2. What is the preanalytical phase?
• It is the phase which includes all activities that occur prior to the sample’s insertion
into the analytical instrument.
• Sub phase of the preanalytical process in blood gas testing includes:
Ordering,
Identification,
Collection,
Transportation,
Separation/preparation
• It has been recognized for years that the phase of testing most prone to errors is the
preanalytical phase.
• Pre-preanalytical Error includes :
• Inappropriate test ordered, order entry error, patient/specimen
misidentification, sample contamination (from indwelling catheter),
sample collection mishap (hemolysis, clotting, insufficient volume),
inappropriate collection container, improper
storage/handling/transport.
2
4. ABG – Procedure and Precautions
Site- (Ideally) RadialArtery
BrachialArtery
FemoralArtery
Ideally - Pre-heparinised ABG syringes
- Syringe should be FLUSHED with 0.5ml
of 1:1000 Heparin solution and emptied.
DO NOT LEAVE EXCESSIVE HEPARIN IN THE
SYRINGE
HEPARIN DILUTIONAL
EFFECT
HCO3
PCO2
Only small 0.5ml Heparin for flushing and discard it
Syringes must have > 50% blood. Use only 2ml or less
syringe.
4
6. Determination of PaO2
2
PaO is dependant upon Age, FiO ,P
2 atm
As Age the expected PaO2
• PaO2 = 109 - 0.4 (Age)
As FiO2 the expected PaO2
• Alveolar Gas Equation:
• PAO2= (PB-P h2o) x FiO2- pCO2/R
O
X
Y
G
E
N
A
T
I
O
N
A 2 B
P O = partial pressure of oxygen in alveolargas, P = barometric pressure
(760mmHg), Ph2o = water vapor pressure (47 mm Hg), FiO2 = fraction of
inspired oxygen, PCO2 = partial pressure of CO2 in the ABG, R = respiratory
quotient (0.8)
6
7. Assessment of ACID BASE Balance
Definitions and Terminology
ACIDOSIS – presence of a process which tends to
pH by virtue of gain of H +
-
or loss of HCO3
ALKALOSIS – presence of a process which tends
3
to pH by virtue of loss of H+ or gain of HCO -
If these changes, change pH, suffix ‘emia’ is added
ACIDEMIA – reduction in arterial pH (pH<7.35)
ALKALEMIA – increase in arterial pH (pH>7.45)
7
8. Simple Acid Base Disorder/ Primary Acid Base
disorder – a single primary process of acidosis or
alkalosis due to an initial change in PCO2 and HCO3.
Compensation - The normal response of
the respiratory system or kidneys to change in pH
induced by a primary acid-base disorder
The Compensatory responses to a primary Acid Base
disturbance are never enough to correct the change in
pH , they only act to reduce the severity.
Mixed Acid Base Disorder – Presence of more than
one acid base disorder simultaneously .
8
10. Normal Values
ANALYTE Normal Value Units
pH 7.35 - 7.45
PCO2 35 - 45 mm Hg
PO2 72 – 104 mm Hg`
[HCO3] 22 – 30 meq/L
SaO2 95-100 %
Anion Gap 12 + 4 meq/L
∆HCO3 +2 to -2 meq/L
10
11. STEP0
STEP1
STEP2
• Is this ABGAuthentic?
• ACIDEMIA or ALKALEMIA?
• RESPIRA
TORY or METABOLIC?
STEP3 • Is COMPENSATION adequate?
STEP4 • If METABOLIC ACIDOSIS – ANION GAP?
STEP5
• If High gap MetabolicAcidosis– GAP GAP?
11
12. Is this ABG authentic ?
pH = - log [H+]
Henderson-Hasselbalch equation
pH = 6.1 + log HCO3
-
0.03 x PCO2
The [HCO3-] mentioned on the ABG is actually calculated
using this equation from measured values of PCO2 ndpH
• [H+] neq/l = 24 X (PCO2 / HCO3)
• pH = -log [ H+]
pHexpected = pHmeasured = ABG is authentic
12
14. Look at pH
<7.35 - acidemia
>7.45 – alkalemia
RULE – An acid base abnormality is present even if
either the pH or PCO2 are Normal.
ACIDEMIA ORALKALEMIA?
STEP 1
14
15. IS PRIMARY DISTURBANCE RESPIRATORY OR
METABOLIC?
pH PCO2 or pH PCO2 METABOLIC
pH PCO2 or pH PCO2 RESPIRA
TORY
RULE- If either the pH or PCO2 is Normal, there is a
mixed metabolic and respiratory acid base disorder.
RESPIRATORY or METABOLIC?
STEP2
15
18. ADEQUATE COMPENSATION?
IS THE COMPENSATORY RESPONSE ADEQUATE OR NOT?
METABOLIC DISORDER
PCO2measured ≠ PCO2expected
PCO2expected
MIXED DISORDER
RESPIRATORYDISORDER pHexpected acute-chronic
pHm ≠ pHe range MIXED DISORDER
18
19. STEP0
STEP1
STEP2
• Is this ABGAuthentic?
• ACIDEMIA or ALKALEMIA?
• RESPIRA
TORY or METABOLIC?
STEP3
STEP4
• If Respiratory – ACUTE or CHRONIC?
• Is COMPENSATION adequate?
STEP4
• If METABOLIC ACIDOSIS – ANION GAP?
STEP6
• If High gap MetabolicAcidosis–
GAP GAP?
19
20. Electrochemical Balance in Blood
80%
70%
60%
50%
40%
30%
20%
10%
0%
100%
90%
CATIONS ANIONS
Sulfate
Phosphate
Mg- OA
K - Proteins
Ca-HCO3
Na- Cl
UA
UC
Na
Cl
HCO3
20
21. Anion Gap
AG based on principle of electroneutrality:
Total Serum Cations = Total Serum Anions
M cations + U cations = M anions + U anions
Na + (K + Ca + Mg) = HCO3 + Cl + (PO4 + SO4
+ Protein + Or
Na + UC
But in Blood there is a
= HCO3 + Cl + UA
relative abundance of Anions,
ganicAcids)
hence
Anions > Cations
Na – (HCO3 + Cl) = UA – UC
Na – (HCO3 + Cl) = Anion Gap
21
22. METABOLIC ACIDOSIS-
ANION GAP?
STEP4
IN METABOLIC ACIDOSIS WHAT IS THE ANION GAP?
ANION GAP(AG) = Na – (HCO3 + Cl)
Normal Value = 12 + 4 ( 8- 16 Meq/l)
Adjusted Anion Gap = Observed AG +2.5(4.5- S.Albumin)
50% in S.Albumin 75% in Anion Gap !!!
High Anion Gap MetabolicAcidosis
MetabolicAcidosis
Normal Anion GapAcidosis 22
23. High Anion Gap Metabolic
Acidosis
M
U
D
P
I
E
METHANOL
UREMIA - ARF/CRF
DIABETIC KETOACIDOSIS & other KETOSIS
PARALDEHYDE, PROPYLENE GLYCOL
ISONIAZIDE, IRON
L
LACTICACIDOSIS
ETHANOL, ETHYLENE GLYCOL
S
SALICYLATE
23
24. CO EXISTANT METABOLIC
DISORDER – “Gap Gap”?
STEP5
C/O HGAG METABOLIC ACIDOSIS,ANOTHER DISORDER?
∆ Anion Gap = Measured AG – NormalAG
Measured AG – 12
∆ HCO3 = Normal HCO3 – MeasuredHCO3
24 – Measured HCO3
Ideally, ∆Anion Gap = ∆HCO3
For each 1 meq/L increase in AG, HCO3 will fall by 1 meq/L
∆AG/ HCO3
- = 1 Pure High AG MetAcidosis
AG/ HCO3
- > 1 Assoc Metabolic Alkalosis
AG/ HCO3
- < 1 Assoc N AG MetAcidosis
24
26. CASE-1
pH-7.28,PaCO2-24, HCO3-12,Na-128, K-3.8,
Step 1: Acidemia
Step 2: Metabolic
Step 3: Appropriate compensation- 26
PCO2= 1.5 (HCO3)+8(winters formula)
Step 4: Calculate AG= Na – (Cl+ HCO3)
128- (98+ 12) = 18
Elevated Anion Gap
If S alb= 2.0
c AG = 18 + 2.5(4-2) = 23
26
27. CASE-2
pH-7.17,PaCO2-65,HCO3-22
Na-136,Cl-98,
Sr.Alb-1.6
Step 1: Acidemia
Step 2: Respirtory
Step 3: Evaluate compensation – no, its decreased- metabolic
acidosis
Step 4: Calculate AG= Na – (Cl+ HCO3)
136- (98+ 22) = 16
If S alb= 1.6
Step 5: c AG = 16 + 2.5(4-1.6) = 22 (elevated gap Met Acidosis
Step 6: Calculate Delta ratio : 22- 12/ 24- 22= 10/ 2= 5
DELTA RATIO > 2, IRRESPECTIVE OF ETIOLOGY- OF
INCREASED AG, IT POINTS TO METABOLIC
ALKALOSIS
48 yr, alcoholic , found unconscious
,soiled with vomitus, last seen at party
6 hrs back
27
28. 17 year old w/severe kyphoscoliosis, admitted
for pneumonia
pH 7. 42
paCO2 29
paO2 72
HCO3 17
Na 140
K 4.0
Cl 109
Step 1: Check pH: Normal (towards alkalosis)
Step 2: Check paCO2 : Alkalotic (respiratory alkalosis)
Step 3: Compensation: (11mmHg CO2 ~ 5 meq HCO3)
Step 4: Calculate Anion Gap AG=140 –(109+17)=14
Step 5: Delta gap= 14-12/24-17= 2/ 5= 0.4
Signifies underlying normal anion gap metabolic acidosis
28
29. 60 year male, smoker, COPD with respiratory
distress
pH 7.26
paCO2 60
paO2 55
HCO3 26
Na 140
K 4.0
Cl 104
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : High (Respiratory)
Step 3: Compensation (2 HCO3 for 20 mmHg CO2)
Step 4: AG=140-(104+26)= 10
Respiratory Acidosis
29
30. 45 yr old male, k/c/o COPD since 10 yrs
pH 7.34
paCO2 60
paO2 55
HCO3 32
Na 140
K 4.0
Cl 100
Step 1: Check pH: Acidemia
Step 2: Check paCO2 : Raised (respiratory )
Step 3: Evaluate compensation : Compensation (8 HCO3 for
20 mmHg CO2) - adequate
AG=140 –(100+30)=10
Respiratory Acidosis (Chronic) compensated
30
31. 75 yr woman, fever & profuse diarrhoea x 2ds,
T 38.5°C, HR 130/min, BP 78/30 mmHg
pH 7.29
paCO2 30
paO2 80
HCO3 14
Na 128
K 3.2
Cl 94
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : decreased ( Primary Metabolic)
Step 3: Check compensation
Expected CO2=1.5xHCO3+8=21+8=29 (appropriate)
Step 4: AG=128 –(94+14)=20, Delta Anion Gap=20-12=8
Step 5: Delta Ratio=▲AG/ ▲HCO3=8/10=0.8
High Anion Gap Metabolic Acidosis, Additional NAG
Metabolic Acidosis 31
32. 40 years,male, CRF, on maintenance haemodialys,
Missed his last appointment .
pH 7.25
paCO2 27
paO2 99
HCO3 14
Na 140
K 4.0
Cl 96
S Albumin 2.5 gm%
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Decreased ( Primary metabolic)
Step 3: Evaluate compensation
Expected CO2=1.5xHCO3+8= 21+8=27 (appropriate)
Step 4: AG=140 –(96+12)=32, Adjusted AG=36
Step 5: Calculate Delta Ratio=▲AG/ ▲HCO3
Delta Anion Gap=36-12=24 ; Delta ratio=24/10= 2.4
High Anion Gap Metabolic Acidosis, Additional Metabolic Alkalosis 32
33. 55-year-old male admitted with a recurring bowel
Obstruction& intractable vomiting forthe last several hours
pH 7.50
paCO2 48
paO2 90
HCO3 38
Na 135
K 3.6
Cl 85
Step 1: Check pH: Alkalotic
Step 2: Check paCO2: increased ( Primary Metabolic)
Step 3: Evaluate compensation: 40= 0.7* (HCO3- 24) = 50
Appropriate so no additional resp disorder
Step 4: AG=135 –(85+38)=12, Delta Anion Gap=12-12= 0
Metabolic Alkalosis
33
35. 12 yearold diabetic presents with Kussmaul breathing
pH 7.05
paCO2 15
paO2 68
HCO3 5
Na 140
K 4.0
Cl 98
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Decreased (Metabolic Acidosis)
Step 3: Evaluate compensation:
Expected CO2 =1.5xHCO3+8=15, so no additional respiratory disorder
Step 4: AG=140 –(98+12)=30, Delta Anion Gap=30-12= 18
Step 5 : Delta Ratio=▲AG/ ▲HCO3=18/19=1
High AG Metabolic Acidosis,
35
36. Pt of severe pneumonia with pulmonary oedema
pH 7.30
paCO2 38
HCO3 18
Na 140
K 4
Cl 102
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : N : Check HCO3- : Acidotic
Step 3: Evaluate compenstaion
Metabolic Acidosis
Expected CO2 =1.5x(HCO3)+8 =35 (slight resp acidosis)
Step 4: AG=140 –(102+18)=20, Delta Anion Gap=20-12= 8
Step 5: Delta Ratio=▲AG/ ▲HCO3=8/6=1.2
High AG Metabolic Acidosis
36
37. 48yr male,Alcoholic, found unconscious, after a party
pH 7.17
paCO2 65
HCO3 22
Na 136
K 3.4
Cl 98 S Alb=1.6 gm%
Step 1: Check pH: Acidotic
Step 2: Check paCO2 : Acidotic (Respiratory Acidosis)
Step 3: Evaluate Compensation
Resp acidosis: Compensation ?? NO
AG=136-(98+22)=16, Adjusted AG=22, Delta Anion Gap=22-12=10
Delta Ratio=▲AG/ ▲HCO3=10/2=5
High Anion Gap Metabolic acidosis with Metabolic Alkalosis
Expected CO2 =1.5x(HCO3)+8 =42
Respiratory Acidosis High AG Metab acidosis , with M Alkalosis
37
39. 55 yrmale,alcoholic,disoriented, c/o recurrent vomiting,
Hematemesis
pH 7.45
paCO2 34
HCO3 23
Na 135
K 4.0
Cl 85
Step 1: Check pH: Normal
Step 2: Check paCO2 : slightly alkalotic
Step 3: evaluate compensation ---- Bicarb……almost normal
ABG Normal ???
Expected CO2 =1.5x(HCO3)+8 =42
Step 4: AG=135-(85+23)=27, Delta Anion Gap=27-12=15
Step 5: Delta Ratio=▲AG/ ▲HCO3=15/1=15
High Anion Gap Metabolic acidosis with Metabolic Alkalosis
High AG Metab acidosis with M Alkalosis with Respiratory
Alkalosis 39
40. 61yr, morbidly obese, COPD, CHF
pH 7.41
paCO2 55
paO2 75
HCO3 34
Na 140
K 4.0
Cl 95
Step 1: Check pH: Normal
Step 2: Check paCO2 : Acidotic ??
Step 3: Evaluate compensation ?? COPD, chronic
Compensation : chronic Respiratory acidosis…6 in this case
HCO3= 34 compensation not appropriate …Metabolic Alkalosis
Step 4: AG=140-(95+34)=11, Delta Anion Gap=11-12=-1
Metabolic alkalosis with Primary resp acidosis (chronic)
40
41. 32 yr pregnant, 4days of non stop vomiting, afebrile,HR
145/min, BP78/62
pH 7.41
paCO2 42
HCO3 26
Na 146
K 3.2
Cl 92
Step 1: Check pH: Normal
Step 2: Check paCO2 : Normal, Check HCO3- : Normal
Step 3: Evaluate Compensation ABG Normal ????
Step 4: AG=146-(92+26)=28, Delta Anion Gap=28-12=16
Step 5: Delta Ratio=▲AG/ ▲HCO3=16/-2=-8 (negative delta ratio
with N pH)
41
42. References
1) Harrison’s Principles of Internal Medicine, 17th edition,
Chap 48 – Acidosis andAlkalosis
2) Paul L.Marino – The ICU Book, 3rd Edition
3) Guyton and Hall – Textbook of Medical Physiology, 11th
edition
4) Davenport – The ABC of Acid Base Chemistry, 6thedition
5) Cohen and Kassirer – Acid Base
6) Hansen JE, Clinics in Chest medicine10(2), 1989, 227-37
7) Lippincott’s-Fluid Balance, NM Metheny
8) World Wide Web
42
Editor's Notes
Is HCO3 increased by 1 for every 10 mm rise of PCO2
Respiratory alkalosis (due to chronic hyperventilation
secondary to hypoxia) with Compensatory MA
improve the ventilation status by , mechanical ventilation, pulmonary toilet or by bronchodilators.
40 years,male,CRF,on maintenance haemodialys,Missed his last appointment Tachypnoeic, febrile
pH 7.32
PaCO2 32 mmHg
HCO3- 18 meq/l
Base deficit -6
Follow the three steps:
1. pH is low (normal 7.35-7.45); therefore acidosis.
2. PaCO2 is low.
lungs, acting as the primary acid-base buffer, are attempting to compensate by “blowing off excessive C02”, therefore increasing the pH.
3. Assess the HCO3. It is low (normal 22-26). We would expect the pH and the HCO3 to move in the same direction, confirming that the primary problem is metabolic.
55-year-old male admitted with a recurring bowel
Obstruction & intractable vomiting for the last several hours
pH 7.50
PaCO2 42
HCO3- 33
Treatment of this patient might include the administration of I.V.fluids and measures to reduce the excess base.
Severe partly compensated metabolic acidosis (due to ketoacidosis) without hypoxemia
Alcoholic KA (inadequate fluid and carb intake)
Met Alk (vomiting)
Alcoholic KA (inadequate fluid and carb intake)
Met Alk (vomiting)