Chapter 13: Diagnostic tests
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Anatomy and physiology
The superficial veins of the upper limb are most commonly chosen for venepuncture. These veins are numerous and accessible, ensuring that the procedure can be performed safely and with minimum discomfort (McCall and Tankersley [138]). In adults, veins located on the dorsal portion of the foot may be selected but there is an increased risk of complications, especially if the patient has diabetes or a history of vascular or coagulation disorders (Hoeltke [88]). Therefore, veins in the lower limbs should be avoided where possible.
Veins
The veins commonly used for venepuncture are those found in the antecubital fossa because they are sizeable veins capable of providing copious and repeated blood specimens (Weinstein and Hagle [252]). However, the venous anatomy of individuals may differ. The main veins of choice are (Figure 13.5):
- median cubital veins
- cephalic vein
- basilic vein
- metacarpal veins (used only when the others are not accessible).
Figure 13.5 (a) Superficial veins of the forearm. (b) Superficial veins of the dorsal aspect of the hand. Green, nerves; red, arteries; blue, veins. Source: Reproduced with permission of Becton, Dickinson and Company.
Median cubital vein
The median cubital vein may not always be visible, but its size and location make it easy to palpate. It is also well supported by subcutaneous tissue, which prevents it from rolling under the needle (Hoeltke [88]).
Cephalic vein
On the lateral aspect of the wrist, the cephalic vein rises from the dorsal veins and flows upwards along the radial border of the forearm as the median cephalic vein and crossing the antecubital fossa as the median cubital vein. Care must be taken to avoid accidental arterial puncture, as this vein crosses the brachial artery. It is also in close proximity to the radial nerve (Dougherty [53], Tortora and Derrickson [243]).
Basilic vein
The basilic vein, which has its origins in the ulnar border of the hand and forearm (Waugh and Grant [250]), is often overlooked as a site for venepuncture. It may well be prominent but it is not well supported by subcutaneous tissue, making it roll easily, which can result in difficult venepuncture. Owing to its position, a haematoma may occur if the patient flexes the arm on removal of the needle, as this squeezes blood from the vein into the surrounding tissues (McCall and Tankersley [138]). Care must also be taken to avoid accidental puncture of the median nerve and brachial artery (Hoeltke [88]).
Metacarpal veins
The metacarpal veins are easily visualized and palpated. However, the use of these veins may not be suitable in the elderly because skin turgor and subcutaneous tissue are diminished, which makes the veins more difficult to anchor (Hoeltke [88]).
Layers of the veins
Veins consist of three layers: the tunica intima, the tunica media and the tunica adventitia.
Tunica intima
The tunica intima is a smooth endothelial lining, which allows the passage of blood cells. If it becomes damaged, the lining may become roughened, leading to an increased risk of thrombus formation (Weinstein and Hagle [252]). Within this layer are folds of endothelium called valves, which keep blood moving towards the heart by preventing backflow. Valves are present in larger vessels and at points of branching and are seen as noticeable bulges in the veins (Tortora and Derrickson [243]). However, when suction is applied during venepuncture, the valve can compress and close the lumen of the vein, preventing the withdrawal of blood (Weinstein and Hagle [252]). Therefore, if detected, venepuncture should be performed above the valve in order to facilitate collection of the sample (Weinstein and Hagle [252]).
Tunica media
The tunica media, the middle layer of the vein wall, is composed of muscular tissue and nerve fibres, both vasoconstrictors and vasodilators, which can stimulate the vein to contract or relax. This layer is not as strong or stiff as the equivalent layer in an artery and therefore veins can distend or collapse as the pressure rises or falls (Waugh and Grant [250], Weinstein and Hagle [252]). Stimulation of this layer by a change in temperature, mechanical or chemical stimulation can produce venous spasm, which can make a venepuncture more difficult.
Tunica adventitia
The tunica adventitia is the outer layer and consists of connective tissue, which surrounds and supports the vessel (Waugh and Grant [250]).
Choosing a vein
The choice of vein must be that which is best for the individual patient. The most prominent vein is not necessarily the most suitable vein for venepuncture (Weinstein and Hagle [252]). There are two stages to locating a vein:
- visual inspection
- palpation.
Visual inspection
Visual inspection is the scrutiny of the veins in both arms and is essential prior to choosing a vein. Veins adjacent to foci of infection, bruising and phlebitis should not be considered, owing to the risk of causing more local tissue damage or systemic infection (Dougherty [53]). An oedematous limb should be avoided as there is danger of stasis of lymph, predisposing the patient to complications such as phlebitis and cellulitis, with increased risk of causing tissue damage from the tourniquet application (Hoeltke [88]). Areas of previous venepuncture should be avoided as a build‐up of scar tissue can cause difficulty in accessing the vein and can result in pain due to repeated trauma (Hoeltke [88]).
Palpation
Palpation is an important assessment technique as it determines the location and condition of the vein, distinguishes veins from arteries and tendons, identifies the presence of valves and detects deeper veins (Dougherty [53]). The nurse should always use the same fingers for palpation as this will increase sensitivity and the ability of the nurse to know what they are feeling. The less dominant hand should be used for palpation so that in the event of a missed vein, the nurse can repalpate and realign the needle (Hoeltke [88]). The thumb should not be used as it is not as sensitive and has a pulse, which may lead to confusion in distinguishing veins from arteries in the patient (Hoeltke [88]).
Thrombosed veins feel hard and cord‐like, and should be avoided along with tortuous, sclerosed, fibrosed, inflamed or fragile veins, which may be unable to accommodate the device to be used and will result in pain and repeated venepunctures (Dougherty [53]). Use of veins that cross over joints or bony prominences and those with little skin or subcutaneous cover (e.g. the inner aspect of the wrist) will also subject the patient to more discomfort (Dougherty [53]). Therefore, preference should be given to a vessel that is unused, easily detected by inspection and palpation, patent and healthy. These veins feel soft and bouncy and will refill when depressed (McCall and Tankersley [138]).
Arteries
Arteries tend to be placed more deeply than veins and can be distinguished by their thicker walls (which do not collapse), the presence of a pulse and the bright red blood they carry. It should be noted that aberrant arteries may be present; these are arteries located superficially in an unusual place (Weinstein and Hagle [252]).
