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I'm I Made Wiradiana
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| Thursday, January 11, 2007 |
| "Arterial and venous blood sampling" |
Venepuncture has traditionally been carried out using an ordinary needle and syringe; The blood is then injected into sample tubes by piercing the rubber bung using the same needle. With this method there is a risk of needle stick injury by a contaminated needle or contamination of work surfaces or yourself with drops of spilt blood. Does this matter?
The answer is yes. It matters because:
•Needle stick injuries can transmit bloodborne viruses such as HIV and hepatitis B and C viruses
•If you contract one of these diseases your career as a doctor may be over before it ever really got started
Anyone who is expected to take or handle blood samples should be immunised against hepatitis B. In fact it is now a condition of employment in most hospitals that you have proof of immunity. If you have not been vaccinated you should consult your occupational heath department immediately. If you ever receive or cause a needle stick injury contact the needle stick hotline in your hospital. This information service will advise you about what action to take.
In an effort to reduce the risk of needle stick injury, closed systems for taking blood (Vacutainer and Monovette) have been introduced. Many hospitals now have teams of phiebotomists who take routine bloods in a recognised safe manner. But despite such services you will inevitably have to take blood from time to time. When you do I recommend that you use one of the closed systems rather than a needle and syringe. There are, however, occasions when such equipment is not available and you may have to carry out traditional venepuncture. I shall describe both a safe traditional method and how to use the Vacutainer closed system.
Traditional venepuncture
PREPARATION
You will need the following equipment:
•Tourniquet
•Alcohol swabs
•Gloves
•21G needle and 10-20 ml syringe
•Cotton wool ball Blood tubes
Before you start wash your hands and put on disposable gloves. Gloves should always be worn to protect yourself. They may make practical procedures a little more awkward at first but persevere - soon you will find that you feel uncomfortable if you do not wear them. Always explain to the patient what you are going to do and why. Make sure you have chosen a syringe big enough to collect sufficient blood for all the tubes. Use a 21G (green) needle as the risk of haemolysing the blood sample is less when drawing blood or injecting it into the bottles.
SITES OF ACCESS
Take time to select a vein of reasonable size. The median cubital vein in the antecubital fossa is most commonly used, although other veins on the forearm or back of the hand may be used. Use a tourniquet 5-10 cm above the intended site of venepuncture. This is safer than getting an assistant to squeeze the arm as needle stick injuries into other people are not uncommon. Samples for calcium estimation should be taken without the aid of a tourniquet. Venous stasis leads to acidaemia and since pH affects binding of calcium to proteins, this can result in a falsely high reading.
If you can't find a vein work through the following options:
•Look at the other arm
•Try gently tapping or rubbing with an alcohol swab over a small vein to see if it dilates
•It may be easier to use a vein that is easily palpated rather than one that is seen Is the patient warm? If not, put the patient's hands in a bowl of warm water for 10 minutes
BLOOD SAMPLING
Clean the skin with an alcohol swab. Using your dominant hand to support the syringe and needle and insert the needle through the skin and into the vein at an angle of 15-30 degrees. You will see a flashback of blood into the hub of the needle if you have successfully entered the vein. Now support the needle and syringe with your other, non-dominant hand, so freeing your dominant hand to draw back on the plunger and aspirate blood into the syringe. It takes practice to make this look easy and avoid dislodging the needle out of the vein when changing hand positions. Once there is enough blood in the syringe remove both needle and syringe from the vein quickly and press on the venepuncture site with a cotton wool ball (don't use the alcohol swab as it causes stinging) Two to three minutes of pressure should be ample. Transfer the blood into the appropriate tubes carefully. Invert the tubes several times to ensure mixing (this is especially important for samples for clotting estimations). Do not attempt to resheath the needle but dispose of it and the contaminated syringe immediately into a sharps box.
QUESTIONS
Should I use local anaesthetic?
Injection of local anaesthetic causes stinging. Although the 25G or 27G needles used to inject local are hardly felt the local anaesthetic itself stings. For simple venepuncture with a 21G needle local anaesthetic is probably unnecessary. If a patient is really needle phobic cream containing lignocaine and prilocaine may help. This is a eutectic mixture of local anaesthetics which can be applied and absorbed topically and sealed by an occlusive dressing. This should be applied for at least 30 minutes to allow the local anaesthetics to take effect.
When should I ask for help?
We all fail from time to time. It is better to ask for help sooner rather than later. If you have damaged all the good veins even the most experienced person may have trouble.
Closed system venepuncture
PREPARATION
You will need the following equipment in addition to that listed above for traditional venepuncture:
•Needle in a sealed shield
•A holder
•Evacuated blood sample tubes
BLOOD SAMPLING
The needle for the Vacutainer system comes with a coloured and white section connected by a tamper proof seal. Holding the coloured section remove the white section to reveal one end of the needle that is covered in grey rubber. This is then screwed into the holder. The blood tubes will be pushed on to this end of the needle when ready to sample blood.
Perform venepuncture as described above. Once you have gained access to the vein use your nondominant hand to support the equipment and use your dominant hand to push the blood tube into the holder and on to the rubber sheathed needle. Blood will flow into the tube until the vacuum is exhausted or if you come out of the vein. Disengage the tube and push the next tube into the holder for sampling if necessary. Once you have finished venepuncture the needle should be disengaged from the holder using the sharps specially designed or by disposing of the whole unit into a universal sharps box.
If the veins are small there is a butterfly collection set with a smaller gauge needle.
Arterial blood sampling
Taking arterial blood requires more skill than venous blood sampling. Patients who need arterial blood sampling are often very ill and require gentle handling. Make sure that you have noted what percentage oxygen concentration the patient is breathing as interpretation of blood gases is dependent on knowing the inspired oxygen concentration.
PREPARATION
You will need the following equipment:
•2 ml syringe
•256 needle
•Lignocaine 1%
•Alcohol swabs
•23G needle or butterfly
•A preheparinised syringe
•Cotton wool ball
•Gloves
•An incontinence pad A sealable polythene bag filled with ice
Before you start wash your hands and put on disposable gloves. Explain to the patient what you are going to do and why.
SITES OF ACCESS AND POSITIONING
The artery most commonly used is the radial artery at the wrist as it is the easiest artery to feel and the most accessible (see fig 1 for anatomical relations).
Fig 1 - Position of radial artery
For radial artery puncture extend the wrist over a rolled towel or 500 ml bag of fluid. Place an Inco pad underneath the area to protect the bedclothes from blood spillage.
Alternative sites of access include the brachial artery at the antecubital fossa or the femoral artery in the groin. These arteries are not as superficial as the radial and may be harder to locate. There are more structures which may be damaged if you do not know your anatomy. Only use these approaches under supervision.
SAMPLING
Clean the skin with an alcohol swab, then inject a subcutaneous bleb of local anaesthetic over the artery at the level of the proximal skin crease. Warn the patient that this will sting a little. After a few seconds take the 236 needle or butterfly attached to the preheparinised syringe in your dominant hand. If you cannot find a preheparinised syringe draw up 1 ml of 1:1000 heparin into a 2 ml syringe and then squirt out all the heparin so that the syringe has just been coated in heparin. This will be sufficient to prevent the sample clotting.
Palpate the longitudinal course of the artery with the index and third fingers of your non-dominant hand proximal to the area of anaesthetised skin (figs 2 & 3)
Fig 2 - Palpation of radial artery to determine the point of puncture. Reproduced with permission of Butterworth-Heinemann from "A Guide to Practical Procedures" by H A F Dudley et al
Fig 3 - Insertion of butterfly needle. Reproduced with permission of Butterworth-Heinemann from "A Guide to Practical Procedures" by H A F Dudley et al
Then with your dominant hand holding the syringe and needle (or butterfly wings) puncture the anaesthetised area at 45 degrees to the skin. The bevel of the needle should be uppermost. Advance the needle slowly a millimetre at a time. By keeping your index finger on the artery you will be able to define the direction in which you should advance the needle.
When you hit the artery there will be a flash of blood into the hub of the needle. If you are in the artery the syringe should fill under arterial pressure, confirming your placement. If you have had the flash of blood but the syringe has failed to fill the bevel may not be completely in the artery and you need to advance the needle a little further. Once you have taken 1-2 ml of blood withdraw the needle and immediately place a cotton wool swab over the site and press hard. Pressure should be maintained for 5 minutes to prevent bleeding and bruising. Ask an assistant or, if appropriate, the patient to help you.
Dispose of the needle carefully. Expel any air bubbles from the syringe and cap it off. Place the sample in the bag containing ice and send it to the lab immediately. Make sure that the biochemist knows to expect the sample as even stored on ice the sample should be analysed swiftly so that continued metabolic activity by the cells does not alter the true result. Most intensive care units have a blood gas analyser and you may be able to analyse the sample there. Unless you have been instructed on the use of such expensive equipment you will require the goodwill of the staff of the intensive care unit so you need to be diplomatic in your approach.
QUESTIONS
Are there any useful tips to help improve your success?
Yes. People often fail at radial puncture by aiming at a point too proximal. If you aim for the point where the pulse is palpable at the level of the proximal skin crease the radius is more superficial and acts as a good back support to the artery. If you keep missing the artery and the pulse begins to fade this may be because of spasm. Do not persist. It is best to ask someone more experienced to have a go - if the other radial artery goes into spasm then the patient will need to have an arterial blood sample taken from a potentially less safe site.
What do I do if I keep hitting a vein?
It is true that sometimes a superficial vein overlies the artery. If you are sure it is venous blood you are aspirating then choose a site more proximal and try again. In sick patients it may be hard to distinguish between venous and arterial blood. Using a butterfly needle with plastic tube extension may help as arterial blood can be seen to pulsate down the tube. The ultimate test is of course blood gas analysis. |
posted by I Made Wiradiana @ 10:02 PM  | Permalink
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