ANSWERS

1. Define anaesthesia.
   Anaesthesia is defined as the loss of sensation in a part or the whole of the body by controlled, reversible suppression of the central nervous system (CNS). Sensations affected by anaesthesia include appreciation of touch, pressure, temperature and pain. Temperature is of particular consequence since an animal that is unable to regulate its body heat is susceptible to hypothermia; this is why it is very important to keep a patient warm during anaesthesia.
2. Describe how an animal is able to appreciate pain and how an anaesthetic drug works in order to block this sensation.
   Peripheral nerve endings that are sensitive to pain within the sense organs are stimulated causing the sensation of pain. The sensory or afferent nerves deliver the sensation to the spinal cord, and from here it travels to the cerebro-cortical projection areas of the brain. An anaesthetic drug blocks this sensory pathway at any point from the peripheral sense organ affected to the projection areas of the brain thus eliminating the sensation. Remember that sensitivity to touch, pressure and temperature are also affected.
3. In addition to the depression of sensation, anaesthetics also depress the function of subcortical areas of the brain that receive information regarding unconscious stimuli. List 3 important unconscious stimuli and state the risks relating to them that may result during deep anaesthesia.
   • Blood pressure - Hypotension is a risk.
   • Blood temperature - Hyothermia is a risk.
   • Blood gas levels (oxygen and carbon dioxide) - Hypoventilation is a risk.
4. List the 3 main needs for anaesthesia.
   • Upon humane grounds; to prevent the awareness of pain in an animal undergoing a surgical procedure.
   • To facilitate examination of a patient; enabling ease of handling and the provision of skeletal and respiratory muscle relaxation where necessary.
   To permit surgery in compliance with the Protection of Animals Act 1964 The aims of anaesthesia may be achieved by the administration of a single agent, however, it is generally safer to use a combination of agents each with a specific action within the body of the patient.
   
   
5. List 2 additional uses of anaesthesia other than those relating to the permission of surgery and subsequent immobilisation and pain relief.
   • To aid the control of seizures such as status epilepticus.
   • To perform euthanasia (commercial solutions for humane destruction are concentrated anaesthetic drugs).
6. What are the 2 main ways in which local anaesthetics work?
   • The blocking of sensation in the peripheral nerves following a conduction block.
   • The blocking of sensation in the spinal cord following an extradural injection.
7. Drugs affecting the central nervous system are either stimulants or depressants; list and define the types of depressants available.
   • General anaesthetics - agents which eliminate sensation by causing unconsciousness.
   • Sedatives - agents which calm the patient and cause drowsiness.
   • Ataractics/Tranquillisers - drugs which allay anxiety and relieve tension without causing drowsiness.
   • Hypnotics/Soporifics - sleep inducing agents from which it is possible to arouse the patient.
   • Narcotics - these produce a drug induced stupor characterised by insensibility and paralysis.
     Narcotics may also have stimulant effects on the CNS.
   • Neurolepts - agents which relieve emotional distress and produce a state of apathy and mental detachment.
   • Neuroleptanalgesics - a combination of agents (a neurolept and an opioid analgesic) which produce deep sedation and analgesia without total loss of consciousness (animals are often hypersensitive to sound).
8. Most depressants fall into more than one of the categories listed above; what is the main reason for this?
   Most agents are dose dependent which means that a low dose is likely to result in quite a different outcome to a high dose.
   ACP is both a sedative and tranquilliser.
9. For what purpose might stimulant drugs be employed in veterinary practice?
   Stimulants increase nervous activity and are used to oppose the effects of anaesthetics.
   Stimulants are categorised according to site of action; medullary stimulants are most common and are known as analeptics. Doxapram is an example of a stimulant licensed for veterinary use.
10. An understanding of drug interactions is important in fully comprehending the use of premedicants and anaesthetic agents; define additive, synergy and antagonism.
    • Additive - 2 drugs which when taken together have an equal effect to that resulting had they been taken individually (inhalation agents act in an additive way).
    • Synergy - 2 drugs which when taken together have a greater effect than if taken separately (neurolepts and opioids act in a synergistic way).
    • Antagonism - the effect of the inhibition of one drug by another.
11. What is the blood-brain barrier, and in what way is it important to the subject of anaesthesia?
    The blood-brain barrier (BBB) is a membranous barrier separating the blood from the brain; it is formed by tight junctions between the endothelial cells of the capillaries and the envelopment of the brain capillaries by the glial cells. The BBB is permeable to water, oxygen, carbon dioxide, glucose, alcohol, some drugs and general anaesthetics. In order for anaesthetic agents to cross the BBB quickly, they must be small, non-ionised, lipid soluble and unbound from albumin.
12. Define analgesia.
    Analgesia is defined as insensibility to pain; an analgesic drug reduces the perception of pain without causing loss of consciousness (analgesics interrupt the ascending pain pathway at various levels and suppress the sensation of pain).
13. The animal's brain is able to respond to pain in 4 main ways; briefly describe each.
    • Spinal response - withdrawal reflex.
    • Medullary response - increased heart rate, blood pressure and respiratory rate.
    • Hypothalamic response - catecholamine release from the adrenal medulla and nerve endings of the sympathetic nervous system causes increased heart rate and blood pressure and piloerection; secretion of releasing factors cause the pituitary gland to release stress hormones.
    • Cortical response - voluntary acts such as vocalising and escaping from or biting the stimulus.
14. List the 6 drug groups that possess the property of analgesia.
    • Benzodiazepines.
    • General anaesthetics.
    • Glucocorticoids.
    • Local anaesthetics.
    • Non-steroidal anti-inflammatory drugs (NSAIDS).
    • Opioids.
15. In what way do glucocorticoids and non-steroidal anti-inflammatory drugs (NSAIDS) suppress the sensation of pain?
    Glucocorticoids and NSAIDS lower nerve ending sensitivity to auticoids (pain sensitising chemicals) from damaged tissue.
16. In what way do benzodiazepines suppress the sensation of pain?
    Benzodiazepines are antagonists with analgesic effects at spinal level.
17. Define surgical anaesthesia.
    Surgical anaesthesia is a state of insensibility enabling the performance of surgery; a combination of unconsciousness, analgesia and muscle relaxation are required.
18. Define balanced anaesthesia.
    Balanced anaesthesia is defined as the use of several drugs in order to achieve unconsciousness, analgesia and muscle relaxation. The use of analgesics and muscle relaxants in addition to general anaesthetic agents means that lower anaesthetic doses may be used, thus preserving vital centre activity.
19. Why is an understanding of the respiratory system during anaesthesia important?
    • Anaesthetics depress ventilation which may contribute to or cause cardiac arrest.
    • Volatile anaesthetics are taken up and eliminated by the respiratory system.
      It is essential that the respiratory system is able to provide enough fresh gas to oxygenate the blood and remove carbon dioxide.
20. Why might flexing the joints of an anaesthetised patient lead to increased depth of respiration or invoke inspiration in an apnoeic animal?
    Increased breathing during exercise is due to the influence of sensors in joints and muscles in addition to increased carbon dioxide production.
21. Carbon dioxide tension in the blood has a powerful effect on respiratory rate which is mediated by a reduction of pH; how is the reduced pH detected?
    • Directly by the respiratory centres of the brain (medullary, apneustic and pneumotaxic centres); these are sensitive to the pH of the surrounding cerebro-spinal fluid - carbon dioxide diffuses from the blood to the CSF thus lowering its pH.
    • Indirectly by the peripheral chemoreceptors in the carotid and aortic bodies located in the sinuses of the carotid artery and aorta; these respond to falling oxygen tensions in the blood.
22. Reduced blood oxygen concentration (hypoxia) may lead to respiratory arrest in severe cases; what part of the anatomy monitors the oxygen tension of the blood?
    The peripheral chemoreceptors in the carotid and aortic bodies.
23. Explain the Hering-Bruer reflex.
    At peak inspiration, impulses from the receptors within the pleurae and alveolar walls travel to the respiratory centres in the vagus nerve causing inspiration to halt. This prevents over-inflation of the lung parenchyma and subsequent trauma.
24. Summarise the normal ventilatory cycle.
    The normal ventilatory cycle is a continuous unconscious process produced by the rhythmical actions of the respiratory centres. The cycle is adjusted by the influence of feedback from various body areas in order to meet the body's needs.
25. What is the normal respiratory rate of a cat and dog?
    • Cat - 20 to 30 breaths per minute.
    • Dog - 10 to 30 breaths per minute (small breeds generally have a higher RR than larger breeds).
26. Define tidal volume and minute volume, and explain their importance in the subject of anaesthesia.
    • Tidal volume is the volume of gas inspired per breath.
    • Minute volume is the volume of gas inspired per minute; ie. tidal volume multiplied by respiratory rate.
      In the calculation of flow rates for gaseous anaesthesia it is essential provide enough gas to enable the oxygenation of the blood. Tidal volume may be estimated at 10-15mls/kg (an accurate weight of the patient must be known), and minute volume is calculated by multiplying the tidal volume by the patient's respiratory rate. The minute volume is multiplied by the circuit factor in order to establish the flow rate in l/min. This subject is covered fully later.
27. What is dead space?
    Inspired gas that does not reach the alveoli and therefore is not participant in gaseous exchange. Excessive dead space causes the reduction of alveolar ventilation.
28. What is anatomic dead space volume?
    The portion of inspired gas left at the end of inspiration residing at the level of conducting bronchioles which does not participate in gaseous exchange.
29. What is mechanical (or apparatus) dead space volume?
    Excessive space within anaesthetic equipment at the proximal airway resulting in decreased efficiency of ventilation; an overly long endotracheal tube is an example.
30. What is alveolar dead space?
    Inspired gas that reaches the alveoli but is not perfused with blood and therefore not participant in gaseous exchange.
    Anatomic dead space plus alveolar dead space together make up physiologic dead space.
31. What is the normal measurement of carbon dioxide in arterial blood?
    5.33 kPa or 40 mm Hg. The elimination of carbon dioxide is directly proportional to the alveolar volume (volume of inspired gas reaching the alveoli), therefore a normal measurement of blood carbon dioxide is an indicator of adequate ventilation.
32. Name 2 conditions that may cause hypercapnia/hypercarbia (a build up of carbon dioxide in the blood).
    • Pyrexia.
    • Malignant hyperthermia.
33. What is hypoventilation?
    Inadequate ventilation resultant from shallow respiration; this may cause hypercapnia/hypercarbia (a build up of carbon dioxide in the blood) and respiratory acidosis. Causes of hypoventilation are:
    • Anaesthetic overdose resulting in an insufficient respiratory rate.
    • Compression of the chest wall resulting in reduced tidal volume.
    • Mechanical dead space resulting in reduced alveolar perfusion.
34. What is hyperventilation?
    Over-ventilation causing hypocapnia/hypocarbia (a deficiency of carbon dioxide in the blood) and respiratory alkalosis. Causes of hyperventilation are:
    • Light anaesthesia.
    • Pain.
    • Excessive manual or mechanical ventilation.
35. Define respiratory acidosis.
    Pulmonary retention of carbon dioxide as a result of impaired ventilation or respiratory arrest.
36. Define respiratory alkalosis.
    Excessive loss of carbon dioxide as a result of hyperventilation.
37. Hypoxia describes an abnormally low oxygen tension in arterial blood while tissue hypoxia describes active tissues deprived of oxygen as a result of the inadequate oxygenation of haemoglobin; list the possible causes of reduced oxygen delivery to the body tissues:
    • Reduced oxygen in inspired gas - perhaps resultant of an empty oxygen cylinder during gaseous anaesthesia.
    • Reduced alveolar ventilation - may be caused by excessive dead space, reduced respiratory rate or diminished tidal volume.
    • Structural or functional changes of the lungs - neoplasia or pneumonia may severely impair lung function.
    • Insufficient haemoglobin available to carry oxygenated blood to the body tissues - anaemic animals are at risk.
    • Increased tissue demand - strenuous exercise may cause hypoxia of the myocardium (heart muscle tissue).
38. Summarise the components of the cardiovascular system and the purpose of cardiovascular activity.
    The cardiovascular system comprises:
    • Heart.
    • Blood vessels.
    • Blood.
    • The elements of the autonomic nervous system that control the heart's activity.
      The purpose of the cardiovascular system is to transport sufficient volumes of blood containing metabolic reagents through the tissue capillary beds; this is known as perfusion.
39. What are baroreceptors and where are they located?
    Receptors that constantly monitor blood pressure by response to stretch. They are located within the walls of the aortic arch, carotid artery sinus and the large arteries of the neck and thorax.
    When blood pressure rises, the artery walls are stretched; the baroreceptors respond resulting in relaxation of the smooth muscle of the blood vessel walls and a subsequent fall in blood pressure to normal levels. When blood pressure falls, the artery walls are less stretched; this results in vasoconstriction causing blood pressure to increase to normal levels.
40. Anaesthetics depress many facets of cardiovascular function; careful monitoring is therefore essential during anaesthesia. State the normal heart rates for a cat and dog.
    • Cat - 110 to 180 bpm.
    • Dog - 60 to 180 bpm.
41. Explain why cardiac output is reduced if the heart rate is very high.
    If the heart rate is rapid, there is less time available for the ventricles to fill with blood and therefore a lower stroke volume (the volume of blood ejected per beat); this means that the volume of blood ejected by the heart per minute ie. cardiac output, is lowered.
42. Define the following terms and explain their significance in the subject of anaesthesia: i) Hypotension. ii) Hypertension. iii) Tachycardia. iv) Bradycardia. v) Oxygen Flux.
    
    • Hypotension = consistently low blood pressure readings as a result of decreased cardiac output and systemic vascular resistance; most anaesthetics depress heart rate and contractility and volatile agents cause vasodilation.
    • Hypertension = consistently high blood pressure readings as a result of increased cardiac output and systemic vascular resistance; inadequate anaesthesia causes adrenaline release, and pain on recovery may also contribute.
    • Tachycardia = increased heart rate; moderate increases in response to hypotension, hypercapnia and hypoglycaemia are desirable since blood pressure is preserved, however an increase in response to pain may cause the heart to become hypoxic.
    • Bradycardia = decreased heart rate; often desirable since ventricular filling and stroke volume are increased while cardiac work is lowered, however, very slow rates may cause hypotension.
    • Oxygen flux = the volume of oxygen reaching peripheral tissue per minute I.E cardiac output multiplied by the oxygen content of the blood; anaesthetics reduce oxygen flux and the subsequent lowered percentage saturation of oxygen by haemoglobin may cause lung disease, while direct reduction of haemoglobin may cause severe anaemia.