blood gases analysis

1. ABG:- INTRODUCTION
Dr. Bikal Lamichhane
1st year Resident

2. INTRODUCTION
• An arterial blood gas (ABG) is a test that
measures the oxygen tension (PaO2), carbon
dioxide tension (PaCO2), acidity (pH),
oxyhemoglobin saturation (SaO2), and
bicarbonate (HCO3) concentration in arterial
blood

3. Indications:-
●Identification and monitoring of acid-base disturbances
●Measurement of the partial pressures of oxygen (PaO2) and carbon
dioxide (PaCO2)
●Assessment of the response to therapeutic interventions (eg, insulin
in patients with diabetic ketoacidosis).
●Detection and quantification of the levels of abnormal hemoglobins
(eg, carboxyhemoglobin and methemoglobin).
●Procurement of a blood sample in an acute emergency setting when
venous sampling is not feasible (most tests can be performed from
an arterial sample)

4. Absolute contraindications:-
●An abnormal modified Allen's test.
●Local infection, thrombus, or distorted anatomy at the
puncture site (eg, previous surgical interventions,
congenital or acquired malformations, burns, aneurysm,
stent, arteriovenous fistula, vascular graft).
●Severe peripheral vascular disease of the artery selected for
sampling.
●Active Raynaud's syndrome (particularly sampling at the
radial site)

5. • relative contraindications
• Supra therapeutic coagulopathy and infusion of
thrombolytic agents (eg, during streptokinase or
tissue plasminogen activator infusion) are to
arterial needle stick.
• Repeated arterial needle sticks are avoided when
the international normalized ratio is ≥3 and/or
the activated partial thromboplastin time is ≥100
seconds.

6. TECHNICAL CHALLENGES
• Uncooperative patient.
• Difficulty in finding pulses (eg, shock, vasopressor infusion,
arteriosclerosis from end-stage kidney disease, calcification of the
vessel wall).
• Inability to position patient appropriately (eg, cannot fully extend
the wrists for radial artery access due to tremor or joint
contractures).
• Obese or edematous patients with large amounts of subcutaneous
tissue which may obscure the usual anatomic landmarks.

7. PROCEDURE OF ARTERIAL SAMPLING
• It can be obtained by percutaneous needle
puncture or from an indwelling arterial
catheter.
• The risks and benefits of each procedure
should be explained to the patient.

8. • Needle puncture —
• Percutaneous needle puncture refers to the withdrawal of
arterial blood via a needle stick.
• It is suitable for patients who require a limited number of
arterial draws (eg, daily or less than once daily during an
admission to hospital).
• If recurrent sampling (eg, more than four draws in 24
hours) is required, clinicians should, at minimum, rotate
puncture sites (eg, right and left radial) or consider placing
an indwelling catheter

9. • Site selection —
• The initial step in percutaneous needle puncture is locating a
palpable artery.
• Common sites include the radial, femoral, brachial, dorsalis pedis,
or axillary artery.
• There is no evidence that any site is superior to the others.
However, the radial artery is used most often because it is
accessible and more comfortable for the patient than the
alternative sites.
• The radial artery is also typically used for outpatients, while all sites
can be used for inpatients who require an ABG.
• .

10. Radial artery –
• The radial artery is best palpated between the distal radius
and the tendon of the flexor carpi radialis when the wrist is
extended.
• Although infrequently performed, the arm can be taped (at
the level of the forearm and palm) to an armboard with the
palm facing upward; a large roll of gauze also can be placed
between the wrist and the armboard in a position that
extends the wrist.
• Over extension should be avoided as extension of the
overlying flexor tendons may make the pulse difficult to
detect

11. Ensuring collateral circulation: :- The modified Allen's test
• One of the risks associated with arterial puncture is
ischemia distal to the puncture site .
• Although rarely performed in practice, identifying
collateral flow to the region supplied by the artery can
be used by clinicians prior to puncture.
• Radial and dorsalis pedis artery puncture are at highest
risk of this complication (because they are small in
diameter). They receive collateral supply from the
ulnar and lateral plantar artery, respectively.

12. Modified Allen's test –
The patient's hand is initially held high with the fist clenched.
Both the radial and ulnar arteries are compressed firmly by the two thumbs
of the investigator . This allows the blood to drain from the hand.
The hand is then lowered and the fist is opened (the palm will appear white).
The pressure is released from the ulnar artery while occlusion is maintained
on the radial artery.
A pink color should return to the palm, usually within six seconds, indicating
that the ulnar artery is patent and the superficial palmar arch is intact.
Although the timing of return of circulation to the palm varies considerably,
the test is generally considered abnormal if ten seconds or more elapses
before color returns to the hand

13. The Allen's test
• •The Allen's test (from which the modified Allen's
test evolved) is performed identically, except
these steps are executed twice:
• once with release of pressure from the ulnar
artery while occlusion is maintained on the radial
artery, and
• once with release of pressure from the radial
artery while occlusion is maintained on the ulnar
artery.

15. Femoral artery –
• The femoral artery is best palpated just below the
midpoint of the inguinal ligament, when the lower
extremity is extended and the patient is lying supine.
• The needle should be inserted at a 90 degree angle
just below the inguinal ligament.

16. Equipment —
• the equipment necessary should be brought to the bedside prior to
the procedure.
• This includes:
• Non sterile gloves
• Antiseptic skin solution (eg, chlorhexidine and povidone-iodine are
solutions)
• ABG kit OR a pre-heparinized 3 mL ABG syringe with a 22 to 25-
gauge needle and syringe cap
• 2 × 2 inch sterile gauze
• Adhesive bandage
• Plastic hazard bag with ice (if not provided in the kit)
• Sharp object container

17. • The planned puncture site should be sterilely
prepared.
• Local analgesia with injectable 1 to 2
percent lidocaine can be administered but is
not usually performed.

18. ● One or two fingers should be used to gently palpate the
artery while holding the needle in the other hand.
• Both fingers should be proximal to the desired
puncture site.
• The artery should be punctured with the needle at a 30
to 45 degree angle (radial, brachial, axillary, dorsalis
pedis) or at a 90 degree angle (femoral artery) relative
to the skin.

20. • The syringe fills on its own (ie, pulling the plunger is
usually unnecessary).
• Approximately 2 to 3 mL of blood should be
removed.
• For patients with poor distal perfusion (eg,
hypovolemia, shock, vasopressor therapy) who may
exhibit a weak arterial pulse, the operator may need to
pull back the syringe plunger, although this increases
the risk of venous blood sampling.

21. • If arterial flow is lost during the arterial draw, the
needle may have moved outside the vessel lumen.
• The needle may be pulled back slightly and
repositioned to a point just below the skin; subsequent
redirection using the maneuver described above
should be attempted to re-access the artery.
• Multiple blind or stabbing movements of the needle
while it is inserted deeply in the patient's limb should
be avoided since this increases the risk of local injury
and pain.

22. • After withdrawing a sufficient volume of blood, the needle should
be removed while simultaneously applying pressure to the
puncture site with sterile gauze until hemostasis is achieved.
• This usually takes five minutes in a non-anticoagulated patient;
avoid checking the puncture site until local pressure has been
maintained for at least this period as this increases the risk of
hemorrhage or a hematoma.
• In patients who have a coagulopathy or are on anticoagulation
therapy, it may be necessary to apply local pressure for a longer
time.
• Once hemostasis is achieved, apply an adhesive bandage over the
puncture site.

23. • When ABG kits are used, apply the needle protective sleeve then
untwist the sleeve and place it in the sharp object container.
• When an ABG syringe is used, recap, remove, and discard the
needle, being careful to avoid a needle stick injury.
• After discarding the needle, remove the excess air in the syringe by
holding it upright and gently tapping it, allowing any air bubbles
present to reach the top of the syringe, from where they can then
be expelled.
• Cap the syringe, roll it between the hands for a few seconds to
allow blood to mix with the heparin (prevents clotting), then place
on ice in the hazard bag and send it for analysis.

24. Postprocedural care:-
• Patients should be monitored for new symptoms
such as skin color changes, persistent or
worsening pain, active bleeding, and impaired
movement or sensation of the limb.
• Monitoring is particularly important in patients
who are subsequently supra therapeutic on
anticoagulants or are given thrombolytics, as
bleeding may be observed in such patients even
though the needle stick occurred a few hours
prior

25. • Complications :-
• Common complications of ABG sampling include the
following:-
●Local pain and paresthesia
●Bruising
●Local minor bleeding
• Less common complications include:-
●Vasovagal response
●Local hematoma from moderate or major bleeding
●Artery vasospasm

26. Rare complications include:-
●Infection at the puncture site
●Arterial occlusion from a local hematoma
●Air or thrombus embolism
●Local anesthetic anaphylactic reaction
●Local nerve injury
●Needle stick injury to health care personnel (limited due to use of
ABG kits)
●Pseudoaneurysm formation
●Vessel laceration
Should local bleeding, hematoma, vasospasm, and/or arterial
thrombus be severe, compartment syndrome and limb ischemia
can occur

27. TRANSPORT AND ANALYSIS
• The arterial blood sample should be placed on ice during
transport to the lab and then analyzed as quickly as
possible.
• This reduces oxygen consumption by leukocytes or platelets
(ie, leukocyte or platelet larceny), which can cause a
factitiously low partial pressure of arterial oxygen (PaO2).
• This effect is most pronounced in patients whose
leukocytosis or thrombocytosis is profound. In addition, it
reduces the likelihood of error due to gas diffusion through
the plastic syringe or the presence of air bubbles.

28. Sources of error:-
• Gas diffusion through the plastic syringe and consumption
of oxygen by leukocytes is a potential source of error that
results in a falsely low PaO2 when the sample is left for
prolonged periods at room temperature.
• However, the clinical significance of this error is minimal if
the sample is placed on ice and analyzed within 15 minutes.
• The heparin that is added to the syringe as an
anticoagulant can decrease the pH if acidic heparin is used
and the dismissal of heparin from the syringe is incomplete

29. DEFINITIONS OF ACID-BASE DISORDERS
●Acidemia – An arterial pH below the normal range (less than 7.35).
●Alkalemia – An arterial pH above the normal range (greater than
7.45).
●Acidosis – A process that tends to lower the extracellular fluid pH
(hydrogen ion concentration increases). This can be caused by a fall
in the serum bicarbonate (HCO3) concentration and/or an elevation
in PCO2.
●Alkalosis – A process that tends to raise the extracellular fluid pH
(hydrogen ion concentration decreases). This can be caused by an
elevation in the serum HCO3 concentration and/or a fall in PCO2.

30. • ●Metabolic acidosis – A disorder that reduces the serum HCO3
concentration and pH.
• ●Metabolic alkalosis – A disorder that elevates the serum HCO3
concentration and pH.
• ●Respiratory acidosis – A disorder that elevates the arterial PCO2
and reduces the pH.
• ●Respiratory alkalosis – A disorder that reduces the arterial PCO2
and elevates the pH.
• ●Simple acid-base disorder – The presence of one of the above
disorders with the appropriate respiratory or renal compensation
for that disorder.
• ●Mixed acid-base disorder – The simultaneous presence of more
than one acid-base disorder.

31. four-step approach:-
• ●Step 1: Establish the primary diagnosis:-
• •Metabolic acidosis is characterized by a low serum HCO3 and a low arterial pH; the serum
anion gap may be increased or normal.
• •Metabolic alkalosis is characterized by an elevated serum HCO3 and an elevated arterial pH.
• •Respiratory acidosis is characterized by an elevated arterial PCO2 and a low arterial pH.
• •Respiratory alkalosis is characterized by low arterial PCO2 and an elevated arterial pH.
• •With the exception of chronic respiratory alkalosis and mild to moderate respiratory
acidosis compensatory responses do not usually return the arterial pH to normal.
• Thus, a normal arterial pH in the presence of substantial changes in both serum HCO3 and
arterial PCO2 is usually indicative of a mixed acid-base disorder

32. ●Step 2: Assess the degree of compensation as
defined above for the individual disorders.
• A substantially reduced or excessive level of compensation
is indicative of a mixed acid-base disorder.
• The compensatory response must be correlated with the
history.
• This is particularly true in respiratory acid-base disorders
since the renal compensation occurs over three to five
days.
• Thus, the expected level of compensation is smaller with
acute respiratory disorders compared with chronic
respiratory disorders.

33. ●Step 3: Determine whether or not the anion gap is
elevated.
• This is especially important for patients with
metabolic acidosis.
• If the anion gap is increased, then analyze the
ratio of the increase in anion gap to the decrease
in the HCO3 concentration.
• This is called the delta anion gap/delta HCO3
ratio

34. Step 4: The fourth and final step is to
establish the clinical diagnosis.
• Once the acid-base disorder, or disorders, is
identified, the underlying cause or causes of
each disorder should be determined and
addressed

36. •Thank you