Anaesthetic procedures



Anaesthetic procedures


Denise Prisk



INTRODUCTION


Anaesthesia may be defined as the production of a controlled, reversible state of unconsciousness. By using certain drugs designed to have an effect on the nervous system, anaesthesia may be described as being general, i.e. the animal is unconscious and the entire nervous system is rendered insensitive to stimuli, or local, whereby a specific area is rendered insensitive to stimuli. Anaesthesia is used for welfare reasons, as it is obviously unpleasant for painful procedures to be performed on a fully conscious animal. It may also be used as a means of restraint, for example, when performing radiography or to examine an aggressive animal.


Local anaesthesia is used in small animal practice either to perform superficial surgery, such as suturing a small skin wound, or as an adjunct to other anaesthetic protocols, including sedation and general anaesthesia.


General anaesthesia can be achieved with the use of injectable agents alone or with inhalant agents. It is frequently the veterinary nurse who is responsible for setting up and maintaining anaesthetic equipment, as well as monitoring the depth of anaesthesia in the patient. It is vital that the veterinary nurse has a thorough understanding of the anaesthetic process if the patient is to survive the procedure and make a good recovery.


This chapter describes the procedures involved in preparing the anaesthetic equipment and in caring for the patient perioperatively (before, during and after the procedure). It also describes the different types of anaesthetic breathing systems (circuits) in common veterinary usage.



THE ANAESTHETIC MACHINE


Anaesthetic machines are designed to deliver accurate amounts of carrier gases and volatile liquids in a vapour form to the patient to produce anaesthesia (Figs 6.1 and 6.2). Table 6.1 describes the parts of the anaesthetic machine.



Table 6.1


Parts of the anaesthetic machine












































Component Description Function
Gas supply In the UK, medical gas cylinders are identified by the colour of the shoulder (top part). Oxygen cylinders are black with a white shoulder, or all white. Nitrous oxide cylinders are blue. Cylinders may either be attached to the anaesthetic machine (sizes E and F) or kept remotely and gas piped to the machine (size J) Supplies fresh oxygen and nitrous oxide to the patient
Pressure reducing valve or regulator Placed between the cylinder/pipe and the flowmeter. Often not visible as may be incorporated into the yoke or positioned underneath the machine Reduces the pressure of the gas leaving the cylinder to a safe working pressure
Pressure gauges Colour-coded for different gases, these may be attached to the anaesthetic machine near the gas supply (cylinder or pipe) or incorporated into the front panel of the machine (Fig. 6.2A). As the pressure in the cylinder of non-liquid gases falls, so does the pressure reading, indicating the amount of gas remaining in the cylinder. The pressure gauge of piped gases shows the pressure in the pipelines only, not in the cylinder. A separate gauge is attached to large cylinders.
The gauge of nitrous oxide cylinders always reads full until the cylinder is empty, as it shows the pressure of the vapour above the liquid. Therefore, the pressure gauge cannot be used to assess the amount of nitrous oxide remaining in a cylinder – it must be weighed		 For non-liquid gases, such as oxygen, the volume of gas remaining in the cylinder is proportional to the pressure reading on the gauge
Flowmeters (or rotameters) Consist of a tapered glass or plastic tube, calibrated and colour-coded for different gases. A bobbin (oblong) or ball (round) should rotate freely within the tube, providing an accurate reading of the flow rate. The reading is taken from the top of a bobbin or the middle of a ball. The dot in the centre of the bobbin is to ensure it is spinning in the gas Control and measure the flow of gas. Calibrations are in l/min. Some machines also have flowmeters which are calibrated in 100 ml/min, up to 1 litre, in addition to those in l/min (Fig. 6.2B)
Vapourizers Modern vapourizers, such as the TEC and Penlon type, are designed to remain accurate in output despite temperature and gas flow changes. They are agent-specific Deliver a known concentration of anaesthetic vapour to the patient
Back bar Flowmeters and vapourizers can be attached to the back bar. This allows more than one volatile agent to be available Supports the flowmeters and vapourizers
Common gas outlet Location varies between anaesthetic machines Enables connection to anaesthetic breathing systems and ventilators
Oxygen flush valve A button or valve that allows pure oxygen to be released at high pressure and high flow, bypassing the vapourizer. Care must be taken when using this valve to deliver oxygen to a patient, to avoid barotrauma to the lungs Provides oxygen in emergency situations and purges anaesthetic gases from the circuit before disconnection, to minimize pollution
Low-oxygen alarm An alarm sounds when oxygen levels become low. In modern anaesthetic machines, the delivery of nitrous oxide stops when the oxygen alarm sounds, activating the nitrous oxide alarm also Warns the anaesthetist of low oxygen levels

image
Figure 6.1 The parts of the anaesthetic machine. Reproduced with permission from Anderson Moores Veterinary Specialists.

image
Figure 6.2A Pressure gauges set into the front panel of an anaesthetic machine. Reproduced with permission from Anderson Moores Veterinary Specialists.

image
Figure 6.2B Flowmeters calibrated in ml/min and litres/min. Reproduced with permission from Anderson Moores Veterinary Specialists.



Procedure: Checking the anaesthetic machine before use



1. Action: Turn on the spare oxygen cylinder and check that it is full.


    Rationale: The contents of the spare cylinder must be noted to ensure a constant supply of oxygen is available throughout the anaesthetic.


2. Action: Turn the cylinder off and label it as full.


    Rationale: If the in-use cylinder and the spare cylinder are both open, they will empty at the same time.


3. Action: Turn on the in-use cylinder or piped oxygen supply, check the contents and replace if necessary. Label the cylinder as in-use.


    Rationale: If the pressure reading is in the red area of the gauge, the cylinder should be changed.


4. Action: Repeat the process for the nitrous oxide cylinders or piped supply.


5. Action: Open and close the oxygen flowmeter valve.


    Rationale: This confirms that the ball or bobbin can move and rotate freely in its column of gas.


6. Action: Check the low oxygen alarm by turning the oxygen cylinder off and pressing the oxygen flush valve. Turn the oxygen back on.


    Rationale: The alarm should sound as the oxygen pressure falls to a dangerously low level. The flow of fresh gas through the oxygen flush valve is also confirmed.


7. Action: Check that the correct vapourizer is properly fitted and that it is full.


    Rationale: The dial should move freely and the vapourizer should contain sufficient agent to avoid having to fill it during anaesthesia.


8. Action: Connect the correct circuit, having checked it carefully for faults.


    Rationale: Leaks may result from disconnected inner tubes in coaxial circuits or from leaking reservoir bags.


9. Action: Connect the scavenging system. Switch active systems on.


    Rationale: It is a legal requirement that waste gases are scavenged to avoid pollution of the environment.




PATIENT PREPARATION




Procedure: Pre-anaesthetic instructions



1. Action: Food should be withheld for the required period of time, as necessary, depending on species, age and condition. For routine procedures in healthy adult dogs and cats, food is generally withheld for 6–12 hours. Water should be available up until the time of premedication.


    Rationale: If food is present in the stomach of animals that are able to vomit, regurgitation or vomiting may occur. This may lead to fatal aspiration, as the swallowing reflex is reduced or lost during anaesthesia.


2. Action: Cats should be kept inside overnight with a litter tray until ready to be taken to the practice the following morning.


    Rationale: This prevents the cat from disappearing and possibly missing its scheduled procedure.


3. Action: Dogs should be walked prior to admittance.


    Rationale: This allows the patient to urinate and defecate before admission.


4. Action: Cats must be brought to the surgery in a secure cage or basket. Dogs must wear a secure collar and be on a lead.


    Rationale: This minimizes the risk of escape.


5. Action: The client is given a time to arrive at the surgery.


    Rationale: This enables the nursing team to plan the surgery list and allow time for each patient to be admitted.



Procedure: Admitting the patient



1. Action: Check that the patient is included on the day’s list and confirm the procedure.


    Rationale: A consent form should have been prepared in advance. The owner must be fully aware of what the procedure entails.


2. Action: Take the client and pet into a consulting room.


    Rationale: This is more professional than dealing with the client in a busy waiting room.


3. Action: Weigh the patient.


    Rationale: The exact weight is essential for accurate administration of all anaesthetic and analgesic drugs.


4. Action: Obtain a complete history (Table 6.2).



Table 6.2


Questions to ask when obtaining a history































Question Significance
1. How old is the animal? The animal’s age should be on the client records but should be checked: old and very young patients may pose a higher anaesthetic risk
2. When did the animal last eat? If food has been recently consumed, there is an increased risk of vomiting or regurgitation during anaesthesia
3. Has the animal had any previous illnesses and if so, what treatment was given? Client records should supplement this information. Any condition involving the major body systems may increase the anaesthetic risk
4. Have there been any signs of illness in the past 24 hours? If so, what were the symptoms and has the animal recovered? Anaesthetic risk might be increased due to dehydration, fever or electrolyte imbalance. Pathogens may also be introduced to the environment
5. How well does the animal tolerate exercise? Poor exercise tolerance may indicate cardiovascular or respiratory problems
6. Is the patient on any medication? If so, has it been administered today? Some drugs can alter the effects of anaesthetic agents
7. Is there any history of allergies or drug reactions? Prolonged recovery from a previous anaesthetic or anaphylactic reactions to any medication should be noted
8. Is the patient entire or has it been neutered? If an entire female, is she in season or pregnant? If ovariohysterectomy is to be performed on a pregnant female, surgery time may be prolonged and there might be an increased risk of haemorrhage

    Rationale: An accurate history is vital in order to evaluate the patient’s anaesthetic risk.


5. Action: For small mammals and exotic species, it is especially important to find out the animal’s normal routine, including its usual diet and preferred drinking method.


    Rationale: This ensures the post-operative period is as stress-free as possible and that a diet likely to be eaten is offered. It is preferable for clients to bring the usual food with them to minimize the risk of post-operative anorexia.


6. Action: Check the sex of patients admitted for neutering procedures.


    Rationale: It is far better to discover a mistake prior to surgery.


7. Action: Identify a cryptorchid patient prior to castration.


    Rationale: The owner must give consent for any additional surgery and can also be informed of an increased fee.


8. Action: Obtain a contact telephone number for the duration of the patient’s stay.


    Rationale: It is vital to be able to contact the owner or an agent of the owner in the event of an emergency.


9. Action: Check the owner understands all the details, then obtain a signature on the consent form.


    Rationale: The owner or agent must read, understand and sign the consent form. The owner or agent must be over 18.


10. Action: Transfer the patient to a kennel, making sure it cannot escape on the way.


    Rationale: It is best to ask the owner to leave before the dog is taken away as it is more likely to leave its owner willingly. Cats should be transferred in a secure basket.



Procedure: Pre-anaesthetic check

Assessment should be carried out before anaesthesia and surgery. This is usually performed by the veterinary surgeon.



1. Action: Assess the function of the cardiovascular system by auscultation of the heart and palpation of the pulse. (For normal values see Table 2.1.)


    Rationale: The cardiovascular system is affected by anaesthesia. Note the heart rate and rhythm, abnormal heart or lung sounds, pulse rate and presence of a pulse deficit.


2. Action: Assess the function of the respiratory system.


    Rationale: This is also affected by anaesthesia. Note the respiratory rate and any abnormalities.


3. Action: Palpate the abdomen.


    Rationale: This may detect the presence of an enlarged liver or abnormally small kidneys, either of which may lead to inefficient excretion of anaesthetic agents.


4. Action: Palpate superficial lymph nodes.


    Rationale: Enlarged lymph nodes may indicate the presence of infection, allergy or neoplasia.


5. Action: Take the patient’s temperature.


    Rationale: Note any temperature outside the normal range. Premedicant and anaesthetic agents all decrease body temperature.


6. Action: Assess pain, if relevant, prior to the administration of a premedicant.


    Rationale: Most premedicants contain an analgesic, which may mask signs of pain in conditions such as lameness, rendering further physical examination useless.


7. Action: Take a blood sample to perform a pre-anaesthetic blood screen, if required.


    Rationale: Pre-existing conditions may be evaluated, along with the patient’s hydration status and function of the liver and kidneys.


8. Action: Administer a premedicant.


    Rationale: Premedication is performed to sedate the patient, to relieve anxiety, to enable reduced doses of anaesthetic drugs to be used and to allow a smooth recovery. In addition, analgesia is provided with the inclusion of analgesic drugs. The patient should be kept warm after premedication as many drugs used for sedation cause a drop in body temperature. Common premedicant agents are shown in Table 6.3.



Table 6.3


Common premedicant drugs








































Drug Type/group Use Warnings
Acepromazine Sedative/ataractic: phenothiazine Calms the patient prior to induction and allows reduced doses of induction agent
Used with opioid analgesics to produce neuroleptanalgesia		 May reduce the seizure threshold. Its use may be avoided in known epileptic patients
Boxers are very sensitive to the effects of acepromazine and it is used with caution at low dose rates in this breed
Diazepam Sedative/ataractic: benzodiazepine Calms the patient prior to induction and allows reduced doses of induction agent
Used as a premedicant for sick patients and those prone to seizures		 Usually only produces sedation in very young or very sick patients
Medetomidine and dexmedetomidine Sedative:
alpha-2 agonist		 Used as a premedicant or in combination with other drugs to produce sedation for minor procedures. It can reduce the dose of induction agent by up to 80%. These agents also have analgesic properties. The sedative and analgesic effects are antagonized with atipamezole Alpha-2 agonist agents cause cardiovascular and respiratory depression. They should only be used in healthy patients. Spontaneous arousal is possible and care should be exercised when using in aggressive animals
Morphine, methadone, pethidine Analgesic:
full opioid agonist		 Produce pain relief. Given prior to surgery to provide pre-emptive analgesia which leads to decreased requirements in the post-operative period Full opioid agonist agents may cause respiratory depression, although this is rarely a problem in veterinary medicine. Morphine causes vomiting in healthy dogs
Buprenorphine, butorphanol Analgesic:
partial opioid agonist		 As above Butorphanol provides better sedation but less analgesia than buprenorphine. Its effects are also shorter-acting
Carprofen, meloxicam Analgesic:
non-steroidal anti-inflammatory drug
(NSAID)		 May be given at premedication, after induction or during recovery, to provide multimodal analgesia Renal toxicity and gastric irritation


image



GENERAL ANAESTHESIA


In modern practice, stages and planes of general anaesthesia are no longer referred to as surgical anaesthesia can be attained with the use of several different drugs working together (a process known as synergy). This means that patients do not need to be at a great depth of anaesthesia for painful surgical procedures to be performed. The time of greatest risk to the patient is said to be at induction and during recovery, and close monitoring at all times is essential.





Induction

During the induction period the patient passes from consciousness to unconsciousness. Induction is usually performed by intravenous injection, using one of the induction agents described in Table 6.4. Other methods are also possible and the advantages and disadvantages of each method are discussed in Table 6.5.



Table 6.4


Common induction agents




















Agent Effects Warnings
Propofol A propyl phenol agent
Rapidly metabolized in the dog by the liver, lungs and muscles. Its effects are therefore not cumulative in dogs. Anaesthesia in the dog can be maintained by giving incremental doses or by constant rate infusion – a technique known as total intravenous anaesthesia (TIVA). It is cumulative in cats, causing prolonged recovery, and should not be topped up or infused continuously		 Muscle twitches are sometimes seen at induction or during recovery
A brief period of apnoea and a fall in blood pressure may be seen on induction, especially if administered quickly
Two preparations are currently available: Preservative-free, which should be discarded 6 hours after opening.
Preservative with benzyl alcohol, giving a broached shelf-life of 28 days
Alfaxalone A neurosteroid anaesthetic agent
Non-cumulative in both dogs and cats, so can be used for TIVA without prolonging recovery time		 Post induction apnoea is sometimes seen. Its occurrence can be minimized by administering the dose slowly, over 1 minute. Induction time may therefore be prolonged
Ketamine A dissociative agent
Produces anaesthesia and profound analgesia
Combined with alpha-2 agonists, opioid analgesics and benzodiazepines to produce anaesthesia in cats, dogs, rabbits, small rodents, birds and reptiles		 Cannot be used alone as it causes muscle rigidity and central nervous system stimulation
Hallucinations and emergence phenomena are often seen during recovery in the cat


Table 6.5


Methods of induction
























Method Advantages Disadvantages
Mask induction
A tightly fitting clear plastic mask with a diaphragm is placed over the animal’s face. 100% oxygen is administered for several minutes; this maximizes tissue perfusion and allows the patient to adjust to the mask. The anaesthetic concentration is then gradually introduced		 Used to administer oxygen and inhalant agents when endotracheal intubation is not possible
Useful for small mammals and birds		 May be very distressing for the patient
The airway is not protected and obstruction can occur
Masks increase mechanical dead space
Atmospheric pollution is a significant hazard
Chamber induction
The conscious patient is placed inside an anaesthetic induction chamber, which should be large enough for the animal to lie with its neck extended. Oxygen and the inhalant agent are delivered via a gas inlet. Waste gases are scavenged via an appropriate outlet. The patient is removed when it loses the ability to stand		 Induction chambers are useful for small mammals and uncooperative patients Only suitable for small patients
Risk of vomiting
Cardiopulmonary function cannot be monitored
Atmospheric pollution occurs when the chamber is opened
Intravenous
The induction agent is injected into a peripheral vein, over the recommended period of time, depending on the agent being used		 Smooth and usually rapid induction
Most induction agents can be given to effect so minimal quantities are used		 Patients must be restrained well
Perivascular injection can be avoided by placing an intravenous catheter before induction
Intramuscular
This method can only be used for certain drugs such as those used in combination mixtures		 Technically easier than intravenous injection
Useful in fractious patients when intravenous access is not possible		 Dosage is according to weight; it is therefore not easy or possible to dose to effect
Slower onset of anaesthesia


Maintenance

During this period the state of unconsciousness is usually maintained with the use of an inhalant agent, delivered by means of an endotracheal tube connected to an anaesthetic breathing system and machine. In some cases an anaesthetic mask may be necessary, for example, if intubation is not possible. Total intravenous anaesthesia (TIVA) is a method of maintaining anaesthesia with the constant infusion of a suitable intravenous agent or by giving incremental doses of the agent. It is preferable that such patients undergoing TIVA are intubated to ensure the airway is maintained and protected. This also allows the administration of oxygen. The common inhalant agents are described in Table 6.6.




ENDOTRACHEAL TUBES


After induction, an endotracheal tube should be placed in the patient’s airway (tracheal intubation). This tube conducts anaesthetic gases and oxygen from the anaesthetic machine to the lungs, bypassing the nasal passages and pharynx. Tubes come in a variety of sizes, may be made of rubber or silicone and may be cuffed or uncuffed.




Procedure: Intubation of a patient

The patient must be sufficiently anaesthetized in order to carry out intubation. This is indicated by the following signs:




Equipment required

Selection of suitably sized endotracheal tubes, lubricant to facilitate intubation, laryngoscope, local anaesthetic spray to prevent laryngeal spasm in cats, syringe or cuff inflator, stylet if necessary, suitable tie to secure the tube in place.



1. Action: Select several endotracheal tubes of varying sizes and measure the required length against the patient’s head and neck.


    Rationale: This will enable selection of the most suitable tube for the patient. The largest diameter tube possible should be used, as long as tracheal damage does not occur. This avoids resistance to ventilation, which occurs when tubes which are too small are used; it also decreases mechanical dead space.


2. Action: Inflate the cuff of cuffed tubes and leave for several minutes; inspect for excessive wear and check the patency of the tube.


    Rationale: Rubber tubing can perish over time, causing malfunction of the cuff. If the cuff does not remain inflated, anaesthetic gases may leak around it during anaesthesia and fluid and debris may be inhaled. Patency is essential for adequate patient ventilation and for the delivery of oxygen and anaesthetic gases.


3. Action: Lubricate the tube with water-soluble lubricant.


    Rationale: This allows smooth, atraumatic introduction of the tube; it also enhances the seal produced by the cuff.


4. Action: The patient is restrained in sternal recumbency. An assistant extends the neck and holds the head so the nose is pointing upwards (Fig. 6.3).


image
Figure 6.3 Position of an animal for intubation. Reproduced with permission from Anderson Moores Veterinary Specialists.

    Rationale: Intubation in sternal recumbency reduces the risk of regurgitation and aspiration. Intubation in lateral or dorsal recumbency is also possible.


5. Action: The upper jaw is held still whilst the tongue is pulled out and down so that it lies between the lower canines. Pull the lower jaw downwards by pulling the tongue down until the epiglottis can be clearly seen.


    Rationale: In this position, visibility of the anatomy of the pharynx is maximized (Figs 6.4 and 6.5).


image
Figure 6.4 Anatomy of the pharynx. Inset: When the epiglottis is depressed, the glottis is exposed. The endotracheal tube is advanced through the glottis. Reproduced with permission from McKelvey and Hollingshead (2003), Mosby.

Only gold members can continue reading. Log In or Register to continue

Related posts:

  1. Administration of fluid therapy
  2. Measuring clinical parameters
  3. Surgical nursing procedures
  4. Diagnostic imaging

Stay updated, free articles. Join our Telegram channel
Tags:
Jan 8, 2017 | Posted by in NURSING & ANIMAL CARE | Comments Off on Anaesthetic procedures

Full access? Get Clinical Tree
Get Clinical Tree app for offline access