Anaesthesia - Diagnosis

CURRENT CONCEPTS IN PHARMACOLOGY OF ANAESTHETIC AGENTS IN RELATION TO DIAGNOSTIC PROCEDURES IN ANIMALS K.ADILAXMAMMA*, E.MADAN MOHAN AND M.ALPHA RAJ, DEPARTMENT OF PHARMACOLOGY & TOXICOLOGY, COLLEGE OF VETERINARY SCIENCE, TIRUPATI

Anaesthesia is used for a wider range of circumstances in animals than in humans due to unwillingness of animals to co-operate with certain diagnostic / therapeutic procedures. The diagnostic procedures requiring anaesthetic procedures are •

Endoscopy

•

Bone marrow aspiration

•

Ultrasonography

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Magnetic resonance imaging

•

X-Ray imaging

•

Aggressive animals

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Exotic and wild animals

Differences between surgery and diagnostic procedures Diagnostic procedures differ from classical surgery in the following aspects i.

Elective

ii.

Minimally invasive

iii.

Short duration

iv.

General anaesthesia or sedation is usually required to assure patient immobility during these procedures.

v.

Anaesthetic should provide rapid recovery

vi.

Not life saving

Principles of anaesthesia for diagnostic procedures i.

Identification and correction of underlying patient problem to minimize anaesthetic risk.

ii.

Formulation of an anaesthetic protocol feasible with existing environment and that can be readily adapted to individual patient and its disease process.

iii.

Application of effective and adequate monitoring provision of appropriate supportive therapy.

iv. *

Presenting author

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Anaesthetics commonly used in large animals include – Alpha – 2 receptor agonists like xylazine, romifidine, detomidine, medetomidine. Propofol is rarely used in large animals due to cost factors. Butorphanol is used in all species. Ketamine is used in many species. Etomidate and desflurane are used only in university hospitals. Patient Preparation Pre-anaesthetic patient assessment including physical examination and laboratory evaluation (complete blood count, chemistry panel and urine analysis) is essential for successful outcome. For marrow and organ biopsy, a coagulation profile should also be evaluated. System specific diseases and interactions should be considered. Protocol Selection When formulating an anesthetic plan, reversibility, familiarity and maintenance of homeostasis by provision of adequate supportive care and monitoring should be addressed. The plan should meet majority of patients and should be compatible with the facility. •

The drugs should be short acting and or reversible

•

For premedication, an opioid alone or with a benzodiazepine has relatively mild cardiopulmonary effects and will provide sedation and reduce induction and maintenance requirements. The opioid can be reversed with naloxone and benzodiazepine with flumazenil, if necessary.

•

Acepromazine premedication may be beneficial for healthy animals that are difficult to restrain or aggressive, if there are no patient contraindications. Acepromazine facilitates a smooth but prolonged recovery, so the benefits should be weighed against the disadvantage.

•

Ketamine with an opioid–benzodiazepine combination may be beneficial in fractious cats to provide sedation and immobilization. Induction options include propofol, etomidate, ketamine and inhalant agents. Propofol is probably the most frequent used because of its short duration of action, but can be detrimental in animals with underlying hypotension that remains uncorrected prior to induction. Etomidate may provide a better alternative in cardiovascular –compromised patients.

•

Ketamine with benzodiazepine represents an alternative method for induction but recovery may be prolonged. Contraindications for ketamine use such as a history of seizures and the presence of increased intracranial pressure are particularly dangerous to patients undergoing MRI and CT. Induction with an inhalant agent via facemask may be used if other options present

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unacceptable patient risk. A major disadvantage with mask induction is waste gas pollution and subsequent exposure of personnel. Monitoring One or more monitoring devices should be applied to every individual patient to help assure its well-being. Monitoring techniques could be Continuous ECG Pulse oximetry Capnography Blood pressure monitoring Measurement of end – tidal anaesthetic gas concentration The use of montoring should be tailored to individual procedure. Pulse oximetery is especially important for assessing patients at risk for developing hypoxemia such as during bronchoscopy and thoracostomy. Capnography is useful for intubated patients at risk of hypoventilating (thorocostomy tube placementa). Availability of mechanical ventilation to provide intermittent positive pressure ventilation (IPPV) is important during diagnostic imaging. IPPV in combination with capnography will enhance patient management by assuring ventilatory homeostasis. Increased intracranial pressure caused by trauma or brain tumor is a common sign in patients undergoing CT or MRI imaging. Maintain these patients in a mildly hypocapneic state (PaCO2 = 30 mm Hg) will improve anesthetic out come by minimizing the detrimental effects of increased CO2 on intra-cranial pressure. Anaesthesia in different diagnostic procedures Endoscopy The visual examination of internal organs with out invasive surgery and performed with a rigid / flexible fiberoptic instrument is termed as endoscopy. a. Respiratory Endoscopy: This is one of the most valuable diagnostic procedure to evaluate air way diseases in dogs and cats. Respiratory endoscopy includes rhinoscopy, laryngoscopy, bronchoscopy. General anaesthesia is required. Generally, gas anaesthetics are used for rhinoscopy and injectible anaesthetics are used for laryngoscopy and bronchoscopy. i.

Rhinoscopy : It is the visual assessment of nasal cavity , nasopharynx and in some cases paranasal sinus. A complete examination requires general anesthesia, due to strong airway protective reflexes (sneezing and 3

gagging). Rhinoscopy requires a deep plane anaesthesia especially for posterior rhinoscopy. Topical lidocaine sprayed on mucosa may blunt some of the reflexes. ii.

Laryngoscopy: It is a gold standard for assessing laryngeal diseases and it allows for evaluation of both anatomical as well as disorders of intrinsic laryngeal function / motion, specifically, laryngeal paralysis. Light plane of anaesthesia is required so that animal maintains gagging reflex. Sedation is required to facilitate relaxation of the jaw, but anaesthetic should be carefully selected to have a minimal effect on laryngeal function. It can be challenging to keep patients adequately sedated without affecting laryngeal function. A neuroleptanalgesic combination of an opioid and benzodiazepine is adequate for most patients. These drugs have the advantage of preserving laryngeal function and being reversible if respiratory distress develops. Challenge with doxaparam 1mg/kg i.v to increase respiratory activity may be used if evaluation is hindered by drug induced respiratory depression.

iii.

Bronchoscopy : It is the gold standard for diagnosis of lower respiratory tract diseases in small animals. It may be used for diagnosis, therapeutic and prognostic purposes. General anaesthesia is necessary to control the reflexes during bronchoscopy there by preventing trauma to airways and protecting the endoscope. The ideal anaesthetic should provide good patient restraint, minimal effects on cardio-pulmonary effects, reversible or short duration of action and should facilitate smooth recovery. The general anaesthetic propofol is used with either atropine or glycopyrrolate or either acepromazine or butorphanol. This anesthetic is adequate and allows rapid patient recovery. Bronchoscopy is performed in dogs and cats for evaluation of airway disease and to perform bronchoalveolar lavage. Many of the patients presented for laryngoscopy and bronchoscopy are at increased risk for development of hypoxemia. Preoxygenation should accompany both the procedures and the anaesthetist should always be prepared to take control of the airway by intubation and application of ventilatory support. During the bronchoscopy, oxygen may be delivered to smaller patients via an endoscope-working channel. Larger patients may be intubated and connected to oxygen by a breathing system: a Y-piece aperture may be used to pass the endoscope into the trachea. An opiodbenzodiazepine combination for sedation, followed by administration of low-dose propofol to effect, is commonly used to facilitate bronchoscopy.

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b. Gastrointestinal Endoscopy Gastroduodenoscopy and proctoscopy may be performed in relatively healthy dogs or in dogs with a history of chronic weight loss. The decreased serum protein concentration may result in decreased anaesthetic requirement for thiopental or propofol and serious loss of fat and muscle will result in prolonged recovery from thiopental. Premedication with atropine and morphine is reported to enhance the chances of passing of the endoscope. Bone Marrow Aspiration This is performed to evaluate bone marrow disease and to stage certain cancer patients. Protocol selection should be based on the individual patient; depending on the level of an animal’s activity, bone marrow aspiration may be performed in dogs with local infiltration alone or in combination with sedation. Uncooperative dogs and the majority of cats may require general anaesthesia. Reversible agents with mild cardiopulmonary effects, such as combination of a benzodiazepine with butorphanol or puprenorphine provide adequate sedation in most patients. Propofol, benzodiazepine–ketamine combination or etomidate are good induction choices, providing for relatively rapid recoveries. Laproscopy This is an operative procedure designed for the visual inspection and biopsy of peritoneal cavity and its organs. General anesthesia with gaseous agents like isoflurane and sevoflurane are preferred. Propofol is commonly used for the induction of anesthesia. In depressed patients, local anesthetic agents may be sufficient used either alone or in conjecture with a combination of diazepam and butorphanol or propofol. Oxymorphine or butorphanol provides analgesia, which is often beneficial to the patient during recovery. Ultrasonography It is a routine diagnostic procedure for animals. Most animals tolerate us evaluation without sedation or anaesthesia, but sedation may be necessary in fractious, aggressive or painful animals. Ultrasonography guided organ or tissue biopsy is becoming a common method for collecting samples less invasively. Small animals usually tolerate the procedure well with sedation and local anaesthetic infiltration, but general anaesthesia may be more effective in some individuals. Local anaesthetic infiltration alone or with mild sedation, combined with proper restraint is effective for collecting biopsy samples in standing horses, cows and other large animal species. 5

X-Ray Imaging Patient must remain motion less and be precisely positioned for x-ray. A profound sedation or general anaesthetic protocol should be used that is easily administered and has a quick recovery time. During x-ray, personnel cannot be present in the X-ray room to directly monitor the animal because of radiation safety. Remote monitors or cameras focused on the patient and in-room monitors should be used to assess the patient. Computed Tomography And Magnetic Resonance Imaging Anaesthetic management for CT and MRI presents unique challenges •

There is no painful stimulation during the anaesthetic period. Thus, response to a noxious stimulus does not provide a method for assessing anaesthetic depth.

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The majority of the patients of scans are performed with dorsal recumbency, which has the most significant detrimental effects on ventilation and perfusion matching.

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Access to the patient is limited during CT and MRI. Hence subjective evaluation such as assessment of pulse quality and mucous membrane colour may not be feasible. With CT although patient access is not problematic, exposure of the anaesthetist to radiation is an issue and direct patient assessment during scan is discouraged.

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Reaction to contrast media administration in animals is relatively uncommon but potential complications can arise.

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Traditional anaesthetic equipment and monitors may be unsafe, may be damaged, may malfunction or may interfere with the image generation when used in MRI suite.

Anaesthesia for CT and MRI is most commonly maintained with an inhalant agent. Isoflurane and sevoflurane both facilitate a rapid recovery. If Anaesthesia machines and ventilators, which are not MRI compatible, are present, anaesthesia can be maintained by propofol by using a constant rate infusion or intermittent bolus technique.

Maintenance

with

propofol

should

always

include

intubation,

supplemented with oxygen and a method for ventilatory support. Patients anesthetized for MRI and CT require i.v. fluid administration to maintain adequate perfusion and application of monitoring devices to identify and address alterations in homeostasis proactively.

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Premedication : Opiod – benzodiazepine combination Opioids (IV, IM or SC; IV administration of morphine is not recommended) Butorphanol

0.2 – 0.4 mg/kg

Buprenorphine Morphine Hydromorphine Oxymorphone

0.01 – 0.04 mg/kg 0.4 – 1.0 mg/kg 0.1 – 0.2 mg/kg 0.05 – 0.1 mg/kg

Benzodiazepines (Diazepam: IV only; Midazolam : IV, IM or SC) Diazepam 0.1 – 0.4 mg/kg Midazolam 0.1 – 0.3 mg/kg Antagonists Naloxone (opioid) Flumazenil

0.001 mg/kg 0.01 – 0.02 mg/kg

(Benzodiazepine) Induction Propofol Etomidate

2-6 mg/kg, IV. 0.5-2.0 mg/kg, IV.

Ketamine 2-5 mg/kg, IV, IM. Maintenance Inhalation anaesthesia : Isoflurane or Sevoflurane Propofol, CRI 0.4 mg/kg/min Propofol, IB

0.5 – 2.0 mg/kg

CRI = Continuous rate infusion; IB = Intermittent bolus REFERENCES Adams H.R (2001) Veterinary pharmacology and therapeutics. 8th Edition, Iowa State University Press / Ames. Hall L.W., Clarke K.W and Trim C.M (2001). Veterinary anaesthesia, 10th Edition, Saunders Ltd. McKiernan B (2001). Respiratory Endoscopy—A Visual Assessment of the Respiratory Tract. Proceedings of World Small Animal Vetrinary Association World Congress – Vancouver. Sarah P (2007). Endoscopic Investigation - How to Prepare and What to Expect. Veterinary Nurse Times. Schoeffler P (2008). Anaesthesia for gynecological endoscopy. Practical training and research in gynaecologic endoscopy. Geneva foundation for medical education and research. William J.T, John C.T and Kurt A.G (2007). Lumb and Jones Veterinary anaesthesia and analgesia. 4th Edition, Wiley-Blackwell publishers.

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