BLOOD GAS ANALYSIS - Newbornwhocc

41 Pages·2010·0.66 MB·English

by  Dr. Ashok Deorari

Deorari , AIIMS 2008

BLOOD GAS ANALYSIS
Deorari , AIIMS 2008

Contents
1. Introduction, indications and sources of errors
2. Terminology and normal arterial blood gases
3. Understanding the print outs
4. Details about
(i) pH
(ii) Oxygenation, oxygen saturation, oxygen content, alveolar gas equation, indices of
oxygenation
(iii) Carbon dioxide transport, Pco total CO content, and bicarbonate levels
2 2
(iv) Base excess and buffer base
5. Simple and mixed disorders
6. Compensation mechanisms
7. Anion Gap
8. Approach to arterial blood gases and exercises
9. Arterial blood gases decision tree
10. Practical tips for sampling for ABG.

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Abbreviations
ABE Actual base excess
ABG Arterial blood gas
AaDO Alveolar to arterial oxygen gradient
2
Baro/PB Barometric pressure
BB Buffer base
BE Base excess
BEecf Base excess in extracellular fluid
BPD Bronchopulmonary dysplasia
CH+ Concentration of hydrogen ion
CO Carbon dioxide
2
ECMO Extra corporeal membrane oxygenation
FiO Fraction of inspired oxygen
2
HCO Bicarbonate
3
H CO Carbonic acid
2 3
MAP Mean airway pressure
O CT Oxygen content of blood
2
PaCO Partial pressure of carbon dioxide in arterial blood
2
PaO Partial pressure of oxygen in arterial blood
2
pAO Partial pressure of oxygen in alveoli
2
pH O Water vapour pressure
2
PPHN Persistent pulmonary hypertension in newborn
RBC Red blood corpuscles
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RQ Respiratory quotient
Sat Saturation
SBE Standard base excess
St HCO- /SBC Standard bicarbonate
3
TCO Total carbon dioxide content of blood
2
THbA Total haemoglobin concentration
UAC Umbilical artery catheter

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The terminology of arterial blood gas (ABG) is complex and confusing. It is made
worse by the printouts generated by recent microprocessors. Basically the machines

Total-CO mMol/I Plasma Pco
2 2
mm Hg
60
60 HCO3 mEq/I Plasma 10
Base Excess
50 mEq/I blood or plasma
50
+ 3
0
40 + 2 pH
40 5 8 . 15
0
35 35 +0 2 7 .
9
30 30 + 1 7 .
5 8 20
+ 1 7 .
20 25 0 7
0 +5 7 .
6 25

0 7 .
20 5
20 -
5 7 . 30
4
7 .
-10 3 35
15
7 .
15 2 40
7 .

1
-15
7 .
0 50
10 6 .
25 9
10 9 -20 20 6 . 60
8
15
8
6 .
9 10 7 70
7 6 .
8 Hm eo 6 80
6 -25 5glo
90
bin
7 g/1
100
00
ml 110
6 120
4 130
0
-30 140
150
5
3

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measure pH, carbon dioxide and oxygen. All other parameters are derived based on
software in machine which can be obtained manually if one knows how to use Siggaard
ndersen Nomograms (given above).
Goals of ABG in newborn is to characterize the type of disorder, quantify the
magnitude and assess the nature and extent of compensation.
Indications for ABG
(1) Severe respiratory or metabolic disorders
(2) Clinical features of hypoxia or hypercarbia
(3) Shock
(4) Sepsis
(5) Decreased cardiac output
(6) Renal failure
(7) Ideally any baby on oxygen therapy
(8) Inborn errors of metabolism
Collection of Samples
Ideal artery for sampling in newborn is radial or umbilical artery. One must perform
“Allen Test” to ensure collateral blood supply by ulnar artery before puncturing radial
artery. If sample from umbilical artery catheter (UAC) is being taken, one should assure
free flow of blood and remove three to four times dead space volume before sample is
taken. Indwelling arterial line may only be put if round the clock facilities for ABG
estimation are available considering this as a potent source of infection.
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Arterialised capillary samples are comparable to arterial blood (Table I). If capillary
sample (100-150 micro L) is being taken from prewarmed heel, let the capillary fill from
the tissue site from where blood is oozing out (figure I). Avoid squeezing and first drop
of blood. Rotate the capillary in palm . to mix anticoagulant with blood. Care should be
taken not to include any air bubble in the capillary.
Venous blood is good for HCO- estimation but bad for pH, pCO and pO . While
3 2 2
drawing venous sample make sure that no tourniquet is applied, artery is not compressed
and sample is drawn against the flow of blood towards heart.
Table I : Comparison of Blood Gas Analysis at different sites
Arterial Capillary Venous
PH Same ---------- Lower
PO Higher Lower
2
PCO Lower Higher
2
HCO- Same ---------- Same
3
Recommendation Good Fair Bad
Precautions for collection of blood sample
(1) Heparin is acidic and lowers pH. Use heparin of lower strength (1000 units per ml
instead of 5000 units per ml) or heplock solution.
(2) Use small volume of heparinised saline just for lubricating syringe and plunger. If
volume is more, dissolved oxygen in haparinised saline may increase pO .
2
(3) Avoid air bubble and let syringe fill spontaneously.
(4) It is desirable to use a glass syringe as plastic syringes are permeable to air.
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(5) Sample may be collected in a heparinised capillary from hub of needle used to
puncture artery.
The sample should be processed immediately, preferably within 30 minutes. Blood is
a living medium. The cells consume oxygen and produce CO . Drop in pO depends
2 2
on initial pO . If the latter is very high, significant drop may be noticed. The changes
2
are as depicted in Table II. Slush of ice (not cubes) should be used for storing samples
till processing. The sample should be shaken, homogenised before putting in
machine.
Table II: Changes in ABG every 10 minutes in vitro
37C 4C
pH 0.01 0.001
pCO 0.1 mm Hg 0.01 mm Hg
2
pO 0.1 mm Hg 0.01 mm Hg
2
* It is obvious that blood sample should be stored at 4C, if it cannot be processed
immediately for minimal error.
Terminology of ABG
Acidosis pH <7.3
Alkalosis pH>7.5
Hypercapnia pCO > 50 mm Hg
2
Hypocapnia pCO <30 mm Hg
2
Hypoxia pO < 50 mm Hg
2
Hyperoxia pO > 70 mm Hg
2
*Acidemia and alkalemia refer to blood while acidosis, alkalosis to tissue pH.
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Normal Neonatal ABG values
PH 7.35 – 7.45
pCO 35 – 45 mm Hg
2
pO 50 – 70 mm Hg
2
HCO 20 – 24 mEq/L
3
BE  5
ABG values vary with age of neonate and even with gestational age (Table III, IV).
Table III: ABG values based on neonatal age
Pre-birth 5 min 1-7 days
(Scalp) after birth after birth

pH >7.20 7.20-7.34 7.35-7.45
pCO <50 35-45 35-45
2
pO 25-40 49-73 70-75
2
Sat% >50 >80 >90
HCO >15 16-19 20
3

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Printout generated by blood gas machine
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