Manual of Equine Anesthesia and Analgesia. Группа авторов

creating a large submucosal pocket.

       When using a Dyson self‐retaining, tracheostomy tube, one of the tongues may be inserted inadvertently into the peri‐tracheal tissue, rather than into the tracheal lumen. This results in insufficient movement of air through the tube into the tracheal lumen and increases the likelihood of infection at the site of tracheostomy.

       Subcutaneous and peri‐tracheal infection.Placing gauze sponges, to which an antimicrobial ointment has been applied, between the faceplate of the tracheostomy tube and the cutaneous incision decreases the likelihood of infection at the wound. Gauze sponges also absorb exudate, preventing excoriation of skin from accumulation of exudate.

       Subcutaneous emphysema often surrounds the site of tracheostomy. This can extend proximally, to involve the head, and caudally, to involve the trunk.Compressing the faceplate of the tracheostomy tube to the wound with elastic adhesive tape decreases the likelihood of subcutaneous emphysema developing at the wound. (see Figure 4.17)Severe subcutaneous emphysema is likely to develop if the tracheostomy tube is insufficient in cross‐sectional area to relieve high negative intrathoracic pressure. Exaggerated inspiratory effort pulls air through the cutaneous incision.

       Pneumothorax can result from migration of subcutaneous air into the mediastinal space and then into the pleural cavities.Inserting a tracheostomy tube insufficient in cross‐sectional area to prevent high intrathoracic pressure may contribute to development of pneumothorax.

       Completely transecting a tracheal ring leads to enfolding of the ring into the tracheal lumen, because the tracheal rings are incomplete dorsally.

       Leaving the cuff of a cuffed tracheostomy tube inflated for more than three hours may lead to mucosal damage, which in turn, may lead to a stricturing cicatrix.

Suggested Reading

      1 Chesen, A.B. and Rakestraw, P.C. (2008). Indications for and short‐ and long‐term outcome of permanent tracheostomy performed in standing horses: 82 cases (1995‐2005). J. Am Vet. Med. Assoc. 232: 1352–1356.

      2 Saulez, M.N., Slovis, N.M., and Louden, A.T. (2005). Tracheal perforation managed by temporary tracheostomy in a horse. J. S. Afr. Vet. Assoc. 76: 113–115.

5 The Renal System

       Natalie S. Chow

      I Role of the kidney

       The functions of the kidney include the following:Elimination of nitrogenous and organic waste.Regulation of body water and electrolytes.Regulation of blood pressure.Maintenance of acid–base status.Production of hormones, including erythropoietin, calcitriol, and renin.

      II Normal anatomy and physiology

       The equine kidney is unilobar.

       The kidney can be divided into the cortex, medulla, and renal pelvis.The cortex is the outer portion between the renal capsule and the medulla.The medulla is the inner portion that is responsible for maintaining the salt and water balance of the blood.The pelvis is a continuation of the proximal portion of the ureters, carrying urine from the kidney to the urinary bladder.

      A The nephron

       The nephron is the functional unit of the kidney.It is composed of the renal corpuscle and the renal tubule.

       The renal corpuscle consists of the glomerulus and Bowman's capsule. These structures are responsible for filtration.

       The renal tubule consists of:

       The proximal convoluted tubule (PCT).The loop of Henle (LOH) (composed of descending and ascending limbs).The distal convoluted tubule.The collecting duct.These structures are responsible for reabsorbing and secreting electrolytes and water across the tubular lumen.

       Approximately 95% of urine is water; the remainder is organic and inorganic solutes.

       The glomerulus

       It consists of a network of specialized capillaries, which is surrounded by Bowman's capsule.

       It allows fluid, very small proteins, and electrolytes to be filtered and enter the PCT.

       The glomerular filtration rate (GFR) can be measured to determine renal function.

       Proximal convoluted tubule

       After leaving the glomerulus, the filtrate enters the PCT.

       The PCT is responsible for the active reabsorption of more than 60% of the filtered substances.

       Major substances reabsorbed include sodium, potassium, calcium, phosphate, magnesium, chloride, bicarbonate, urea, glucose, and amino acids.

       The PCT is permeable to water and follows the reabsorption of the electrolytes.

       The PCT is the major site of ammonia production.

       Loop of Henle

       The descending limb of the LOH receives filtrate from the PCT.Little to no active transport processes occur here.This part of the LOH is only permeable to water.Water is reabsorbed via osmosis, making the filtrate hypertonic compared to plasma.

       The ascending limb of the LOH receives filtrate from the descending limb.Electrolyte reabsorption via active transport occurs here.It is impermeable to water.This is the major area of solute reabsorption.The filtrate becomes hypotonic relative to plasma.

      Distal convoluted tubule (DCT)

       Electrolyte and water reabsorption occurs here.

       This is the site of the majority of K+ and H+ secretion via active transport.

       Collecting duct

       This is the final segment of the nephron, and it has an important role in electrolyte and water absorption.

       The collecting duct is under the influence of arginine vasopressin (anti‐diuretic hormone – ADH), which controls the final urine osmolality (concentration).

       During states of dehydration, ADH is released from the posterior pituitary, resulting in the insertion of aquaporin channels into the membrane of the cells of the collecting duct.These channels allow the absorption of water, resulting in a concentrated urine.

       Equine urine

       Equine urine can be very cloudy and foamy, and rather viscous, depending on the horse's diet. (see Figure 5.1)

       The cloudiness results from the high concentration of calcium carbonate crystals.Diets high in calcium (e.g. alfalfa) are associated with higher concentrations of calcium carbonate crystals in the urine.Excess dietary calcium is eliminated in the urine and feces.The majority of equine uroliths are composed of calcium carbonate.Figure 5.1 Image of a sample of horse urine. The urine appears cloudy and foamy which is a normal finding.

        The foamy and viscous nature of