Managing gunshot wounds and serious bite wounds (Proceedings)

Gunshot Wounds

Gunshot wounds can be a common injury in veterinary medicine varying from shotgun and rifle injuries in rural areas to handguns and air powered weapons in cosmopolitan areas. The severity of injury resulting from gunshots is variable as well based on the type and composition of the bullet, the projectile (flight) characteristics, kinetic energy absorbed on impact and tissues involved. A basic understanding in ballistics and tissue reaction can be helpful in determining the extent of injury and the appropriate treatment necessary for a given projective wound.

The characteristics of the bullet and weapon influence the type and severity of injury inflicted. These can be classified into Internal Ballistics, External Ballistics, and Terminal Ballistics. Internal Ballistics refers to the characteristics within the weapon such as bullet design (bullet, shotgun pellets, air-powered pellets), weapon design, and barrel effects. External Ballistics refers to the in flight projectile characteristics such as projectile design, wind velocity, drag, and gravity. Terminal Ballistics are the characteristics that affect impact including the projectile velocity, kinetic energy of the projectile and absorption by the particular tissue, and projectile movement/ position on impact.

Bullets typically have a lead core, however they vary in size, shape, and weight. The shape and casing of the bullets (fully jacketed, partially jacketed, hollow point) affects the degree the bullet is deformed/ fragmented on impact, which slows the velocity of the projectile causing soft tissue destruction along its path as well as increasing the kinetic energy absorbed by the soft tissues and thereby increasing the amount of soft tissue destruction. For example, a partially jacketed hunting bullet designed to deform and mushroom on impact can create 40X more soft tissue destruction than a military bullet of similar size and mass.

The bullet and barrel design also affects the flight characteristics of the projectile. Typically, most modern bullets are fired though barrels which are rifled with helical grooves which elicits a spin on the bullet improving its motion through flight and accuracy. Bullets can become unstable during flight resulting in a yaw or tumble of the bullet as it travels through the air. This altered motion slows the velocity of the projectile but overall increases the soft tissue damage by increasing the area of the bullet on contact and may facilitate fragmentation of the bullet further enhancing the amount of tissue destruction.

Shotgun pellets are available in steel and lead and come in a variety of pellet sizes and number. BB and pellet guns fire low mass and low velocity projectiles. Shotguns pellets are designed to disperse increasing the target area. The pattern of dispersion can be changed by modifying the barrel length and diameter with a cone shaped dispersion of pellets extending from the barrel. Even though they are fired at a low velocity, the typical hourglass or cone shape of these pellets can incur severe soft tissue injury at a close range. The closer the range the more destructive this pattern becomes as a concentrated density of pellets with high kinetic energy strikes a small target area.

The Kinetic Energy of a projectile is a measure of its destructive potential as is measured by its mass and velocity.

Kinetic energy (KE) = Mass X Velocity2 / 2

Looking at this equation, doubling the mass of the projectile doubles its kinetic energy whereas doubling the velocity will quadruple its kinetic energy. As a result, a smaller projectile with a higher velocity may have a greater kinetic energy and destructive potential compared to a slower projectile with greater mass at impact. This is important, as the velocity of a projectile is the most important factor or determinant for wound severity.

Low Velocity: less than 1000ft/s (<400m/s)

Medium Velocity: 1000 – 2000 ft/s (400-825m/s)

High Velocity: greater than 2000ft/s (>825m/s)

Wound Severity is determined also by the target tissue type and its ability to absorb or resist kinetic energy, which is a function of the tissues specific gravity, elasticity, and relative cohesive properties. Tissues with a lower specific gravity and greater elasticity, such as skin and lung, are better able to absorb a portion of the kinetic energy of a projectile, compared to bone. Although liver and muscle have similar specific gravities, liver has more relative tissue destruction because the tissue is less cohesive and less resilient. Dense tissues with limited compliance, such as bone, absorb a larger amount of kinetic energy and can shatter.

When a bullet interacts with tissues several mechanisms of tissue injury can occur including Lacerations and Crushing injuries, Shockwaves, Cavitation, and Secondary Missiles. Crushing injuries and lacerations are the primary method of injury with low velocity projectiles. Shockwaves are waves of high energy and short duration which propagate in all directions and tend to move ahead of the projectile causing not only adjacent but tissue damage distant to the path of the projectile. Cavitation refers to the transient rapid expansion of tissues adjacent to the bullet. This distension can be up to 30X the diameter of the bullet. The marked tissue distension creates a negative pressure resulting in a vacuum into which debris can be sucked deep into the wound it its entry and exit points. The result of this rapid expansion of tissues is a permanent cavity that can harbor necrotic and contaminated tissues. Tissues that have a lower specific gravity and greater elasticity are less affected by cavitation. Secondary Missiles are either primary projectile fragments or bone fragments that arise from the shattering of bone and these missiles are driven into adjacent tissues resulting in further tissue injury.

Treatment of specific gunshot wounds

In the treatment of gunshot wounds, it is important to remember that the size and severity of the entry wound may have no correlation to the damage to the underlying tissues and exit wound. It is also important to note, that the projectile might not have maintained a linear trajectory throughout its path through the animal making identification of the exit wound difficult.

Once the animal is stabilized initial wound management is started. In general this includes an overall physical examination, analgesia, clipping and cleaning and copious lavage of the wounds and stabilization if indicated. In the case of open fractures, the management of the soft tissue injuries is the most important factor predicting the final outcome of healing. After culturing the wound, broad-spectrum antibiotics should be started. Gram-positive cocci are the most commonly cultured bacteria from gunshot wounds. Even if the resulting wounds in the skin are small, the wounds are still considered contaminated from the introduction of skin, hair and other debris with the entrance of the projectile and cavitation of the wound. Contrary to popular belief, he heat of the bullet does not sterilize the wound.

Radiographs should be taken of the general area involved and other diagnostics including ultrasound, CT, myelography, and endoscopy may be indicated based on the wound location and clinical findings. Soft tissue (skin/muscle) wounds are usually left open and allowed to heal by second intention with the aid of sterile dressings such as wet-to-dry bandages to allow for debridement of the wound as needed while promoting healthy granulation tissue formation. Open (Penrose) and closed suction drains (Jackson-Pratt) can also be used to allow for decontamination of wounds closed primarily. Locating and removing all particulate debris is not indicated unless it is easily achieved being careful not to induce more soft tissue injury in the removal of those fragments. Penetration of fragments intra-articularly however does require arthrotomy for fragment removal as lead is slowly leached from the fragments and absorbed systemically. Gunshot wounds penetrating the abdomen and thorax, particularly in cases with unrelenting pneumo- or hemothorax after chest tube placement, require surgical exploration. Trauma to the cervical region also required exploration and potential endoscopy to assess for trauma to the esophagus or trachea. In cases of severe cervical soft tissue damage, tracheostomy may be indicated to restore the airway.

Evidence

It is important when treating animals with gunshot injuries, for potential litigation reasons that all medical records are complete including wound descriptions, treatment and owner conversations as well as photo documentation if possible. All fragments of the projectile retrieved should be saved and given to the appropriate authorities.

Bite Wounds

Bite wounds can cause significant injury not only in the skin, but in the tearing and crushing of underlying tissues. Compression of the canine teeth results in the classic puncture wounds with crushing wounds from the molars and premolars. Shearing during compression as the biting animal shakes its head results in tensile forces creating avulsions of the skin, herniation and devitalization of underlying tissues. The canine jaw on compression can exert a force between 150 to 450psi causing significant damage. Under the classic small innocuous puncture wound is often a much larger area of soft tissue damage, referred to commonly as the "Iceberg" effect. It is important to keep this in mind when evaluating animals with bite wounds and formulating a treatment plan.

Small (<10kg) intact male dogs, particularly the small terrier breeds are overrepresented as victims of bite wounds. The head, neck, and limbs are reported to be the most frequent sites affected followed by the thorax and abdomen. Smaller dogs may have an increased incidence of bite wounds to the thorax and abdomen compared to affected larger dogs.

Bite wounds depending on their number and severity can result in various life threatening issues. As with any severe injury, all patients with bite wounds should be critically evaluated and treated for shock, respiratory and cardiovascular compromise, and hemorrhage accordingly. Bite wounds to the cervical or thoracic area may require intubation, tracheostomy, or chest tube placement to re-establish an airway or treat a pneumothorax/hemothorax. Oxygen supplementation either via mask or nasal O2 can be helpful in the treatment of initial shock, but also to improve oxygenation of injured soft tissues.

All bite wounds, whether penetrating or not should be considered contaminated. The animal mouth is laden with anaerobic, aerobic, Gram negative and Gram-positive bacteria. Cats in particular have large numbers of Pasteurella multocida. Even in non-penetrating wounds, the local devitalized environment created from the crushing injury is an ideal substrate for harboring bacteria. Once cultured, a broad-spectrum antibiotic should be initiated. The number and severity of bite wounds can initiate a systemic inflammatory response in which excessive local inflammation results in a generalized inflammatory response.

Thorough wound cleaning, lavage, debridement and exploration is indicated for every bite wound. Surgical exploration within the abdominal and thoracic cavities is also indicted to assess the degree of organ injury for the prevention/treatment of sepsis, hemorrhage, air leakage as well as to re-establish the body wall if its integrity has been compromised. Bite wounds to the perineal area may require diligent evaluation of the colon/ rectum and urinary tract. Superficial bite wounds can be closed or allowed to heal by second intention with or without drains depending on the severity of the wounds.