RSP 1220: Arterial Sampling - Blood Gas Analysis & Interpretation

Blood Gas Sampling - risk, invasive procedure (going into the body!)

Pg. 361 & Pg. 365 Box 19.3

• Indications:

- Need to evaluate ventilation and oxygenation

- Need to assess the patient’s response to therapy

- Need to monitor the severity and progression of a documented disease process

⬆

emphysema

Blood Gas Sampling:

Pg. 365 Box 19.2

⭐️ Radial artery

⭐️ Brachial artery: babies, young children

Procedure:

• CHECK THE ORDER!

• Anticoagulant therapy - clot platelets, anticoag drugs ➡ heparin, warfarin

• Therapeutic dose?

• Clotting factors

⭐️ Determine puncture site:

- Radial, brachial, femoral, dorsalis pedis

- Test for collateral circulation

• Modified Allen Test

⭐️ Can also do brachial ABGs + femoral = less common!

Modified Allen Test:

• Occlude the radial & ulnar arteries ➡ situation dependent ➡ code blue, less likely!

• Have patient make a fist

• Raise the arm for 5 seconds

• Lower the arm & release the ulnar

• Palm should flush within 5-10 seconds

Blood Gas Sampling:

Pg. 364 Box 19.1

• Equipment

⭐️ Obtain patient label

⭐️ Obtain ice - biohazard bag

⭐️ PPE (gloves & goggles)

⭐️ ABG Kit

• Syringe/needle

• Alcohol & gauze

• Needle guard

• Syringe cap

Blood Gas Sampling:

ABG = Arterial Blood Gas

• Arterial blood is more oxygenated

• Usually need about 1 cc of blood

⭐️ Procedure

1. Position wrist at about 30°

2. Clean the site

3. Palpate the artery

   • Use your non-dominant hand

4. Hold syringe/needle at about 45°

5. Insert needle bevel up

   • Advance slowly into the artery

6. Allow arterial pressure to fill the syringe

   • Do not forcefully pull back on the plunger

7. Remove the needle

   • Immediately apply pressure with gauze

8. Hold pressure for 3–5 minutes

   • Hold longer if the patient has a higher bleeding risk

9. Activate the needle safety guard

10. Cap the syringe securely

⭐ ️Important Notes:

• Fill first → then gently mix/roll the sample

• Avoid air bubbles

• Proper handling helps keep the ABG results accurate

ABG Interpretation:

• Oxygenation:

- PaO2 - Pressure of Oxygen in the Plasma

- SaO2 - Saturated of the Hb in a percent

• Acid-base:

- PaCO2 - Pressure of Carbon Dioxide in the Plasma

- pH - H+ concentration in the blood

- HCO3 - Bicarbonate

- B.E. - Base Excess (used in calculations)

Arterial Blood Gas Analysis:

• Purpose of a blood gas

- Determine:

⭐️ PaO2 - Measured value

⭐️ SaO2 - Calculated value

⭐️ PaCO2 - Measured value

⭐️ pH - Measured Values

⭐️ HCO3 - Calculated value

⭐️ Base Excess (B.E.) - Calculated value

ABG Interpretation:

• Oxygenation - Normals

- PaO2 - 80 - 100 mm Hg

- SaO2 - 93-97% (not used in interpretation)

• Acid-Base - Normals

- PaCO2 - 35-45 mm Hg

- pH - 7.35 - 7.45

- HCO3 - 22 - 26 mEq/L

- B.E. + 2

↪ Base Excess!

ABG Interpretation:

• pH definition:

“Physiological mechanisms that keep that hydrogen concentration of body fluids in a range that is compatible with life”

ABG Interpretation:

Pg. 290 Table 14.3

• Acidosis vs. Alkalosis

- Acidosis = pH < 7.35

- Alkalosis = pH > 7.45

• Respiratory vs. Metabolic

- Lung problem vs. kidney problem

⭐️ lungs: paCO2 (regulate min by min = minute ventilation)

⭐️ kidneys: bicarbonate HCO3

↪ retaining or excreting amounts of hydrogen!

ABG Interpretation:

Normal ranges

• pH: 7.35–7.45

  • ↓ = acid

  • ↑ = alkaline

• PaCO2: 35–45 mmHg

  • ↑ = acid

  • ↓ = alkaline

• HCO3: 22–26 mEq/L

  • ↓ = acid

  • ↑ = alkaline

⭐️ Steps to Interpret:

1. Check the pH

• < 7.35 = acidosis

• 7.45 = alkalosis

2. Check PaCO2 → Respiratory

• CO2 moves opposite the pH.

• ↑ CO2 → ↓ pH = respiratory acidosis

• ↓ CO2 → ↑ pH = respiratory alkalosis

3. Check HCO3 → Metabolic

• HCO3 moves in the same direction as pH.

• ↓ HCO3⁻ → ↓ pH = metabolic acidosis

• ↑ HCO3 → ↑ pH = metabolic alkalosis

4. Check compensation

• Uncompensated: only the cause is abnormal

• Partially compensated: pH is still abnormal, but the other system is trying to correct it

• Fully compensated: pH is back in the normal range

• Mixed disorder: more than one acid-base problem is occurring

Compensation:

- Uncompensated

• pH = abnormal

• Cause = abnormal

• Other value = normal

⭐️ Example:
pH 7.30 ↓ acid
PaCO2 48 ↑ acid
HCO3 24 = normal

→ Uncompensated respiratory acidosis

- Compensated

• pH = normal

• Cause = abnormal

• Other value = abnormal too because it is compensating

⭐️ Example:
pH 7.38 = normal, but on the acid side of normal
PaCO2 48 ↑ acid
HCO3 30 ↑ alkaline

→ CO2 is the cause
→ HCO3⁻ is compensating

→ Compensated respiratory acidosis

Compensation:

Compensation

⭐️ Partially Compensated:

• pH = abnormal

• Cause = abnormal

• Other value = abnormal because it is compensating

• Basically: all 3 are abnormal

Example:
pH 7.34 ↓ acid
PaCO2 48 ↑ acid
HCO3 27 ↑ alkaline

→ CO2 is causing the acidosis
→ HCO3⁻ is trying to compensate
→ Partially compensated respiratory acidosis

⭐️ Mixed Disorder

• pH = abnormal

• PaCO2 = abnormal

• HCO3 = abnormal

• Both PaCO2 and HCO3 are pushing the pH in the same direction

• That means it is not compensation

Example:
pH 7.25 ↓ acid
PaCO2 55 ↑ acid
HCO3⁻ 15 ↓ acid

→ PaCO2 = acid
→ HCO3⁻ = acid
→ Both are causing acidosis
→ Mixed respiratory + metabolic acidosis

Interpretation of Oxygenation:

• PaO2 - dissolved oxygen (plasma)

- 80-100 mm Hg

• SaO2 - oxyhemoglobin

- 93-97%

• SpO2 - Non-invasive read of oxyhemoglobin

- Pulse oximeter ➡ extremely accurate!

Terminology:

• Hyperventilation - ⬇ CO2 < 35

• Hypoventilation - ⬆ CO2 > 45

• Hyperoxemia - ⬆ O2 > 100

• Hypoxemia - ⬇ O2 < 80

• Tachypnea - rapid frequency > 35

• Bradypnea - slow frequency < 12

• CO2 = metabolic needs