1. Blood Gas Analysis Teguh Triyono Bagian Patologi Klinik
FKUGM/ RSUP Dr Sardjito Yogyakarta

2. Normal acid-base balance
Maintained through a number of buffer system but the most important is the bicarbonate system. The organs responsible are the kidneys and the lungs
The body maintain pH or the hydrogen ion concentration (H+) of the blood

3. Assessment of acid-base balance is obtained by measuring the components of bicarbonate buffer system which can be expressed as the following modified Hendersen-Hasselbalch equation:
pH= (HCO3)/ pCO2

4. Components of a Blood Gas
Oxygenation: lungs/ECMO
HCO3
Base: metabolic
pCO2
Acid: lungs/ECMO pH
The sum total of the acid/base balance,
on a log scale (pH=-log[H+])

5. Normal Values pH pCO2 pO2 HCO3 BE Arterial 7.35-7.45 35-45 80-100
22-26
-2 to +2
Venous
43-50
~45

6. Aims of Blood Gas Analysis
Determine if pH is acidotic or alkalotic
Determine cause:
Respiratory
Metabolic
Mixed
3. Check oxygenation

7. Why Arterial Blood?
Firmly establish the severity of an oxygenation abnormality
To evaluate hyper- or hypoventilation
Currently no convenient noninvasive way of evaluating pCO2
To determine acid-base status, particularly in patients with metabolic acidosis (e.g., diabetic ketoacidosis)
To monitor the application of mechanical ventilation

8. The pH defines the (H+) of blood:
increased pH = alkalaemia
decreased pH = acidemia
The (HCO3) defines metabolic component:
increased (HCO3) = metabolic alkalosis
decreased (HCO3) = metabolic acidosis
The pCO2 defines respiratory component:
increased pCO2 = respiratory acidosis
decreased pCO2 = respiratory alkalosis

9. Acidaemia : pH < 7.35
metabolic acidosis : (HCO3) < 23 mmol/L
respiratory acidosis : pCO2 > 45 mmHg
Alkalaemia : pH > 7.45
metabolic alkalosis : (HCO3) > 33 mmol/L
respiratory alkalosis : pCO2 < 35 mmHg

10. Simple metabolic acidosis
Low blood bicarbonate
The primary event is either an increased in hydrogen ion production or a decreased in its rate of excretion. Loss of bicarbonate via the gut (diarrhea) or the kidney is equivalent to increased H+ production
Causes: renal failure, diarrhea, lactic acidosis, diabetic ketoacidosis

11. Simple metabolic alkalosis
high blood bicarbonate
The primary event is either an decreased in hydrogen ion production or a increased in its rate of excretion. Loss of hydrogen ions via the gut (vomiting) or the kidney (diuretic therapy) is equivalent to increased bicarbonate production
Causes: diuretic therapy, vomiting, mineralocorticoid excess

12. Respiratory acidosis high blood pCO2
The primary event is retention of CO2 which can be due to lung disease or hypoventilation by a subject with a normal respiratory tract
Causes: CNS depression(trauma, drugs), neuromuscular disorders(poliomyelitis, GBS, MG), thoracic disorder(hydrothorax, pneumothorax),lung disorders(bronchial obstruction, emphysema, oedema), mechanical ventilation

13. Respiratory alkalosis
low blood pCO2
The primary event is hyperventilation resulting in a low pCO2
Causes: CNS disturbances (hypoxamia,trauma, infection), pulmonary disorders(embolus,oedema, asthma, pneumonia), mechanical ventilation

14. Acid-Base Regulation Three mechanisms to maintain pH Respiratory (CO2)
Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb)
Renal (HCO3-)

15. Compensation of acid-base disorders
Occurs in response to simple acid-base disturbances whereby homeostatic mechanism attempt to shift an abnormal pH back towards normal.
A metabolic acidosis (low HCO3) is compensated by a respiratory alkalosis (low pCO2)

16. Practical-1
Every change in CO2 of 10 mEq/L causes pH to change by 0.08 (or Δ1 = 0.007)
Increased CO2 causes a decreases in pH
Decreased CO2 causes an increase in pH

17. Respiratory Acidosis Hypercarbia from hypoventilation Findings:
pCO2 increased therefore… pH decreases
Example:
ABG : /50/ /25

18. Respiratory Alkalosis
Hypocarbia from hyperventilation
Findings:
pCO2 decreased… therefore pH increases
Example:
ABG – 7.45/32/ /25

19. Metabolic Changes
Remember normal HCO3- is 22-26

20. Practical-2
Every change in HCO3- of 10 mEq/L causes pH to change by 0.15
Increased HCO3- causes an increase in pH
Decreased HCO3- causes a decrease in pH

21. Metabolic Acidosis
Inability to excrete acid – e.g. renal tubular acidosis
Loss of base – e.g. diarrhea
Example:
ABG – 7.25/40/ /15

22. Metabolic Alkalosis
Loss of acid – e.g. vomiting (low Cl and kidney retains HCO3-)
Gain of base – e.g. contraction alkalosis (lasix)
Example:
ABG – 7.55/40/ /35

23. Mixed pH depends on the type, severity, and acuity of each disorder
Over-correction of the pH does not occur

24. Practical Application
Check pH
Check pCO2
Every change in CO2 of 10 mEq/L causes pH to change by 0.08
Every change in HCO3- of 10 mEq/L causes pH to change by 0.15

25. Example -1 ABG- 7.30/48/ /22 Acidotic or Alkalotic? pCO2 High or Low?
pH change = pCO2 change?

26. Example -1 ABG- 7.30/48/ /22 Acidotic or Alkalotic? pCO2 High or Low?
pH change = pCO2 change?
Combined respiratory and metabolic acidosis

27. Example -2 ABG- 7.42/50/ /32 Acidotic or Alkalotic? pCO2 High or Low?
pH change = pCO2 change?

28. Example -2 ABG- 7.42/50/ /32 Acidotic or Alkalotic? pCO2 High or Low?
pH change = pCO2 change?
Metabolic alkalosis with respiratory compensation

29. For respiratory abnormalities, is the condition acute or chronic?
Acute respiratory disturbances change pH 0.08 units for every 10 mmHg deviation from normal
Therefore, in acute respiratory acidosis, the pH will fall by x [(PCO2 - 40)/10]
In acute respiratory alkalosis, the
pH will rise by 0.08 x [(40-PCO2)/10]

30. For respiratory abnormalities, is the condition acute or chronic?
Chronic respiratory disturbances only change pH 0.03 units for every 10 mmHg deviation from normal
Therefore, in chronic respiratory acidosis, pH will fall by x [(PCO2 - 40)/10]
In chronic respiratory alkalosis, the pH will rise by 0.03 x [(40-PCO2)/10]

31. Limitations of ABGs ABGs measure gas partial pressures (tensions)
Remember: PO2 is not the same as content! A severely anemic patient may have an oxygen content reduced by half while maintaining perfectly acceptable gas exchange and therefore maintaining pO2
Technical issues
They hurt
Sampling from a vein by mistake
Finding an arterial pulse can be difficult in very hypotensive patients
Complications such as arterial thrombosis are possible, but awfully rare

32. Arterial blood sampling
Choice of site
Alternative sites
Alternative sites
Alternative sites
Fisrt choice

33. Terimakasih