Physiological Haemostasis

Haemostasis requires interaction between the vessel walls, platelets and procoagulants. These interactions are usually prevented from forming unnecessary clots in the bloodstream by limiting reactions (Figure 1).

When a small vessel is damaged it initially undergoes reflex vasoconstriction which is later maintained by the platelets and surrounding tissue mediators. The damage exposes collagen within the vessel wall causing platelets to become activated and also triggering the intrinsic coagulation cascade. Platelet activation results in formation of a loose primary 'plug' of platelets (the primary or temporary haemostatic plug). In order to maintain effective haemostasis the primary clot needs to mature into a secondary and more stable plug. Formation of the secondary clot requires the production of thrombin by the clotting cascades. Thrombin converts fibrinogen to fibrin which interlaces between the platelets of the primary plug to strengthen the clot. Factor XIII cross-links the fibrin to further strengthen the structure of the plug (see Figure 1). Surgical equipment used in haemostasis has two objectives:

 Cessation of blood loss by the direct action of the equipment

 Directly or indirectly to facilitate a more permanent method of haemostasis by physiological or artificial means

This lecture concentrates on equipment available to the surgeon to reduce or prevent bleeding during surgery.

Figure 1. Diagram illustrating the basic steps of physiological haemostasis.

Pressure Pad Haemostasis

Low-pressure haemorrhage from small vessels may be effectively controlled by pressure for several minutes using swabs. Tissues should be blotted rather than wiped to avoid disruption of newly formed blood clots. Used correctly, this method leaves no foreign material in the surgical site and is atraumatic.

Haemostatic Forceps

The forceps close the vessel lumen in order to arrest haemorrhage and in doing so crush the vessel wall which activates the physiological clotting mechanism (Figure 1). Definitive haemostasis by crushing alone is limited to low-pressure vessels and requires sufficient time and effective physiological clotting activity. For larger vessels, haemostatic forceps are used temporarily to facilitate ligation, electrocoagulation or other method of haemostasis.

Larger forceps (e.g., Roberts) may be used as a 'crush clamp' on large pedicles containing fat and other tissue (e.g., the ovarian pedicle). Placing a ligature in the crush zone adds to ligature security and reduces the possibility for delayed haemorrhage following fat necrosis.

Electrosurgery

Electrosurgery (often referred to as diathermy although this term is strictly incorrect) is a term used to describe the induction of heat to cause coagulation, desiccation or fulguration within tissue using flow of electricity. It can be used without decreasing wound healing potential if used correctly, though overall security is generally lower for coagulated than for ligated vessels. Improper use has been associated with burns, delayed healing, increased risk of infection, fires and explosions.

 Monopolar coagulation: Current is passed between a handheld instrument and a base plate in contact with the animal. A distinct advantage of monopolar electrosurgery is the ability to cut at the same time as coagulate tissue. Haemostasis may also be carried out by applying the handpiece to a haemostat in which the vessel is held, though care must be taken to avoid damage to surrounding tissues. Monopolar electrosurgery is not effective in a wet surgical field (e.g., during ongoing bleeding) because the energy is not focused on the intended area.

 Bipolar coagulation involves current passing between the tips of the forceps when they are held about 1 mm apart. The flow of current between the tips rather than through the animal eliminates the need for a ground plate under the animal. Bipolar electrosurgery has some advantages over monopolar electrosurgery:

 Less current is required

 Less risk of damage to surrounding tissues

 Effective haemostasis can be achieved in a wet surgical field

Ligasure^TM (Valleylab^TM; Covidien)

The Ligasure^TM device uses a combination of pressure applied by a handpiece and thermal energy to melt collagen and elastin within blood vessel walls and create a permanent seal. Unlike conventional electrosurgery it does not rely on a proximal thrombus (blood clot) to achieve haemostasis because the vessel walls are permanently fused together. This effect is achieved by the application of significantly higher current than a standard electrosurgery generator, and much lower voltage. The device uses feedback assessment to automatically administer the correct amount of energy and therefore achieve a reliable result with 2 mm or less spread of damage to surrounding tissues.

Vibration Coagulation - Harmonic® Scalpel (Ethicon; Johnson & Johnson)

This technology involves the application of vibration at extremely high frequencies to cut and simultaneously coagulate tissue using a Harmonic® scalpel. The vibration causes protein denaturation during the cutting procedure to limit bleeding and seal vessels but cannot be applied to provide haemostasis without cutting.

Vascular Clips

Vascular clips are available in various sizes and can be easy to apply in deep locations where access is limited. Disadvantages include cost (though this may be offset by decreased surgery time), dislodging of clips by further surgical manipulation and the fact that they persist in the wound as foreign material (possibly affecting future radiographs, magnetic resonance imaging (MRI) or radiotherapy depending on the metal used). Most clips are titanium, which reduces their likelihood of contributing significantly to infection risk and advanced imaging interference.

Surgical Staples

Stapling devices are becoming increasingly common in veterinary surgical theatres and usually apply rows of 'B'- shaped titanium staples in two or three row patterns. The design of the staple and offset rows is such as to crush and hold larger vessels closed to allow a physiological clot to form but still allow capillary flow to the edges of the pedicle and therefore avoid an area of devitalised tissue. Staples are available in different sizes and configurations depending on their intended use and in some applicators have been combined with blades that are deployed during the stapling procedure.