Abstract

Computed tomography (CT) and plain film radiography of three adult and one sub-adult ringed seals were used as adjuncts to dissection to determine alternative sites for intravenous and intramuscular injections. Sites for intramuscular injection by both manual and remote injection methods are described including external recognition of the proposed injection site with the underlying structures. The largest muscle groups identified were the epaxial muscles of the trunk, the pectoral muscles, the gluteal muscles, biceps femoris muscle, muscles of the brachium and shoulder, and the muscles of the crust. The caudal branch of the medial saphenous vein was identified as a large bore vessel suitable for venipuncture in restrained seals. The anatomical landmarks for performing a vena puncture are listed. Blubber thickness of the three adult and one juvenile ringed seal was measured from CT scan images and/or by manual soundings. Blubber thickness over the lateral aspects of the thoracic and pelvic limbs, as well as, the dorsal portion of the crus were less than the thickness of blubber of the dorsal and ventral midlines and right and left lateral margins of the seal. These areas of decreased blubber thickness coincide with proposed injection sites into the larger muscles previously listed.

Introduction

Administration of xenobiotics to phocids for purposes of management and research in the field or for treatment and prophylaxis of disease in the clinical setting are an everyday occurrence. In many instances however, the surety of delivery of the xenobiotic to the appropriate area may be in question. Several factors, concerning both the method of delivery of the chosen xenobiotic and the animal in its environment, contribute to the difficulty of such a commonplace task. Choice of xenobiotic can be limited by the agent's availability in a particular vehicle and the efficacy of the agent in that form. Other limiting factors in the choice of a xenobiotic are purpose for use, method of administration, volume per dose, and cost of the agent. Factors related to the animal include size and state of nutrition. The disposition of the animal: submissive vs. aggressive, eating vs anorectic, or healthy vs compromised, must also be considered when selecting an agent to be used and the route of administration.

In the clinical setting, the oral route is most often used to administer medication. Oral administration is easy and safe for the animal, as well as, the person administering the drug. Difficulty arises when the animal is anorectic and is incapable of eating on its own. Forced feeding can be stressful on the animal and dangerous for the handler.

A major complication in the administration of intramuscular and intravenous agents to phocids is the thickness of the blubber coat. A well fed adult animal can have a blubber coat of 10 cm at its thickest point.^2,3,6 Due to this blanket of fat, few external anatomic landmarks are observed, making visualization of the proposed injection site difficult if not impossible.

Many of the drugs manufactured in recent years are labeled for intramuscular or intravenous use. Their increased efficacy and decreased cost per unit volume have made them popular. Two major categories of xenobiotics with increased usage today in pinnipeds are antibacterials and chemical immobilizers. Most immobilizing agents are labeled for intramuscular administration, making field administration difficult unless the animal is restrained. Remote injection via either a jab pole or the remote apparatus used by Ryding, can be compromised by the need for rapid delivery to a discrete area, especially in an uncooperative or unpredictable subject.¹⁴ In order to dart accurately, via a blowgun or darting gun, needle length and volume of drug must be considered.⁷ Subcutaneous deposition of a chemical immobilizer can result in variability of the onset of restraint and the duration of restraint, which is a potential hazard to the subject, as well as, to the handlers.¹⁷

Accessing a vessel for intravenous deposition of a drug is a greater challenge than that posed by intramuscular injections. Intravenous injection sites in seals have been described by Geraci, Ridgway and Sweeney.^4,5,12,16 Superficial blood vessels of phocids are generally small and few in number.^1,13,15 Stress incurred during manual restraint may elicit sympathetic constriction mimicking the dive response.^8,10

The objective of this study was to identify additional intravenous and intramuscular injection sites. Knowledge of the myology and superficial vasculature of the phocid will facilitate the administration of parenterals both in field and clinical settings. This is a preliminary report and a more detailed report with figures will follow.

Methods and Materials

Three adult ringed seal specimens, 2 males (RSB-2-1-90 A, RSB-3-4-91), one female (RSB-2-1-90 B); and one sub-adult male (RSB-1-28-90) ringed seal specimen were examined after collection by Eskimo hunters near Point Barrow, Alaska. All seals were scanned at one centimeter intervals via computed tomography (CT) in order to compare in situ morphology with dissection data. The resulting images were used as adjuncts in evaluating blubber thickness and muscle depths at various sites on the specimens. Plain film radiographs were taken of all four seals.

Blubber thickness and muscle depth were recorded from three adult seals using the images from the CT scans and by manual soundings. Measurements of blubber thickness were taken directly from the CT scan. Using a spinal needle inserted to depth, soundings were performed at selected intervals along the dorsal and ventral midlines, along the lateral margins of the trunk, and over the limbs. At each site, at least four measurements were obtained and the mean value used. Drawings were made and photographs taken of the specimens in order to illustrate salient anatomical features. More sites will be sampled from the specimens and the data will be compiled at a later date.

Results and Discussion

All four seals appeared to be in good flesh and compared favorably with published data3 on length, weight, and blubber thickness for the species (Table 1). With this in mind, the sample animals appeared to be representative of the species for the season. From cranial to caudal, blubber thicknesses of the trunk were shallowest immediately caudal to the occiput (1.7 cm), but increased rapidly to a maximal thickness between the scapulae (5.3 cm). From this point, a slow decline was maintained to the region of the tail (3.2 cm). The leanest areas of the trunk were caudal to the skull and medial to the metacarpals of the pectoral flippers (3.5 cm). Blubber thickness over the pelvic limbs decreased markedly toward the calcaneus (1 4 cm).

Table 1. Relationship of sex, weight and length of ringed seal specimens

Three large muscle groups associated with the thorax were identified: 1) extrinsic muscles of the thoracic limb [trapezius, rhomboideus, latissimus dorsi, serratus ventralis (magnus)], 2) epaxial muscles (longissimus and iliocostalis muscles), and 3) muscles of the scapulohumeral association (supraspinatus, deltoideus, infraspinatus, lateral and long heads of the triceps brachii). The scapulohumeral group of muscles represented an area of significant muscle mass, underlying an area of the blubber coat that was thinner than the dorsal and ventral midline blubber. These features suggest that this area would be an appropriate location for intramuscular injection especially when using a remote injection technique. The area best suited for intramuscular injection into this group of muscles was immediately caudal to the spine of the scapula and proximal to the antebrachium.

The pectoral muscles may also be considered as a site for intramuscular injection. This large muscle mass lies approximately 5 cm below the surface of the skin with the greatest thickness located 7-10 cm lateral to the sternal midline at the level of the second sternebra. Access to the pectoral muscle mass would necessitate the use of a 2 ½ - 3 inch (6 - 7 ½ cm) needle inserted to the hub. From a ventral approach, the needle should be directed perpendicular to the long axis of the animal, straight into the muscle. To use this site, the animal must be turned on its side, however it provides ventral drainage should an abscess form following injection.

The muscles of the pelvic limb suitable for intramuscular injection by both remote and manual methods include the middle gluteal, biceps femoris, gastrocnemius, peroneus longus, and cranial tibial muscles. The two largest muscles in the group are the middle gluteal muscle and the cranial portion of the biceps femoris muscle and thus represent the best target area for remote injection methods when given a lateral view of the subject. For the manual technique the biceps femoris and the caudal muscles of the crus, especially the peroneus and gastrocnemius muscles, provide the best sites available in this region. Ventrally in the pelvic region, the gracilis muscle is the single largest muscle and also provides a satisfactory site for injection with the advantage of ventral drainage.

Even though they possess visible appendages, the fusiform shape of the phocidae creates difficulty in localization of deeper structures. In the past, the use of imaginary lines imposed superficially to outline deeper non-visible structures has been successful.¹¹ Dividing the animal into equal thirds will help localize muscle groups. The division between the first and second thirds of the animal occurs just Caudal to the front flipper at the level of the fifth thoracic vertebra, and the division between the second and last thirds, which although there is no readily observable external landmark, is directly cranial to the pelvic girdle at the lumbosacral junction.

The muscles of the scapulohumeral association can be localized by proceeding cranially approximately 5 cm from the line separating the first and second thirds of the animal at a level that is half the dorsoventral height of the animal. This area can be more precisely demarcated externally by a series of lines that will form a triangle over the area. From a lateral projection, extend two lines caudally from the external auditory meatus, one parallel to the long axis of the trunk and the other caudo-ventrally to the carpus to form two sides of the triangle as noted in figure 1. The remaining side is formed by extending a line dorso-caudally from the carpus to the line paralleling the long axis of the body so that an equilateral triangle is formed as the lines intersect. By drawing a line from the vertex (located at the carpus) vertically to the base, the triangle can be divided into two right triangles. The caudad triangle overlies the triceps muscles (the largest group of brachial muscles) providing the best injection site of the brachium and shoulder (Figure 1). When inserted to the hub, a 3 inch needle should be sufficient to assure intramuscular deposition at this site.

The muscles of the thigh and crus can be localized externally by proceeding caudally approximately 10 cm from the line separating the middle and caudal thirds of the animal at a level that is approximately half of the dorsoventral height of the animal at that point. This area can be more precisely demarcated by extending a line cranially from the tail along the vertebral column that transects the boundary line of the middle and caudal thirds of the animal. Another line is extended cranially from the calcaneus (which is readily palpable at the proximal end of the pelvic flipper), parallel to the long axis of the trunk which transects the boundary of the middle and caudal thirds of the body. A final line is directed vertically, transecting the two horizontal lines approximately 5 cm cranial to the calcaneus, thereby yielding a rectangular shaped region outlining the area as in Figure 1. When inserted to the hub, a three inch needle is ideal for a deep intramuscular injection into this muscle group.

Figure 1. Lateral view of a ringed seal with imaginary lines superimposed and with muscles labeled.

Historically, recommended sites for venipuncture in phocids are the extradural intervertebral vein and the planter venous plexus of the caudal flippers.^4,5,12 The extradural intervertebral vein extends the length of the spinal column originating at the base of the skull thus representing the principal venous drainage for the cerebrum. Anastomoses with other venous tributaries occurs along the entire length of the system with the major connections being with the azygos vein and the abdominal plexus leading to the pericapsular renal plexuses.^1,13,15 Access to this system is gained by inserting a 3 inch needle to depth in an intervertebral space in the lower lumbar region.^4,12

As previously reported, the main venous drainage from the digits was observed to arise from a confluence of the planter digital veins, the planter venous plexus, at the level of the mid-metatarsals.^1,9,13,15 Leaving the planter venous plexus and coursing proximally, the caudal branch of the medial saphenous vein (commonly called the caudal saphenous vein) continues, from the planter venous plexus of the flipper, medial to the calcaneus, over the tarsal canal where it is bound to the tarsus by heavy connective tissue. In the mid-tibia, this vein turns medially from the crus coursing dorso-medially, between the semitendinosus muscle and caudal slip of the biceps femoris muscle, reaching the trunk of the animal near the tail fold (junction of the tail and the trunk). From this point it courses deep to the cranial portion of the biceps femoris muscle near its origin and enters the extradural intervertebral vein at the third sacral vertebra. Along its route, it receives drainage from the perineal and caudal thigh region and from the superficial musculature of the lower leg, including the biceps (Figure 2).

Figure 2.

In view of the preceding anatomical description, one can inject into or draw blood from the caudal branch of the medial saphenous vein. Using the calcaneus as a landmark and the rear flipper as a hand-hold, place the thumb of the grasping hand just medial to the calcaneus. The tip of the thumb is used to guide a 1½ inch, 20 gauge needle cranially and slightly ventrally (10° -20°) remaining parallel to the axis of the limb, into the caudal branch of the medial saphenous vein whose diameter is 4 - 5 mm at this point. Just proximal to the calcaneus, the vein lies 1 cm below the skin.

Summary

Computed tomography and plain film radiography used as adjuncts to dissection, have provided a means of correlating the limited number of externally visible landmarks with the underlying musculoskeletal system of the ringed seal. Sites identified for intramuscular injection using remote and manual injection techniques include: 1) the muscles of the scapulohumeral association ((deltoideus, supraspinatus, infraspinatus, lateral and long heads of the triceps brachii),), 2) the muscles of the pelvic limb (the middle gluteal, biceps femoris, peroneus longus and gastrocnemius muscles), and 3) the epaxial musculature of the lumbar area. The caudal branch of the medial saphenous vein, described above, may prove to be a viable alternative to venipuncture sites described by other authors. It was hoped that this research might provide a model that could be successfully applied to other phocidae. There are differences in and among genera, however homology within the taxonomic family may allow for the application of a model to other general.

Acknowledgements

We appreciate the cooperation of the Eskimo hunters who provided us with the anatomical specimens examined in this study. The specimens were transported and studied under the authority of permit # 519 issued to Dr. T. F. Albert by the National Marine Fisheries Service. Partial funding was supplied by the North Slope Borough, Department of Wildlife Management, Box 69, Barrow, Alaska 99723, through P. O. #'s 25916 & 29655. Other funding was provided by the Department of Animal Science, College of Veterinary Medicine, The University of Tennessee, Knoxville, TN 37901-1071.

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