Arterial blood gas analysis

23/5/2020 ARTERIAL BLOOD GAS ANALYSIS 2

Indications of ABG
• Assess adequacy of oxygenation & ventilation
• When there is initiation or change in oxygen therapy; or
changes in ventilatory parameters :
- In patients without overt pulmonary disease a steady
state is reached between 3–10 minutes
- In patients with chronic airways obstruction it takes
about 20–30 minutes after changes have been made to
ventilator therapy
• Helps to establish diagnosis and severity of Respiratory
Failure
• Helps guide treatment plan
• Management of ICU patients

Contraindications of Arterial Puncture
• Cellulitis or other local infection
• Distorted anatomy at puncture site
• Absence of palpable arterial pulse
• Negative results of an Allen test/ modified Allen test
• Active Raynaud’s Syndrome
• Coagulopathies or medium-to-high-dose
anticoagulation therapy - relative contraindication
• History of arterial spasms following previous
punctures
• Severe peripheral vascular disease
• Arterial grafts

Allen’s & Modified Allen’s Test
Allen’s :
• Patient elevates hand & makes fist for 20 seconds
• Firm pressure held against radial and ulnar arteries
• Patient opens hand which should be blanched white
• Examiner releases only ulnar compression
• Normal result - hand color returns within 5 - 7
seconds
• Abnormal result: Delayed or absent hand flushing
indicating inadequate collateral circulation

Modified Allen’s Test :
• Elevate patient’s feet
• Occlude dorsalis pedis artery; then blanch the
great toe by compressing for several seconds
• Release pressure on the nail and observe for
flushing (rapid return of color indicates adequate
collateral flow)
• Posterior tibial artery is used for pediatric
population

ABG Precautions
• Remove air bubbles – ↑ pO2 ; ↓ pCO2
• Remove excess Heparin – dec. HCO3 & pCO2
• Cold chain transport in ice bag if analyzed within 10-
15 minutes, stable for 1 Hr. on ice
• Apply pressure for 5 minutes at site after procedure
• Don’t palpate too firmly
• Don’t Reposition a needle without first withdrawing the
tip to subcutaneous tissue

Complications of arterial puncture
• Pain
• Bruising and hematoma
• Nerve damage
• Aneurysm
• Spasms
• AV Fistula
• Infection
• Vasovagal response
• Air or thromboembolism
• Anaphylaxis from local anesthestic

ABG - PROCEDURE
• Radial Artery of non- dominant hand is preferred
• Brachial, femoral, dorsalis pedis can also be used
• 2mL blood
• 21 G needle
• Flush syringe with 0.5 mL of 1:1000 heparin
• Place an arterial line when > 4 samples of arterial
blood in 24 hours are anticipated
• Obtain ABG & serum electrolytes simultaneously

Estimation Of H+ from pH
26
Eg. 7.40 = 40 ; 7.39 = 41 ; 7.38 = 42
7.11 = 78.2 ; 7.12 = 76.4 ; 7.13 = 74.6
• Kassirer & Bliech equation :
Measured HCO3
- = 24 * pCO2 / [H+]
+/- 2 for validation for ABG to be compatible
Derived from Modified Henderson-Hasselbach Equation :
pH = pKa + [ log (HCO3
- )/ 0.03 * PaCO2 ] ,
where pKa = 6.1
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
130 100 80 62 50 40 32 25 20 16

Calculation of Bicarbonate
• Actual bicarbonate — value obtained from blood
gas sample
• Standard/corrected bicarbonate - value of
bicarbonate had the sample been corrected to 40
mmHg & at room temperature
• Base deficit/excess - amount of alkali or acid that
must be added to a solution to restore its pH to 7.4
after it has been equilibrated to a PCO2 of 40 mm
Hg. It is amount of deviation of the standard
bicarbonate from the normal

• Normal Base Excess is -2 to +2
• If more negative, metabolic acidosis
• If more positive, metabolic alkalosis
Normal SID = 40
If < 40, BE is negative = metabolic acidosis
If > 40, BE is positive = metabolic alkalosis

Normal Values of ABG
• pH – 7.36 to 7.44 (7.40)
• HCO3
- - 22 to 26 (24)
• pCO2 - 35 to 45 (40)
• H+ - 36 to 44 (40)
• PaO2 - 80 to 100 mmHg
• Base Excess - +/- 2 mEq/L
• O2 Saturation - 95 – 100 %
• Anion Gap - 8 to 14 (12)
• Osmolar Gap - 10
• PaO2 / FiO2 = > 300

Compensation
• For compensation, HCO3
- & CO2 always travel in same
direction as the 1’ factor
1. For Metabolic Acidosis –
when HCO3
- ↓ by 1mEq/L → pCO2 ↓ by 1.2 mmHg
or Winter’s Formula –
pCO2 = (1.5* HCO3
-) + 8 (range : +/- 2)
2. For Metabolic Alkalosis –
when HCO3
- ↑ by 1mEq/L → pCO2 ↑ by 0.7 mmHg

3. For Respiratory Acidosis –
Acute - pCO2 ↑ by 10mmHg → HCO3
- ↑ by 1 mEq/L
Chronic - pCO2 ↑ by 10mmHg → HCO3
- ↑ by 4 mEq/L
4. For Respiratory Alkalosis –
Acute - pCO2 ↓ by 10mmHg → HCO3
- ↓ by 2 mEq/L
Chronic - pCO2 ↓ by 10mmHg → HCO3
- ↓ by 4 mEq/L

* Few Terms :-
• Over compensated – 2 opposite acid-base disorder
• Fully Compensated/simple Acid- base disorder
• Uncompensated/ partially compensated
• Compensatory parameter has value in direction
opposite to compensation – adds up to the primary
disorder

Calculation Of Anion Gap(AG)
• Serum Anion Gap = Na+ - (Cl- + HCO3
-) =
Unmeasured Anions – Unmeasured Cations
• Normal Anion Gap – 8 to 14 (12) mEq/L
• In Metabolic Acidosis –
 if fall in HCO3
- is compensated by rise in Cl-,
then AG will not change – NAGMA
 if fall in HCO3
- is compensated by some ion
other than Cl-, then AG will increase – HAGMA

• In the presence of low serum albumin, anion
gap is reduced by approximately 2.5 mEq/L for
every 1g/dL fall in albumin
• Corrected anion gap = Calculated anion gap +
2.5 x (Normal albumin–observed albumin)
• Normal albumin is taken as 4.4 g/dL

Delta Ratio/ Gap Gap Ratio
• In HAGMA, we always calculate Delta ratio
• Delta Ratio = delta Anion Gap / delta HCO3
-
• Delta Ratio = anion gap – 12 / 24 - HCO3
-
• If it is < 0.4 → Hyperchloremic normal AG acidosis
• If it is < 1 → ∆HCO3
- increases disproportionately,
i.e. HCO3
- levels fall drastically, therefore 1 more
cause for Acidosis, so HAGMA + NAGMA
• If it is 1- 2 → it is HAGMA alone
• If it is >2 → ∆HCO3
- decreases, i.e. HCO3
- levels
increase, so causes alkalosis, so HAGMA + Metabolic
Alkalosis

Urine Anion Gap
• In NAGMA, we calculate Urine Anion Gap to
know the cause of Metabolic Acidosis
• Urine Anion Gap = Urinary [ (Na+ + K+) – Cl- ] =
Urinary Anion – Urinary cation
• If U.A.G. is positive, cause is R.T.A.
• If U.A.G. is negative, cause is G.I. loss of HCO3
-

Osmolality
• Calculated Osmolality = [ 2*Na+ + Blood Glucose/18 + Blood
Urea Nitrogen/2.8 ] = 285-290 mOsm/kg
• Or, Osmolality = [ 2*Na+ + Blood Glucose/18 + Blood Urea/6 ]
• Measured osmolarity by Osmometer
• Osmolar Gap = (Measured – Calculated) Osmolality
• If > 10, Methanol or Ethylene Glycol poisoning
• If < 10, Salicyclates or Paraldehyde poisoning
• B.U.N.(mg/dL) = blood urea (mg/dL) / 2.142
Other Causes for >10 :
Ethanol, Isopropyl alcohol ,Glycine, Mannitol, Glycerol, Chronic renal
failure

Strong Ion Difference
• Difference between measured strong cations & anions
• S.I.D. = ( Na+ + K+ + Ca2+ + Mg2+ ) – ( Cl- + Lactate- )
• Normal S.I.D. – 40 to 44 mEq/L
• S.I.D. = HCO3
- + Albumin + Phosphate
• Met./Hypochloremic Alkalosis → SID ↑
• Met. / Hyperchloremic Acidosis → SID ↓
• Dehydration (Na+) → SID ↑
• Keto/ Lactic acidosis → SID ↓
• Hypoalbuminemia → SID ↓
• Hyperalbuminemia → SID ↑
Na+
Ca2+
K+
Mg2+
S.I.D.
Lactate
Cl-

Eg. In 2L plasma, we add Normal Saline :
• Na+ = 140 + 154/ 2 = 147
• Cl - = 102 + 154/ 2 = 128 → Hyperchloremia
• SID = 19↓ → pH↓ → more acidosis
&
When Ringer Lactate is added to 2L plasma
• Na+ = 140 + 137/ 2 = 139
• Cl - = 102 + 109/ 2 = 105
• Lactate (metabolize) = 0 (organic strong anion
undergo rapid metabolism after infusion)
• SID = 34, close to normal to 38

Metabolic Acidosis
A. Non-Anion Gap Metabolic Acidosis (NAGMA)
1. Renal Acidification Effects –
a. RTA I, IV
b. A.T.N.
c. Hypoaldosteronism
2. G.I. loss of HCO3-
a. Diarrhoea
b. Small bowel losses (ileostomy)
c. Ureteral diversions
d. Anion exchange resins (glue sniffing)
e. RTA II
B. Low-Anion Gap Metabolic Acidosis
a. Multiple Myeloma
b. Hyperparathyroidism
c. Hypoalbuminemia
d. Hypermagnesemia.

B. High Anion Gap Metabolic Acidosis (HAGMA)
(KULT)
1. Endogenous Acid Load -
a. Ketoacidosis – DM(DKA), Alcoholism, Starvation
b. Uremia – Uremic acidosis (chronic renal failure)
c. Lactic Acidosis
2. Exogenous Toxins –
a. Osmolar Gap present – Methanol, Ethylene glycol
b. Osmolar Gap absent – Salicyclates, Paraldehyde

Metabolic Alkalosis
1. Loss of H+ ions (e.g. vomiting, diuretics)
2. Increased reabsorption of bicarbonate
– Low intravascular volume
– Hypokalemia
– High pCO2
– Increased mineralocorticoids (aldosterone).
3. Administration of alkali (in setting of renal
impairment) e.g. Ringer’s lactate where lactate
gets metabolised to bicarbonates in liver adding
to alkali pool.

Respiratory Acidosis
Conditions causing hypoventilation :
1. Airway/pulmonary parenchymal disease
a. Upper airway obstruction
b. Lower airway obstruction
c. Pulmonary alveolar process:
i. Cardiogenic pulmonary edema
ii. Pneumonia
iii. ARDS
iv. Pulmonary perfusion defect—PE—air/fat/tumor
2. Normal airway/pulmonary parenchymal
a. CNS depression
b. Central nervous system depression related to head injury
c. C.N.S. depression d/t narcotics, sedatives or anesthesia
3. Neuromuscular disease and impairment
4. Ventilatory restriction— d/t pain, chest wall injury/deformity, or
abdominal distension

Types of Respiratory Failures
• Type I respiratory failure; is also known as
hypoxemic failures and type II is called
hypercapneic failure

Respiratory Alkalosis
1. CNS stimulation: Fever, pain,
thyrotoxicosis, cerebrovascular accidents.
2. Hypoxemia: pneumonia, pulmonary edema,
severe anemia.
3. Drugs/hormones: Medroxyprogesterone,
catecholamines, salicylates.
4. Miscellaneous: Sepsis, pregnancy
5. Psychological responses, such as anxiety or
fear.

10 STEPS :
Step 1 : Always Get Serum electrolytes with ABG
Step 2 : Calculate HCO3- = 24* pCO2/ H+ (+/-2)
Step 3 : Look at pO2 (<80 mmHg) & O2 saturation
(<90 %) for hypoxemia
Step 4 : Look for pH
< 7.4 – acidosis
> 7.4 – alkalosis
7.4 – normal or mixed ABG
Step 5 : Identify primary cause/problem (see change
pCO2 & HCO3- )

HCO3 more – metabolic Alkalosis
HCO3 less – metabolic acidosis
pCO2 more – respiratory acidosis
pCO2 less – respiratory alkalosis
Step 6 : Calculate compensation, see if it is partially or
fully compensated
Step 7 : In Metabolic Acidosis, calculate Anion Gap
HAGMA or NAGMA

Step 8 : In HAGMA, Calculate Delta Ratio
< 0.4 – NAGMA only
< 1 – HAGMA + NAGMA
1-2 – pure HAGMA only
>2 – HAGMA + Metabolic Alkalosis
Step 9 : In HAGMA, if exogenous cause(toxins) is present, calculate
Osmolar Gap
• If > 10, Methanol or Ethylene Glycol poisoning
• If < 10, Salicyclates or Paraldehyde poisoning
Step 10 : In NAGMA, Calculate Urine Anion Gap
If UAG is positive – cause is RTA
If UAG is negative – cause is GI loss of HCO3
FORMULATE DIFFERENTIAL DIAGNOSIS

Treatment with HCO3
-
Pros – good for myocardium
Cons –
• Hypocalcemia
• Hypokalemia
• Intracellular acidosis
• Volume overload
• Hypernatremia
• Hyperosmolarity
• Overshoot alkalosis
• Shift of oxyhemoglobin dissociation curve to left
Replace HCO3
-, otherwise treat primary cause :
• If pH < 7.1, HCO3
- < 5 mEq/L
• NAGMA (HCO3
- loss )

Arterial blood gas analysis

Arterial blood gas analysis

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