Emergency anesthesia: What should we do? (Proceedings)

The risk of anesthesia is higher in emergency cases compared with elective cases. Most of these patients are compromised and this increases the danger of anesthesia, especially in cases when there is no time to optimize the condition of the patient before anesthesia. Some of these cases are performed at night and the increased risk may be related to inadequate personnel help during this time. The personnel may also be tired as a result of the long day resulting in inattentiveness to problems.

Emergency cases are those that involved diseases or injury that pose an immediate risk to an animal's life or long term health. Some of these cases will need anesthesia. Cases of note in small animal practice include: acute diaphragmatic rupture, gastric dilatation-volvulus, fracture repair due to trauma, septic abdomen, hemoabdomen, blocked urethra, ruptured urinary bladder and C-section.

When these cases are presented, oxygen supplementation and fluid therapy should be instituted immediately while the initial assessment is being done. Pertinent blood work should be done following the initial examination. The most important problems that will complicate anesthesia should be identified and stabilization should be done before anesthesia.

Preoperative preparation

Emergency cases can have one specific problem or a combination of different problems. The cardiovascular compromise is a common problem in emergency cases. They can have hypovolemia, dehydration, shock, and arrhythmias. Fluid resuscitation should be performed in these animals. With continuous bleeding, the goal of fluid resuscitation should be to maintain a mean arterial pressure (mmHg) in the 60s. Arrhythmias can be present in animals involved in vehicular accident, patients with electrolyte imbalances, and in gastric dilatation-volvulus. Arrhythmias are usually controlled with lidocaine.

The following are problems that should be identified, managed, and corrected before anesthesia:

     1. Pneumothorax:

        Perform thoracocentesis once or twice. If signs of respiratory compromise persist, chest tube should be placed before anesthesia.

     2. Hemothorax:

        If the hemothorax results in respiratory compromise, thoracocentesis should be performed and blood aspirated from the pleural cavity.

     3. Head trauma:

        The goal of therapy is to maintain cerebral perfusion. This can be achieved by optimizing mean arterial pressure (fluid therapy) and reducing intracranial pressure. For fluid therapy, isotonic crystalloid solutions like lactated Ringer's, 0.9% NaCl, or Normosol are usually suitable. Hypertension and overhydration should be avoided. Do not use crystalloid solution with dextrose unless the patient is hypoglycemic. Mannitol (20%) and 7.5% hypertonic saline are indicated in patients with deteriorating neurologic signs. These preparations can worsen cerebral hemorrhage. Colloids can be also be used to decrease cerebral edema and increase cerebral perfusion. The use of corticosteroids (methyprednisolone sodium succinate and dexamethasone sodium phosphate) is controversial.

        Supplemental oxygen should be provided.

     4. Hyperkalemia, acidosis, dehydration, and uremia

        These are related to ruptured urinary bladder. Hyperkalemia increases the risk of anesthesia. Hyperkalemia and dehydration should be corrected before anesthesia.

     5. Shock

        Emergency patients in a state of shock should be treated before anesthesia. Crystalloids and colloids are administered to restore vascular volume. If there is an infection or if there is a suspicion of infection, an antibiotic should be given. Oxygen supplementation should be done routinely.

Premedication

The three main reasons for premedication in emergency patient are: a) to continue the provision of analgesia if the animal is painful, b) to minimize anxiety and stress, and c) to decrease the dose needed to induce anesthesia. For minor procedures like gastric decompression and chest tube placement, an opioid can be used for sedation.

Opioids

The use of opioids as premedicant is appropriate for emergency patients. Opioids have both analgesic and sedative properties. The condition, size, and age of the patient will dictate the dose and type of opioid used. Patients in severe pain need a full agonist like morphine, hydromorphone, and methadone. The advantage of methadone over the other full opioid agonist is that it rarely, if ever, causes vomiting. Buprenorphine, a partial agonist, appears to provide acceptable analgesia in cats. The disadvantage of using buprenorphine as premedicant is when the cat does poorly during anesthesia, the use of a full agonist like fentanyl to balance the technique will be less effective. Buprenorphine has a strong affinity at the μ-opioid receptors leading for the less efficacious action of fentanyl. Hydromorphone, morphine, and methadone can be used in cats at a lower dose. In pediatric (<4 weeks of age) and geriatric patients, lower dose of opioid should be used first. Based on the response of the patient, the dose can be adjusted later. Patients with head trauma should be given lower doses of opioid to prevent severe hypoventilation. High carbon dioxide level will increase the intracranial pressure worsening cerebral perfusion. These patients may require anesthesia for diagnostic imaging and repair of traumatized areas of the body.

Hydromorphone, methadone, and fentanyl can be given intravenously without the danger of histamine release. Morphine and meperidine will cause the release of histamine if given IV, especially at a rapid rate.

For C-section, the author prefers to give the opioid once the pups and kittens are delivered.

Table 1. Premedicants for emergency patients (the dose given should be based on the clinical condition of the animal.)

Sedatives/tranquilizers

Most emergency patients are depressed upon presentation. They do not need a sedative or tranquilizer as premedicant especially those that are more critical. Stable patients that are more difficult to handle will benefit from a dose of a sedative. In these cases, acepromazine, diazepam or midazolam can be given as part of premedication. Diazepam or midazolam is preferably given to the more compromised patients. If a patient is aggressive and extremely difficult to handle, dexmedetomidine at a lower dose will be suitable. Additional doses can be given as needed.

Table 2. Choices of sedatives for trauma patients

Anticholinergic

Atropine or glycopyrrolate can be given if the patient has sinus bradycardia upon presentation. Bradycardia due to hypothermia will not respond to anticholinergic. Cold patients should be warmed before anesthesia. The use of anticholinergic is contraindicated in cases of myocardial contusion with ventricular premature complexes showing on ECG. If needed, atropine can be given at 0.02-0.04 mg/kg IM or IV. Glycopyrrolate's dose is 0.005-0.01 mg/kg IM or IV. Use the lower range of the dose for IV administration.

Induction

The appropriate choice of induction agent(s) becomes more important as the patient's condition becomes more critical. As the patient's medical condition worsens, the ability to compensate for cardiovascular and respiratory depression becomes limited resulting in morbidity or mortality. Knowing the properties, advantages and disadvantages of the different induction agents is helpful in choosing the appropriate anesthetic regime for emergency cases.

Propofol

IV administration of propofol results in rapid induction and recovery. It is rapidly distributed and metabolized extensively. It has minimal cumulative effects in dogs after repeated or continuous administration. In cats, it will accumulate when given repeatedly leading to longer recovery time. It causes myocardial and respiratory depression, peripheral vasodilation, and hypotension. It decreases cerebral metabolic activity, cerebral blood flow, and intracranial pressure. It is a suitable agent for head trauma, however it does not possess any analgesic property.

It can be used in emergency cases when the cardiovascular stability has been re-established. Do not use in hypotensive and hypovolemic patients. If there are no other choices, it should be given slowly following premedication with an opioid and/or benzodiazepine. When used in C-section, propofol resulted in better puppy vigor compared with thiopental.

The calculated dose is 4.0 mg/kg IV; half of the dose should be given over 40-60 seconds and the remaining dose is given to effect. To further reduce the cardiopulmonary depressant effect of propofol, a benzodiazepine can be incorporated during the induction process. A common technique is administering ¼ of the calculated dose (1.0 mg/kg) first. It should be given slowly. Then, a dose of benzodiazepine (midazolam or diazepam, 0.1-.02 mg/kg) is given IV. The rest of the propofol is given to effect, i.e., until the patient can be intubated. This technique results in a much lower amount of propofol needed for induction.

Thiopental

It is not commercially available anymore in the US. The initial bolus dose of thiopental can cause myocardial depression, hypotension, and respiratory depression. It is interesting to note that in study of hypovolemic dogs, thiopental was less hypotensive than propofol. If available, its use in emergency cases should be limited to those that are hemodynamically stable.

Ketamine-benzodiazepine combination

Midazolam or diazepam is added to the induction protocol to minimize muscle rigidity, spontaneous movement, and seizure caused by ketamine. Addition of the benzodiazepine to the protocol also reduces the amount of ketamine to induce anesthesia. Having minimal cardiopulmonary depression, benzodiazepine does not exacerbate the overall hemodynamic state of the animal. Ketamine stimulates the sympathetic system indirectly. It increases heart rate, blood pressure, and cardiac output. Despite the sympathetic stimulation, it has a minor effect on systemic vascular resistance. Its respiratory depressant effect is transient. Ketamine depresses the myocardium directly. In healthy and less compromised patients, this depressant effect is negated by the indirect sympathetic stimulation. In compromised patients, this direct myocardial depression may become prominent resulting in hypotension. The myocardial depression may also occur when larger dosages of ketamine are used.

Ketamine increases cerebral blood flow and intracranial pressure. It is not a good choice for patients with head trauma. It has some analgesic property and is suitable for trauma patients that are very painful.

The doses of diazepam or midazolam and ketamine are 0.25 mg/kg IV and 5.0 mg/kg IV, respectively. They can be combined in a single syringe before administration. Use lower doses (2/3 of the calculated dose) in dogs and cats that have profound sedation from the premedicants.

Telazol®, which is combination of tiletamine (dissociative) and zolazepam (benzodiazepine), can be used in the same fashion as a diazepam-ketamine combination. The cardiovascular and respiratory effects will be similar. The duration of action of Telazol® is longer than diazepam-ketamine. The dose of Telazol® is 1-3 mg/kg IV. Use the lower range of the dose in compromised patients.

The ketamine-diazepam combination is recommended for patients prone to hypotension. For Caesarean section, this combination maintains a better hemodynamic state for the dam compared with propofol; however, it was associated with less puppy vigor.

Etomidate

IV administration of etomidate results in rapid induction and recovery. Repeated administration does not result in accumulation, however, prolonged CRI is not recommended because it is formulated in 35% propylene glycol which has a very high osmolality. This can result in hemolysis, hyperosmolality, and lactic acidosis. Pain on injection, phlebitis and thrombosis may occur due to the propylene glycol. It causes minimal myocardial and respiratory depression. It is recommended for emergency cases that are more critically ill, e.g., hypovolemic and septic. It can be used for C-section.

It decreases cerebral blood flow and intracranial pressure making it a good choice for patients with head trauma. It suppresses adrenal response to ACTH and this may persist for several hours after induction. It should not be used in animals in Addisonian's crisis. It does not have analgesic property. During or right after induction, myoclonic muscle twitching and gagging may be observed. The incidence of these side effects is higher in patients that are not premedicated. The quality of induction with propofol is definitely better than that seen with etomidate.

The calculated dose is 2.0 mg/kg IV; 1/4 of the dose should be given over 30-40 seconds and the remaining dose is given to effect. To reduce the side effects seen during etomidate induction, a benzodiazepine can be incorporated during the induction process. The technique involves administering ¼ of the calculated dose (0.5 mg/kg) of etomidate first. It should be given slowly. Then, a dose of benzodiazepine (midazolam or diazepam, 0.1-.02 mg/kg) is given IV. The rest of the etomidate is given to effect until the patient can be intubated. This technique also results in a much lower amount of etomidate needed for induction.

To minimize the pain during injection, it is advisable to administer etomidate with fluids. This means that the crystalloid is being infused while the etomidate is being administered. The idea behind this is to dilute the etomidate as it is entering the vein of the patient.

Neuroleptanalgesic combination (opioid and benzodiazepine)

Administration of neuroleptanalgesic agents is a suitable induction method for critically ill patients. This is not the preferred technique if rapid control of the airway (e.g., diaphragmatic hernia, airway obstruction, and high potential for vomiting) is important. Emergency cases with continuous internal bleeding, in shock, or that are septic are excellent candidates for a neuroleptanalgesic combination. The author prefers not to use this combination for C-section because of the high dose of the opioid needed for induction.

Opioids and benzodiazepines cause minimal myocardial depression. In general, opioids maintain cardiac output, blood pressure, and tissue perfusion. They are both reversible. The opioid component produces analgesia.

Neuroleptanalgesic combination results in slower induction and the patient may still be swallowing during the induction process.

The higher dose of the opioid is needed in this combination to induce deep narcosis. It is important that the heart rate is monitored closely during the induction process. The author does not routinely administer anticholinergic before induction. The anticholinergic is drawn up for quicker administration when sinus bradycardia occurs.

Opioids and benzodiazepines are not complete anesthetics in dogs and cats. The patient can respond to sharp noise and will move. It is important that a quiet environment is maintained during induction and the initial phase of anesthetic maintenance.

Diazepam and hydromorphone are given at 0.25 mg/kg IV and 0.2 mg/kg IV, respectively. If midazolam is used, the same dose is used. Fentanyl or remifentanil can substitute for hydromorphone. The dose for fentanyl and remifentanil is 10 ug/kg IV. The IV catheter should be flushed before and after diazepam administration to prevent precipitation brought about by the chemical incompatibility of diazepam with the other drugs. Another way of preventing precipitation is to administer crystalloid fluid during the induction process. The fluid that is flowing through the IV catheter becomes the carrier of the drugs. Midazolam, a water soluble agent, does not cause precipitation when mixed with other drugs.

Maintenance

Anesthesia for emergency cases can be maintained using isoflurane or sevoflurane. Cases that are stable can be maintained on inhalant alone; however, cases that are critical need a balanced anesthetic technique. Critical patients appear to respond poorly to inhalant anesthetic. They develop hypotension despite fluid and inotropic support. When the vaporizer is reduced to bring the blood pressure up, they show signs of very light plane of anesthesia. Balanced anesthesia involves lowering the requirement for inhalant anesthetic using other drugs to produce analgesia and CNS depression. The most common drug we use in this setting is fentanyl. It is given as CRI using a dose range of 0.2-2.0 ug/kg/min. This is a much higher dose of fentanyl compared with the dose used in postoperative analgesia. By doing this, the vaporizer setting can be reduced to less than 1.0% with isoflurane and less than 2.3% with sevoflurane. An alternative to fentanyl is remifentanil. Remifentanil's major advantages are its quick onset of action and rapid clearance when the infusion is stopped. It is broken down by esterases and does not depend on hepatic metabolism and renal excretion. The dose range for remifentanil is similar to fentanyl.

If the fentanyl or remifentanil CRI will continue until the recovery period, it is prudent to reduce the dose range to 0.03-0.08 ug/kg/min about 30 minutes before the end of anesthesia with fentanyl and about 5 minutes for remifentanil. If the infusion will not be used in recovery, it can be stopped at times indicated above for fentanyl and remifentanil. A dose of a longer acting opioid should be ready and administered once the patient is extubated when using remifentanil and about 30 minutes to an hour after the stoppage of fentanyl. The patient's presentation after extubation will also dictate if a dose of a longer acting opioid is needed sooner than the time indicated above.

In very painful patients, an opioid (e.g., morphine), ketamine, and lidocaine can be infused during anesthesia to balance the technique.

Adjunct to general anesthesia including local anesthesia

Any drug given during general anesthesia to provide analgesia and reduce inhalant requirement is considered as an adjunct. The drugs that we commonly use are opioids, ketamine, and lidocaine. They are administered in most cases as CRI. The dosages for these drugs are in one of the CVC proceedings paper this year entitled "Constant rate infusion analgesia."

Local anesthesia or administration of opioid (morphine) epidurally may be part of the anesthetic management of emergency cases that are painful. Contraindications for epidural injection are septicemia, coagulopathy, skin problem on the pelvic area, and pelvic fractures (where landmarks are difficult to ascertain). There are specific nerve block techniques that can be used depending upon the area that is affected. Here are some examples: mandibular block for mandibular fracture, intercostal block for lateral thoracotomy, line infiltration for abdominal procedure, brachial plexus block for radio-ulnar fracture. Epidural administration of morphine is an option for chest trauma, abdominal wound, and front leg fracture. The dose for epidural morphine is 0.1 mg/kg. Epidural administration of local anesthetic to trauma patients (especially hypovolemic ones) should be avoided. Epidural local anesthetic blocks the sympathetic system at the spinal level resulting in hypotension.

Intraoperative support

Fluid

Isotonic electrolyte solution (crystalloid) is the most common preparation to maintain intravascular volume and replace the extracellular volume deficit. Emergency cases may need crystalloid given at a higher rate (20-40 mL/kg/hour) before anesthetic induction. This is to prevent severe hypotension after anesthetic induction. Anesthetic drugs will blunt the compensatory mechanism of the body and this will result in the unmasking of the underlying hypovolemia.

Available crystalloid solutions for volume replacement and expansion are lactated Ringer's solution, Normosol, Plasmalyte, and 0.9% saline. In case of acute intraoperative hemorrhage, hypertonic saline at 4 ml/kg IV over 5 minutes can be given right away and then followed by isotonic crystalloid solution.

During anesthesia, patients may remain hypotensive despite repeated boluses of crystalloid. When this happens, the next step is to use a colloid solution. Hetastarch is the one we commonly use. It is given initially at 5.0 ml/kg over 10 minutes. As a rule of thumb, Hetastarch should not be given at dose of more than 20 ml/kg/day in dogs and 12-15 ml/kg/day in cats. While the Hetastach is being given, the administration of crystalloid solution continues at 10 ml/kg/hour if the PCV and plasma protein levels are still within the acceptable limit during anesthesia. PCV and plasma protein should stay above 20% and 4.0 mg/dl, respectively.

Inotropes and pressors

A patient under anesthesia may require inotropic support when volume replacement is not enough to maintain acceptable blood pressure (mean arterial pressure ≥60 mmHg).

Dopamine is the initial inotrope we administer to hypotensive patients. It is administered as a CRI without giving a bolus dose. Its cardiovascular effect is dose-dependent. The beta effect (increased myocardial contractility) is expected following a dose of 5-10 μg/kg/min. The alpha effect (peripheral vasoconstriction) is produced using a higher dose of 10-20 μg/kg/min. A good starting dose is 5.0 μg/kg/min. A positive response should be seen in 3-5 minutes. If the blood pressure does not increase at this dose, the rate can be increased by 50%. Very sick patient may not respond to dopamine and another inotrope or a pressor may have to be used.

Dobutamine is primarily a beta agonist. In small animals, it appears that the beta-2 effect (peripheral vasodilation) exists resulting in less significant increase in blood pressure. Patients that are vasodilated (state of shock) will not respond to dobutamine in terms of increasing blood pressure. However, it is possible that there is an increase in cardiac output. Since we do not measure cardiac output in clinical setting, this effect will not be apparent. There are patients with preexisting conditions that will benefit from dobutamine's peripheral vascular effect. These conditions include dilated cardiomyopathy, chronic renal failure, and mitral regurgitation (chronic valvular disease). The dose of dobutamine is 2-10 μg/kg/min. A bolus dose should not be administered before starting the infusion.

Ephedrine is a useful alternative to dopamine. Following a bolus dose of 0.03-0.1 mg/kg, the beta and alpha receptors are stimulated resulting in increased myocardial contractility and vasoconstriction. Patients that are vasodilated will respond positively to ephedrine. Its onset of action is less than a minute while its effect lasts for about 20 minutes. Repeated administration of ephedrine will result in less effect (tachyphylaxis). A CRI dose of 0.01-0.03 μg/kg/min has been reported in the literature. The author has not used ephedrine as a CRI.

Patients in shock (vasodilatory shock) rarely respond to dopamine, dobutamine and even ephedrine. If there is no response to these agents, a potent vasoconstrictor is needed. This is assuming that attempts have been made to replace the fluid deficit. Norepinephrine and vasopressin are vasoconstrictors that can increase blood pressure. Norepinephrine is primarily an alpha agonist. It is given as a CRI at 0.5-2 μg/kg/min. The author prefers to stop the administration of the other inotrope while norepinephrine infusion is being given. An alternative to norepinephrine is vasopressin. It has been shown that patients in vasodilatory shock have a deficiency in vasopressin. Their adrenergic receptors are downregulated and the response to the usual adrenergic stimulation is decreased. By giving vasopressin, the adrenergic receptors become sensitive again to the adrenergic agonist. Vasopressin is given at 0.001-0.004 U/kg/min. It is best to continue the adrenergic agonist while the vasopressin is being administered. Its effect is not instantaneous. Sometimes, it may take 10 minutes or more before significant effect is seen.

Ventilation and oxygenation

Ventilation and oxygenation of these patients should be monitored closely. Adequacy of ventilation can be assessed using capnography and blood gases. If inadequate, ventilation should be supported either by mechanical or manual ventilation. Those trauma patients with lung pathology should be ventilated conservatively using lower peak inspiratory pressure.

Oxygenation can be assessed using pulse oximetry and blood gases. SpO2 should be maintained above 96%. When the anesthetized patient is receiving a high concentration of oxygen, the PaO2 should be above 400 mmHg. In patients with lung lesion, it is expected to have a PaO2 below than the expected levels. In practice, as long as the PaO2 is above 90 mmHg, the patient should have enough oxygen content. If the patient is hypoxemic, ensure that the patient is receiving high concentration of oxygen. Ventilate the patient to open up some collapsed alveoli. Ensure that there is no evidence of pneumothorax. Septic animals with respiratory compromise are the most difficult cases to manage. Despite changing ventilatory strategies, they will stay hypoxemic. Adding a PEEP (positive end-expiratory pressure) valve to the ventilator should be tried if the patient does not respond to the typical IPPV (intermittent positive pressure ventilation).

Body temperature

Patient's body temperature should be monitored and kept at normal levels. Based on our experience, a forced-air warming system, e.g., Bair Hugger®, maintains body temperature well. There are other ways of warming the trauma patients which include warm-water blankets and HotDog® conductive fabric warming device. Fluids, blood and blood products should be warmed before administration. It is important to treat hypothermia because it can decrease metabolism of drugs, impair glucose utilization, decrease renal function, and affect platelet function. During recovery, hypothermia will lead to shivering increasing the oxygen consumption of the trauma patients.

Renal function

Maintaining normal renal function during anesthesia can be achieved by appropriate fluid therapy and supporting systemic blood pressure. Monitoring blood pressure during anesthesia is essential in these patients.

Recovery

The following should be managed and addressed during the recovery period: cardiovascular, respiratory, and bowel function, mentation, and pain. The animal should recover in a quiet environment. The room temperature should be optimal. The patient should be kept clean, free of dried blood, feces, and urine.