The pre-operative anaesthetic evaluation is an extremely important part of the work of the anaesthetist and should be carried out with the diligence and concentration that are more often reserved to other steps of anaesthesia. The number of procedures in medium-to-large veterinary clinics is such that anaesthetists often work under pressure and are forced to dedicate less time to the assessment of each case. Misunderstandings or a superficial approach during this phase may lead to mistakes in planning the anaesthesia, which can have serious consequences.
Each animal to be anaesthetised or sedated should undergo an anaesthesiological evaluation by an anaesthetist. During this evaluation the anaesthetist must collect data on the patient in order to resolve two fundamental issues: the first is ascertaining that the animal is in the best possible condition of health for the procedure; the second is assessing the risks/benefits resulting from delaying the procedure in order to carry out further tests or to start therapies to improve the animal’s condition.
The results of these assessments may lead to scenarios which appear paradoxical but which are perfectly logical, such as deciding to postpone surgery in a young dog with a mild respiratory infection and instead going ahead with surgery in another with chronic heart failure and possibly even respiratory failure.
It is only by having a thorough understanding of the case in its entirety that appropriate solutions to these issues can be reached. This implies not only an accurate pre-anaesthesia assessment, with all the necessary additional tests, but also a detailed understanding of the type of surgery in question. This means that complex operations or cases requiring multiple procedures within the same anaesthesiological session must be planned well in advance and the exchange of information between the surgeon and anaesthetist must clearly be as complete as possible and, ideally, should take place before the anaesthesiological assessment.
The need for further tests and therapies should always be considered as a possible option. In view of this, to avoid causing stress, disappointment or anxiety in the owner, the date of the surgical procedure should be decided only after receiving approval from the anaesthetist.
The “operating list” should be organized giving paramount consideration to the patients’ safety. Cases which could require greater post-operative care should come first in the daily list and be operated on at the beginning of the week rather than just before the weekend. Based on the foregoing, it is once again clear that collaboration and agreement between the anaesthetist and surgeon are essential in order to achieve the highest possible degree of safety.
The pre-anaesthesia assessment not only has major implications for the case being examined but also, more in general, represents an extraordinary opportunity for the anaesthetist to become “visible” to owners. Without this occasion the anaesthetist risks being “inexistent”. Therefore, while it is important to gain the trust of the owner so as to acquire as much information as possible on the case, it is just as important to make the owners understand that the anaesthetist is not simply someone who administers anaesthetic agents, but rather is the person who controls and guarantees that each step of the procedure is carried out with the highest possible safety standards for the patient. This process of accreditation is clearly crucial for the “category” of anaesthetists, is beneficial for the veterinary clinics that employ an anaesthetist and is stimulating for all the others.
THE CLINICAL EXAMINATION
The pre-operative anaesthesiological examination does not differ much from a normal physical examination. What does differ is how the results of the examination are viewed and considered. Just like in a normal assessment the anaesthetist looks for any indications of a possible disease, which may perhaps be investigated further. It is more complex, and at times arbitrary, to determine the impact that any disease features found to be present may have on the anaesthesiological management of the patient.
Signalment
In veterinary anaesthesiology signalment is of extraordinary importance, in particular with regards to the species, breed and age which have major impacts on the anaesthesiological management of the patient.
Species
Among the few certainties that have emerged from studies on mortality during veterinary anaesthesia of companion animals (dogs, cats and exotic animals) is the demonstration that the anaesthesiological risk is species-dependent. In the most recent study on peri-operative mortality, carried out by Brodbelt et al. in the United Kingdom in 2008, the mortality risk for dogs undergoing anaesthesia was 0.17%, while that for rabbits was 1.39%. The mortality risk for cats undergoing anaesthesia was 0.24% while the risk for guinea pigs was 3.80%. Hamsters and chinchilla did not fare much better than guinea pigs which remain the most difficult mammals to anaesthetise. Budgerigars and birds in general seem to die of anaesthesiological causes just as frequently as small mammals. Since the mortality risk of dogs in American Society of Anesthesiologists (ASA) class III (and higher) was found to be 1.33%, it should be appreciated that anaesthetising a small mammal requires the same attention and skills as anaesthetising dogs with severe heart disease.
Breed
Breed is another extremely important factor, especially in view of the fact that the species with which we deal most, the dog, is characterized by an incredible variety of breeds, with extreme variations in terms of size, character, morphology and, above all, incidences of diseases. This means that the same, apparently asymptomatic heart murmur should “ring different bells” depending on whether it is found in a small, active mongrel or in a stately Great Dane. Difficulties with breathing, establishing a vascular access or giving an epidural injection are all additional problems which the anaesthetist may encounter when dealing with specific breeds of dogs.
Age
Extremely young and elderly patients may reserve unpleasant surprises for the anaesthetist. The reasons underlying this statement are mainly physiological, in the sense that both these categories are more sensitive to the negative effects of anaesthetics. Additionally, the physical examination is often difficult in extremely young patients and some important findings may be missed, while elderly patients are often affected by multiple diseases which the anaesthesia may worsen or decompensate with a domino-like effect on the entire organism.
History
The clinical history taking must accurate, uncovering all the events in the life of the animal, with a specific focus on potential signs of a disorder, if present. A specific goal of the clinical history it to assess, within the limits of what is possible, the patient’s capacity of dealing with stressful events, such as with anaesthesia. This is well summarised by the concept of physiological reserve, meaning the physiological capacity that is potentially always present but that is exploited during physical effort or a disease in order to maintain homeostasis of the biological system.
In other words, the combination of surgery and anaesthesia induces rapid and dramatic changes not only in the cardio-respiratory system but also in hormones and organ function which may lead to decompensation in those animals not capable of reacting to and compensating for the changes underway. The events that may take place during anaesthesia are not always predictable from studies carried out in resting conditions, such as echocardiography. Furthermore, the final result is not dependent on the activity of a single organ but rather on the interaction between many organs and apparatuses working together.
The anaesthetist should determine the degree of physical activity the animal is accustomed to and whether there have ever been any reports of episodes of collapse or sudden disorientation, particularly on hot days, during such activity. Any decrease in the level of physical activity, not due to joint or muscle problems, may be a warning sign of reduced cardiorespiratory reserves. The assessment of the cardiorespiratory reserve of certain species, such as the cat, is much more difficult. This means that the anaesthetist should treat such species with particular attention and caution.
Sensorial state and temperament
The sensorial state and temperament have major impacts on the result of anaesthesia. An animal which is depressed, for whatever reason, will be more sensitive to the action of anaesthetics.
In addition, the temperament of an animal often influences the type of reaction to sedatives and tranquillisers. These differences must be taken into consideration because otherwise, as an example, induction and recovery of conciousness could be problematic.
When dealing with an aggressive animal it is imperative to guarantee one’s own safety as well as the safety of all those working on the case. This said, the goal of the first step of anaesthesia should be to sedate the patient in order to catheterise a vein in safe conditions without, however, depressing the cardiovascular and respiratory systems to the extent of compromising the rest of the anaesthesia.
Obesity
Obesity increases anaesthesiological risk in humans and it is probable that the same holds true for veterinary patients. Obese patients often have problems in maintaining adequate ventilation and obesity causes additional problems when using loco-regional anaesthetic techniques and often makes the work of surgeons more difficult, with operations taking a longer time.
Examination of the cardiovascular and respiratory systems
This is the most important part of the examination. The assessment of the mucosae, capillary filling time and pulse as well as careful auscultation of the chest cannot be done in less than 5 minutes. Any doubt or perplexity regarding the presence or not of an underlying disease should be resolved by performing additional tests. It must be emphasized that the presence of a heart murmur is not, in itself, sufficient to define a patient as cardiopathic.
Unfortunately auscultation may be extremely difficult in very agitated, aggressive animals or in purring cats. Consequently, if the presence of heart or respiratory disease is suspected for any reason, additional tests must be carried out.
Emergencies
The physical examination of a severely decompensated patient is based on the Advanced Cardiac Life Support (ACLS) guidelines. The A-B-C sequence must be followed. A is for airways, that is, ensuring that the upper airways are patent so that the patient can breathe, otherwise endotracheal intubation must be rapidly carried out. B is for breathing, that is, ensuring that the patient is capable of breathing without support, otherwise ventilation with 100% oxygen should be started (something which should actually be done in every case of decompensation). C is for the cardiovascular system, that is, ascertaining the functionality of this system and starting cardiac massage if the patient has definitely had a cardiac arrest (in such a case the massage should be started as early as possible). Another extremely important procedure in these patients is the placement of a venous catheter.
In general, whenever a patient in an emergency condition must be anaesthetised the assessment of vital parameters and the beginning of stabilisation procedures must actually take place at almost the same time. It is, therefore, extremely important to treat these patients in fully equipped practices, ideally with enough staff so that each procedure can be achieved in the minimum time possible. On the other hand there are conditions in which stabilisation is not possible unless surgery is performed (internal haemorrhages, severe dyspnoea due to a diaphragmatic hernia, etc.) and consequently in these cases the anaesthesiologist is “simply” asked to find a compromise.
In emergencies in patients not requiring extreme resuscitation procedures it is important to remember: (i) the absolute necessity of supplementing inspired air with oxygen; (ii) the need to establish a venous access as soon as possible; and (iii) that the stress resulting from containment may cause much more damage than well-managed sedation.
The management of patients with multiple trauma is a complex topic which cannot be dealt with here. It may, however, be useful to recall that in these patients the number of clinical, systemic and organ signs is such that the pre-operative approach differs from that necessary in other patients. Rather than carrying out one examination suggested by a clinical sign, which may have multiple causes, a series of tests should be performed with the purpose of determining which areas of the body, such as the chest, abdomen, spine and pelvis, have or have not been involved by the trauma. This is also done in accordance with a general principle in trauma care: each patient that has suffered trauma should be considered as a having multiple trauma until proven otherwise.
THE ASA CLASSIFICATION AND ANAESTHETIC RISK
The ASA classification, which appears in all textbooks of anaesthesiology (to which the reader is referred for consultation), was developed in 1963, when the American Society of Anesthesiologists understood the need to classify patients undergoing anaesthesia based on their clinical condition. This enabled comparison of the results of different trials, setting the basis for greater homogeneity of the populations involved in the various studies. This classification is now often erroneously considered as an evaluation index for anaesthetic risk. Nevertheless, epidemiological studies in both human and veterinary medicine have shown a close correlation between anaesthesiological risk and clinical condition.
The Confidential Enquiry into Perioperative Small Animal Fatality (CEPSAF) study (Brodbelt et al., 2008) showed that while the mortality risk of ASA I-II dogs is 0.05% (0.11% in ASA I-II cats), the risk in ASA III (or greater) dogs is 1.33% (1.40% in cats).
Studies on mortality during anaesthesia are extremely important because they can give useful information on the anaesthesiological risk in veterinary patients; however, the results must be interpreted with caution due to the innumerable, inherent limitations of such studies. The veterinary practices which collaborate in such studies participate on a voluntary basis and are often motivated by a special interest in anaesthesia, meaning that the anaesthesiological standards of these centres is above average. As an example Brodbelt himself recently stated that all the centres included in his study had a referent veterinarian for anaesthesia, something which in reality is not so common even in the United Kingdom. It is, therefore, questionable that these studies credibly represent the average reality. The factors which have a marked impact on the risk of anaesthesia are the training and availability of an anaesthetist, the degree of monitoring as well as the experience of the surgeon and of the expert in internal medicine who has followed the case. Within the variegated world of veterinary medicine, these factors can vary greatly. The anaesthetist is once again the person who is in the best possible position to assess all of these factors and to decide on the anaesthesiological risk for each patient in the specific veterinary clinic in which he or she must provide anaesthesia.
Suggested readings
- Green D, Ervine M, White S (2003) Principles of preoperative assessement, optimisation and management. In Fundamentals of perioperative management. GMM San Francisco USA pp 39-54
- BrodbeltDC, Blissitt KJ, Hammond RA et al (2008) The risk of death: the confidential enquiry into perioperative small animal fatalities. Vet Anaesth Analg.35, 365-73.