other than 37o C Measured values should be considered And Corrected values should be discarded
Slide 19 : Bicarbonate is calculated on the basis of the Henderson equation: [H+] = 24 pCO2 / [HCO3-] or for the Mathematically inclined… Act Bicarbonate: -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0C pH 7.436 pCO2 47.6 mm Hg pO2 122.4 mm Hg Calculated Data HCO3 act 31.2 mmol / L HCO3 std 30.5 mmol / L B E 6.6 mmol / L O2 ct 15.8 mL / dl O2 Sat 98.4 % ct CO2 32.5 mmol / L pO2 (A -a) 30.2 mm Hg ? pO2 (a/A) 0.78 Entered Data Temp 38.6 0C FiO2 30.0 % ct Hb 10.5 gm/dl
Slide 20 : -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0C pH 7.436 pCO2 47.6 mm Hg pO2 122.4 mm Hg Calculated Data HCO3 act 31.2 mmol / L HCO3 std 30.5 mmol / L B E 6.6 mmol / L O2 ct 15.8 mL / dl O2 Sat 98.4 % ct CO2 32.5 mmol / L pO2 (A -a) 30.2 mm Hg ? pO2 (a/A) 0.78 Entered Data Temp 38.6 0C FiO2 30.0 % ct Hb 10.5 gm/dl
Slide 21 : Oxygenation Parameters: /limitations O2 Content of blood: (Hb x1.34x O2 Sat + 0.003x Dissolved O2 ) Remember Hemoglobin Oxygen Saturation: ( remember this is calculated …error prone) Alveolar / arterial gradient: ( classify respiratory failure) Arterial / alveolar ratio: Proposed to be less variable Same limitations as A-a gradient -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0C pH 7.436 pCO2 47.6 mm Hg pO2 122.4 mm Hg Calculated Data HCO3 act 31.2 mmol / L HCO3 std 30.5 mmol / L B E 6.6 mmol / L O2 ct 15.8 mL / dl O2 Sat 98.4 % ct CO2 32.5 mmol / L pO2 (A -a) 30.2 mm Hg ? pO2 (a/A) 0.78 Entered Data Temp 38.6 0C FiO2 30.0 % ct Hb 10.5 gm/dl
Slide 22 : Alveolar-arterial O2 Difference * When FiO2 = 21 % : PiO2 = (760-45) x .21= 150 mmHg O2 CO2 ?PAO2 = 150 – 1.2 (PCO2) = 150 – 1.2 ? 40 = 150 – 50 = 100 mm Hg ?PaO2 = 90 mmHg ………..PAO2 – PaO2 = ? PAO2 = PiO2* -(PCO2/0.8) PAO2 – PaO2 = 10 mmHg PaO2 PAO2 Classify Respiratory Failure Ventilation–perfusion mismatch ……FiO2 dependant derivation
Slide 23 : Alveolar-arterial Difference O2 CO2 Alveolar – arterial G. 100 - 45 = 55 ……………….Wide A-a Oxygenation Failure Wide Gap PCO2 = 40 PaO2 = 45 PAO2 = 150 – 1.2 (40) = 150 - 50 = 100 Ventilation Failure Normal Gap PCO2 = 80 PaO2 = 45 PAO2 = 150-1.2(80) = 150-100 = 50 Alveolar arterial G. 50 – 45 = 5 …………….Normal A-a
Slide 24 : 20 × 5 = 100 Expected PaO2 = FiO2 × 5 = PaO2 Normal
Slide 25 : The Blood Gas Report: pH 7.40 (7.35-7.45) PCO2 40 (35-45) mm Hg PO2 80 - 100 mm Hg HCO3 24 (22-26) mEq/L O2 Sat >95 O2 ct >18 Always mention and see FiO2 ct Hb The essentials -----XXXX Diagnostics---- Blood Gas Report Measured 37.0 0C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Calculated Data HCO3 act 31.2 mmol / L O2 Sat 98.4 % O2 ct 15.8 pO2 (A -a) 30.2 mm Hg ? pO2 (a/A) 0.78 Entered Data FiO2 % Ct Hb gm/dl
Slide 26 : A Systematic and pointed ………approach
Slide 27 : Steps for Successful Blood Gas Analysis 7
Slide 28 : 1. 2. Look at pH? 3. Who is the culprit ?...Metabolic / Respiratory 4. If respiratory…… acute and /or chronic 5. If metabolic acidosis, Anion gap ?ed and/or normal or both? Is more than one disorder present? Correlate clinically Consider the clinical settings! Anticipate the disorder 7 steps to analyze ABG
Slide 29 : Step 2 Look at the pH Is the patient acidemic pH < 7.35 or alkalemic pH > 7.45 If pH = 7.4 …… Normal Mixed or Fully compensated
Slide 30 : Step 3 ……. CULPRIT? HCO3…… METABOLIC > 26 ….. Met. Alkalosis < 22 ……Met. Acidosis PCO2 ……RESPIRATORY > 45 …… Resp. Acidosis < 35 …… Resp. Alkalosis HCO3 = Base Normal…22-26 CO2 = ACID Normal…35-45
Slide 31 : Step 4 If there is a primary Respiratory disturbance, is it acute ? .08 change in pH ( Acute ) .03 change in pH (Chronic) 10 mm Change PaCO2 = Remember………… relation of CO2 and pH
Slide 32 : PCO2 of 10 pH Acute change .08 Chronic change .03
Slide 33 : Step 5 If it is a primary Metabolic disturbance, whether respiratory compensation appropriate? For metabolic acidosis:Expected PCO2 = (1.5 x [HCO3]) + 8 + 2 (Winter’s equation) For metabolic alkalosis: Expected PCO2 = 6 mm… for 10 mEq. rise in Bicarb. ………UNCERTAIN COMPENSATION CO2 is equal to Last two digits of pH Remember If : Suspect ............. actual PaCO2 is more than expected : additional ...respiratory acidosis actual PaCO2 is less than expected : additional...respiratory alkalosis
Slide 34 : Step 5 cont. If metabolic acidosis is there How is anion gap ? Is it wide ... Na - (Cl-+ HCO3-) = Anion Gap usually <12 If >12, Anion Gap Acidosis : M ethanol U remia D iabetic Ketoacidosis P araldehyde I nfection (lactic acid) E thylene Glycol S alicylate Common pediatric causes Lactic acidosis Metabolic disorders Renal failure
Slide 35 : Step 6… Is more than one DISORDER present?
Slide 36 : Mixed Acid-Base Disorders : Clues -- Clinical history -- pH normal, abnormal PCO2 n HCO3 -- PCO2 n HCO3 moving opposite directions -- Degree of compensation for primary disorder is inappropriate -- Rise of anion gap and Fall of HCO3….. ……..R/F equation
Slide 37 : th step Clinical correlation 7
Slide :
Slide 39 :
Slide 40 : THANKS For any queries : vbuche@rediffmail.com

 


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