Physiology

1. Acid -Base PhysiologyAcid -Base Physiology
Dr.L. Thomas

2. Hydrogen ionsHydrogen ions
Very low in conc in ECF.
40 nano eq/L
Very 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)

3. Regulation of H+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

5. IntroductionIntroduction


6. The inverse relation of pH &HThe inverse relation of pH &H
Ph 7.80 H+ 16
Ph 7.40 H+ 40
Ph 6.80 H+ 160

7. Measurement of pHMeasurement of pH
Blood drawn anaerobically.
In to a heparinized syringe.
Using electrodes which measure H+ & Co2
Venous blood can also be used to measure
pH if it is drawn from well perfused area
without a tourniquet.

8. pitfallspitfalls
Blood 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.(<5% of the volume of blood).

9. Pitfalls.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 Hco3
Arterial 7.37-7.43 36-44 22-26
Venous 7.32-7.38 42-50 23-27

10. Regulation of Acid-BaseRegulation of Acid-Base
balance.balance.
By kidneys – Change in the rate of H+
secretion.
By Lungs- Elimination of Co2 by hypo or
hyperventilation.

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

12. Primary abnormalitiesPrimary abnormalities
Primary abnormality in PCO2.
 Resp acidosis (High PCO2)
 Resp alkalosis (Low Pco2)
Primary abnormality in plasma Hco3.
 Met.acidosis (low Hco3)
 Met.alkalosis (high Hco3)

13. Compensation.Compensation.
COMPENSATORY RESPONSE
ALWAYS IN THE SAME DIRECTION
AS PRIMARY DISTURBANCE.

14. Characterestic of primary acidCharacterestic of primary acid
base disturbancesbase disturbances
 Ph Pri.Dist Compen
Met. Acidosis ↓ ↓Hoc3 ↓Pco2
Met.Alkal ↑ ↑Hco3 ↑Pco2
Resp.Acidosis ↓ ↑Pco2 ↑Hco3.
Resp.Alkalosis ↑ ↓Pco2 ↓Hco3

15. Metabolic acidosisMetabolic acidosis
Fall in plasma Hco3
Low pH.
Compensatory response- Hyperventilation
and drop in Pco2.
Ultimate restoration in Ph by renal
excretion of excess acid (that take
few days)

16. Metabolic alkalosis.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.

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

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

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

20. Mixed acid-base disorders.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.

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

22. Metabolic acidosisMetabolic acidosis
Primary - 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.

23. Metabolic AlkalosisMetabolic Alkalosis
Primary –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

24. Respiratory acidosisRespiratory acidosis
Primary – 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. 2×1 =2,
24+2=26) So bicarb should be 26

25. Chronic resp acidosisChronic resp acidosis
Pco2 high.
Compensation – 3.5 meq/L increase in
Hco3 for every 10 mmof Hg rise in Pco2
So a Pco2 60 bicarb should be (3.5×2 =7.
And 24+7= 31) 31.

26. Acute resp alkalosis.Acute resp alkalosis.
Primary Pco2 low.
Compensation – 2 meq/l reduction in Hco3
for every 10 mmof Hg fall in Pco2
Ex- Pco2 20 , (40-20=20, 2×2=4, 24-
4=20) so bicarb should be 20.

27. Chronic resp alkalosisChronic resp alkalosis
Pco2 low.
Compensation – 4meq /l reduction inHco3
for every 10 mmof Hg reduction in Pco2.
Ex- Pco2 20 ,then bicarb should be 2×4=8,
24-8 =16

28. Mixed disorders.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.

29. Case-1Case-1
A pt with salicylate overdose ABG, Ph
7.45, pc02- 20, bicarb- 13
Alkalemic- (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)

30. But here the bicarb is 13 ,
So a combined metabolic acidosis and resp
alkalosis present.

31. Case -2Case -2
ABG 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.

32. Case-3Case-3
pH 7.32, pco2-28, bicarb 14.
24-14 =10, 10×1.2= 12,
40-12= 28.
So pure metabolic acidosis.

33. Case 4Case 4
7.47 , Pc02 – 20, bicarb 14.
Alkalosis, respiratory.
Compensation ,chronic, 4×2 =8
24-8 = 16

34. Case 5Case 5
7.08 , pc02 –49 , bicarb- 14
Acidotic, metabolic.
Compensation should be (24-14 =
10×1.2=12 ) Pco2 should be 40-12=28.
But here it is 49.
So combined resp and metabolic acidosis.

35. Case 6Case 6
7.51 , pco2 49 , Hco3 38.
Here metabolic alkalosis,
Compensation should be (38-24=
14×0.7=9.8 ) Pco2 shpuld be 40+9.8.
So here pure metabolic alkalosis.

36. Case 7Case 7
6.98, Pco2- 13, Hco3 – 3.
What is the acid base disturbance here?.

37.  THANKYOU.