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Practical Procedures
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Arterial blood gas (ABG) measurement

Equipment
Method
Pitfalls
Other sites

 
 
  Equipment
 
  • ABG syringe (almost always preheparinized)
  • Alcohol swab
  • Gloves
  • Hazardous material container
  • Gauze or cotton
  • Adhesive tape
  • 27-gauge (orange) needle

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  Method
   
 
  • Tell the patient what you are about to do.
  • Palpate the site to be sampled. In general, the radial artery is best: it is most accessible, less susceptible to venous sampling, and easiest to clean (see below).
  • Swab the skin.
  • Discard the needle from the ABG pack and attach a 27-gauge needle (orange). This is much less painful, eliminating the need for local anesthetic.
  • Hold the syringe like a pencil and enter the skin over the site of the palpated pulse, aiming proximally at an angle of 30° to the vertical (Fig. 4).


Fig. 4 Radial artery sampling. Use one hand to feel the radial pulse and insert the syringe at a 30° angle to the skin surface.

  • Advance slowly along the line of the artery until you see a flashback of blood. If you fail to see a flashback, withdraw the needle slowly. Try palpating the artery again.
  • Hold the syringe steady and allow the syringe to fill. You might need to draw back gently on the plunger if using a small needle.
  • Once you have 0.5 ml of blood, withdraw the syringe and press firmly over the puncture site with a gauze pad for 2–3 min. Attach the gauze pad tightly to the skin with tape.
  • Take the needle off the syringe and dispose of it in a hazardous material container. Carefully expel any air from the syringe (some packs have a filter that you can attach to the syringe to help with this).
  • Cap the syringe with the cap provided and tip upside down a few times to mix well.
  • Take the sample to the blood gas machine yourself or, if it needs to be sent to the biochemistry lab, place the sample in a plastic bag with a few ice cubes. Don't forget to put a label on the syringe.

Note: If you don't know how to use the blood gas analyzer, ask someone. They are expensive and a broken machine is extremely inconvenient.

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  Pitfalls
   
 

Unable to obtain blood: try a larger needle (blue or green); unless it is an emergency, you will have to inject local anesthetic around the artery (be very careful to aspirate before injecting so as to avoid injecting lidocaine into the artery). A larger needle usually allows the blood gas syringe to fill without pulling back because of the arterial pressure.

Venous sampling: you might suspect this if the sample looks darker than expected; however, samples from hypoxic patients may well be dark. A better guide is if the pO2 or SaO2 are much lower than the pulse oximetry readings – this suggests venous sampling.

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  Other sites
   
 

If you cannot obtain a sample from a radial artery, try a femoral artery sample.

  • Use a standard 21-gauge (green) needle; this is long enough to get to the femoral artery.
  • Palpate the femoral pulse. In obese individuals, ask the patient to externally rotate the hip – this brings the artery closer to the surface.
  • Clean the skin thoroughly.
  • Place a finger on either side of the femoral pulse and insert the needle between the fingers, perpendicular to the skin. Beware – it is easy to give yourself a needlestick injury here.
  • Advance until flashback of blood occurs and the syringe starts to fill.

If neither the radial nor femoral artery site yields a sample, you could try a brachial artery sample. The technique is similar to the two above but the artery is often hard to find and can be quite mobile; this technique is also more uncomfortable for the patient. Try to use a 27-gauge (orange) needle and consider using local anesthetic.

If you can't get a sample after two or three attempts, call your chief resident.

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