acid base physiology .1
TRANSCRIPT
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Acid -Base PhysiologyDr.L. Thomas
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Hydrogen ionsVery low in conc in ECF.40 nano eq/LVery highly reactive.Small fluctuation in conc can affect cellular enzyme reactions.H+ conc compatible with life is 16-160nanoeq/L (Ph 7.8 6.8)
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Regulation of H+ By buffers.Buffers are either weak acids or their ionized salts.Weak acids release H+ ions and ionised salts take up H+H + HCO3 ---- H2 CO3----H20 + CO2
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Introduction
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The inverse relation of pH &HPh 7.80 H+ 16
Ph 7.40 H+ 40
Ph 6.80 H+ 160
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Measurement of pHBlood drawn anaerobically.In to a heparinized syringe.Using electrodes which measure H+ & Co2Venous blood can also be used to measure pH if it is drawn from well perfused area without a tourniquet.
- pitfallsBlood should be drawn anareobically to prevent Co2 loss in to air.Rapid measurement or cooling to 4 C.-if metabolism continued there could be fall in pH due to production of acids.Heparin should be enough to coat the syringe.(
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Pitfalls.If drawing from A-line discard first 8-10 ml.The arterial pH is not always the pH at the tissue level especially in pts with circulatory failure or cardiac arrest. Normal Values. pH Pco2 Hco3Arterial 7.37-7.43 36-44 22-26Venous 7.32-7.38 42-50 23-27
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Regulation of Acid-Base balance.
By kidneys Change in the rate of H+ secretion.
By Lungs- Elimination of Co2 by hypo or hyperventilation.
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Acidemia Decrease in blood pHAlkalemia Increase in blood Ph.Alkalosis and acidosis are the process that tend to raise or reduce pH respectively.
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Primary abnormalitiesPrimary abnormality in PCO2. Resp acidosis (High PCO2) Resp alkalosis (Low Pco2)Primary abnormality in plasma Hco3. Met.acidosis (low Hco3) Met.alkalosis (high Hco3)
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Compensation.
COMPENSATORY RESPONSE ALWAYS IN THE SAME DIRECTION AS PRIMARY DISTURBANCE.
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Characterestic of primary acid base disturbances Ph Pri.Dist CompenMet. Acidosis Hoc3 Pco2Met.Alkal Hco3 Pco2Resp.Acidosis Pco2 Hco3.Resp.Alkalosis Pco2 Hco3
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Metabolic acidosisFall in plasma Hco3Low pH.Compensatory response- Hyperventilation and drop in Pco2.Ultimate restoration in Ph by renal excretion of excess acid (that take few days)
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Metabolic alkalosis.Increase in plasma bicarbonate.Increase in pH.Compensation hypoventilation and increase in Pco2.Renal excretion of excess Hco3 to restore Ph, but due to concomitant volume depletion this usually does not happen.
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Respiratory acidosisIncreased Co2.Decreased pH.Renal compensation by increasing H+ excretion thus increased plasma Hco3.Renal compensation takes 3-5 days to reach completion.
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Acute resp acidosis with dramatic fall in Ph.Chronic resp acidosis with well protected Ph. (with well protected Ph)
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Respiratory alkalosis.Decreased pCo2.Increased pH.Renal compensation time dependant- diminished H+ secretion and increased bicarbonate loss.So acute and chronic resp alkalosis.
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Mixed acid-base disorders.Suppose a pt has low pH = Acidemia.Serum bicarbonate low = metabolic acidosis.ABG showing a high PCO2 for the same patient = suggestive of resp acidosis.So possibility of combined metabolic and resp acidosis.
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Knowledge of the extent of renal and respiratory compensation allows more complex disturbances to be diagnosed.
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Metabolic acidosisPrimary - decrease in Hco3.Compensation 1.2 mmof hg reduction in Pco2 for every 1 meq/l fall in Hco3.Ex- Bicarbonate 10, so P02 should be (24-10 =14 1.2 = 16.8) 40-17 = 23.
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Metabolic AlkalosisPrimary increase in Hco3.Compensation 0.7 mmof Hg elevation in Pco2 for every I meq/L rise in Hco3.ABG with bicarb 35 (35-24= 11 0.7=7.7) so pco2 should be 40+7 = 47
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Respiratory acidosisPrimary Pco2 high.In acute resp acidosis compensation is 1 meq/ L increase in Hco3 for every 10 mm of Hg rise in the Pco2.Ex- PCO2 60 (60-40= 20. 21 =2, 24+2=26) So bicarb should be 26
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Chronic resp acidosisPco2 high.Compensation 3.5 meq/L increase in Hco3 for every 10 mmof Hg rise in Pco2So a Pco2 60 bicarb should be (3.52 =7. And 24+7= 31) 31.
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Acute resp alkalosis.Primary Pco2 low.Compensation 2 meq/l reduction in Hco3 for every 10 mmof Hg fall in Pco2Ex- Pco2 20 , (40-20=20, 22=4, 24-4=20) so bicarb should be 20.
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Chronic resp alkalosisPco2 low.Compensation 4meq /l reduction inHco3 for every 10 mmof Hg reduction in Pco2.
Ex- Pco2 20 ,then bicarb should be 24=8, 24-8 =16
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Mixed disorders.Renal and resp compensation return the Ph towards normal, but rarely to normal.So a normal pH with changes in bicarb and Pco2 immediately suggests a mixed disorder.
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Case-1A pt with salicylate overdose ABG, Ph 7.45, pc02- 20, bicarb- 13Alkalemic- (Ph)(Low pco2 or high bicarb can cause it)Here low Pco2 ,so respiratory, from history it is acute. So in acute resp alkalosis what should be the compensated bicarb (24-4 =20)
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But here the bicarb is 13 ,
So a combined metabolic acidosis and resp alkalosis present.
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Case -2ABG with pH 7.40 ,pCo2 60, bicarb- 36.Here Normal pH.Pco2 high (resp acidosis)Even if it is chronic resp acidosis bicarb should be 24+7 = 31.So here there is a combined met alkalosis and resp acidosis.
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Case-3pH 7.32, pco2-28, bicarb 14.
24-14 =10, 101.2= 12, 40-12= 28.
So pure metabolic acidosis.
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Case 47.47 , Pc02 20, bicarb 14.
Alkalosis, respiratory.
Compensation ,chronic, 42 =824-8 = 16
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Case 57.08 , pc02 49 , bicarb- 14Acidotic, metabolic.Compensation should be (24-14 = 101.2=12 ) Pco2 should be 40-12=28.But here it is 49.So combined resp and metabolic acidosis.
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Case 67.51 , pco2 49 , Hco3 38.Here metabolic alkalosis,Compensation should be (38-24= 140.7=9.8 ) Pco2 shpuld be 40+9.8.
So here pure metabolic alkalosis.
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Case 76.98, Pco2- 13, Hco3 3.
What is the acid base disturbance here?.
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