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Respiratory compromise

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Abstract

Causes of respiratory compromise can range from stenotic nares to sever alveolar collapse, and anaesthesia of patients with these conditions can exacerbate or even create further problems. The choice of anaesthetic agents will be mainly dictated by the overall health of the patient, but the chosen anaesthetic technique is often critical. This chapter looks at the general patient assessment, choice of drugs for patients with respiratory compromise and anaesthesia for these patients.

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Figures

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22.2 A cat receiving oxygen supplementation. ‘Flow-by’ or ‘blow-by’ oxygen can be delivered from an anaesthetic machine or other oxygen source. The tube delivering the oxygen should be held as close to the patient’s nares or mouth as possible, otherwise the technique will be inefficient. This method of oxygen delivery may increase inspired oxygen from 21% (the proportion in room air) to 30–40%.
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22.5 Positioning of the patient for endotracheal intubation. (a) The larynx is easiest to visualize if the head and neck are stretched out from the body at approximately a 45 degree angle with the tongue gently pulled forward. (b) Under-extension or (c) over-extension of the head and neck will make the larynx difficult to visualize.
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22.6 Equipment that should be prepared before intubation: (i) several sizes of endotracheal tubes; (ii) laryngoscope with working light source; (iii) rigid stylet; (iv) soft stylet; (v) sterile lubricant to lubricate the tube; (vi) a means of securing the tube (e.g. tubing); (vii) lidocaine in a syringe with a catheter attached.
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22.7 Proper inflation of the endotracheal tube (ETT) will just prevent a leak when the pressure relief (or ‘pop-off’) valve is closed and the system is pressurized to 20 cmHO as measured by the manometer. Pressurizing the system at pressures higher than 20 cmHO is not necessary and may result in overinflation of the ETT cuff and excessive pressure on the tracheal mucosa, which could damage the mucosa and lead to tracheal constriction.
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22.10 Placement of an endotracheal tube through a pharyngotomy site. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
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22.11 A manually operated high-frequency jet ventilator (Manujet; VBM). The insufflation tube (top of the image) is inserted into the airway. The unit is connected to a source of pressurized oxygen (400 kPa) and the black knob allows adjustment of delivered pressure in the range 50–350 kPa. (Courtesy of Chris Seymour, Royal Veterinary College, London, UK)
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22.12 Oxygen–haemoglobin dissociation curve, with the partial pressure of oxygen in arterial blood ( O) on the horizontal axis and the percentage of haemoglobin saturated with oxygen ( O) on the vertical axis. Note that at values of O <90%, further decreases in O result in a rapid decline in oxygen saturation (dashed line). At this point, the saturation can be a sensitive indicator of ventilation because decreased ventilation will cause immediate desaturation. Once 100% saturation is reached, as will occur with supplemental oxygen therapy, the O can continue to increase but O cannot increase beyond 100% (open arrows). At this point, the O is a very insensitive indicator of ventilation because decreased ventilation can occur with no decrease in saturation.
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22.13 Capnographic waveforms can be used to diagnose some types of respiratory compromise. Waveform 1 is normal. Slope A represents the beginning of expiration when carbon dioxide (CO) in the exhaled breath increases rapidly. Slope B represents the slower rise as CO is exhaled from the alveoli. Slope C results from the switch to inhalation, when the CO drops back to zero. The intersection of slopes B and C is the point where end-tidal carbon dioxide ( CO) is measured by the capnometer; this value should be approximately 40 mmHg (5.3 kPa), as indicated by the dotted line. Waveform 2 represents hypoventilation. The waveform shape is normal but CO is elevated. Waveform 3 represents an obstructed airway, which is indicated by the longer, steeper slope B. The airway obstruction may be in either the upper airway (e.g. plugged or kinked endotracheal tube) or the lower airway (bronchoconstrictive disease, e.g. asthma) (see also Chapter 7).
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22.15 If the tongue is not available, the pulse oximeter probe can be placed on any relatively thin, non-pigmented skin. Sites that may be suitable include the (a) lip, (b) toe and (c) prepuce. Other suitable sites include the ear, vaginal fold and flank.
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22.16 Brachycephalic breeds, like this Pug, generally tolerate an endotracheal tube in the trachea until they are completely conscious.
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22.17 English Bull Terrier anaesthetized by means of a propofol infusion for removal of a bronchial foreign body. Oxygen supplementation is being provided by a urinary catheter placed in the trachea and connected via a 2.5 ml syringe to the anaesthetic breathing system. (Courtesy of Marieke de Vries, Davies Veterinary Specialists, Higham Gobion, UK)
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22.18 Tracheal collapse that extends beyond the location that can be reached by an endotracheal tube, as seen in this radiograph, may necessitate IPPV during the maintenance phase of anaesthesia.
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22.19 Diagram and picture of an adapter used for bronchoscopy. The double diaphragm used with this arrangement reduces the amount of anaesthetic spilling into the room while allowing the delivery of inhalant anaesthetics with positive pressure ventilation if needed.
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22.20 (a) Sagittal computed tomography image from a dog which had developed sudden onset of dyspnoea and spontaneous pneumothorax. Note the fluid-filled bulla (arrowed) in the left cranial lung lobe. (b) Appearance of the bulla after lung lobectomy. (Courtesy of Chris Seymour, Royal Veterinary College, London, UK)
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22.21 Pneumothorax and pulmonary contusions in a dog following blunt trauma. Patients with this presentation should have air removed from the thorax before anaesthesia.
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22.22 (a) Feline patient with pleural haemorrhagic effusion. After sedation with butorphanol and local infiltration with lidocaine, the chest is drained in a sterile manner by means of a butterfly needle with the cat in sternal recumbency. Extra oxygen is provided and an ECG is attached to monitor heart rate and rhythm. (b) Close up of the connection of the syringe, 3-way tap, butterfly needle and extension set. During aspiration, the 3-way tap blocks off the connection to the extension line. To empty the syringe, the 3-way tap is closed towards the butterfly needle to avoid injecting aspirate back into the chest.

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