Brenda K. Feller
CVT, RVT, VTS (Anesthesia and Analgesia)
Brenda graduated from Michigan State University, one of the first veterinary technician programs in the United States. She has worked in private practice, a university anesthesia department, and specialty practices during her career. She is not only a board member at large of the Academy of Veterinary Technicians in Anesthesia and Analgesia, but also a member of the academy’s examination, preapplication, and conference committees. She is married to Doug, a retired veterinarian, with three grown children and a growing number of grandchildren! Doug and Brenda share their house with a rescue Westie mix.
Brenda is a frequent speaker at major conferences and teaches online anesthesia classes. In her spare time, she likes to rollerblade and read nonfiction.Read Articles Written by Brenda K. Feller
Compared with young adult patients—and with each other—geriatric and pediatric patients have differences in physiology, pharmacology, and anatomy. Yet their anesthetic needs are often very similar to each other. These cases should be considered challenges for the anesthetist. Veterinary technicians should be knowledgeable not only about how to provide the optimum anesthetic experience, but also about the reasons behind these actions.
This article outlines special concerns for pediatric and geriatric patients, but it is important to note that all comorbidities in an individual patient must be taken into consideration when assessing anesthetic risk and designing an anesthetic protocol for any patient, regardless of age. A full examination of all aspects of anesthesia is beyond the scope of a single article; interested readers are recommended to consult the cited references for further details.
Classifying Anesthetic Risk in Pediatric and Geriatric Patients
Most veterinary technicians realize that geriatric patients need specialized care under anesthesia. Often, pediatric patients are considered healthy and expected to handle anesthesia well. But nothing could be further from the truth. In reality, the anesthetic needs of both pediatric and geriatric patients are more closely related to those of critical patients, but often for very different reasons.
The reason for anesthesia is one factor to consider in determining a patient’s anesthetic risk. Underlying disease that could affect the patient’s reaction to anesthesia is another. All patients scheduled for surgery should be assessed to determine their physical status and corresponding anesthesia risk classification according to the American Society of Anesthesiologists (ASA) classification system (BOX 1). While age is not a disease and, unlike organ dysfunction or comorbid disease, does not automatically place a patient in a higher ASA class, it is a risk factor in morbidity and mortality.1 The argument can be made to classify healthy pediatric and geriatric patients as either ASA I or ASA II, but because of their special needs, pediatric and geriatric patients presenting for routine surgery without any comorbidities are generally classified as ASA II.
Differences Between Pediatric and Young Adult Patients
Pediatric patient is a general term used to describe a puppy or kitten from weaning age (approximately 8 weeks) to around 12 weeks old. Because different breeds mature at different rates, this is a general range. The neonatal period—from birth to weaning, or at least 4 weeks of age—is not covered in this article.
The pediatric thoracic cage is still pliable, and the intercostal muscles are immature and weak. As a result, pediatric patients have to work harder and use more pressure to maintain a normal tidal volume.1 In addition, immature respiratory chemoreceptors do not respond to increased carbon dioxide or decreased oxygen levels as mature ones in adult patients do. Pediatric patients are therefore at a respiratory disadvantage. Compared with healthy adults, they have increased respiratory rates, lower functional residual capacity (FRC), and an increased oxygen demand, which makes them prone to respiratory fatigue and hypoxemia while under anesthesia.1
Pediatric patients have naturally lower blood pressure, blood volume, and systemic peripheral resistance than adult patients. Immature baroreceptors depress the ability of the vascular system to constrict in response to lower heart rates to maintain cardiac output, so these patients rely more on heart rate to maintain blood pressure and tissue perfusion than adults.1
Pediatric patients also have a large percentage of noncontractile cardiac mass and low ventricular compliance.2 These characteristics result in a fixed stroke volume, causing increases in preload and afterload to be poorly tolerated.
Immature hepatic metabolism has several implications for anesthesia in pediatric patients. First, these patients are often hypoalbuminemic; therefore, drugs that are highly protein bound, such as ketamine, opioids, and propofol, have a more profound effect.3 Second, the immature liver decreases the capacity for drug distribution, which may prolong the effect of the drug.3 Because most drugs used in anesthesia are either protein bound or eliminated by the liver, lower dosages of all drugs should be used in pediatric patients. Third, the liver plays a role in regulating insulin,3 so while most pediatric patients have a normal blood glucose level, their glycogen stores are low. If a pediatric patient becomes stressed or is fasted, it can quickly become hypoglycemic.1
The kidneys of pediatric patients have not yet reached the adult glomerular filtration rate (GFR), which decreases their ability to concentrate urine.3 Decreased GFR calls for a higher fluid rate during anesthesia, but pediatric patients may also have limited renal clearance and, as mentioned, preexisting hypoalbuminemia. These two factors require restriction of high fluid rates during surgery because the kidneys may not be able to clear the urine and overhydration can cause a dilution of serum protein. Ensuring a normal fluid rate and refraining from administering fluid boluses to combat hypotension are good rules to follow with pediatric patients.2 The American Animal Hospital Association and the American Association of Feline Practitioners have published excellent fluid therapy guidelines for all patients (BOX 2).4
Pediatric patients have a naturally lower hematocrit, creating the risk of mild hemorrhage resulting in anemia.2 They also have a high metabolic rate, a low body fat percentage, an immature thermoregulatory system, a high body surface area to mass ratio, and poor vasomotor control.2,5 All of these factors make them very susceptible to hypothermia, which leads to a slower recovery in all patients. Prevention is the key. Steps to avoid hypothermia are outlined during the discussion of anesthetic protocols for geriatric patients.
Differences Between Geriatric and Young Adult Patients
At the other end of the life cycle, a patient is considered geriatric when it has completed 75% to 80% of its expected life span.3 Life spans vary greatly between species and among breeds of dogs. A cat or small dog is generally expected to have a longer life span than a giant breed of dog. Therefore, the age at which a patient should be considered geriatric depends on the individual.
Geriatric patients often require surgery because of a disease process, and this factor needs to be taken into consideration when crafting the anesthesia protocol. Think of a geriatric patient like an old car. It may still be running just fine, but under the hood, there will be some wear and tear. A minor issue can result in a major disturbance.
Two of the most common respiratory issues in geriatric patients are weakening of the intercostal muscles and loss of lung elasticity. In addition to pulmonary fibrosis, which is common, this results in decreased elastic recoil of the lungs and chest wall compliance.1 This, in turn, results in a lower lung capacity and FRC and predisposes the patient to atelectasis during prolonged recumbency.1 Atelectasis is a reduced volume or lack of air in all or part of the lung due to collapsed alveoli. Atelectatic lung tissue therefore contains very little oxygen. Some degree of atelectasis is common in anesthetized patients, but because atelectasis can result in hypoxemia, efforts should be made to avoid it.
Geriatric patients may also have decreased pulmonary diffusion capacity and capillary blood volume and an increase in airway resistance.1 These changes may lead to hypoxemia due to reduced efficiency for expiration and gas exchange.
The heart undergoes changes during the aging process that may include ventricular thickening, valvular calcification, and myocardial fibrosis.1 These changes prevent the heart from moving blood as efficiently as it once did and predispose geriatric patients to cardiac arrhythmias.1 Decreased baroreceptor activity, circulation time, blood volume, and cardiac output all lead to decreased cardiac reserve in these patients.1 Therefore, the potential for cardiopulmonary disease should be high on the list of concerns. Valvular heart disease, dilated cardiomyopathy, pericardial disease, arrhythmias, and systemic hypertension are common in geriatric patients.1 Geriatric cats are prone to hypertrophic cardiomyopathy.
Blood pressure concerns in geriatric patients include both hypotension and hypertension. During the history and physical examination, it is important to note any exercise intolerance, arrhythmias, abnormal pulses, murmurs, or other cardiac-related signs. Electrocardiography is advisable if any cardiac abnormalities are noted during the examination. It may be prudent for some patients to undergo a cardiac workup with a cardiologist before anesthesia.
Geriatric patients may have a decrease in liver mass, hepatic blood flow (a result of reduced cardiac output), albumin production, and clotting factors.3 These deficiencies can result in hypoproteinemia, hypoglycemia, hypothermia, and coagulopathies.
Hypertension in cats is often associated with chronic renal disease6; therefore, obtaining a preoperative blood pressure is imperative for geriatric cats. Because the kidney has no regenerative capacity, renal reserve decreases with age. Reduced renal blood flow due to cardiovascular changes may result in a lower number of glomeruli and nephrons. This decrease can cause a 50% decrease in GFR. Physiologic tubular changes lead to a decreased ability to concentrate urine, making the patient more dependent on fluid intake and unable to tolerate fluid excess or deficit.6
Geriatric patients have sarcopenia (a decrease in muscle mass) and an increase in the fat to lean muscle ratio. This change in body composition is seen in all geriatric patients regardless of body condition score.3,5 Obesity in geriatric patients poses a challenge because landmarks for epidurals may be hidden under a layer of adipose tissue. If drug doses are not decreased to reflect ideal body weight, the increased fat to lean muscle ratio may prolong drug redistribution.3 Osteoarthritis and spondylosis can also distort anatomy, making it a challenge to find the epidural space and adding to the need for gentle positioning during surgery.3
Geriatric patients should be considered to have arthritis regardless of whether it is recognized and being currently treated.2 They should be given extra padding in cages at all times, and during positioning for surgery, the joints should be handled with care and gentle restraint. The surgery table should be padded, and recovery should take place in a warm area with lots of padding.
Many geriatric patients have some degree of cognitive dysfunction that can be exacerbated by unusual surroundings, such as the animal hospital.
Like all patients undergoing anesthesia, both pediatric and geriatric patients require patient-specific anesthetic protocols. One size does not fit all. TABLE 1 summarizes specific considerations that should be taken into account for these patients.
Pediatric patients have a high metabolic rate; therefore, they should not be fasted for longer than 2 hours.2,5 When they arrive at the clinic, the time of their last meal must be determined, and if surgery will not take place within 2 hours of the last meal, they should be fed a small amount every 2 hours until surgery. Water should not be withheld. The blood glucose level should be measured as part of the preoperative blood work, then followed up every 60 minutes during anesthesia and postoperatively. If the veterinarian determines that the patient is hypoglycemic, dextrose can be added to IV fluids to create a 2.5% or 5% solution. It is important that these patients be fed as soon as they are up and alert.
Pediatric patients should be weighed accurately on the day of surgery so drug doses are accurate. Puppies and kittens can grow at an incredible rate during this stage of life.
An IV or intraosseous catheter should be placed before surgery.
Pediatric patients feel pain and should receive analgesia in the form of pure mu opioids, local blocks, and nonsteroidal anti-inflammatory drugs (NSAIDs). Pure mu opioids provide excellent analgesia and may provide sedation alone in pediatric patients owing to incomplete metabolism by the immature liver.3,5 Opioids are protein-bound drugs, so the low end of the dosage range should be considered and the patient carefully monitored for a reduction in heart and/or respiratory rate. The pros outweigh the cons for this class of drugs, but side effects may need to be addressed.
When local blocks are used in small patients, care must be taken to avoid overdosing. NSAID use is acceptable in pediatric patients at 25% to 50% of the adult dose; however, age restrictions for individual drugs should be checked before use.
When considering preanesthetic drugs, keep doses on the low end of the dosage range, attempt to avoid drugs that reduce heart rate, and use drugs that have a reversal agent. Additional doses can always be administered to achieve the desired effect if needed. Unlike their effect in adult patients, benzodiazepines often produce sedation. They have little effect on the cardiovascular system and provide good skeletal muscle relaxation. Flumazenil is the reversal agent for benzodiazepines. Many other drugs, such as acepromazine, are not contraindicated, but their side effects must be weighed against their benefits. For example, acepromazine is metabolized by the liver, has no reversal agent, and causes vasodilation, which can lead to hypotension and hypothermia.
In the past, it has been recommended that an anticholinergic agent be used as part of the premedication for pediatric patients, but this practice is now under debate. While it is not incorrect to administer an anticholinergic to a pediatric patient, there is currently very little research to support it. If not used in premedication, an anticholinergic can be considered for use during surgery to counteract bradycardia, with a dose precalculated and all equipment ready for administration at a moment’s notice.
Inhalant agents are preferred3 because they are noncumulative and anesthetic depth can be easily adjusted.2 Anesthetic depth can change rapidly, and the patient should be monitored vigilantly to ensure adequate depth.2 There has been little research in veterinary patients, but human infants have a larger ratio of alveolar ventilation to FRC, which results in faster induction when gas inhalants are used.1
Pediatric patients have large tongues, can be difficult to intubate, and have a naturally low hemoglobin concentration, so preoxygenation is recommended if it does not stress the patient.2,3 A fast-acting, rapidly distributed induction drug should be used to obtain rapid control of the airway. Commonly used agents include ketamine/midazolam, propofol, alfaxalone, or etomidate. In general, no induction drugs are contraindicated for use in healthy pediatric patients. If the patient has a comorbid condition, this must be taken into consideration when creating the anesthetic protocol.
A cuffed endotracheal tube is always recommended to protect the patient’s airway. A 2- to 3-mm endotracheal tube should be adequate for most small pediatric patients.2 The tracheal tissue and larynx are fragile, so care should be taken when intubating these patients.1
Pediatric patients benefit from the use of non-rebreathing systems, which provide little resistance to breathing by eliminating one-way valves and the carbon dioxide absorbent canister. Dead space should be minimized by trimming the endotracheal tube to size (the ideal placement of an endotracheal tube is for the distal end to be at the thoracic inlet and proximal end at the level of the incisors) and eliminating nonessential extras in the anesthesia circuit such as elbows and respiratory monitors. However, non-rebreathing systems require high oxygen flow rates (150–500 mL/kg/min). This is in contrast to rebreathing, or circle, systems, which typically require an oxygen flow rate of 20 to 22 mL/kg/min. Higher oxygen flow rates can dry and cool the respiratory tract and accelerate temperature loss.
Pediatric patients often require intermittent positive pressure ventilation (IPPV).3 They are prone to respiratory fatigue because the thoracic cage is still pliable and the intercostal muscles are not fully developed. Mechanical ventilation is not required, but as few as 1 to 4 assisted breaths per minute can provide adequate oxygen, minimize atelectasis, and remove carbon dioxide from the lungs. Only gentle pressure is needed. If a rebreathing system with a manometer is being used, ensure that the pressure does not exceed 10 to 12 cm H2O. If a manometer is not used, during surgical prep, give the patient a small breath while watching the chest expand to determine an adequate breath, taking care to not overextend the lungs. Monitoring end-tidal carbon dioxide (ETCO2) enables the anesthetist to determine the adequacy of IPPV.
Minimize heat loss! It is much harder to regain body temperature than it is to maintain it. Pediatric patients have little body fat, a high body surface area to mass ratio, and a high metabolic rate. They should have a heat source available before surgery and be on a veterinary-approved heat support system during prep, while in surgery, and postoperatively. An adult patient can lose up to 2°F in body temperature during surgical prep, and pediatric patients can lose even more. Ways to can minimize heat loss include wrapping extremities in bubble wrap, warming prep solutions, using appropriate warming devices from premedication through recovery, keeping anesthesia time to a minimum, and warming IV fluids close to the body with inline fluid warmers.
Diligent monitoring is crucial to a positive outcome. Puppies and kittens have higher heart rates (~200 bpm) and respiratory rates (15–35 breaths/min) but lower blood pressures than adult patients.5 A mean arterial pressure of 55 mm Hg is acceptable in pediatric patients.2 It is important to obtain a heart rate and respiratory rate before administration of any drugs to establish what is normal for the individual patient.
It can be a challenge to obtain blood pressures in pediatric patients. Doppler readings may show something closer to mean arterial pressure, and many oscillometric units can be unreliable in smaller patients. Cuff size has a dramatic effect on accurate blood pressure reading; using a cuff with a width that is 30% to 40% of the circumference of the limb will provide a more accurate reading. Unless a direct arterial blood pressure is obtained, blood pressure readings are estimates and should be viewed as trends rather than exact numbers.
ETCO2 readings can be difficult to obtain because of the small tidal volume and high oxygen flow rates used in pediatric patients. An artificially low ETCO2 is often observed when using high oxygen flow rates or adult monitors on pediatric patients. If possible, pediatric adapters, which produce a smaller dead space volume and deliver a more accurate reading, should be used.
All patients require IV fluids under anesthesia, including pediatric patients. However, because pediatric patients have low hemoglobin concentrations and immature kidneys, it is especially important to not over- or underhydrate them.2 The heart, which has a stiff myocardium at this stage, cannot respond to fluid loading as an adult heart can.3 Overhydration can therefore lead to pulmonary edema.
Monitor the patient, not the monitor. Assess pulses, mucous membranes, and capillary refill time regularly. Always monitor more than one parameter before making a judgment. An esophageal stethoscope can be a very effective monitor to auscult heart and respiratory rates in these small patients.
Postoperative care is essential to a positive outcome. Keeping patients warm during recovery is crucial. Incubators are a good option for pediatric patients; some also supplement oxygen. Other approved veterinary warming devices, such as circulating warm water blankets or warm forced air blankets, should be used if the patient is hypothermic, but care must be taken to avoid thermal burns, which can occur quickly in pediatric patients because of their thin skin and lack of subcutaneous fat. Once the patient is fully alert and able to move about the cage, warm water bottles or similar devices can be used. Treatment of pain is imperative. Food should be offered as soon as the patient is alert and able to eat in a normal manner and at regular intervals until the patient is sent home.
At the other end of the spectrum, geriatric patients have their own concerns. These patients often do not hear or see as well as they once did, so they should be approached slowly to avoid startling them. They must be handled gently and require extra time and effort owing to a variety of issues, such as cognitive disorder. Clues to cognitive disorder include owners stating that the pet is suddenly “cranky,” having accidents in house, or becoming lost in its regular surroundings.
Geriatric patients thrive on routine, and if possible, these patients benefit from being in the hospital for as little time as medically necessary. They do best in their home surroundings with their normal routine and may recover better at home,3 if their release is not contraindicated because of their individual medical needs. Having them come in early in the morning and scheduling their surgery as the first procedure of the day may allow them to return home the same day.
Because of their age, the organs of geriatric patients may not work as efficiently as they once did and may have some degree of irreversible dysfunction. Organ dysfunction that could affect anesthesia should be identified by obtaining preoperative blood test results. A sobering fact to remember is that 60% of kidney function must be lost before renal dysfunction shows up in blood work.5
Preanesthetic liver function tests are essential for geriatric patients. A low total protein necessitates a reduction in doses of protein-bound drugs. If low blood glucose is noted, it may be prudent to start fluids containing dextrose to prevent hypoglycemia. Because reduced cardiac output already reduces hepatic blood flow in these patients, hypotension should be avoided to prevent further liver damage.
It is best to stabilize these patients before anesthesia. Drugs with minimal cardiovascular and respiratory side effects are preferable. Reducing drug dosages based on the suspicion of at least some hepatic dysfunction is warranted. Additional drug can be administered if required.
Patients with laryngeal paralysis and patients that are nervous and pant are prone to hyperthermia.3 These patients may benefit from a maintenance dose of IV fluids to prevent dehydration while waiting for surgery.
IV access in these patients is a must to allow titration of drugs to effect and rapid response in an emergency.
As with any patient, balanced anesthesia is the best option, and no one protocol is perfect for every geriatric patient. The patient’s history, physical examination findings, blood work results, hydration status, and blood pressure must be taken into account to design an individual protocol. Decreases in brain size, loss of neurons, increased cerebrospinal fluid volume, and depleted neurotransmitters in geriatric patients require reduced doses for most drugs used in anesthesia.1 Local and regional blocks allow for lower doses of other analgesic and maintenance drugs.
Premedication is a must. Extremely low doses of acepromazine (0.01 mg/kg)3 may be acceptable in some anxious pets. However, acepromazine has no reversal agent and can cause splenic engorgement, so it is not the best option for patients undergoing abdominal surgery.2 Another potential adverse effect is vasodilation, which may lead to hypotension, so its use should be evaluated carefully.
Although benzodiazepines can provide sedation in some geriatric patients, paradoxical excitement may also occur. Benzodiazepines have a reversal agent, so adverse reactions can be managed. Opioids are a good choice, and some debilitated patients can be sedated with only an opioid premedication. Alpha2-adrenergic agonists, such as dexmedetomidine, are not absolutely contraindicated in geriatric patients, but they do produce profound cardiovascular effects at any dosage, so their benefits must be weighed against any potential adverse effects.5
Inhalant agents are used for most surgeries. These agents have a dose-dependent negative effect on blood pressure, ventilation, cardiac contractility, and body temperature, so keeping the vaporizer at the lowest possible setting during surgery is beneficial. Administration of additional analgesics during surgery either IV or via constant rate infusion may assist in this goal. As a patient’s body temperature drops, so does the need for gas inhalants. If a patient becomes hypotensive, a reduction of the vaporizer setting should be the first consideration. In very critical cases, the inhalant may be discontinued and the patient kept under a surgical plane of anesthesia using total intravenous anesthesia.
Anticholinergics are not warranted as a premedication in adult patients.6 If given, they may cause tachycardia, causing excessive work for the heart and an increase in oxygen consumption.
As with pediatric patients, preoxygenation is advised if it will not cause stress to the patient. Geriatric patients may also require IPPV, mechanical or manual, in order to prevent atelectasis and respiratory fatigue, and they benefit from postoperative oxygen supplementation.
At induction, drugs should be titrated to effect to quickly secure the airway. When the patient is on inhalant agents, remember that the reduced blood–brain permeability causes rapid changes is anesthetic depth. A variety of induction drugs are appropriate for geriatric patients. The key is to titrate and use only what is needed for intubation.
One consideration of concern in geriatric patients is aspiration of fluids into the lungs, especially in breeds that are prone to laryngeal paralysis. While nothing can be done to prevent laryngeal paralysis, close monitoring after premedication and quick induction to get control of the airway are essential to prevent fluids from entering the lungs. These patients may have a diminished ability to protect their own airways, so securing the airway in an efficient manner and ensuring the endotracheal tube cuff is properly inflated will help protect them.
Hypothermia is a major concern with geriatric patients. Hypothermia has a negative myocardial outcome for high-risk patients, increases surgical wound infection, delays recovery, and can change the pharmacokinetics of anesthetic agents.1 Hypothermia from a weakened thermoregulatory system can lead to bradyarrhythmias.1 The act of shivering can increase oxygen consumption by 200% to 300%,5 so oxygen is often required postoperatively in these patients.
Geriatric patients should be monitored diligently and with equipment appropriate to their specific needs. Electrocardiography, pulse oximetry, noninvasive blood pressure monitoring, and temperature monitoring should be the minimum; ETCO2 monitoring should be used if available. It is often beneficial to employ extensive monitoring equipment on these patients during surgical prep.
Renal clearance may be limited in geriatric patients, so careful monitoring of fluid administration is needed to avoid fluid overload. If not carefully monitored, overhydration can lead to pulmonary edema and congestive heart failure in patients that have difficulty excreting salt and electrolytes.1
Some cardiovascular depression and hypotension is normal during anesthesia. While young, healthy patients may tolerate these changes well, such effects can be detrimental to geriatric patients. Geriatric patients have a decreased cardiac reserve, which is another reason to avoid fluid overload. Cardiac electrical activity and blood pressure should be closely monitored so any minor changes can be addressed while they are easier to correct.
Recovery is a critical time for the geriatric patient. Postoperative temperature monitoring is essential, and additional heating sources may be required. Remember to provide lots of padding for old joints. Patients with osteoarthritis, diagnosed or not, may need assistance in standing postoperatively. A maintenance rate of IV fluids may benefit these patients until they are able to drink on their own.
One often-overlooked postoperative treatment is expression of the bladder at the end of surgery, before the patient is awake. These patients are usually litter/house trained and become agitated if their bladders are full after surgery.
Geriatric patients are prone to emergence delirium, due to age-related cognitive dysfunction, but some extra hands-on time postoperatively can work wonders for these patients.3 Returning geriatric patients home as quickly as medically feasible is essential.3 If it is possible for them to go home the same day of the surgery, it is important to contact the owners before the last staff member leaves for the night to ensure these patients are doing well at home.
So what is the bottom line? Is there a “magic,” specialized anesthesia protocol for pediatric and geriatric patients? Not really. As for any patient, their individual protocols should take into account their special needs and comorbid conditions. However, their needs will also reflect their stage of life, and they require more careful monitoring as a result. Just because a patient wakes up and goes home does not necessarily mean that the anesthesia was successful or optimal.
This profession is constantly changing and we must change with it. Adjusting our anesthesia protocols as we educate ourselves is how we provide the best medical care for our patients.
Next time you see a geriatric or pediatric patient, remember their needs and treat them with tender loving care.
- da Cunha AF. Neonatal, pediatric, and geriatric concerns. In: Canine and Feline Anesthesia and Co-Existing Disease. Ames, IA: Wiley Blackwell; 2015:310-317.
- Duke-Novakovski T, deVries M, Seymour C. BSAVA Manual of Canine and Feline Anaesthesia and Analgesia. 3rd ed. Quedgeley; 2016:418-427.
- Dugdale A. Veterinary Anaesthesia, Principles to Practice. Ames, IA: Wiley-Blackwell; 2010:312-317.
- Harold BA, Johnson T, Johnson, A, et al. AAHA/AAFP Fluid Therapy Guidelines for Dogs and Cats. 2013. catvets.com/guidelines/practice-guidelines/fluid-therapy-guidelines. Accessed September 2016.
- Grimm KA, Tranquilli WJ, Lamont LA. Essentials of Small Animal Anesthesia and Analgesia. 2nd ed. Ames, IA: Wiley-Blackwell; 2011:490-496.
- Tranquilli WJ, Thurmon JC, Grimm KA. Lumb & Jones’ Veterinary Anesthesia and Analgesia. 4th ed. Ames, IA: Blackwell; 2007:988.