HOW TO INTERPRETATE BLOOD GAS

2. Learning outcomes:
Interpret arterial blood gas results in a stepwise and logical manner to
make a correct diagnosis
Know the causes of metabolic and respiratory acidosis and alkalosis

3.  There are four primary acid base disorders:
 Respiratory acidosis
 Metabolic acidosis
 Respiratory alkalosis
 Metabolic alkalosis
 They are usually accompanied by compensatory changes
 These changes rarely compensate fully for the primary disorder
 In a chronic disorder the magnitude of compensation is greater with
subsequent better protection of the pH

4.  Always look at the clinical picture when
interpreting blood gas

5. There are 3 mechanisms by which the body controls the blood
acid-base balance within the narrow range (7.34 – 7.45 ):
Intracellular and extracellular buffers .
Regulation by the kidneys.(regulate the conc. Of HCO3 by adjusting renal excretion of
carbonic acid and the reabsorbtion of bicarbonate)
Regulation by the lungs.(Regulate Pco2 by adjusting rate of alveolar ventilation)
The most important buffer system involve :
Hemoglobin
Carbonic acid (weak acid formed from the dissolved CO2)
Bicarbonate (weak base )

6.  Blood gas analysers directly measures pH and Pco2.
 HCO3 is calculated from the Henderson-Hasselbalch equation.
 This equation shows that the pH is determined by the ratio of HCO3
concentration to pCO2, not by the value of either one alone.
 pH=(6.1) + log
𝐻𝐶𝑂3
(0.03)×𝑝𝐶𝑂2
 The simplified version from the equation will help you to understand
the compensatory changes
 pH∽
𝐻𝐶𝑂3
𝑝𝐶𝑂2

7. determine acid base balance
Determine oxygenation (arterial pO2 gives information about gas exchange)
Diagnose and establish the severity of respiratory failure (Pco2 gives
information about ventilation)
Guide therapy for example oxygen or non invasive ventilation in patients with
chronic obstructive pulmonary disease or therapy in patients with DKA,

8. Acidemia : this occurs when pH is < 7.35
Alkalemia : this occurs when pH is > 7.45
Acidosis :
• This is a process that cause acid to accumulate
• It does not necessarily result in abnormal pH
• from the Henderson-Hasselbalch equation you can see acidosis can be
induced by fall in HCO3 conc or rise in Pco2
• pH∽
𝐻𝐶𝑂3
𝑝𝐶𝑂2
• Occuring alone it tends to cause acidemia
• Occuring at the same time as an alkalosis the resulting pH may be normal
,high or low

9.  Alkalosis
this is the process that cause alkali to accumulate
It does not necessarily result in abnormal pH
From th equation you can see the alkalosis can be induced by a rise in HCO3
conc. Or a fall in pCO2
pH∽
𝐻𝐶𝑂3
𝑝𝐶𝑂2
When it occurs alone it tends to cause alkalemia
When it occurs at the same time as an acidosis the resulting pH may normal
,high or low.

10.  base excess
 Is the quantity of base or acid needed to titrate one litre of blood to pH
 7.4 with the pCO2 held constant at 5.3 kPa

11. Step 1 is there is an acidemia or an alkalemia ?
Step 2 is the primary disturbance respiratory or metabolic ?
Step 3 for metabolic acidosis is there a high anion gas ?
Step 4 is there compensation ? If there is ,is it appropriate ?

12. Arterial pCO2 : 35 – 45 mmHg (4.5 – 6.0 kPa)
Arterial pO2 : 80 – 100 mmHg ( 11 – 13 kPa)
HCO3 : 22 – 28 mmol/L
Base excess : -2 - +2
Anion gap : 8 – 16 mmol/L
CL : 98 – 107 mmol/L

13.  Look at the pH if it is:
 < 7.35 the patient is acidaemic
 > 7.45 the patient is alkalaemic
 If the pH is normal look at the pCO2nn & the concentration of HCO3 IF either
one or both is abnormal the patient may have mixed disorder

14. Look at the pH , pCO2 and the concentration of HCO3:
If the pH is< 7.35 an acidosis is causing acidemia and:
* if pCO2 is increased there is a primary respiratory acidosis
* if the conc, of HCO3 is decreased there is primary metabolic
acidosis
If pH is> 7.45 an alkalosis is causing alkalemia and :
* if pCO2nn is decreased there is a primary respiratory alkalosis
* if the conc. Of HCO3 is increased there is a primary metabolic
alkalosis

15.  you are called to see a 12 years old girl on the orthopedic unit who had a
right fracture femur 1 week ago . She become breathless . Her arterial blood
gas results are as follows:
pH : 7.48
pO2 : 62 mmHg
pCO2 : 25 mmHg
HCO3 : 25 mmol/L
 What is her acid base disturbance ?
 Step 1------- there is an alkalemia
 Step 2-------her pCO2 is decreased so this is a primary respiratory
alkalosis . The DD would include pulmonary embolus and hospital acquired
pneumonia

16.  You see 11 years boy in emergency departement . He has been vomiting for
the last 24 hours and feels unwell . His arterial blood gas results as follows :
 Na : 138 mmol/L
 K : 3 mmol/L
 Urea : 7.8 mmol/L
 Creatinine : 130 mmol/L
 pH : 7.49
 pO2 : 98 mmHg
 pCO2 : 38.5 mmHg
 HCO3 : 31 mmol/L
 Step 1 : there is alkalemia
 Step 2 : his conc. Of HCO3 is increased,so it is a primary metabolic alkalosis
 ( due to GIT loss of H ions from vomiting)(hypokalemia may also contributing
to the metabolic alkalosis)

17.  for the metabolic acidosis is there a high anion gap
 Identifying the type of acidosis will help you to narrow down the possible
underlying causes
 What is the anion gap?
 in the body the number of cations and anions are equal .blood tests measure
most cations but only a few anions ,therefore adding all the measured anions
and cations together leaves a gap that reflects unmeasured anions such as
plasma protein albumin
 Because Na is the primary measured cation and CL&HCO3 are the primary
measured anions the anion gap is calculated using the following formula :
 Na – ( HCO3 + CL )
 The normal anion gap is ( 8- 16 ) mmol/l
 Some hospital laboratories include K when caculating the anion gap , when K
is included the normal range is ( 12 -20) mmol /L

18.  MAIN CAUSES OF HIGH ANION GAS ACIDOSIS
INABILITY TO EXCRETE ACIDSINCREASED EXOGENOUS ACID SINCREASED ENDOGENOUS ACID
PRODUCTION
Chronic renal failureMethanol
Ethylene glycol
Aspirin
KETOACIDOSIS
LACTIC ACIDOSIS
TYPE A: impaired tissue O2
TYPE B:tissue O2 not impaired

19. Impaired renal acid excretionLoss of bicarbonate
• Type 1( distal) RTA
• Type 4 RTA (hypoaldosteronism)
• GIT :
o diarrhoea
o ileostomy
o pancreatic ,biliary,intestinal
fistula
• Renal :
o type 2 proximal RTA
o carbonic anhydrase inhibitors

20.  A patient with low albumin concentration may have a normal anion gap in the
presence of of a disorder that ususally produces a high anion gap
 The anion gap is reduced by about 2.5 mmol/L for every 10 g/L fall in
albumin concentration

21.  a 12 years old boy with chronic liver disease is admitted following an upper
GIT bleed. His blood pressure is 70/30 mm .. His arterial blood gas results as
follows:
 Albumin : 20 g/L (n=40 g/L0
 Na : 135 mmol/L
 K : 3.5 mmol/L
 CL : 100 mmol/L
 pH : 7.3
 pCO2 : 25.4 mmhg
 HCO3 : 20 mmol/L
 Lactate : 5 iu/L
 What is the anion gap and what acid base disturbance does he have ?

22. Ist calculate the anion gap Na – (CL + HCO3)
135- (100+200) = 15 mmol/L
This is within the normal range of (8-16)
Next coreect the anion gap
Gap= 15
Albumin = 20 g/l
The anion gap is reduced by about 2.5 mmol/L for every 10 g/L fall in albumin
concentration.
Therefore the anion gap is reduced by 5 mmol/l
Anion gap = 15 +5 = 20
This patient therefore has a high anion gap metabolic acidosis
In view of high lactate and hypotension this is likely secondary to lactic acidosis
type 1

23.  9 years odgirl feels unwell .she is thirsty and drinking lots of fluids. Her
arterial blood gas results are as follows:
 Glucose : 30 mmol/L
 pH : 7.32
 pO2 : 88.5 mmhg
 pCO2 : 23.1 mmhg
 HCO3 : 18 mmol/L
 Na : 148 mmol/L
 K : 3.5 mmol/L
 CL : 100 mmol/L
 What acid base disturbance does he have ?

24. Step 1 : there is acidemia
Step 2 : his concentration of HCO3 is decreased
so this is primary metabolic acidosis
Step 3 : the anion gap = Na – (CL + HCO3 ) = 148-118 = 30 mmol/L ⇡⇡⇡
This patient has a high anion gas acidosis most likely due to DKA.

25.  10 years old boy with ulcerative colitis has had severe diarrhoea for the past
2 days . His labs as follows:
 Creatinine : 200 mmol/L
 Urea : 17 mmol/L
 pH : 7.31
 pO2 : 96 mmhg
 Pco2 : 30.7 mmhg
 HCO3 : 14 mmol/L
 Na : 135 mmol/L
 K : 3.1 mmol/ L
 CL : 113 mmol/L
 What acid base disturbance does he have ?

26.  Step 1 : acidemia
 Step 2 : HCO3 ⇣⇣( primary metabolic acidosis )
 Step 3 : the anion gap 135 – (113 + 14 ) = 8 mmol/L
 This patient has normal anion gap metabolic acidosis most likely secondary
to loss o HCO3 from severe diarrhoea .

27.  Compensation refer to the action taken by the body to restore the correct
acid base balance . The normal comensatory measures are:
 Buffers : which include hemoglobin , plasma proteins ,bicarbonate and
phosphate . This response occurs in minutes.
 Ventilatory response , which occurs in minutes to hours
 Renal response which may take up to several days.
 Recognising compensation will help you to separate primary disorders from
derangement that exist only because of 1ry disorder .
 A useful aid is to be aware of the expected degree of compensation for the
primary disorder,

28. magnitude of compensationCompensatory
response
Initial chemical
change
Acid base disturbance
For every 10 mmhg increase in pCO2above
40 mmhg in acute respiratory acidosis :
The HCO3 increases by 1 mmol/L
The pH decreases by 0.07
For every 10 mmhg icrease in pCO2 above 40
mmhg in chronic respiratory acidosis
the HCO3 increases by 3.5 mmol/L
The pH decreases by 0.03
↑ HCO3↑ pCO2Respiratory
acidosis
for every 10 mmhg decrease in pCO2 below
40 mmhg in acute respiratory alkalosis
The HCO3 decreases by 2 mmol/L
The pHnn increses by 0.08
For every 10 mmhg decrease in pCO2nn below
40 mmhg in chronic respiratory alkalosis :
The HCO3 decreases by 5 mmol/L
The pHnn increases by 0.03
↓ HCO3↓ pCO2
Respiratory
alkalosis

29. Magnitude of compensationCompensatory
response
initial chemical
changes
Acid base disorder
Stimulation of the central and peripheral
chemoreceptors that control respiration results in
an increase in alveolar ventilation this in turn
causes a compensatory respiratory alkalosis
↓ pCO2↓ HCO3Metabolic acidosis
It is difficult to hypo ventilate to compensate
oxygenation also compromised by hypoventilation .
The respiratory system therefor rarely retain
pCO2nn to > 58 mmhg. A value greater than this
suggests a mixed disorder .that is metabolic
alkalosis and respiratory acidosis rather than a
compensated metabolic alkalosis
↑ pCO2↑ HCO3Metabolic alkalosis

30. You should suspect a mixed acid-base disorder when:-
The compensatory response occurs but the level of compensation is in
adequate or too extreme.
 As a rule of thumb
 When the pCO2 is ↑and the HCO3 CONC is↓respiratory acidosis and
metabolic acidosis coexist.
 When the pCO2 is ↓HCO3 IS ↑respiratory alkalosis and metabolic alkalosis
coexist.

31.  3 years old boy presented to ER after accidental ingestion of benzodiazepine
his mother had depression on medication.
 His ABG as follows:
 pH : 7.3
 pO2 : 85 mmhg
 pCO2 : 60 mmhg
 HCO3 : 25 mmol/L
Step1: acidemia
Step 2: pCO2nn is raied so this is primary respiratory acidosis
Step 4: his concentration of bicarbonate is normal,so there is no compensation.
The history is acute . Metabolic compensation takes days
He has acute respiratory acidosis 2ry to CNS depression of his respiratory drive
by the benzodiazepine overdose

32.  ABG
 pH : 7.34 ↓
 pO2: 69 mmhg ↓
 pCO2 : 60 mmhg ↑
 HCO3 : 32 mmol/L ↑
 What is the acid base disturbance

33.  17 years old with Duchene muscular dystrophy is admitted with UTI . He has
a temp of 39 c .he feels warm and peripherally vasodilated with a blood
pressure of 90/60 mmhg . Since being catheterized one hour ago he passed
5 ml urine. His ABG
 pH : 7.28 ↓
 pO2 :83mmhg ℕ
 pCO2 : 45 mmhg ↑ℕ
 HCO3 : 18 ↓
 Na : 146
 K : 4.5
 CL : 101

34.  pH : 7.28 ↓
 pO2 :83mmhg ℕ
 pCO2 : 46 mmhg ↑ℕ
 HCO3 : 18 ↓
 Na : 146
 K : 4.5
 CL : 101
 Step 1 : acidemia
 Step 2 : pCO2nn upper normal and his HCO3 concentration is decreased
 Step 3 ; the anion gap 146 – ( 18 + 101 ) = 27 mmol/L ↑↑
 Step 4 : for metabolic acidosis you would expect the pCO2 to be reduced .
For respiratory acidosis you would expect the HCO3 to be increased .
 Therefore he had mixed acid-base disorder . He has high anion gap
metabolic acidosis most likely from septic shock and a respiratory acidosis
rom DMD.

35.  12 years girl is admitted following a mixed overdose of ibuprofen and
paracetamol. She currently appears well sitting up in bed and conversing
normally. (HR 79 bpm, RR 16 pm ,blood pressure 112/71 ,saturation 98 %,
temp 36.5 c) her blood gas shows
 pH : 7.37
 pO2 : 51.6 mmhg
 pCO2 : 47 mmhg
 HCO3 : 28 mmol/L
 Saturation 76%
 What does it show ?
A. A fully compensated respiratory acidosis
B. Acute hypoxia type 1 respiratory failure
C. A venous blood gas
D. A fully compensated metabolic alkalosis

36.  pH : 7.40
 pO2 : 65 mmhg
 pCO2 : 60 mmhg
 HCO3 : 44 mmol/L
 What is the acid base balance ?
A. Mixed respiratory acidosis and metabolic alkalosis
B. Metabolic alkalosis with appropriate respiratory compensation
C. Respiratory acidosis with appropriate metabolic compensation

37. o pH : 7.33
o pO2 : 90 mmhg
o pCO2 : 34.6 mmhg
o HCO3 : 18 mmol/L
o Na : 135
o K : 3.5 mmol/L
o CL : 110 mmol/L
o Urea : 6.8 mmol/L
o Creatinine : 125 umol/L
o Albumin : 30 g /L
 What is the acid base disturbance ?
A. High anion gap metabolic acidosis
B. Normal anion gap metabolic acidosis

38. Thank you