Arterial Blood Gas (ABG) Collection and Analysis

Arterial Blood Gas (ABG) Instrumentation and Measurement Accuracy

• Measurement Thresholds and Accuracy:

  • If a machine measures a $p_aO_2$ as $103$ when the actual value is $100$, this is considered an unacceptable result.

  • If measurement errors occur, the immediate corrective action is to recalibrate the machine.

• Blood Gas Electrodes:

  • Clark Electrode: This electrode is specifically designed to measure the partial pressure of arterial oxygen ($p_aO_2$). It can also be utilized to measure the fraction of inspired oxygen ($F_iO_2$).

  • Severinghaus Electrode: This electrode is used to measure the partial pressure of arterial carbon dioxide ($p_aCO_2$).

  • Mnemonics for Recall:

    • For the Severinghaus electrode, associate it with the "Slytherin House" from Harry Potter (as $CO_2$ is often viewed as "bad" or something to be eliminated, similar to the reputation of the house).

    • For the Clark electrode, associate it with "Lois and Clark" (Superman), because oxygen is "good."

Preparation and Necessary Supplies for Drawing an ABG

• Mandatory Preliminary Step: Before proceeding with any draw, the clinician must ensure they have a specific medical order for the procedure. One should never perform a draw without an official order.

• Required Equipment:

  • ABG Kit: Most hospitals provide pre-packaged kits containing several necessary items.

  • Needle and Syringe: Usually part of the kit.

  • Cleaning Agents: Alcohol is standard. Iodine can be used but requires screening for patient allergies.

  • Anticoagulant: Kits typically contain aerosolized heparin. This is crucial to prevent the blood from clotting within the syringe before the sample can be analyzed, especially if the draw is difficult and takes longer.

  • Gauze and Tape: Needed to stop bleeding and secure the site post-puncture.

  • Band-Aid: Used for the final dressing, though gauze is still recommended underneath due to arterial pressure.

  • Ice: Required only if the sample cannot be analyzed within a thirty-minute window.

Site Selection and Collateral Circulation Assessment

• Primary Site: The radial artery is the preferred site for an ABG draw.

• Anatomical Location: The radial artery is located on the thumb side of the wrist.

• Rationale for Site Choice:

  • The radial artery is preferred because it is the most superficial artery (closest to the surface).

  • It allows for the assessment of collateral perfusion, meaning circulation is supplied by more than one vessel.

• The Modified Allen’s Test:

  • This test verifies collateral circulation from the ulnar artery.

  • Procedure: Apply pressure to both the radial and ulnar arteries to blanch the hand. Release the pressure on the ulnar artery specifically to see if blood flow returns to the hand.

  • Assessment in Unresponsive Patients: If the patient cannot make a fist, still apply pressure to the arteries and raise the hand above the heart. Gravity will cause the hand to blanch. Release the ulnar artery and observe for the return of color.

• Handling Negative Results:

  • If the Allen’s test is negative on one arm, try the other arm.

  • If both arms are negative, try a different artery (e.g., brachial or femoral).

  • If all standard sites are compromised, notify the physician. Often, the benefit of the ABG data outweighs the risk of limited collateral circulation, and the physician may order the draw regardless.

Procedural Technique for Radial Artery Puncture

• Patient History and Screening:

  • Check if the patient is on blood thinners (anticoagulants). Patients on these medications will bleed longer, requiring extended manual pressure post-draw.

• Positioning for Success:

  • Adjust the bed to a comfortable height or use a chair.

  • Hyperextend the patient’s wrist (bend it back) to pull the radial artery closer to the skin surface.

  • Stabilize the hand, perhaps by placing a rolled-up towel underneath the wrist. Avoid a "free-floating" hand to prevent needle-stick injuries if the patient flinches.

• Syringe Types:

  • Self-filling: These syringes automatically push the plunger back as the arterial pressure fills the barrel.

  • Non-self-filling: The clinician must manually pull the plunger back to the desired volume (typically $1\,ml$; a minimum of $0.5\,ml$ is required) before inserting the needle.

• Asepsic Technique:

  • Clean the skin using circular motions, starting at the intended puncture site and moving outward to pull bacteria away from the site.

  • Allow the site to dry completely. If the alcohol is wet, it has not killed the bacteria (often Staphylococcus), risking the introduction of bacteria into the bloodstream.

• Needle Insertion:

  • Bevel Orientation: The bevel (the slanted opening at the tip) must be pointed upward. This ensures the needle is sharp enough to pierce the skin easily.

  • Angle: Insert the needle at a $45^{\circ}$ angle.

  • Grip: Hold the syringe like a pencil.

  • Stability: Rest your hand on the patient’s hand to provide a stable base and prevent movement.

• Obtaining the Sample:

  • Look for a "flash" of blood in the hub of the needle, which indicates you have entered the artery. The hub will fill quickly in sync with the patient’s heartbeat.

  • Redirection: If you miss the artery, pull the needle back toward the surface (but not out) before making very small, incremental corrections. Large swings at depth can slice the artery (arterial dissection).

Post-Procedure Care and Potential Complications

• Manual Pressure Requirements:

  • Normal Patient: Apply firm pressure with gauze for $5$ to $10$ minutes.

  • Patient on Blood Thinners: Apply pressure for more than $10$ minutes.

  • Modified Pressure Dressing: After manual pressure, fold a $2 \times 2$ gauze pad and tape it tightly over the site to maintain pressure for an additional $20$ minutes.

• Immediate Complications:

  • Pain: The radial artery is flanked by the radial nerve and a tendon. Contact with either during the draw is very painful.

  • Hematoma: A collection of blood under the skin/in the third space. It appears as a raised bruise.

  • Arterial Dissection: Slicing the artery with the needle tip.

  • Clotting: If heparin is not properly mixed or the draw is too slow, the sample may clot.

Sample Handling and Laboratory Integrity

• Removing Air Bubbles:

  • After removing the needle and capping the syringe, all air bubbles must be expelled. Flick the syringe to move bubbles to the top and push the plunger until they are gone.

  • Effect of Air Bubbles on Results: Room air contains approximately $21\%$ oxygen and almost no carbon dioxide ($\text{less than } 0.03\%$).

  • An air bubble will cause the $p_aCO_2$ to decrease falsely.

  • A decrease in $p_aCO_2$ will cause the $pH$ to increase falsely.

  • An air bubble will cause the $p_aO_2$ to increase falsely.

• Storage and Temperature:

  • If the sample will be run within $30$ minutes, it can stay at room temperature with a patient label.

  • If the analysis will take longer than $30$ minutes, the sample must be placed on ice. This prevents cellular metabolism (where blood cells consume oxygen and produce $CO_2$), which would otherwise alter the ABG results.

Questions & Discussion

• Question: What if the patient is unresponsive and cannot make a fist for the Allen's test?

• Response: Apply pressure to the radial and ulnar arteries, then raise the hand up. Gravity will cause the hand to blanch. Then release the ulnar artery to check for the return of circulation.

• Question: How much blood is needed for a sample?

• Response: Ideally, you want $1\,ml$. This allows you to run the test twice if the first result is questionable. However, a minimum of $0.5\,ml$ is required for a successful run.

• Discussion on Patient Communication: The instructor advises being honest with patients, noting that the procedure will hurt, but emphasizing its medical necessity for checking oxygenation levels.