Introduction

Many veterinarians sample patient's blood prior anesthesia and surgery with the aim of discovering if they have covert disease. Parameters routinely assessed include PCV (hematocrit), total protein, urea, creatinine and liver enzymes with further tests carried out if the patient has a history or clinical signs of a disease. This is an important concept-good history and clinical examination are of equal if not greater importance in determining suitability to undergo anesthesia.

Once blood-work has been assessed the question arises "Should the anesthetic technique be altered if abnormal results are unexpectedly found in an animal that has no history or clinical signs of disease?" To put some rationality into the decision-making it is important to know when results are really abnormal.

Reference ranges

Reference ranges are determined by sampling a number of animals within a population and calculating mean and standard deviation for each parameter. The reference range is estimated out by taking 2 standard deviations either side of the mean i.e., 95% of animals within a population are included within the reference range. What about the remaining 5%? It means that 1 in 20 healthy animals is expected to have a value outside the reference interval i.e., in the "abnormal" range. Admittedly this "abnormal" value is expected to be close to the reference range. A marked difference from the reference interval probably truly represents a pathologic state. When multiple tests are carried out there is a greater chance of finding an abnormal result in at least one of the parameters. For example when a panel of 10 tests is assessed there is a 40% change that at least one parameter will be outside the reference range increasing to 64% when 20 tests are run. This is important to consider when laboratory tests are requested for clinically healthy animals for a pre-anaesthetic check. The other factor to bear in mind is that abnormal results do not necessarily represent what is occurring in the animal but may arise when sample handling, analysis of the sample or transcription of results is incorrect.

Urinary function tests and anesthetic protocol

Urea and creatinine are often assessed pre-operatively in geriatric cats because chronic renal disease is so common. If urea and creatinine concentrations are within normal limits should we assume that the cats have normal renal function? No, we should not. A cat may appear clinically healthy with no sign of azotemia and still have lost greater than 50% of its renal mass. The kidneys may be subclinical for disease but they do not tolerate additional insults such as anesthetic induced hypotension. Some studies indicate that some animals may lose up to 90% of their renal mass before azotemia and anemia appear. This doesn't give the clinician much room for error. Consequently we should perhaps assume that all geriatric cats have some degree of renal insufficiency and treat them accordingly. In most cases this doesn't mean that the anesthetic protocol (i.e., drugs) needs to be radically changed but appropriate monitoring and support is very important. It is vital to ensure that dehydration is not present prior to administering the anesthetic agent. Although we advise owners not to feed their pet from the evening before the procedure we should ensure that water will be available at all times. Once the patient is in the hospital water should be provided in the cage up to 2-3 hours prior to induction. After induction it is vital to ensure that systolic blood pressure (BP) does not fall below 95 mmHg because hypotension lasting longer than 10 minutes is sufficient to cause irreversible tissue damage. In most cases parenteral fluids should be administered during the anesthetic period. For short procedures (< 20-30 minutes) that are minimally invasive in cats with normal renal parameters (urea, creatinine, urine SG) a bolus of 10 ml/kg LRS given subcutaneously is OK. For longer procedures, invasive procedures and animals with obvious renal disease a catheter should be placed and fluids (LRS) given IV. Provided that support and monitoring is adequate anesthetic technique is of lesser importance.

ACP, xylazine, medetomidine and other α2 agonists can markedly decrease tissue perfusion and should be avoided. This is in contrast to the opioids and benzodiazepines (diazepam, midazolam) that have minimal effect on blood pressure and tissue perfusion. A technique that provides rapid recovery is desirable because the animal can begin to drink soon after the procedure. If recovery is prolonged then fluids either IV or SQ must be provided until voluntary intake is possible.

We less commonly have to anesthetize dogs with renal disease. Unlike cats, dogs typically lose concentrating ability (and so have polyuria) before azotemia develops. Consequently dogs with history or clinical signs of polyuria should be managed as though they have potential renal compromise. The features of anesthetic management discussed above for cats also apply to dogs.

Prerenal azotaemia

Any process that reduces renal blood flow (RBF) decreased glomerular filtration rate (GFR) and clearance of urea and creatinine consequently azotemia is not always related to renal disease. Distinction between the two must be made prior to anesthesia. If pre-renal azotemia is severe and persistent it may lead to renal hypoxia and acute renal damage. Prolonged hypotension during anesthesia may induce acute renal insufficiency or failure and so appropriate measures must be taken to ensure that BP is monitored and supported. Suspect prerenal azotemia when USG > 1.030 (dog) or 1.035 (cat) with history/signs of dehydration, hemorrhage, shock or decreased cardiac output and decreased urinary volume

Liver function tests and their effect on anesthetic protocol

In combination decreased urea, hypoalbuminemia, hypocholesterolemia and raised serum bile acids a severe decrease in liver function. This warrants careful thought about the anesthetic management because many agents are metabolised in the liver and a prolonged recovery can be anticipated. Long acting agents such as ACP and those that can markedly alter hepatic blood flow such as the α2 agonists should be avoided. Similarly, agents that require hepatic metabolization for recovery such as thiopental and ketamine should not be considered.

Halothane can induce formation of hepatic free radicals in the presence of hypoxia and so has the potential to cause further liver damage. Short acting opioids such as meperidine (pethidine) and fentanyl are desirable for sedation but longer acting agents such as morphine can be given safely because they are reversible Opioids have minimal effect on cardiovascular function and they can be reversed should recovery by excessively prolonged. Ensure that by reversing sedation that analgesia is not also reversed because a sudden increase in sympathetic tone accompanying nociceptive transmission may cause alterations in hepatic blood flow. Induction agents such as propofol or etomidate with short duration of action are useful as are the inhalational agents (isoflurane, sevoflurane) and reversible agents (opioids). Appropriate fluid administration is also important. See Perioperative Fluids.

An increase in serum ALT may indicate hepatocellular damage, reparative stage of liver disease, hyperthyroidism (cats) and glucocorticoid hepatopathy (dogs--Cushing's disease, iatrogenic). Unless there are concurrent signs consistent with decreased hepatic function it is not necessary to alter anesthetic technique other than to avoid halothane. It would be prudent to provide parenteral fluids to ensure that hepatic perfusion remained adequate and to monitor BP to avoid hypotension.

Alkaline phosphatase (ALP) is a not a leakage enzyme and so does not denote hepatocellular damage. Cells with high ALP activity are found in several tissues of the body but those of most significance are in bone and in cells lining bile canniculi. Exposure to various endogenous and exogenous chemicals stimulates increased production and release, consequently raised serum ALP activity can be due to a number of possible causes. In very young animals serum ALP activity may be up to 30 times the upper reference if the sample is taken post-suckling. ALP remains high compared to adult values in young animals because of extensive bone remodeling as they grow. These animals do not have cholestasis and require no special anesthetic protocol other than that taking account of their age. Older animals with osteosarcoma or major trauma to the musculoskeletal system may also have raised ALP. A number of drugs will also increase ALP activity in the absence of cholestasis. Corticosteroids are the most well known and commonly encountered but other drugs include anti-convulsants can increase ALP. Corticosteroid induced increases in ALP activity is unique to dogs and is not noted in cats.

Cholestatic liver disease necessitates care in anesthetic management as discussed above.

Anemia and or hypoproteinemia prior to anesthesia necessitates consideration of peri-operative fluids rather than a change in anesthetic protocol. PCV less than 25 % (dogs) and 20% (cats) is reason to consider transfusion before anesthesia as all anesthetic techniques cause some fall in PCV and TP. Below critical values tissue oxygen delivery is reduced to a point where hypoxia will occur. Cardiovascular mechanisms that compensate for anemia are reduced by anesthetic agents, so whereas a dog may cope with a PCV of 20% when conscious, it will not do so when anesthetized. This is covered in detail in Perioperative Fluids. Agents such as ACP that can have a profound effect on BP or thiopental that increases splenic volume can have a marked effect on PCV also.

Abnormalities in electrolytes should be addressed before anesthetic induction. Hyperkalemia and hypocalcemia can be life threatening and so must be corrected as soon as possible whether anesthesia is contemplated or not. Other electrolyte abnormalities are not so critical and can be addressed before and during anesthesia.

Summary

Blood chemistry and hematological parameters are frequently measured before anesthesia to determine an animal's fitness to undergo a procedure. Interpretation of the results can be fraught with difficulty and should be considered in light of the animal's history and clinical signs.