Cannulation should be considered when repeated samples are required as it avoids multiple needle entries at any one site. It is suitable for use in all strains of rats and can be used to take blood from the femoral artery and vein, carotid artery, jugular vein, vena cava and dorsal aorta. Surgery is required and appropriate anaesthesia and analgesia should be used to minimise any pain caused. Rats should be allowed to regain their pre-operative body weight before blood samples are taken. See this technique below.

The cannula is exteriorised at the nape of the neck (through a jacket and tether system) or the base of the tail (via a tail cuff and tether system). The use of a tail cuff is preferable to a jacket as the percentage of adverse effects has been found to be 1% with the tail cuff compared to 5-10% for jacketed models. The tail cuff model also has minimal weight loss after surgery, whilst in the jacketed model the rat takes at least 24 hours to regain pre-operative weight. The use of a subcutaneous access port should be explored and may eliminate the need for tethering systems during periods when animals are not being sampled from.

The jacket and tether system may restrict free movement and rats may need to be housed singly after surgery. The caging, bedding and environmental enrichment need to be appropriate to prevent the tether becoming entangled and the wound contaminated. In addition the bedding needs to be sange free.

The cannula used is small which may promote blood clotting (larger cannulae can abrade the blood vessel wall). To prevent this, the cannula requires regular maintenance, e.g. flushing with an anticoagulant.

Blood should be collected aseptically. Usually 0.1 - 0.2 ml can be taken per sample, and depending on the sample volume, up to six samples over a 2-hour period or up to 20 samples over a 24-hour period. Sterile saline with anticoagulant should be flushed into the cannula after blood sampling to prevent the blood from clotting. A pin is then inserted into the exteriorised end of the cannula, which stops the blood from flowing. A sterile locking solution can be use to lock the cannula after a series of samples have been taken and thereby avoid flushing for a number of days.

The following should be checked daily - 

• The skin in contact with the jackets should be checked for abrasion
• The jacket should be checked for tightness
• The tail cuff should be checked for tightness and the tail for swelling
• Wound sites should be checked for infection/bruising/swelling/haemorrhage
• The cannula should be checked for patency
• The weight of the rat (for recovery work)

Number of samples:

Depending on sample volume, up to six samples in a 2-hour period or up to 20 samples over a 24-hour period. 

Sample volume:

0.1 - 0.2 ml

Equipment:

23G - 25G cannula

Staff resource:

One person is required to take the blood sample. However, further staff resource may be required for surgery, post-operative care for up to 5 days after surgery, and daily animal observations post-surgery.

Adverse effects :

• Infection: 5-10% with jacketed rats, 1% with tail cuffs
• Haemorrhage: 5-10% with jacketed rats, 1% with tail cuffs
• Blocked cannula: 5-10% with jacketed rats, 1% with tail cuffs
• Swelling around the jacket: 5-10% with jacketed rats, 1% with tail cuffs
• Skin sores from the jacket: 5-10% with jacketed rats, 1% with tail cuffs