Acidbasebalance presensetation in surgery

acid base balance

1. Arterial blood gases and
Acid – Base balance
Dr Ahsan Shafiq
MBBS FCPS (Surgical Oncology)
Assistant Professor of Surgery, SMDC
Applied Sciences Lecture Course

2. Learning outcomes
•Importance of acid base balance
•pH Vs [H
+
]
•Regulation of pH or H+
–Buffers
–Respiration
–Renal
•Anion gap
•Pragmatic approach to Blood gases
–Oxygenation
–Acid-base balance

3. [H
+
] in ECF is very tightly regulated
•Why is a precise regulation essential?
•Compared to other ions, [H
+
] in ECF is low
•Na
+
: 145mEq/l Vs H
+
:0.00004mEq/l
•Normal variations in [H
+
] are also of much smaller magnitude
•-Log[H+]: Normal pH: 7.4……..40nEq/l
•A small change in pH represents a large change in [H
+
]
•http://www.purchon.com/chemistry/ph.htm

4. Definitions:
•Acid: H
+
donor
–Strong Vs Weak acid
•Base or alkali: acceptors of H
+
•Acidosis
•Alkalosis
Normal pH range: 7.35-7.45

5. Intracellular pH is lower and ranges
between 6-7.4
Arterial blood: 7.4
Venous blood 7.35
Interstitial fluid7.35
Intracellular fluid:6-7.4
Urine: 4.5-8
Gastric juice:0.8
The optimal
buffer within
each
compartment is
dependant on
this operating pH

6. pH
•Measure of [H
+
] in plasma
•Normal pH 7.35-7.45
•Narrow normal range
•Small changes in pH reflect a large change in [H
+
]
•Compatible with life 6.5 - 8.5

7. Regulation of Acid-base balance
•Immediate: Buffers
–Bicarbonate
–Phosphate
–Proteins
•Intermediate: Respiration
–Regulate the removal of CO
2
and therefore HCO
3
-
•Delayed: Renal
–Removal or addition of HCO3-
Do not add or eliminate H
+
from body

8. Efficiency of Buffers
•Concentration
•pK
•HCO
3
-
, PO
4
3-
, proteins

9. Titration curve for the bicarbonate
buffer
CO
2+H
2O↔H
2CO
3↔H
+
+ HCO
3
-

10. The Henderson-Hasselbalch equation
6.1
[HCO
3
-
]
PCO
2
CO
2+H
2O ↔ H
2CO
3 ↔ H
+
+ HCO
3
-
Kidneys
Lungs

11. Bicarbonate buffer is the most
physiologically important ECF buffer
•Concentration is relatively higher
•The normal physiological requirement is to
deal with acid production
•Interacts with the lungs and kidneys
CO
2 HCO
3
-

12. Respiratory regulation of pH
Acidosis is a potent stimulant on the
respiratory centre
Acidosis causes hyperventilation
Within the physiological range
changes in alveolar ventilation has
a very dramatic effect on
arterial/ECF pH as we are operating
on the vertical part of the curve

13. The Henderson-Hasselbalch equation
6.1
[HCO
3
-
]
PCO
2
CO
2+H
2O ↔ H
2CO
3 ↔ H
+
+ HCO
3
-
Kidneys
Lungs

14. Kidneys and acid-base balance
Kidneys regulate acid-base balance by excreting
either acidic or alkaline urine
Slow
response
Infinite
capacity
H
+
excretion , HCO
3
-

reabsorption and
formation of new HCO
3
-

are closely linked

15. The Henderson-Hasselbalch equation
6.1
[HCO
3
-
]
PCO
2
CO
2+H
2O ↔ H
2CO
3 ↔ H
+
+ HCO
3
-
Kidneys
Lungs

16. Anion Gap
•Electrical neutrality: in all fluid compartments
•Positive ions = negative ions
•Only certain ions are routinely measured
•Na
+
, K
+
, Cl
-
, HCO
3
-
•Anion gap is a diagnostic concept
•Lactate, albumin, sulphate, other organic ions

17. Anion Gap
•If an acidosis is present work out the anion
gap to help determine cause.
–Anion Gap = [Na
+
+ K
+
] – [Cl
-
+ HCO
3
-
]
–Normal anion gap 10-18 mMol/l
•If high anion gap present consider
–lactic acidosis, ketoacidosis
–kidney injury
–drugs (salicylate, metformin)
–toxins (ethanol, methanol, ethylene glycol)

18. Stepwise approach
•Assess Oxygenation
–Is the patient hypoxic
–Is there a significant A-a Gradient
•Determine Acid-Base Deficit
–pH>7.45 (H+ < 35) alkalaemia
–pH<7.35 (H+ > 45) acidaemia
•Determine the respiratory component, and
does this explain the acid-base deficit
–PaCO2: >6.0 kPa - respiratory acidosis
– <4.7kPa - respiratory alkalosis

19. Stepwise approach
•Determine the metabolic component, and does this
explain the acid-base deficit
–HCO3 -: <22 mmols/l - metabolic acidosis
– >26 mmols/l - metabolic alkalosis
•Which is primary and which is secondary:
compensation rarely restores pH to the normal
range, and a mixed picture may be present
•If an acidosis is present work out the anion gap to
help determine cause.
–Normal anion gap 10-18
–Anion Gap = [Na + K] –[Cl
-
+HCO
3
-
]

20. Case 1
Young female admitted with overdose of unknown tablets
pO
2
15 kPa on air
pH7.24 H
+
58 nmol/l
PaCO
2
2.5 kPa
HCO
3
8 mmol/l
Na
+
135 K
+
5, Cl
-
103, lactate 1.1
Interpret these gases and what do they show?
What differential diagnosis would you consider?

21. Case 1
•Metabolic acidosis with respiratory
compensation
•Anion gap is high suggesting added acid…
TCA
•DKA, lactic acidosis or other poisoning
could be considered

22. Case 2
Elderly male admitted from nursing home with
one week history of fever and vomiting
PaO
2 12 kPa on 4l by mask
pH7.49 H
+
32
PaCO
2 6.1
HCO
3 35
Na
+
135 K
+
2.8 Cl
-
88lactate 2.1

23. Case 2
•The patient is dehydrated due to the vomiting
•This patient’s PaO
2
should be greater than 12kPa
•Metabolic alkalosis with respiratory compensation has occurred
due to loss of gastric acid
•Note that the patient is profoundly hypokalaemic and this must
be corrected as rehydration occurs
•The patient had pyloric outlet obstruction secondary to chronic
duodenal ulcer… now increasingly rare due to PPI prescription

24. Case 3
Middle aged man admitted with cough sputum
and haemoptysis. Life-long smoker
pO
2
4 kPa on air
pH7.19H+65
PaCO
2 9.7
HCO
3 28
Na+ 145 K+ 3.8Cl - 108lactate
3.1

25. Case 3
•Acute respiratory acidosis with dehydration
•Treatment?
•Acute respiratory acidosis due to
respiratory failure
–no time for metabolic compensation
•The patient should receive a higher FiO
2
•Consider NIV

26. Case 4
Middle aged man post cardiac arrest.
Breathing spontaneously on ET tube
pO
2 35 kPa on 15l via resus bag
pH6.9 H
+
126 nmol/l
PaCO
2 8.9 kPa
HCO
313 mmol/l
Na 135 K 5.0 Chloride 99,lactate 6

27. Case 4
•Deranged oxygenation
•Mixed metabolic and respiratory acidosis
•Very sick… ICU ventilated
•Mixed metabolic and respiratory acidosis
following cardiac arrest
•Patient needs to remain ventilated despite
the good PaO
2
, to optimise acid-base
balance before extubation
•Anion gap 28…..added acid…lactic acidosis

28. Case 5
Young diabetic male admitted with chest
infection, vomiting and drowsiness
pO
2 12 on air
pH 7.31 H
+
49
PaCO
2
1.6
HCO
3 6.0
Na 132 K 4.2 chloride 101
lactate 5.2

29. Case 5
•Acute metabolic acidosis
•DKA most likely
•Consider
–overdose
–lactic acidosis due to sepsis
•Anion gap 29

30. Summary
•Importance of acid base balance
•pH Vs [H
+
]
•Regulation of pH or H+
–Buffers
–Respiration
–Renal
•Anion gap
•Pragmatic approach to Blood gases
–Oxygenation
–Acid-base balance

31. Acknowledgement
Dr Magnus Garrioch: Consultant, Critical Care Medicine
CMFT

32. Questions
?