Useful procedures in respiratory medicine (Proceedings)

Treating animals with respiratory distress may be very challenging. It is essential for the practitioner to have a strong knowledge base of available therapeutic and diagnostic techniques. It is also prudent to be prepared for any potential complications that may develop during diagnostic or therapeutic interventions. Respiratory distress may be divided into canine or feline, and upper or lower airway disorders. It is often practical as well to consider distinguishing animals with known causes for distress (eg. Trauma) from those that present with acute difficulties of unknown origin.

Supplemental oxygen

Supplemental oxygen is beneficial in all types of respiratory distress. Oxygen may be administered via a face mask, oxygen cage, via a nasal catheter or hood, or with an endotracheal tube and intermittent positive pressure ventilation. Flow-by or face mask oxygen requires and external source of oxygen and connecting tubing. Additionally, unless the pet is very obtunded, a person is required to help hold the oxygen source near the animal. Some animals do not tolerate this very well and may actually increase their work of breathing by struggling to avoid oxygen. Nasal oxygen is well-tolerated in larger non-brachycephalic dogs. Nasal oxygen placement requires a local anesthetic, soft red rubber catheter (5- 8 Fr), suture and an oxygen source (ideally humidified). For placement, the local anesthetic is infused into the selected nostril and permitted time for efficacy. The catheter is inserted to the level of the medial canthus of the ipsilateral eye and sutured in place. Oxygen flow rates of 100 ml/kg/min are recommended. The actual inspired concentration of oxygen will reflect the dog's breathing strategy as panting will result in a lower inspired oxygen concentration than quiet nasal breathing. If the potential need for supplemental oxygen can be predicted for a patient undergoing anesthesia, it is often much easier to place nasal oxygen while the pet is recovering from anesthesia. Some animals require placement of an Elizabethan collar to prevent premature dislodgement of the catheter. An oxygen hood can be easily constructed with an E-collar and cellophane wrap to cover the front. The oxygen can flow in from tubing attached at the collar. A small area can be left open to allow for venting. Dogs that pant excessively may overheat with an oxygen hood, but in the majority of cases it is a well-tolerated and useful technique. In animals with severe respiratory distress or marked upper airway obstruction, the best option may be to anesthetize the animal and provide positive pressure ventilation or perform a tracheostomy. This may be performed short-term to permit adequate time for diagnostic and stabilization such as for a dog with a large volume pleural effusion or may be required for a longer time as part of a therapeutic intervention. It is important to realize that animals with severe respiratory distress are typically not going to rapidly improve without an intervention AND that respiratory distress is very frightening to all animals.

Laryngeal and complete upper airway examination

Laryngeal and complete upper airway examination should be a part of the diagnostic evaluation for any dog or cat that presents with signs compatible with upper airway obstruction. Due to the routine nature of endotracheal intubation or the delegation of such a task to support staff, the normal anatomy and function of the upper airway may be underappreciated by some practitioners. Additionally, when given in standard dose to permit endotracheal intubation, laryngeal function is often substantially depressed.

Difficult endotracheal intubation

Difficult endotracheal intubation should be anticipated in dogs and cats with brachycephalic airway configuration, masses, bleeding (either oral or airway) or anything else that may distort the anatomy. Additionally, in neonates, the size of the tongue may be a particular impediment to intubation due to difficulties in visualization. In preparation for elective intubation, it is prudent to have a variety of sizes of endotracheal tubes and a good light source. Additionally, other equipment may be modified to help provide an airway. An 8 – 14 Fr red rubber catheter may be used to provide supplemental oxygen, although the resistance of the tube is typically too great to permit adequate ventilation. In some dogs with large airway masses, digital intubation is much easier that trying to visualize the larynx. Clearly, general anesthesia is required to prevent either being bitten or triggering a strong gag reflex. It is a good idea to practice digital intubation in a normal dog so the anatomy is easily recognized. If a small bronchoscope is available, an endotracheal tube may be threaded over the scope, and then the scope advanced into the airway and then the tube fed over the scope.

In some cases, an animal with a temporary tracheostomy has a disease process that requires a permanent tracheostomy. (eg. Laryngeal tumor). Surgically, it is much easier to create a permanent tracheostomy with an endotracheal tube already in place. In these cases, after induction of anesthesia, a small endotracheal tube may be advanced through the tracheostomy site into the mouth, then another larger tube passed over the guide tube and directed into the desired location.

Tracheostomy

Tracheostomy may be required to by-pass an upper airway obstruction. In our practice, tracheostomies are most commonly required in dogs due to laryngeal paralysis, severe airway swelling or for ease of mechanical ventilation and in cats for neoplastic processes. A tracheostomy is performed by aseptically preparing the cervical region. A midline incision is made. The cervical muscles are blunting dissected. A natural plane of separation is visible between the right and the left muscles. The trachea is isolated. Care should be taken to avoid damage to the vagosympathetic trunk. Two stay sutures- one cranial and one caudal of a heavy suture (eg 1-0 Prolene) should be placed. An incision is made into the trachea- it may either be made transverse or horizontal. In our practice, most incisions are transverse, perhaps with a small additional horizontal component to permit intubation. Standard tracheostomy tubes are commonly used, although regular oral endotracheal tubes will also suffice. The cuff should never be inflated unless the pet is receiving mechanical ventilation. Additionally, the tube should not be sutured in place or covered with a bandage. At Tufts, we prepare an "emergency trach kit" comprised of propofol, syringes, a new endotracheal tube and a light source and leave it attached by a basket to the kennel to save time in the event of a crisis. When removing the trach tube, it is essential to carefully watch the animal to insure that recurrent obstruction does not occur. The tracheostomy site should be allowed to heal by second intention, as if closed primarily a large amount of subcutaneous emphysema may develop.

Thoracocentesis

Thoracocentesis is another technique that is essential in management of the pet with pleural effusion or pneumothorax. Large volumes of pleural effusion can interfere with adequate ventilation through direct compression of the lungs and limitation in the amount of lung expansion. Small volume pleural effusions are unlikely to compromise respiration, but may be very useful in completing the identifying the source of the problem. Pleural effusion may be suspected based upon physical examination, and may be confirmed with either thoracic radiographs or ultrasonograpy. Thoracocentesis is performed by aseptically preparing the lateral chest at the 7-9th intercostal space, at approximately the costochondral arch. The intercostals vessels and nerves run along the caudal aspect of the rib. A butterfly catheter (Terumo Medical Corp, Elkton, MD)) is convenient to use in small dogs and cats. In larger animals (including BIG cats), a 1.5 inch or longer needle or catheter may be required to reach the pleural space. The pleural space should be completely drained if possible. Samples should be submitted for cytological evaluation and bacterial culture and sensitivity testing (if indicated). As small animals do not have a complete mediastinum, it is rare for effusion to be unilateral. Complications of thoracocentesis may include hemorrhage or pneumothorax. Pneumothorax may be severe in animals with chronic pleuritis, which prevents the lungs from rapidly sealing. In animals that are being referred to another hospital, is it advisable to remove as much pleural effusion or air as possible before transport.

Chest tube (Thoracostomy tube) placement is frequently necessary to stabilize patients with a large volume pneumothorax and for therapy for some intrapleural conditions such as pyothorax. In animals with traumatic pneumothorax, placement of a chest tube commonly follows the "three strike rule", meaning that it is often wise to tap the chest three times prior to placement of a chest tube, due to the normal propensity for the lung to heal on its own. Clearly, some clinical judgement is required, some dogs will require an emergent chest tube, while other may be treated with periodic needle thoracocentesis. For dogs with a spontaneous pneumothorax, early rather than later chest tube placement as well as surgical intervention appears warranted.

Placement of a chest tube is easiest under general anesthesia, although in obtunded animals a local block may be sufficient. Commonly, in our practice we will induce general anesthesia with a narcotic (eg hydromorphone -0.1 mg/kg) combined with diazepam or midazolam and propofol followed by intubation and conservative ventilation with isoflurane in oxygen. An individual should be dedicated to patient monitoring. The patient should be placed in either lateral or sternal recumbency. The lateral thorax should be clipped and aseptically prepared. A red rubber style or trocar chest tube may be used. In busy emergency practice, the trocar type (Tyco Healthcare Group LP, Mansfield, MA) is often more reliably placed. The assistant should pull the skin cranial (forward) about 2-4 cm. An incision is made through the skin, subcutaneous space, muscle and pleura and the chest tube introduced. The skin is then released, creating a tunnel to prevent leakage of air when the tube is removed. The tube may be either immediately connected to a syringe and evacuated, or left open to air while the assistant ventilates the dog. The tube should be firmly sutured to the pet and lightly bandaged. Thoracic radiography is strongly recommended after tube placement to document location. Systemic or local analgesics (bupavacaine 1.5 mg/kg via tube -dog 0.5 mg/kg -cat, diluted with saline and flushed) are warranted in most animals.

Intercostal local anesthetic blocks are a useful adjuvant to systemic analgesia in dogs and cats with rib fractures. A local block is performed by injecting bupavacaine or lidocaine caudal to the desired rib at the dorsal aspect of the thoracic cavity. Rib fracture pain is reportedly extreme in people so it seems appropriate to try to control pain in dogs and cats as well.

Collection of cytological samples from the airways and lungs for evaluation and culture are essential for optimum patient care. Tracheal washes may be performed either via an oral or a transtracheal route. The transoral route is usually used in cats, small dogs and brachycephalic dogs. To perform a transoral tracheal wash, the pet is first pre-oxygenated, then general anesthesia is induced with propfol (or other agent). A sterile endotracheal tube is passed and aliquots of saline infused and aspirated. It is often helpful to have the tube drain into a sterile specimen cup as it may be challenging to aspirate all the fluid. Two to three ml aliquots (up to 4) are recommended for small dogs and cats, 5 ml for medium dogs and 10 ml for larger dogs. Supplemental oxygen should be administered during patient recovery.

To perform a transtracheal wash, the pet is usually not or only mildly sedated. The larynx and trachea are palpated. The insertion point may either be the cricothyroid ligament or in-between two adjacent tracheal rings. The selected area is blocked with a lidocaine. The area is aseptically prepared. A through-the-needle catheter (Venocath-Venosystems-Abbott Ireland, Ireland or Intracath-Becton-Dickinson, Sandy, Utah) should be inserted through the skin and then directed down the trachea. Aliquots of sterile saline can be similarly injected and aspirated. Coughing will enhance return.

Bronchoalveolar lavage (BAL) is performed either via a bronchoscope or blindly. Bronchoscopically, the scope is wedged into an airway and saline is infused and aspirated back. Non-bronchoscopically, a modified stomach tube technique has been described (Hawkins and Berry, JAVMA , 1999, 1635-1639).

Cytological samples may also be collected via a fine needle lung aspirate. This technique is most appropriate for diffuse interstitial disease or isolated large mass lesions. The skin overlying the area of interest is aseptically prepared. For a diffuse lesion, an 1.5 inch 22 gauge needle is inserted into the lung parenchyma and an aspirate performed. More localized lesions may better aspirated via ultrasound or CT guidance. A recent retrospective study documented a high degree of correlation of aspirate with eventual lung biopsy (DeBerry JD, Norris CR, Samii VF, Griffey SM, Almy FS. Correlation between fine-needle aspiration cytopathology and histopathology of the lung in dogs and cats. J Am Anim Hosp Assoc 2002 Jul-Aug;38(4):327-36) Reported complications include pneumothorax (which may be severe) and hemorrhage.

Tru-cut lung biopsies may also be collected via ultrasound although this is risky (pneumothorax). Additionally key-hole or thorascopic –guided biopsies may also be useful and may minimize morbidity. In some cases of small animals with diffuse interstitial disease, by the time the clinical signs are apparent, there may be significant morbidity or even mortality during surgical attempts to perform a biopsy.