Patient health assessment

The preoperative assessment of an animal’s health status is valuable to acknowledge preanesthetic risks, to identify management priorities, and to advise clients appropriately prior to anesthesia and surgery. Health status has been consistently reported to be associated with anesthetic death in humans and in the spectrum of species commonly seen in veterinary anesthesia. Increased American Society of Anesthesiologists (ASA) physical status grade [1,2] (see Table 2.1) has been associated with an increased risk of death in a number of anesthetic studies in small animals [3–14], horses [15–18], and humans [19–39].

Anesthetic agents cause cardiopulmonary depression, and the presence of concurrent pathology involving the major body systems is likely to predispose to greater anesthetic‐induced physiologic disturbance [40]. Pre‐existing cardiopulmonary pathology is particularly relevant in the immediate preoperative period, as anesthetic‐related mortality is likely to involve respiratory or cardiovascular compromise, and most anesthetics depress one or both systems at clinical levels of anesthesia [40].

Hematologic and biochemical abnormalities may also be a significant consideration. In particular, anemia will reduce oxygen‐carrying capacity and predispose to hypoxia, and hypoproteinemia has been theorized to increase the response of the patient to highly protein‐bound drugs and result in relative overdose [40]. Renal disease is also important, particularly if dehydration or uremia is present, as under these conditions, the renal system will have a lower tolerance to anesthesia and the patient may be more sensitive to some anesthetics and perioperative drugs such as non‐steroidal anti‐inflammatory agents. Neurologic disease may be relevant with respect to the occurrence of postoperative seizures, increased sensitivity to anesthetics, and when cardiopulmonary function is affected, e.g., medullary pathology can depress ventilation and cardiovascular function. Additionally, liver and endocrine disease may influence the response to anesthesia, with diabetes mellitus and potential intraoperative cellular changes in glucose concentrations being particularly relevant [41].

Hence, some form of physical health status assessment is an important preanesthetic consideration. Most frequently, ASA grade [1,2] has been utilized for this purpose, and there is some evidence that this can identify patients at increased risk of mortality until 24–72 h postanesthesia [42]. However, the repeatability and agreement between observers of such scoring systems have been questioned, and evidence suggests that interobserver agreement in ASA health status classification is poor in veterinary anesthesia [43]. Other assessment systems exist in human medicine, including the Acute Physiology and Chronic Health Evaluation (APACHE), the Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM), and, in pediatric practice, the Neurological, Airway, Respiratory, Cardiovascular and Other (NARCO) score, and all were observed to predict perioperative risk [44–46]. However, these systems are complex, require more time to complete, and have yet to be evaluated for agreement between observers in a veterinary context. Hence, at present, there appears to be little consensus as to the optimal method of patient health status assessment for consistent and efficient classification across observers, and caution should be exercised in overinterpreting individual health status assessments. Nonetheless, there is a body of evidence that highlights that sicker patients are more likely to die perioperatively, and therefore, some form of preoperative patient assessment would be advisable to distinguish sick from healthy patients, to identify those at greater risk, and to manage patients appropriately in order to try to minimize risk prior to, during, and after anesthesia.

Preanesthetic blood testing

Given the fact that organ dysfunction and various pathologic conditions such as anemia or hypoproteinemia may contribute to increased anesthetic morbidity or mortality, it would seem sensible to make every effort to detect these prior to general anesthesia. For this reason, routine preanesthetic blood screening is commonly recommended by many veterinary practitioners and, indeed, some anesthesia specialists. However, although there is no doubt that prior biochemical and hematologic analyses are of definite value in certain patient groups, the question remains as to whether their use can be justified for every patient, in particular healthy animals undergoing elective procedures.

An internet search for “Preanesthetic blood screening in animals” (www.google.com, accessed July 2022) returned over 18 million hits, of which a substantial proportion appeared to be veterinary practices each detailing their reasons and prices for carrying out such a procedure; interestingly, the search term returned virtually no scientific papers relating to the practice. In addition, as with much information to be found on the internet, many of the relevant web pages providing advice on the subject were written by people with no apparent scientific background or credentials for discussing such a topic, with the majority of these being pet owner discussion forums. Although there may be no genuine scientific or clinical background behind these types of discussion groups, they almost certainly help perpetuate the need for ubiquitous preanesthetic blood testing, but given that many veterinary professionals also recommend its routine use, it obviously cannot all be dependent on owner perceptions. So, is there actually a sound rationale upon which the need for preanesthetic biochemical and hematologic sampling is based?

There are numerous studies in human anesthesia now questioning the necessity for preanesthetic laboratory testing in healthy patients [47–49], with each of these demonstrating that, for subjects with no demonstrable abnormalities on the basis of history and clinical examination, there appears to be no reduction in perianesthetic complications if prior blood sampling has been carried out. The United Kingdom (UK) National Institute for Health and Care Excellence (NICE) gathers evidence from a variety of sources and then produces recommendations for human clinicians for various medical and surgical interventions. In terms of preanesthetic blood testing, NICE subdivides its recommendations based on both the age of the patient and the grade of surgery the subject is undergoing (minor, intermediate, or major/complex). Based on this system, NICE does not recommend preanesthetic biochemical or hematological screening for ASA 1 or 2 human patients undergoing minor or intermediate grades of surgery, although assessment of renal function should be considered in ASA 2 subjects having intermediate surgery if they have some predisposition to development of possible acute kidney injury; in other words, unless the patient is sick, preanesthetic blood tests would only be recommended for humans undergoing major/complex surgery. Unsurprisingly, NICE does suggest considering preanesthetic blood screening for ASA 3–5 patients undergoing intermediate or major procedures [50].

As a result of the NICE recommendations, the guidelines of the Association of Anaesthetists of Great Britain and Ireland (AAGBI) [51] for human anesthesia conclude: “Routine preoperative investigations are expensive, labor intensive, and of questionable value, especially as they may contribute to morbidity or cause additional delays due to spurious results.”

Aside from the issue of erroneous results impacting on the efficiency of case throughput, it is also important to remember that the reference ranges established for most laboratory tests incorporate only approximately 95% of the population, i.e., around one in 20 animals that are perfectly healthy will return laboratory results that are outside a “normal” range, which may then lead to further unnecessary investigations being carried out, in addition to delaying the planned procedure; the more tests that are run, the greater the likelihood of this occurring. Hence, it is important to carefully interpret test results obtained and to view them as part of the overall assessment of the patient.

The AAGBI also takes the view that history and examination performed by appropriately trained and competent personnel remain the most efficient and accurate way of initially detecting significant morbidity: “Thus, it is important that, where preanesthetic blood screening is carried out, it is seen as an adjunct to a full clinical examination, rather than an alternative.” While this is undoubtedly the case in both veterinary and human anesthesia, the results from human studies relating to preanesthetic blood screening of healthy patients may not be directly applicable to animals. This is because the majority of humans are both cognitive and verbal and are able to self‐report health issues. Veterinary clinicians, on the other hand, obtain the relevant health information by proxy (from the owner), which may mean that important details are not identified. Thus, it is possible that a higher incidence of abnormalities may be detected on preanesthetic screening of animals than has been reported for humans.

Given that the consensus opinion from human anesthesia seems to be that preanesthetic blood sampling appears to be justifiable only in sicker patients, and that healthy individuals undergoing elective procedures do not benefit from this practice, what recommendations should be put in place for veterinary anesthesia? There appear to be only a small number of studies relating to the validity of routine preanesthetic blood screening in animals [52]. Toews and Campbell [53] performed a complete blood count in 102 horses undergoing cryptorchidectomy and then determined whether any abnormalities detected impacted on the risk of surgical complications. They found that 55 animals had results outside the reference range for at least one hematologic parameter, but there was no correlation between those demonstrating abnormal values and the likelihood of either intra‐ or postoperative surgical complications, nor did these abnormalities dictate alterations in patient management. Alef et al. [54] analyzed results from over 1500 dogs undergoing anesthesia at the University of Leipzig and reported that if no potential issues were identified in either the animal’s history or clinical examination, “the changes revealed by preoperative screening were usually of little clinical relevance and did not prompt major changes to the anesthetic technique.” They concluded that preanesthetic blood screening is, therefore, unlikely to yield additional important information in most cases. However, the same study also documented that of those dogs where the history and clinical examination would not normally have resulted in preanesthetic laboratory testing being performed at their institution (equivalent to 84% of the dogs recruited), 8% demonstrated biochemical or hematologic abnormalities that would have reclassified them as a higher ASA status, even if this may not necessarily have altered the anesthetic protocol. In addition, they also identified that surgery would have been postponed due to the laboratory findings in 0.8% of these dogs where preanesthetic blood screening would not usually have been performed, while 1.5% would have received additional preanesthetic therapy. Although the authors concluded that only 0.2% of dogs in the study would have required an alteration to their proposed anesthetic protocol based on the biochemical or hematologic results, the implication that undiagnosed pathology may be detected prior to anesthesia using “routine” screening may have implications for whether the owner decides to proceed with anesthesia/surgery and may also alter the expected prognosis for the animal. Thus, from this study, despite the fact that preanesthetic biochemical and hematologic testing may not necessarily alter how the subsequent anesthetic would actually be performed in most animals, it may, in reality, be the deciding factor as to whether the procedure goes ahead. Davies and Kawaguchi [55] conducted a retrospective study of almost 800 dogs and cats of varying ASA status which underwent preanesthetic blood screening at a UK veterinary group practice and showed that 97% of cats and 95% of dogs had at least one result from the screening panel that was outside the reference interval (although not necessarily clinically significant). Almost 1% of cases had problems identified by the preanesthetic blood results that were not evident from the history or physical examination, while 4% of dogs and 9% of cats had their anesthetic protocol altered based on abnormalities in the blood work, although these cases were being anesthetized by first opinion practitioners rather than specialist anesthesiologists. A more recent study by Mitchell et al. [52], which presented five Diplomates of the American College of Veterinary Anesthesia and Analgesia with the medical records from 100 randomly selected previously presented cases, subsequently followed by the relevant results from preanesthetic blood screening tests for these animals, resulted in a change in the proposed anesthetic protocol in 79% of patients, based on abnormalities in the blood results. Of note is that 64% of changes were made by only one of the anesthesiologists, which suggests that there may be significant variability in whether alterations in certain preanesthetic tests are considered clinically relevant even between specialists.

While it is often said that “old age is not a disease,” it might be intuitive to assume that older patients may require more intensive preanesthetic screening compared to those who are younger. However, with the exception of recommending a preanesthetic electrocardiogram (ECG) in patients greater than 65 years old undergoing major/complex surgery, NICE does not differentiate the requirements for preanesthetic assessment based on age alone, i.e., they do not suggest blood testing older patients of ASA 1 or 2 undergoing minor or intermediate procedures unless there are certain comorbidities (e.g., diabetes mellitus and cardiovascular or renal disease). In animals, Joubert [56] assessed whether hematologic and biochemical analyses were of value in geriatric dogs (> 7 years of age) presented for anesthesia. Of the 101 dogs recruited to the study, 30 new diagnoses (e.g., neoplasia and hyperadrenocorticism) were made on the basis of the blood sample, with 13 animals not undergoing general anesthesia as a result of the new diagnosis. However, similar to the conclusions of the study by Alef et al. [54], Joubert [56] suggested that although preanesthetic screening had revealed the presence of subclinical disease in almost 30% of the dogs in the study, and that screening of geriatric patients is important, “the value of screening before anesthesia is perhaps more questionable in terms of anesthetic practice, but it is an appropriate time to perform such an evaluation.” In other words, although preanesthetic blood testing may be of value in uncovering undiagnosed pathology in geriatric patients, there was little evidence that what was detected would actually impact either how the subsequent anesthetic was managed, or the overall outcome from it. However, this study did identify that over 10% of the dogs had their anesthesia canceled due solely to the findings of the preanesthetic blood screening, which is obviously of significance.

Interestingly, and somewhat in contrast to the previous studies, work within the Confidential Enquiry into Perioperative Small Animal Fatalities (CEPSAF) highlighted a reduction in risk when preoperative blood work was performed in higher ASA grade patients. CEPSAF was a multicenter study undertaken in the UK between 2002 and 2004 and involved over 100 practices and data from approximately 200,000 dogs and cats [7]. When analyzing risk factors for anesthetic death in sick dogs (ASA grade 3–5), having a preoperative blood test was associated with reduced odds of death, particularly in ASA grade 4–5 dogs [57]. This association was not detected in the overall analyses where ASA grade 1–5 dogs were considered together or in cats but does suggest that preoperative biochemistry and hematology are most likely to be merited in the sicker animals that are anesthetized.

Thus, based on the evidence from human anesthesia, and from a smaller number of published veterinary studies, there would appear to be negligible benefit to apparently healthy animals (ASA 1 or 2, regardless of age) of biochemical or hematologic screening prior to anesthesia in terms of either anesthetic risk reduction or alteration of the anesthetic protocol; however, given that a significant percentage of animals may have the procedure canceled based on the results of these tests (due either to a worsened prognosis or the need for further treatment prior to anesthesia), this may counterbalance the preceding argument. Overall, the requirement for preanesthetic blood screening in healthy animals is likely to remain a contentious issue, with valid arguments both for and against.

The situation in animals that are ASA 3 or greater, however, is probably more clear‐cut with the published veterinary studies providing some justification that preanesthetic screening may be of value in terms of potentially altering anesthetic management and outcome.

Aside from the impact (or lack thereof) that preanesthetic screening may have on the subsequent conduct of anesthesia and ultimate outcome for veterinary patients, there is perhaps another factor that may require consideration, namely that of potential litigation. It seems that an increasing number of clients are willing (sometimes overly so) to point the finger of blame at the veterinarian when things go wrong in relation to anesthesia, even when in many cases this may be completely unjustified. Hence, the genuine reason why many veterinary practices carry out routine preanesthetic screening may have more to do with covering one’s back rather than providing the ability to alter anesthetic management suitably if abnormalities are actually detected. It is impossible to say what the legal system may make of a healthy animal undergoing an elective procedure that dies during anesthesia where no preoperative blood sampling had been performed, but based on the recommendations from human anesthesia and the lack of evidence of any benefit in the few veterinary studies that have been carried out, it would appear difficult for them to state that preanesthetic biochemical or hematologic screening is a basic standard of care. Given that there is a more limited evidence base for sicker animals, it may be considered wise to perform preanesthetic screening in patients of ASA 3 or above, from both standard of care and litigation points of view.

Risk of anesthetic death

Mortality, in contrast to morbidity, has been reported extensively in the veterinary literature. In small animal anesthesia, the risk of death has been documented over the last 75 years [84], and trends in the reduction of risk over time have been reported (see Table 2.2). Referral‐center‐ and university‐based studies have generally reported higher death risks, potentially due to the nature of their patients and procedures, whereas practice‐based studies tended to reflect healthier populations and simpler procedures. Direct comparison of risks of death between studies has been limited by a number of factors, including variations in study case definitions, study populations, and procedures performed.

Initial institution‐based studies from the US documented a wide range of relatively high risks of mortality. An early study at the Angell Memorial Animal Hospital in Boston, Massachusetts published risks of anesthetic death of 0.26% in dogs, 0.36% in cats, and 5% in other species [85]. Colorado State University reported higher risks of 1.08% in dogs and 1.79% in cats between 1955 and 1957 [86], and the Wheatridge Animal Hospital in Colorado reported anesthetic death risks of 0.23% in dogs and 0.40% in cats between 1960 and 1969 [86]. At a similar time, the University of Missouri Veterinary Hospital reported mortality risks of 0.8% in dogs and 0.53% in cats [86]. More recent referral‐center studies have reported lower risks of mortality, suggesting that outcomes have improved. Further work at Colorado State University documented risks of 0.43% in dogs and 0.26% in cats between 1979 and 1981 and 0.43% in dogs and 0.35% in cats between 1993 and 1994 [58,87]. Louisiana State University reported higher risks of perioperative death of 1.49% of dogs and 5.80% of cats at their institution between 1995 and 1996, although this related to all deaths, not just anesthetic‐related mortality [5,10]. Work at the Royal Veterinary College in the UK reported an anesthetic‐related mortality risk of 0.58% in dogs between 1999 and 2002 [6]. A subsequent single‐center referral practice study in France estimated the risk of anesthetic death in dogs and cats combined at 1.4% between 2008 and 2010 [12], decreasing to 0.8% between 2010 and 2011 [88]. Consistent with the later dates of this work, a referral practice study from Japan estimated the risk between 2010 and 2011 at 0.65% in dogs [89]. Based on the more recent work described above, the risk of anesthetic‐related death in the referral setting would appear to be of the order of 0.25–0.60% in dogs and cats.

Work undertaken in small animal primary practice has generally documented lower risks of mortality than referral and veterinary university hospital–based studies. An early practice‐based study evaluated feline mortality in Scotland and published a risk of death of 0.31% in cats [90]. This was followed by a further survey of small animal anesthetic practice, undertaken in Vermont in the US, which reported the risk of death to be 0.11% and 0.06% in dogs and cats, respectively [91]. A similar study was undertaken in Finland in 1993 and reported a risk of death of 0.13% in small animals in general [92]. A retrospective study evaluated mortality in a South African practice population in 1999 and estimated a mortality risk of 0.08% in dogs and cats [62]

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