Science and art of reading ABG by Dr Anand.Tiwari

1. ABG skill station
C.M.E Wanowarie
Dr.Anand.Tiwari

2. anand tiwari
WHY ABG????
• PULSE OXIMETER
• VS
• ABG

3. See the bigger picture!
• Problems with readings of pulse oximeters
• Clinical situation Result
• Carboxyhaemoglobin-- Falsely high saturation
• Bilirubin--- Falsely low saturation
• Melanotic skin---- Variable, reduced signal
• Poor peripheral perfusion--- Low signal,
unreliable results

4. Task -1
• Draw oxygen dissociation curve?
• Enumerate causes for right &left shift?

5. Imp fact
anand tiwari

6. ABG Sampling
• Indications
• Sites
• Precautions
• How long should I wait?
• Sample transport

7. Allen*test
• Problems of taking arterial blood samples
Bleeding
• Vessel obstruction
• Infection

8. Normal range of pH : 7.35- 7.45
Why maintain pH in such a narrow range ?

9. pH balance regulated by:
1. Chemical buffer system (acts immediately)
1. Respiratory centre in brain stem (1-3
minutes)
2. Renal mechanisms (hours / days)

10. Renal Mechanisms
• Kidneys alter/replenish H+ by altering plasma
[HCO3
-]
•  [H+] plasma (alkalosis)  kidneys excrete
lots of HCO3
-
•  [H+] plasma (acidosis)  kidneys produce
new HCO3
-

11. Acid Production
• Volatile Acids
CO2 mainly
• Non volatile acids
Glucose
Proteins
Lipids
Sulphur containing a.a -- cysteine,
methionine
Phosphates –proteins
Glucose – lactic / pyurvic acid
Fatty acids --
acetoacetic acid ,
β-hydroxybutric acid

12. As required for ECG
interpretation,
a systematic approach to
ABGs enhances accuracy.
There are NO short-cuts!
A Systematic Approach

13. The Anatomy
of a Blood Gas
Report
----- XXXX Diagnostics ------
Blood Gas Report
248 05:36 Jul 22 2000
Pt ID 2570 / 00
Measured37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected38.6
o
C
pH 7.439
pCO2 47.6 mm Hg
pO2 123.5 mm Hg
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
ct CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Measured Values
Temperature Correction:
Is there any value to it?
Calculated Data:
Which are the useful ones?
Entered Data:
Derived from other sources

14. Traditional Measurements
pH electrode
pCO2 electrode (Severinghaus)
pO2 electrode (Clark)
Additional options include:
Co-oximeter; measures O2 saturation
Na
+
, K
+
, Ca
2+
, Cl
-
Haematocrit

15. ----- XXXX Diagnostics ------
Blood Gas Report
Measured37.0
o
C
pH 7.463
pCO2 44.4 mm Hg
pO2 113.2 mm Hg
Corrected38.6
o
C
Calculated Data
HCO3 act 31.1 mmol / L
HCO3 std 30.5 mmol / L
BE 6.6 mmol / L
O2 CT 14.7 mL / dl
O2 Sat 98.3 %
t CO2 32.4 mmol / L
pO2 (A - a) 32.2 mm Hg
pO2 (a / A) 0.79
Entered Data
Temp 38.6 oC
ct Hb 10.5 g/dl
FiO2 30.0 %
Bicarbonate is calculated on the basis of
the
Henderson equation:
[H
+
] = 24 pCO2 / [HCO3
-
]
or
for the mathematically inclined…
Bicarbonate:
Henderson-Hasselbach equation:
pH = pKc + Log [HCO3
-
]
a pCO2

16. Steps of evaluation
• Step 1:- Check the validity of the report
• Step 2:- Identify the most obvious disorder
• Step 3:- Apply the formulae to determine
whether compensation is adequate. If not a
second disorder coexists.
• Step 4:- Calculate the anion gap

17. Checking the Reports Validity.
• Calculate H+ ion concentration from the
formula.
• H+ = 24 X pCO2/HCO3
-
• This should correspond to the H+ ion
concentration of the pH in the ABG report.

18. anand tiwari

19. anand tiwari
.
Condition HCO3
- pCO2
Metabolic Acidosis Lower Lower
Respiratory Acidosis higher if chronic Higher
Metabolic Alkalosis Higher higher
Respiratory Alkalosis lower if chronic Lower
Step 2:- Identify the most obvious disorder

20. Mixed or simple disorder

21. Appropriate compensation in simple acid
base disorders
• Metabolic acidosis-
Pco2=(1.5XHCO3)+8-/+2
• Metabolic alkalosis
Pco2= o.7x HCo3+21+_1.5
anand tiwari

22. Change in Ph
• For every increase in Paco2 of 20 mm Hg (2.6
kPa) above normal-
• mal the pH falls by 0.1
• x For every decrease of Paco2 of 10 mm Hg
(1.3 kPa) below normal
• normal the pH rises by 0.1.
• *Any change in pH outside these parameters
is therefore metabolic in origin.

23. Respiratory acidosis: < 24 hrs: D [HCO3] = 1/10 D PCO2
> 24 hrs: D [HCO3] = 3/10 D PCO2
Respiratory alkalosis: 1- 2 hrs: D [HCO3] = 2/10 D PCO2
>2 days: D [HCO3] = 6/10 D PCO2
REF-Text book of Hall criticare
anand tiwari

24. Case 1.
• 51 yr old found unresponsive at home past
history of D.M,HT.medications-
Metformin,hydrochlorothiazide,Lisinipril.
• b/p-120/70,HR-96/min.
• ABG-7.11,Paco2-28.4,pao2-86,room air
• Na 137,hco3-9,cl-90,BUN-116,CRE-
7.2mg%,Albumin-3.9gm%,bl gl-906mg%

25. ----- XXXX Diagnostics ------
Blood Gas Report
Measured 37.0
o
C
pH 7.301
pCO2 76.2 mm Hg
pO2 45.5 mm Hg
Calculated Data
HCO3 act 35.1 mmol / L
O2 Sat 78 %
pO2 (A - a) 9.5 mm Hg D
pO2 (a / A) 0.83
Entered Data
FiO2 21 %
Case 2
60 year old male smoker
with progressive
respiratory distress
and somnolence.
D CO2 =76-40=36
Expected D pH = 36/10 x0.08=0.29
Expected pH = 7.40-0.29=7.11
Chronic resp. acidosis
pH <7.35 ; acidemia
pCO2 >45; respiratory acidemia
Limits:
DHCO3 = 3/10 of D pCO2
=3/10x36=10.8
Limits of HCO3 = 24+11=35
Pure Resp Acidosis
Hypoxia
Normal A-a gradient
Due to hypoventilation

26. Case -- 1
• A middle aged female leukemia admitted to Intensive
care with respiratory failure required invasive
ventilation.
• ABG sample collected after one hour of ventilation
on Fio21 MV-8.0
lits, R.r-16/min on siemens ventilator.SpO2 on pulse
ox-96%
anand tiwari

27. Treat the patient!!!
• Ph-7.293
Pco2-39.2mm of hg
Po2-11.1mmof hg
Hco3-act-18.6mmol/l
Hco3-std-16.6mmol/l
Tco2-19.8mmol/l
BEvt=-7.4mmol/l
BE vv=-8.0
anand tiwari
Sr Electrolytes-Na+=127.3
mmol/l
K+=5.16
cl-=88mmol/l
How will you explain low po2
disparity with spo2?
–
LAB
WBC—297000/ul
Hgb-10 gm/dl
Hct-23.4%
Plt--108

28. • Hemogram as follows—
WBC—297000/ul
Hgb-10 gm/dl
Hct-23.4%
Plt--108
anand tiwari

29. Learning point---
• Be aware of “leukocyte larcency” (aka
pseudohypoxemia)—phenomenon of
artificially
• low pO2 on ABG due to markedly increased
O2 consumption by malignant leukocytes
• -in these instances, pulse oximetry often a
better measure of oxygenation
Majhail NS, Lichtin AE. Acute leukemia with a very high leukocyte count: confronting a medical
emergency. Cleve Clin J Med. 2004 Aug;71(8):633-7.anand tiwari

30. Case 2
18yr old asthmatic 3 days of
cough,dyspnea,orthopnea,not
responding to bronchodilators.
ABG-Ph-7.24
PCO2-49.1
PO2-66
HCO3---18 sat 92%
Calculate A-a Gradient

31. Learning point___
• Differentiate hypoventilation from diffusion
defect.

32. Case 3
• A 29-year-old previously healthy female, is
admitted in semiconcious state. She has c/o
fever,sweating and vomitting for last one day.
Following is her ABG (Fio2-.21) and biochemical
report.
anand tiwari
PH - 7.38
> PCO2- 24
> PO2 - 96
> HCO3- 16
> Sr.Sodium - 138 meq/l
> Sr.Potassium - 3.7 meq/l
> Sr.Chloride - 102 meq/l
> Blood Sugar - 48 mg%
> BUN -32 meq/l
> Sr.Creatinine - 1.5 meq

33. • Causes of metabolic acidosis with
hypoglycemia are:
Alcohol intoxication, seizures, Advanced liver
failure, acute adrenal insufficiency, myxedema coma,
severe malaria, starvation ketoacidosis and drug
intoxication such as metformin, salicylates,
anand tiwari

34. Learning point
• the abnormalities here are: High AG Metabolic
acidosis with respiratory alkalosis with
Hypoglycemia In given scenario the likely causes are:
1. severe malaria/cerebral malaria.
2. Quininie toxicity.
3. Salicylate toxicity
anand tiwari

35. Case 4
• ABG was taken from patient who presented
with abdominal pain and vomiting.
• Ph-7.31
• Pco2-45.7 Na-142,k-3.7,cl-91
• Po2-98.1
• Hco3-22.3
• Base excess=3

36. Case 5
• Young male patient of recurrent seizures was
on regular intravenous doses of
benzodiazepine lorazepam for control of
convulsion dose was reduced on day 7 as
spasm came under control
anand tiwari

37. • Biochemical profile of above patient
anand tiwari
Measured
serum
osmolarit
Day1
290
Day4
300
Day7
315
Day10
298
calculated 285 281 284 286
Plasma
lactate
2.1 4.9 2.9
“
Likely explanation for change in osmolar gap

38. Learning point---
• Be Aware of osmolar gap.

39. Case -- 5
• 63 year old female with history of CHF presented to
ER with c/o
• shortness of breath over preceding 2 days
• Fever
• Cough with thick yellow sputum
• Past history hypertension and chf treated with furosemide
20mg/day digoxin 0.25mg/day
anand tiwari

40. • Lung right sided basilar crackles
B.P 140/80
HR-100/MIN
R;R-30/MIN
TEMP 39*
Lab-TLC 14000
HGB 13
NA 139
K-3.4
CI-98
ABG
PH-7.59
PCO2-30
PO2-58
HCO3-29
Chest x ray –lobar infiltrate in right lobe
Sputum exam stain – gpc diplococci and
anand tiwari

41. • Initial presentation is alkalemia pc02 decreased but
bicarbonate is at upper end of normal so combined metabolic
alkalosis +respiratory alkalosis
• History gives a clue that chronic metabolic
alkolosis due to diuretics and acute respiratory
alkalosis is due to pneumonia
anand tiwari

42. • She is started on antibiotic therapy with o2 supplementation
• 12 hours later she complains of increased shortness of breath
ABG
Ph-7.40
Pco2 48
P02 44
Intubate and ventilated tidal vol 600 R.r 16
Fio2 1
Peep-5
ABGg
Ph-7.24
Pco2 38
Po2-86 hco3 16
Anion gap 23
Sr lactate 8 mmol/lit
anand tiwari

43. • As patient developed ventilatory failure ABG showed
normal Ph and respiratory acidosis ie balanced by
chronic increase in bicarbonate level.
• As sepsis developed acidemia with decreased
bicarbonate level and increased anion gap
anand tiwari

44. • Over a period clinical condition improved a she was started on
TPN,dobutamine dicontinued and furosemide 40mg/day
• Her oxygenation continued to improve but
ABG
PH-7.55
PCO2-44
HCO38
Furisemide dicontinued
Acetozolamide begun
ABG
PH-7.6
PCO2 46
HCO3 44
anand tiwari

45. • Her final acid base disorder is severe alkalemia
of metabolic origin
• Was initially thought to be due to diuretic but
found after searching all medication and
substances administrated that patient
parental nutrition contained 70 meq of
acetate /litre
• Acetate –biacarb load –na avidity –
hypochloremia –vol deficit
anand tiwari

46. Final word
 Effects of preexisting disease state may exacerbate
or mitigate degree of Ph change
 Acid base equilbrium in ICU is dynamic system and
multiple disorders may occur simultaneously.
 Look for the underlying cause
 Unexplained disorder should prompt through search
of all administrated substances
anand tiwari