5. H 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
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7. CO 2 CHANGES pH in opposite direction Primary lesion compensation pH HCO 3 CO 2 METABOLIC ACIDOSIS HYPER VENTILATION BICARB CHANGES pH in same direction LOW HCO 3 LOW pH LOW pCO 2 Low Alkali
8. CO 2 CHANGES pH in opposite direction Primary lesion compensation pH HCO 3 CO 2 METABOLIC ALKALOSIS HYPO VENTILATION BICARB CHANGES pH in same direction HIGH HCO 3 HIGH pH HIGH CO 2 High Alkali
9. CO 2 CHANGES pH in opposite direction Primary lesion compensation pH CO 2 BICARB Respiratory acidosis HIGH pCO 2 LOW pH HIGH HCO 3 High CO 2
10. CO 2 CHANGES pH in opposite direction Primary lesion compensation pH CO 2 BICARB Respiratory alkalosis LOW pCO 2 HIGH pH LOW HCO 3 Low CO 2
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13. 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
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16. PaO 2 SaO 2 OXY (Sat) 98% HAEMOGLOBIN 2 % Dissolved Oxygen CaO 2 Content of oxygen Ml/100 of blood Delivery Of Oxygen To Tissues DaO 2 O 2 Cardiac output A.C.I.( Alveolar capillary interface) FiO2….21%....150 mm of Hg O. D. C. PAO2 A.C.I.
17. ----- XXXX Diagnostics ------ Blood Gas Report 248 05:36 Jul 22 2000 Pt ID 2570 / 00 Measured 37.0 o C pH 7.463 pCO 2 44.4 mm Hg pO 2 113.2 mm Hg Corrected 38.6 o C pH 7.439 pCO 2 47.6 mm Hg pO 2 123.5 mm Hg Calculated Data HCO 3 act 31.1 mmol / L HCO 3 std 30.5 mmol / L BE 6.6 mmol / L O 2 CT 14.7 mL / dl O 2 Sat 98.3 % ct CO 2 32.4 mmol / L pO 2 (A - a) 32.2 mm Hg pO 2 (a / A) 0.79 Entered Data Temp 38.6 o C ct Hb 10.5 g/dl FiO 2 30.0 % Now that I have this data, what does it mean? output
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19. -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0 C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0 C 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
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21. Bicarbonate is calculated on the basis of the Henderson equation: [H + ] = 24 pCO 2 / [HCO 3 - ] or for the Mathematically inclined … -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0 C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0 C 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
22. -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0 C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0 C 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 Standard Bicarbonate: Plasma HCO 3 after equilibration to a PCO 2 of 40 mm Hg : reflects non-respiratory acid base change : does not quantify the extent of the buffer base abnormality : does not consider actual buffering capacity of blood Base Excess: D base to normalise HCO 3 (to 24) with PCO 2 at 40 mm Hg (Sigaard-Andersen) : reflects metabolic part of acid base D : no info. over that derived from pH, pCO2 and HCO3 : Misinterpreted in chronic or mixed disorders
23. -----XXXX Diagnostics----- Blood Gas Report 328 03:44 Feb 5 2006 Pt ID 3245 / 00 Measured 37.0 0 C pH 7.452 pCO2 45.1 mm Hg pO2 112.3 mm Hg Corrected 38.6 0 C 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
24. Alveolar-arterial O 2 Difference * When FiO 2 = 21 % : PiO 2 = (760-45) x .21= 150 mmHg O 2 CO 2 PAO 2 = 150 – 1.2 (PCO 2 ) = 150 – 1.2 40 = 150 – 50 = 100 mm Hg P a O 2 = 90 mmHg ……… ..PAO 2 – P a O 2 = ? PAO 2 = PiO 2 * -(PCO 2 /0.8) PAO 2 – P a O 2 = 10 mmHg PaO2 PAO2
25. Alveolar-arterial Difference O 2 CO 2 Alveolar – arterial G. 100 - 45 = 55 ……………… .Wide A-a Oxygenation Failure Wide Gap PCO 2 = 40 PaO 2 = 45 P A O 2 = 150 – 1.2 (40) = 150 - 50 = 100 Ventilation Failure Normal Gap PCO 2 = 80 PaO 2 = 45 PAO 2 = 150-1.2(80) = 150-100 = 50 Alveolar arterial G. 50 – 45 = 5 …………… .Normal A-a
26. Expected PaO 2 = Normal 20 × 5 = 100 FiO 2 × 5 = PaO 2
27. Always mention and see… FiO 2 ct Hb -----XXXX Diagnostics---- Blood Gas Report Measured 37.0 0 C 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
28. Technical Errors Glass vs. plastic syringe: Changes in pO 2 are not clinically important No effect on pH or pCO 2 Heparin (1000 u / ml): Need <0.1 ml / ml of blood pH of heparin is 7.0; pCO 2 trends down Avoided by heparin flushing & drawing 2-4 cc blood Delay in measurement: Rate of changes in pH, pCO 2 and pO 2 can be reduced to 1/10 by cooling in ice slush(4 o C) No major drifts up to 1 hour
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32. 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
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34. 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 PaCO 2 = Remember………… relation of CO 2 and pH
37. Step 5 cont. If metabolic acidosis is there How is anion gap ? Is it wide ... Na - (Cl - + HCO 3 - ) = 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
41. Validity of ABG report… a lab error e.g. pH = 7.30, PCO 2 = 38.1, HCO 3 = 30 By Henderson-Hasselbach H+ = 24 x pCO 2 /HCO 3 = 24 x (38/30) = 30 80 - last two digit pH = H+ 80 - H+ = last two digit pH (after 7) pH should be 7.50 H= 24 x PCO2 HCO3
43. SIMPLE DISORDERS LOOKS LIKE MIXED 1. Not enough time lapsed for compensation 2. 5% out of confidence Bands e.g. pH = 7.20, HCO3 = 18, PCO2 = 33 MIXED DISORDERS LOOKS LIKE SIMPLE e.g. pH =7.24, PCO2 = 65, HCO3 = 26 Chronic Resp acidosis + Metabolic Acidosis Chronic case …. History helps
57. -----XXXX Diagnostics---- Blood Gas Report Measured 37.0 0 C pH 7.301 pCO2 75.1 mm Hg pO2 45.3 mm Hg Calculated Data HCO3 act 35.2 mmol / L O2 Sat 78.4 % O2 ct 15.8 pO2 (A -a) 9.5 mm Hg pO2 (a/A) 0.83 Entered Data FiO2 21 % Ct Hb 12 gm/dl CO 2 =75-40=35 Expected pH ( Acute ) = 7.11 Expected pH ( Chronic ) = 7.30 Chronic resp. acidosis Normal A-a gradient Hypoxia due to Due to hypoventilation Hypoxia….??? Case 1 6 year old male with progressive respiratory distress due to Muscular dystrophy . pH <7.30 …Acidosis Respiratory Acidosis
58. Case 2 8-year-old male asthmatic; 3 days of cough, dyspnea and orthopnea not responding to usual bronchodilators. O/E: Respiratory distress; suprasternal and intercostal retraction; tired looking; on 4 L NC. CO 2 = 49 - 40 = 9 Expected pH ( Acute ) = 9/10 x 0.08 = 0.072 Expected pH ( Acute ) = 7.40 - 0.072 = 7.328 Acute resp. acidosis ----- XXXX Diagnostics ------ Blood Gas Report Measured 37.0 o C pH 7. 24 pCO2 49.1 mm Hg pO2 66.3 mm Hg Calculated Data HCO 3 act 18.0 mmol / L O2 Sat 92 % pO2 (A - a) mm Hg pO2 (a / A) Entered Data FiO2 30 % 153-66= 87 pH <7.35 , acidosis pCO 2 >45; respiratory acidosis Wide A / a gradient Hypoxia WITH INCREASE IN CO2 BICARB MUST RISE ? Metabolic acidosis + respiratory acidosis 30 × 5 = 150
59. Case 3 8 year old diabetic with respi. distress fatigue and loss of appetite. ----- XXXX Diagnostics ------ Blood Gas Report Measured 37.0 o C pH 7.23 pCO2 23 mm Hg pO2 110.5 mm Hg Calculated Data HCO 3 act 14 mmol / L O2 Sat % pO2 (A - a) mm Hg pO2 (a / A) Entered Data FiO2 21.0 % If Na = 130, Cl = 90 Anion Gap = 130 - (90 + 14) = 130 – 104 = 26 pH <7.35 , Acidosis HCO3 <22; metabolic acidemia Last two digits of pH Correspond with co2
60. ----- XXXX Diagnostics ------ Blood Gas Report Measured 37.0 o C pH 7.34 pCO 2 38.1 mm Hg pO 2 90.3 mm Hg Calculated Data HCO 3 act 30 mmol / L O 2 Sat 98.3 % pO 2 (A - a) 10 mm Hg pO2 (a / A) 0.93 Entered Data FiO 2 21.0 % Case 4 16 year old female with sudden onset of dyspnea. No Cough or Chest Pain Vitals normal but RR 56, anxious. Acidosis Low CO2..??? High HCO3…??? LAB ERROR! By Henderson-Hasselbach H+ = 24 x pCO2/HCO3 = 24 x (38/30) = 30 80 - last two digit pH = H+ 80 - H+ = last two digit pH (after 7) pH should be 7.50
61. ----- XXXX Diagnostics ------ Blood Gas Report Measured 37.0 o C pH 7.46 pCO2 28.1 mm Hg pO2 55.3 mm Hg Calculated Data HCO 3 act 19.2 mmol / L O2 Sat % pO2 (A - a) mm Hg pO2 (a / A) Entered Data FiO2 24.0 % Case 5 : 10 year old child with encephalitis pH almost within normal range Mild alkalosis Co2 is low , respiratory Co2 low by around 10 ( Acute ) by .08 (Chronic ) by .03 Bicarb looks low ? Is it expected ?
62. Case 6…… 6 yrs old girl having type 1 Diabetic with H/O persistant vomiting Lab: pH 7.37, pCO 2 35 mm Hg, HCO 3 22 Na 140 , Cl 90 , Blood sugar : 300 Mild Metabolic acidosis ? Should we send her Home?
63. NO !! Anion gap = (140 - 112) = 28 Correlate Rise of Anion Gap with Fall of HCO 3 Anion Gap ↑ed by 18, HCO 3 should ↓ed by 18 , but ↓ed by 2 only HCO 3 retention (production) due to vomiting Metabolic acidosis, metabolic alkalosis
64. 1 month baby having malrotation, (having intermitent vomiting) posted for surgery . His pre-op ABG shows on Room air pH ………. 7.39 pCO 2 ……..l5 paO 2 ……...90 HCO 3 ……..8 b) Metabolic acidosis with compensatory Hypocapnia c) Acute respiratory alkalosis fully compensated. d) Chronic respiratory alkalosis fully compensated. Case 3………. a) Primary metabolic acidosis with respiratory alkalosis What is the probable cause for the above findings ? Are they OK ? As far as oxygenation is concerned ?
65. Patient was hypo- volumic , received Normal Saline bolus... Corrected acidosis He was operated …but post-op… drowsy His ABG……..FiO 2 ….30% pH ……..7.39 PaCO 2 …38 PaO 2 ……60 1) Why hypoxemia ? 2) Were the lungs bad to begin with ? ( Pre OP PaO 2 … 90 mmHg ) 3) Micro atelectesis during surgery ? Anesthetist goofed up the case 4) Pure and simple hypoventilation …..Sedation ?
66. Why hypoxemia ? Lungs were bad to begin with ? Micro atelectesis during surgery Pure and simple hypoventilation ? sedation PRE OP ….ABG on room air pH 7.39 PaCO 2 l5mmHg PaO 2 90 mmHg HCO 3 8mmol/L Pre OP .....A/a gradient P A O 2 = P i O 2 – 1.2 (PaCO 2 ) = 150 – 1.2 x 90 = 150 – 18 = 132 mm Hg 132 – 90= 42 WIDE A / a gradient Oxygenation status good …..? One click
67. Apparently the lungs looked good with PaO 2 of 90… But have a good look at the ABG again With wash out of CO 2 ………. The expected PaO 2 should have been more than 90 . This coupled with correction of acidosis ( normalizing PaCO 2 ) Lowered the PaO 2 …post operatively. Conclusion …….. Lungs were not normal at the beginning
68. Correlate PaO 2 with FiO 2 But please also correlate with PaCO 2 Learning point No click
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71. 5. I shall always take FiO2 into consideration when interpreting pO2 values. I shall also look at the pCO2 values carefully . 6. I shall take the history into consideration before instituting therapy for Chronic respiratory failure. 7. I shall always remember the acronym “ DOPE “ in situations of sudden deterioration of ABG values D- Displacement O- Obstruction P- Pneumothorax E- Equipment failure
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