Читать книгу Farm Animal Anesthesia - Группа авторов - Страница 22

HuiChu Lin

Department of Clinical Sciences, College of Veterinary Medicine, Auburn University, AL, USA

Preanesthetic tranquilization or sedation is rarely necessary in ruminants, but on occasion its use may be required to calm intractable animals for the safety of the animals and also for the safety of the personnel handling the animals. The choice of whether a tranquilizer/sedative is needed for a particular animal is based on that animal's temperament and physical condition. However, a complete physical examination prior to anesthesia in large ruminants often is not feasible because some of these animals are not accustomed to being handled and restrained, particularly free‐ranging animals. In addition to relocation to unfamiliar surroundings for the intended procedure, special equipment like a head catch or restraining chute that may be used for proper restraint of an intractable animal can add another level of stress to the patient. In large aggressive animals, the use of a tranquilizer/sedative will minimize the stress from forceful restraint, ease the anesthetic induction process, and decrease the dose of general anesthetic required, thus preventing the possibility of disastrous hypotension due to the use of large doses of an anesthetic. Camelids and swine are less tolerant of physical restraint; consequently, deep sedation or general anesthesia is often needed to perform even minor surgical procedures. Oral administration of a tranquilizer/sedative may be used to reduce stress associated with physical restraint in free‐ranging animals under field conditions or for animals intolerant of physical restraint. Drugs used for tranquilization and sedation are also used to produce chemical restraint. In ruminants and camelids, oral medication has to pass through the rumen or rumen‐like C1 compartment, respectively. Thus, the absorption and distribution of the drug will be affected by forestomach motility and the pH of the contained fluid [1]. Reticuloruminal motility is controlled by the medullary gastric center of the brain. Physical conditions affecting the patient, such as depression, pain, fear, excitement, and extreme distension of the ruminal wall (e.g. bloating), or administration of analgesics like opioids, α₂ agonists, or anesthetics (e.g. thiopental and propofol) have been reported to decrease gastric motility resulting in prolonged gastric emptying time and increased absorption of a drug [1].

The difference in pH between saliva (8.2), plasma (7.4), and ruminal content (5.5–6.5) also affects the absorption and distribution of the drug by its effect on the degree of ionization of the drug. Nonionized drugs have higher lipid solubility and easily diffuse across cell membranes, whereas ionized drugs have lower lipid solubility and are therefore less capable of diffusion across cell membranes. Weak acidic drugs ionize and accumulate in the alkalinized saliva and thus allow the delivery of large quantities of the drug into the rumen with the flow of saliva. Once in the more acidic environment of the rumen, ionized drugs become nonionized and diffuse across the rumen membrane into the local blood circulation. On the contrary, drugs that are weak bases become ionized and accumulate in the rumen. The degree of ionization and the diffusion of the drug between saliva, ruminal content, and plasma is constantly changing, which increases the difficulty in predicting the bioavailability, distribution, and calming effect of an orally administered tranquilizer/sedative [1]. Drugs administered intramuscularly or intravenously usually produce more predictable and reliable calming effects in ruminants and camelids, and are the preferred routes for administering a tranquilizer/sedative when possible. For monogastric animals like pigs, the pH in the stomach is acidic (1.5–2.5) [2], therefore orally administered acidic drugs will present with a greater percentage in nonionized form and thus better and faster absorption from the stomach and greater bioavailability of the drug. Basic drugs tend to have lower bioavailability due to a higher degree of ionization of the drugs in the stomach.