ARTERIAL BLOOD GAS ANALYSIS
Arterial
blood gas provides information about arterial blood ,pH, partial pressure of
arterial oxygen (PaO2),Partial pressure of arterial carbon dioxide.(PaCO2),arterial
oxygenation saturation ,acid – base balance and to some extend ventilation.
Normal values
PaO2 -
80-100 mmHg
PaCO2
- 35-45 mmHg
pH -
7.35-7.45
HCO3 -
22-26 meq/kg
Oxygenation
Saturation – 96-100%
Nurse
must report to the physician immediately if the values are below or above normal.
i. Check whether it is acidic or alkaline –pH
<7.35 is acidosis and >7.45 is alkalosis.
ii.
Check the PaO2 -low PaO2 indicates respiratory
failure.
iii.
Check PaCO2 and pH to decide whether the problem is basically respiratory or
metabolic.
pH less than 7.35 and PaCO2 greater
than 45mmHg –respiratory acidosis
pH
more than 7.45 and PaCO2 less than 35 mmHg –respiratory alkalosis
Low pH .low PaCO2 and HCO3
-metabolic acidosis
High pH Normal PaCO2 and High HCO3 - metabolic alkalosis
The radial artery at the wrist is the most common puncture site , but sometimes the femoral artery in the groin or other sites are used
INDICATIONS
- Acid base disturbance
- Oxygenation status
- Alveolar ventilation
BASIC TERMINOLOGY
¨ pH signifies free hydrogen ion conc.
¨ Normal pH: 7.35 -7.45
¤ Acidemia pH
<7.35
¤ Alkalosis pH>7.45
¤ PaCO2 40 (35 -45) mm of Hg
¤ HCO3 24 (22-26) Meq/l
1O AB DISORDERS
If the initial
disturbance affects HCO3
¤ Metabolic Acidosis(Fall in bicarbonate)
¤ Metabolic Alkalosis(Rise in bicarbonate)
If the initial
disturbance affects Paco2
¤ Respiratory acidosis ( Rise in Paco2 )
¤ Respiratory Alkalosis ( Fall in Paco2)
So
HCO3
- Metabolic
PaCO2 - Respiratory
CHARACTERISTICS OF 1O
AB DISORDER
Basic Disorder
|
Ph
|
Primary change
|
Secondary change
|
Metabolic Acidosis
|
Low
|
HCO3 low
|
PaCO2 Decreased
|
Metabolic Alkalosis
|
High
|
HCO3 High
|
PaCO2
Increased
|
Respiratory
Acidosis
|
Low
|
PaCO2 High
|
HCO3 Increased
|
Respiratory
Alkalosis
|
High
|
PaCO2 Low
|
HCO3 decreased
|
COMPENSATION
Metabolic Acidosis
|
PaCO2 = (1.5 × HCO3) + 8
PaCO2 = HCO3 + 15
|
Metabolic Alkalosis
|
↑ in PaCO2 = 0.75 × ↑ in HCO3
|
Respiratory Acidosis
Acute (6-24 hr)
Chronic (> 24 hr)
|
↑ in HCO3 -
= 0.1 × ↑ in PaCO2
= 0.4 × ↑ in PaCO2
|
Respiratory Alkalosis
Acute
Chronic
|
↓ in HCO3 –
0.2 ×
↓ in PaCO2
0.4 ×
↓ in PaCO2
|
MIXED AB DISORDERS
Disorders
|
Common causes
|
MA & RA
(↓pH, ↓HCO3, ↑PaCO2)
|
Cardiac arrest (LA + Hypoventilation)
Shock / DKA with Respiratory failure
|
MA & R Alk
(N pH, ↓HCO3 & PaCO2)
|
Salicylate intoxication
Gram-negative Sepsis / Liver Failure
|
M Alk & RA
(N pH, ↑ HCO3 & PaCO2)
|
COAD with Diuretics
|
M Alk & R Alk
(↑ pH & HCO3 & ↓PaCO2)
|
Liver failure with vomiting
|
MA & M Alk
(Near N pH & HCO3)
|
DKA with Vomiting
|
RA & R Alk
|
DO NOT COEXIST
|
BASE EXCESS IN ABG
¨ Normal Value : + 2 to - 2
¨ Base Excess > +2 : Metabolic Alkalosis
¨ Base Deficit < 2 / Base Excess < - 2 :
Metabolic Acidosis
Serum Anion Gap
¨ SAG = Na – (Cl + HCO3)
¨ Purpose: to further classify metabolic acidosis
in aetiological causes
High Anion Gap
|
Normal Anion Gap
|
•
Chronic
Renal Failure
|
•
Diarrhoea
|
•
Lactic
Acidosis
|
•
Renal
Tubular Acidosis
|
•
Diabetic
Ketoacidosis
|
|
•
Methanol
Poisoning
|
PROCEDURE
¨ Site: Radial, Brachial, Femoral are most commonly used
¨ Perform the modified Allen test
¨ Clean the site
¨ Use Needle 24 /25 for <10kg
¨ Use needle 22 for femoral and >10 kg-40 kg
¨ Flush syringe and needle with heparin
¨ Obtain 2-4 ml of blood without aspiration
¨ Firm pressure at the site for 10 min
¨ Air Bubble in the sample will wrongly give high
PO2 values
¨ ABG sample has to be sent on ice pack for
transport
¨
RBC
metabolism produce can produce lactic
acid and may acidify the sample
No comments:
Post a Comment