Chapter 3
Patient Preparation
They should be prepared too!
Carrie Schroeder
Department of Surgical Science, University of Wisconsin School of Veterinary Medicine, USA
- Q. For how long should a patient be fasted prior to anesthesia?
- A. Pre-anesthetic fasting is important in order to decrease the volume of gastric contents as well as decrease the risk of peri-operative regurgitation. It is generally recommended that adult patients be fasted for 8–12 h prior to the administration of anesthetic medications. Most patients will have adequate glycogen stores and can maintain blood glucose throughout this fasting period. Water may be offered until the time any anesthetic or sedative agents are administered.
- Q. Are there exceptions to this rule of thumb in patients with diseases like diabetes or portosystemic shunts?
- A. There are certain disease states in which an animal’s blood glucose cannot be maintained during fasting. Patients with a diminished capacity to maintain normoglycemia, such as those with portosystemic shunt, should be fasted for a shorter period of time based upon their blood glucose. Generally, these patients should be able to tolerate 4–6 h of fasting. Blood glucose should be checked to verify normoglycemia at the time of induction, sooner if the patient has historically been unable to maintain blood glucose within a normal range. Intravenous glucose supplementation (2.5–5% dextrose) should be performed as necessary.
In patients with diabetes, pre-anesthetic fasting should be undertaken with caution as the patient’s insulin dose is typically administered along with food to prevent hypoglycemic episodes. Ideally, surgical procedures should be performed first thing in the morning so that post-operative patients may be monitored closely for the duration of the day and restarted on a regular feeding schedule. Opinions vary on the ideal way to manage blood glucose in diabetic patients, but a common approach is an overnight fast, roughly 6–8 h, followed by administration of one-half the usual insulin dose in the morning. Blood glucose should be monitored every hour following administration of insulin until the time of anesthetic induction, with intravenous glucose supplementation administered as necessary.
- Q. For how long should a young animal be fasted?
- A. Young animals (< 12 weeks) or species with a high metabolism, such as small birds, rodents, and rabbits, should not have food withheld for more than 2–4 h. These patients may become significantly hypoglycemic if fasted for prolonged periods of time. Neonatal patients (< 4 weeks) should be allowed to nurse from the mother until the time of anesthesia.
- Q. What medications should be given prior to anesthesia or anesthetic premedication?
- A. There is no standard recommendation regarding the timing and type of medications that should be administered prior to sedation or anesthetic induction. Common pharmacologic agents administered prior to anesthesia include antibiotics, nonsteroidal anti-inflammatory drugs (NSAIDs), anticholinergics, and antihistamines.
Pre-operative antibiotics such as cefazolin are often administered prior to major orthopedic or soft tissue surgeries. As a general rule, prophylactic antimicrobials should be administered approximately 30–60 min prior to the initial surgical incision [1, 2].
Nonsteroidal anti-inflammatory drugs (NSAIDs) are a highly effective component of a multimodal analgesic plan and are most effective when administered prior to the surgical insult [3, 4]. For maximum analgesic effect, NSAIDs should be given at least 30 min prior to surgery. One must use caution in administering these agents in patients with pre-existing hepatic, gastrointestinal, or renal disease or in patients where peri-operative hypotension is anticipated, as hypotension under anesthesia combined with an NSAID “on board” can lead to renal failure [5].
Antihistamines, such as diphenhydramine, and H2-blockers, such as famotidine, are indicated in patients with mast cell tumors to attenuate the negative effects associated with histamine release that may occur with tumor manipulation. It is important that these agents be administered prior to anesthesia in case of mast cell degranulation. These agents should be given roughly 20 min prior to anesthetic induction and can be administered at the time of intramuscular sedation.
- Q. What are indications for the administration of pre-anesthetic fluids?
- A. While intra-operative fluids are recommended in nearly all patients, the administration of pre-anesthetic intravenous fluids is recommended in selective cases. Patients presenting with renal disease, dehydration, electrolyte abnormalities, and hypovolemic shock are candidates for the administration of pre-anesthetic fluids.
Patients with renal disease, discussed in Chapter 35, should ideally be admitted for intravenous fluid therapy roughly 12–24 h prior to induction of anesthesia. This will allow for stabilization of any possible electrolyte imbalances, correction of dehydration, and optimization of intravenous fluid volume, improving the glomerular filtration rate under anesthesia. Fluid rate should be tailored to each individual patient, based upon the level of dehydration and any concurrent conditions, such as cardiac disease. Patients with renal disease who present on an emergency basis should, at minimum, be administered fluids to replace fluid deficits.
Patients presenting with hypovolemic shock (e.g., gastric dilatation/volvulus), should have fluids administered prior to anesthesia in order to improve cardiac output and tissue perfusion. Ideally, fluid administration rate and amount should be guided by measurement of the patient’s central venous pressure (CVP) in order to prevent fluid overload. In the absence of CVP measurement, patient response to fluid administration can be gauged by pulse rate and quality, capillary refill time, auscultation of lung sounds, and respiratory rate and effort.
- Q. How can I calculate the rate of administration of pre-anesthetic fluids?
- A. The rate of fluid administration for patients with dehydration can be calculated based upon maintenance fluid need (40–60 ml/kg/day or 1.7–2.5 ml/kg/h) plus replacement of any fluid deficit, in addition to fluids to account for ongoing losses such as vomiting, if present. This can be estimated by assessing the patient’s level of dehydration and replacing the deficit over 4–6 h or longer if time allows.
For example, a 5 kg patient presenting with 7% dehydration should have an initial fluid rate calculated as:
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- Q. For how long should a young animal be fasted?
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