Anatomy of Current and Potential Blood Sampling Sites in the Florida Manatee (Trichechus manatus latirostris)
There are few detailed anatomic descriptions of the Florida manatee. This lack of information may create a challenge for veterinarians when trying to perform diagnostic procedures. Large superficial blood vessels that have reliable landmarks for venipuncture are rare in manatees. Thick skin combined with thick blubber and superficial muscle layers limits the number of possible sites for collecting blood. Recent publications of methods for drawing blood do not address some of the anatomic limitations that might be encountered. The purposes of this study were to illustrate aspects of the manatee’s unique anatomy, review current methods of blood collection, and propose alternate sites for blood collection.
The flipper is a region where the skin, blubber, and muscles are relatively thin. The radioulnar interosseous space, olecranon, and well-defined muscles are palpable landmarks. These features aid in finding the correct site to collect blood. Typically, blood is obtained from the brachial vascular bundle, an arteriovenous plexus located on the palmar aspect of the flipper. The brachial vascular bundle originates in the cervical region and extends past the humerus and continues between the radius and ulna. This bundle of vessels is a network of small parallel arteries and veins, each of which is less than 1 mm in diameter.
The standard procedure for obtaining a blood sample is to rotate the palmar side of the flipper dorsally in order to access the brachial vascular bundle. Although this is an excellent technique, there are potential complications with its use. The median nerve extends along the center of the interosseous space on the palmar surface of the vascular bundle. Avoiding this nerve is difficult while performing a blind stick. However, no motor damage has been reported with the use of this technique.
An alternative sampling site is a large arteriovenous plexus in the tail, the caudal vascular bundle, which extends within the chevron canal on the ventral aspects of the caudal vertebrae. The caudal vascular bundle is a plexus of hundreds of arteries and veins each less than a millimeter in diameter. Due to the inherent risk involved to the operator, this technique is probably limited to immature manatees.
The first successful alternative technique for collecting blood from the flipper is a lateral approach to the brachial vascular bundle. The use of this site has been incorporated in the husbandry procedures of some facilities. Advantages of this technique are the ability to avoid the median nerve and excessive manipulation of the flipper. Furthermore, if an animal’s flipper has excessive scar tissue on the palmar surface or while collecting blood a hematoma forms, this technique provides an excellent alternative. This site is particularly useful for catheterization as well. Disadvantages of this technique include penetrating slightly thicker skin and muscle layers. Additionally, some animals respond as though more pain is induced.
A fourth possible site for blood collection is just proximal to the cubital joint. The brachial vascular bundle is much larger at this point than it is distal to the cubital joint. This approach would also be a blind stick. Even so, there are landmarks to aid in locating the plexus. The disadvantages of this technique are potential penetration of the joint capsule and damage to nerves located in this area.
Finally, one publication suggests collecting blood from large veins located on the cranial and caudal edges of the lateral surface of the flipper. However, reliable landmarks are not evident and the volume of blood in these veins will depend on thermoregulatory conditions. When the manatee needs to conserve heat, little or no venous blood may be available from these veins.