Peripheral venous cannulation

From Ganfyd

Cannulating a vein is a skill so basic no doctor should lose it.

As soon as no longer needed remove it

Gaining peripheral venous access refers to the insertion of a cannula into a vein to allow administration of intravenous drugs and fluids. The term venous cannulation, if not otherwise qualified, usually refers to peripheral cannulation, but strictly speaking applies to any type of venous access including central venous access or a venous cutdown.

The most common way to cannulate a peripheral vein is using a 'cannula over needle'. Here, the lumen of plastic cannula contains a needle that allows insertion through the skin. Once introduced, the needle is removed leaving the cannula in the vein.

Indications

Intravenous fluids
NBM
Drugs requiring parenteral administration

Choice of Size

An application of Poiseuille's law is that small changes in the internal diameter of the lumen can make a big difference to the rate of flow (see table below). For rapid infusion of fluids, e.g. in shock or trauma, the ideal cannula is 'short and fat'. For slow infusions, including blood transfusions, a 20G venflon (pink) will suffice.

Theoretical Maximum Flow Rates
Colour	Gauge	Flow
Yellow	24G	13 ml/min
Blue	22G	30 ml/min
Pink	20G	55 ml/min
Green	18G	80-100 ml/min
White	17G	135 ml/min
Grey	16G	180 ml/min
Orange or Brown	14G	270 ml/min

Preparation

Venous cannulation is performed after obtaining patient's verbal consent. A brief explanation should be made of the procedure, and patient's cooperation will make cannulation a non-traumatic experience to the patient.

It is important to rule out drug allergies and allergies to adhesive tape. It is also necessary to rule out latex allergy and if the patient is found to have latex allergy, non-latex gloves, tourniquet and venous cannulae should be used through out.

Local Anaesthetic

Local anaesthetic makes it hurt less.^[1] ^[2]^[3]

Aseptic precautions have to be observed through out and infiltration of local anaesthetic (lignocaine 0.5% to 2.0%) will secure patient's cooperation and make it extremely comfortable to the patient. Local anaesthetic will also reduce the stress level in the patient and contribute to his well being. A very small volume is used.

An unconscious patient (coma, major accidents), can be cannulated without local anaesthetic as securing venous access early is more important. Patients in shock may be less sensitive.

Paedatric Cannulation

Children in particular will benefit from an application of EMLA or Ametop cream over the site of injection. EMLA cream takes at least 30 minutes to work and it numbs the area without any injections. A few precautions are taken when EMLA cream is applied. The sites where EMLA is applied are tied by a bandage to prevent young children from licking the cream and aspirating the dressing.

A trained paediatric nurse can be invaluable, performing several roles: as a human tourniquet, distracting and comforting the child, and, if unavoidable, restraining the child.

Equipment

cannula - the size and site of insertion of the cannula is judged by the clinical need.
Flush (syringe + a few mls of 0.9% saline)
Adaptor?
Extra cannula (in case of failure of first attempt)
Dressing or tape

Procedure

Locating Veins

Put on a tourniquet. Several options are available:

Purpose-made tourniquets: Often made of elastic with an adjustable plastic or metal catch.
A latex rubber glove: Easily available, disposable and unlikely to transmit infections. If not too tightly applied, it is no more uncomfortable than a conventional tourniquet.
A human hand: Can be just as useful, particularly in children.
A blood pressure cuff: Can be inflated manually, but can also be used in automatic non-invasive blood pressure monitors, which have a 'venous stasis' programme that inflates the cuff for a brief period.

It may sound obvious, but the pressure should be less than the systolic pressure to ensure arterial blood flow to the limb! Pressure higher than systolic may not help in distending the veins and also the lack of arterial pulsations can predispose to accidental arterial cannulation.

Veins on the extremities are distended by applying a tourniquet proximal to the puncture site. For instance, veins over the dorsum of the hand distend on applying pressure over the arm.

Skin preparation

Ensure skin clean
By analogy with clinical trials with central catheters alcohol based chlorhexidine solutions are preferable to povidone iodine ^[4], although one followed by the other is superior to either alone^[5].

Securing Cannula

The effect of adhesive tape on securing a plastic tube depends on the area of tape and tube in contact. Maximise this by making the tape conform to the curve of the tube, rather than just placing it across the top. File:Needsmall.jpg

Double adhesive
Loop tubing
Specific dressings are often available

Writing the date on the dressing or tape ensures that everyone is aware of when a cannula was inserted.

Troubleshooting

Flash back, but unable to advance: cannula and needle through other side of vein. Drawing back slowly until flashback resumes can sometimes salvage the situation.
Flashback and haematoma: vein has been punctured. Like above, drawing back slowly until flashback resumes can sometimes salvage the situation, but this is harder as flashback will now be due to blood from the haematoma rather than the vein.
No flashback and vein very mobile: Immobilise vein with downwards stretching of the skin and pierce vein with a rapid, swift movement (slow movements displace rather than puncture the vein)
No flashback, but able to advance: You may be in the vein, but the vein may be too small, too constricted or the tip of the cannula may be lodged in a valve. Flush the vein to confirm. If unable to flush, you are most likely in the subcutaneous space.
Peripheral oedema: Compress skin over area where vein might be expected. This may reveal the vein.
Unable to find veins: look in unusual sites - ulnar/dorsal aspect of forearm or feet.

Very fine veins on the forearm cannot be palpable, but can be cannulated by vision, keeping the angle of the needle nearly horizontal so as not to go through the other wall of the vein.

The long saphenous vein may be hard to palpate, generally found halfway between the medial malleolus and the medial edge of the tibia. It may be localised using the tap test - place one finger ~2cm superiorly and tap lower down - transmission of the tap suggests a column of fluid is present.

Other adjuncts include illumination with a cold light or portable ultrasound.

Alternatives

Changing the Cannula

As soon as no longer needed remove it!
- Point incidence of infection is 0.5 per 1000 device-days for peripheral venous cannula in trials, about a fifth of central lines^[6]
- Every audit has shown poor compliance with this basic rule by healthcare staff^[7]
Administration sets should be changed by 96 hours to minimize risk of infection and 24 hours if lipid containing substances infused^[8].
Regrettably device associated phlebitis is not a clear function of time ^[9], and risk of infection may be a function of technique so the common guideline practice of between 24 to 72 hour replacement of peripheral cannulae has some evidence base weaknesses^[10]
As the majority of cases of staphylococcus aureus and hence MRSA bacteraemia are hospital-acquired, and most are associated with infected intravenous catheters (not just central)^[11] targeted reduction of this complication requires policies to minimise iv cannula use.