1. Acid/base balance
Medhat Hashem, MD, FCARCSI, FRCA
Professor of Anaesthesia, Cairo University
Director of Surgical ICU

2. Objectives




Basic physiology
Definitions
How to read ABG
Examples

3. Normal concentration in ECF
(m mol/L)
Na
140




H+
0.00004
Normal H+ concentration: 40 n mol/L
Viable limits (20-160 n mol/L)
Na ion conc is 1000 times higher.
ECF Electroneutrality essential for cellular activity.

4. pH

[H+] concentration 40 n mol/L (20-160)

100 = (10)2
Log10 100 = 2
Log10 1000 = 3
pH; negative logarithm of reciprocal [H+]
concentration.
pH = - Log10 1/[H+]





5. pH
[H+] (nmol/L)
7.6
25
7.5
32
7.4
40
7.3
50
7.2
63
7.1
80
7.0
100
6.9
125
6.8
160

6. pH of body fluids
Plasma
7.4
Gastric HCl
0.8
Urine
4.5
Pancreatic juice
8

7. Definitions






Acid: A substance that dissociates in water to H+.
Acidosis: A process that causes acids to
accumulate.
Acidaemia; if pH<7.36
Alkali: a substance that accepts H+
Alkalosis; A process that causes bases to
accumulate.
Alkalaemia: if pH > 7.44

8. Buffers and compensation

Buffers: Hb, proteins, H2PO4 , H2CO3
H2O + CO2 → H2CO3 → HCO3- + H+

Respiratory compensation:
Immediate
 Hypo/hyperventilation to ↑or↓ CO
2


Renal compensation:
After 24-48 hours
 Excretion or retention of filtered HCO
3


9. Henderson Hasselbach Equation
pH ≈ HCO3/CO2

pH
>7.44 → Alkalosis (alkalaemia)
 <7.36 → Acidosis (acidaemia)


PaCO2 and HCO3 (and Base deficit)

PaCO2 → Respiratory

HCO3 → Metabolic

10. Blood gas analysis
Blood gas machine measures
 pH (7.36 - 7.44)
 PaO (75 -100 mmHg), room air
2

PaCO2 (35 – 45 mmHg)

HCO3 calculated (22 – 26 mmol/L)

11. Metabolic acidosis
↓pH (↑H+)
 ↓HCO (22-26 mmol/l) (↑Base deficit)
3
 PaCO → normal
2
 Compensation: Hyperventilation to wash CO
2
pH ≈ HCO3/CO2
pH HCO3PaCo2
Compensation

↓
↓↓
↓
Hyperventilation
↓PaCO2

12. Metabolic acidosis

Overproduction of organic acids
Lactic acid: shock, infection, hypoxia
 Urate: renal failure
 Ketones: DM, alcohol
 Drugs/toxins: (salicylate, biguanide, methanol)


Excessive loss of bicarbonate

13. Metabolic acidosis


Overproduction of organic acids
Excessive loss of bicarbonate:
Diarrhoea
 Pancreatic or small intestinal fistula
 Renal tubular acidosis
 Uretero-sigmodostomy
 Addison’s disease


14. Metabolic acidosis (CP)




of the cause
Tachypnea (↑rate and depth)
Severe acidosis → cardiovascular collapse →
cardiac failure (↓BP)
Death

15. Rx of metabolic acidosis
Rx of the cause (e.g., insulin if DKA)
 Mild to moderate:
beneficial (shift O2 dissociation curve to Rt → ↑O2
delivery at tissue level).
 Severe (↓pH < 7.2) → iv NaHCO
3


Amount of HCO3= Bwt x base deficit x 0.3
Start with half correction
 Repeat assessment


16. 35 yrs, M, RTA, bilateral # femur &
pelvis,
pH ≈ HCO3/CO2






SaO2 98%
pH 7.25
PaCO2 37 mmHg
HCO3 17 mmol/L
BE -7
PaO2 130 mmHg (O2 by face mask)
Hb 4.1 gm/dl
Rx: Blood Tx, ( open book #pelvis)

17. 17 yrs, M, unconscious, syringe next to him
pH ≈ HCO3/CO2





SaO2 81%
pH 7.2
PaCO2 80 mmHg
HCO3 22 mmol/L
BE 0
PaO2 45 mmHg (O2 40%)
Hb 14.5 gm/dl
Respiratory acidosis (uncompensated)

18. Respiratory acidosis


↓pH (↑H+)
↑PaCO2 (35-45 mmHg)

HCO3 (22-26 mmol/l) normal

Renal compensation (after 24 hours):

pH
↓
Reabsorption of filtered HCO3
HCO3-
↑
pH ≈ HCO3/CO2
PaCo2 Compensation
↑↑
Renal retention of HCO3-

19. Respiratory Acidosis


Always associated with hypoxia
↑PaCO2 → ↑respiratory depression (CO2
narcosis) → more hypoxia

20. Respiratory Acidosis (causes)

↓Respiratory center
Drugs (opioids-anaesthesia)
 ICT brain tumours-head injury






Cervical spinal cord lesions: trans-section of SC
AHC: e.g. poliomyelitis
Chest wall: #ribs (flail), obesity
Intercostal ms: Myasthenia Gravis
Lung disease: severe COPD (blue bloater)

21. Respiratory Acidosis (CP)

Acute CO2 retention:
restlessness
 flapping tremors


Further CO2 rise:
VD (warm limbs, papilloedema)
 Drowsiness, confusion, coma


Chronic CO2 retention:


poor sleeping →day somnolence
Cyanosis

22. Respiratory Acidosis (treatment)




Depends on the cause and timescale
Patent airway
Oxygen therapy
Rx of the cause e.g.,
Naloxone (opioid overdose)
 Drainage of hydrocephalus
 fixation of flail ribs


Mechanical ventilation

23. 69 yrs, M, smoker, COPD, SOB
pH ≈ HCO3/CO2





SaO2 91%
pH 7.36
PaCO2 61 mmHg
HCO3 32 mmol/L
BE 7
PaO2 79 mmHg (O2 40%)
Hb 21.5 gm/dl
Compensated respiratory acidosis

24. Nurse story, post colecystectomy, 40 yrs, fit
pH ≈ HCO3/CO2





SaO2 98%
pH 7.55
PaCO2 36 mmHg
HCO3 31 mmol/L
BE 6
PaO2 111 mmHg (O2 40%)
Hb 10.1 gm/dl
Metabolic alkalosis (uncompensated)

25. Metabolic alkalosis


↑pH (↓H+)
PaCO2 (35-45 mmHg) normal

↑ HCO3 (22-26 mmol/l)

Compensation: Respiratory inhibition
but hypoxia limit rise of PaCO2 50 mmHg
pH
HCO3-
↑
↑↑
pH ≈ HCO3/CO2
PaCo2 Compensation
↑
Hypoventilation
↑ PaCO2

26. Metabolic alkalosis

Loss of gastric HCl
Repeated vomiting (pyloric stenosis)
 Gastric suction


Hypokalaemia (intracellular K moves out
exchange with K (maintain electro-neutrality)


diuretics
Bicarbonate retention
NaHCO3 administration
 Milk alkali syndrome


27. Metabolic alkalosis (CP)



The cuase
Bradypnea (Chyne Stokes respiration)
Tetany (↓ionized Ca)

28. Metabolic alkalosis (treatment)





Rx of the cause
Cl replacement (Normal Saline 0.9%)
Correcion of hypokalaemia (irritant, CVC)
Iv ammonium chloride????
Rx of tetany: Ca (gluconate-chloride) 10ml
SLOWLY

29. 22 yrs, M, soldier, spasticity
pH ≈ HCO3/CO2





SaO2 98%
pH 7.55
PaCO2 25 mmHg
HCO3 25 mmol/L
BE 1
PaO2 189 mmHg (O2 by face mask)
Hb 13.6 gm/dl
Respiratory alkalosis (uncompensated)

30. Respiratory alkalosis

↑ pH (↓H+)
 ↓PaCO2 (35-45 mmHg)
2

HCO3 (22-26 mmol/l) normal

Renal compensation (after 24 hours):

Increased renal excretion of HCO3
pH
HCO3-
↑
↓
pH ≈ HCO3/CO2
PaCo2 Compensation
↓↓
↑Renal excretion of HCO3-

31. Respiratory alkalosis (causes)

Hyperventilation (wash out of CO2)
Hysteria
 Hyperpyrexia


Iatrogenic hyperventilation

32. Respiratory alkalosis (CP)




The cause
Short lived, well tolerated
↓Ca+ → carpopedal spasm, parasthesia
Severe respiratory alkalosis → respiratory arrest

33. Respiratory alkalosis (treatment)



Breath into paper bag (rebreathing)
Readjust ventilator parameters
Addition of CO2 ????

34. Base Excess

“The amount of acid or base (mmol/l) required
to return the pH of 1L blood to normal at a
PaCO2 40 mmHg.”

It is a measure of the magnitude of the
metabolic component to the acid-base disorder

35. Standard Bicarbonate


Not the actual bicarbonate in the sample
“Estimate of bicarbonate concentration after
elimination of any abnormal respiratory
contribution to HCO3 (i.e., an estimate of HCO3
at PaCO2 40 mmHg).

36. Thank You

37. QUESTIONS?
medhashem@yahoo.com
medhashem@yahoo.com