Acid -Base PhysiologyDr.L. Thomas

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)

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

Introduction

The inverse relation of pH &HPh 7.80 H+ 16

Ph 7.40 H+ 40

Ph 6.80 H+ 160

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.(

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

Regulation of Acid-Base balance.

By kidneys Change in the rate of H+ secretion.

By Lungs- Elimination of Co2 by hypo or hyperventilation.

Acidemia Decrease in blood pHAlkalemia Increase in blood Ph.Alkalosis and acidosis are the process that tend to raise or reduce pH respectively.

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)

Compensation.

COMPENSATORY RESPONSE ALWAYS IN THE SAME DIRECTION AS PRIMARY DISTURBANCE.

Characterestic of primary acid base disturbances Ph Pri.Dist CompenMet. Acidosis Hoc3 Pco2Met.Alkal Hco3 Pco2Resp.Acidosis Pco2 Hco3.Resp.Alkalosis Pco2 Hco3

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)

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.

Respiratory acidosisIncreased Co2.Decreased pH.Renal compensation by increasing H+ excretion thus increased plasma Hco3.Renal compensation takes 3-5 days to reach completion.

Acute resp acidosis with dramatic fall in Ph.Chronic resp acidosis with well protected Ph. (with well protected Ph)

Respiratory alkalosis.Decreased pCo2.Increased pH.Renal compensation time dependant- diminished H+ secretion and increased bicarbonate loss.So acute and chronic resp alkalosis.

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.

Knowledge of the extent of renal and respiratory compensation allows more complex disturbances to be diagnosed.

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.

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

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

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.

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.

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

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.

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)

But here the bicarb is 13 ,

So a combined metabolic acidosis and resp alkalosis present.

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.

Case-3pH 7.32, pco2-28, bicarb 14.

24-14 =10, 101.2= 12, 40-12= 28.

So pure metabolic acidosis.

Case 47.47 , Pc02 20, bicarb 14.

Alkalosis, respiratory.

Compensation ,chronic, 42 =824-8 = 16

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.

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.

Case 76.98, Pco2- 13, Hco3 3.

What is the acid base disturbance here?.

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