Anesthesia and Care of the Large Animal for Survival Studies: Pig
Pigs have been used for many years in cardiovascular research and have been especially valuable because of the similarity of pig heart physiology to that of a human heart. The size of the animal can vary enormously but most pigs used in cardiovascular research are young adults approximately 50-90 kg in weight. As pigs mature the heart becomes relatively small in relation to the total body weight with the heart weighing 0.3% of the total body weight in a 100 kg pig1. Cardiac anomalies are not uncommon in pigs and have been reported to occur at an incidence of 4.35% 2 to 14.6%.3 The more common anomalies are listed in Table 1.
Table 1. Common congenital cardiac malformations in pigs
A major consideration in swine anesthesia is that certain breeds are susceptible to malignant hyperthermia triggered by stress and excitement or by certain anesthetic drugs, especially the inhalant agents and ketamine. However the mutation that leads to the susceptibility of the malignant hyperthermia, lies within a specific calcium channel in skeletal muscle, and has largely been selectively bred out of many pig populations because of the enormous economic impact felt when the disease was more prevalent years ago. Most vendors now are able to supply pigs without this mutation and the attending veterinarian can recommend reliable vendors for purchase of research pigs.
Fat stores constitute a large proportion of a pig’s body weight and act as a drug reservoir for lipophilic drugs. It also means that injecting into the lumbar muscles of a pig requires a long needle such as a spinal needle to ensure that muscle is reached. Intramuscular injections are best done in the neck behind the ear where the skin is thinner and there is minimal body fat, although sometimes it is necessary to use the hindquarters for injection. For this, the semi-membranous or semi-tendinous muscles are often used, although the quadriceps group would be preferable to avoid potential damage to the sciatic nerve. Another problem posed by the fat of pigs is that many vessels easily accessed in other species such as the carotid artery and jugular vein are much deeper in pigs and usually require a cut-down.
A housekeeping tip for pigs is to bathe them the day before the study begins. Pigs love water and do not resent being hosed off to remove any fecal material from the skin and feet. A brush with very soft bristles and shampoo can be used to clean the pig including the feet and ears. If brushing is done too aggressively the skin will become inflamed. Even with clean housing in a research facility and slats in the floor to remove urine and feces pigs can become aromatic relatively quickly.
Because the inborn nature of pigs is to snuffle along the ground and use their considerable jaw strength to root up material they generally do not resent having a facemask applied. Using a fruity flavor on the mask helps with acceptance, especially berry or watermelon (not citrus). This is not an ideal method of induction because of pollution of the environment and the time required for unconsciousness but a mask is useful if oxygenation is required before or after surgery.
Something to consider in cardiovascular research is the risk of sudden death several days to weeks after certain types of cardiac surgery that have created compromised myocardial oxygenation. Because pigs love food so much they become quite excited at feeding time. A pig with cardiac compromise can develop ventricular fibrillation leading to death from the excitement caused by the anticipation of a meal and the noise created by the other pigs.
Pigs resent being restrained and have an exceptional ability to vocalize their displeasure at a very high number of decibels (4,5). Personnel working with pigs should always be supplied with hearing protection. In spite of this distressing feature of pigs they are intelligent animals and can be trained relatively easily to step into specially designed slings that can be supported on a frame. This allows for catheterization, injections, blood sampling, wound dressings, etc. with much less stress to the pig or the attendants. Gentle training daily for an hour or two with treats provided will usually have an amenable pig ready for a study within a few days to a week. It also means a more relaxed pig that is easier to manage on a daily basis.
A common but less humane method of restraint is a nose snare. This consists of a long handle with a retractable metal cord that is placed around the snout and quickly pulled tight to entrap the snout. This causes tissue damage and bleeding and extremely loud vocalization by the pig. This method does not encourage the pig to allow people to approach on subsequent occasions.
For simple tasks such as intramuscular injection it may be sufficient to use a large board to herd the pig into a corner and restrain it there.
C. Physical Examination
This can be a challenge in untrained pigs as they are difficult to restrain and have very strong jaws and can bite when annoyed. Sometimes it is easier to do some of the examination from a distance, e.g. respiratory rate and pattern, etc.
Obtaining samples for pre-operative blood work may be difficult due to a lack of accessible peripheral veins. The most easily accessible vein in pigs is the dorsal auricular vein that is easy to see in Yorkshires but not quite as easy in some other breeds such as Yucatans. The skin is very thin and sensitive so a hematoma easily forms if the pig reacts to the first sensation of the needle tip. If time allows a proprietary eutectic mixture of the local anesthetics lidocaine and prilocaine (EMLA cream®) can be applied approximately 30 minutes before blood sampling or catheterization to desensitize the skin and make catheter placement easier for pig and handler. This may not be successful if the pig shakes the ear sufficiently to remove the dressing with the EMLA cream.
Table 2 lists normal vital signs for pigs. Tables 3, 4, and 5 list hematology and chemistry values. Normal body temperature is a little higher than for some other species but the large fat stores may help to maintain some of this in the earlier stages of anesthesia until the chest is opened. Auscultation of the heart is done at the fourth intercostal space located just posterior to the left elbow in a standing pig. Lung sounds are sometimes difficult to ascertain if the pig is grunting.
Table 2. Normal vital signs for resting adult pigs.
Table 3. Erythrocyte parameters in pigs (17,18).
Table 4. Leukocyte parameters, plasma protein, and fibrinogen in pigs.
Table 5. Serum chemistry in pigs.
Pigs tend to vomit easily so they should be fasted overnight (12-18 hours) and water withheld for 1 to 2 hours prior to anesthesia.
D. Preparation of Equipment
A breathing circuit including a 3 L rebreathing bag designed for adult humans is suitable for a pig. The anesthetic circuit and ventilator should be pressure checked to 25-30 cm H2O to ensure there are no leaks and the endotracheal tube cuffs should be inflated and left undisturbed for 10-15 minutes to ensure there are no slow leaks. The most suitable size of tube for a 50-90 kg pig is 9 to 12 mm ID. Damage to the cuffs by sharp molars may occur during intubation so it is wise to have two or three tubes ready for use in case one has to be discarded. Lubricant should be used on the cuffs and lidocaine jelly is most suitable as pigs develop laryngospasm easily. A laryngoscope with a long straight blade and a bright light is essential as well as a tie or rope for holding the jaws open for intubation and for tying the tube in place. One to two milliliters of 20% lidocaine should be sprayed onto the vocal cords before the tube is inserted into the trachea.
A peripheral nerve stimulator is essential if the pig is to be paralyzed. Infusion pumps and printouts of varying infusion rates for the animal should be prepared and an anesthetic record form prepared. Label all syringes with the names of drugs to be used. A bland eye ointment will protect the corneas from scratches, dessication, spray solutions, etc.
There are several different agents that can be used for premedication prior to general anesthesia and a combination is preferable to one drug alone. Heavy sedation will allow easier handling of the pig for catheter placement and better relaxation for intubation.
The most convenient locations for injection are the dorso-lateral neck muscles or the semi-membranous/semi-tendinous group, using a needle at least two inches long such as a spinal needle. Pigs do not enjoy being restrained or injected so a method that is often helpful is to attach an extension set to the needle. When the needle is inserted the pig will move abruptly or try to run but the extension tubing will allow the handler to move with the pig while injecting without the needle becoming dislodged. Another method is to use a butterfly injection set but this is suitable only for small pigs as the needle is usually too short to reach muscle reliably. Table 6 lists suitable dose rates for premedication, induction, and maintenance of anesthesia in pigs.
Table 6. Drug doses for anesthesia of pigs (20-26).
Atropine or glycopyrrolate
These are useful in pigs for decreasing salivary secretions and preventing bradycardia. Laryngospasm is easily provoked in pigs and a reduction in saliva is helpful in this regard.
Azaperone (not available in U.S.)
Azaperone is a member of the butyrophenone group of tranquilizers that has been used extensively in Australia, Canada, Great Britain and Europe for preventing aggressiveness in newly mixed batches of pigs, and as a general sedative for a variety of veterinary procedures. It is highly effective in the pig, producing dose-related sedation with minimal adverse effects, these being primarily salivation and a slight decrease in arterial blood pressure. For maximum effect, animals should be left undisturbed immediately following administration because excitement (especially noise) during this period will delay or prevent sedative effects.
The cyclohexamine ketamine is commonly used in swine for its rapid onset and reliable sedation. If used alone at higher doses muscle tone is increased and salivation with teeth grinding is common. Ketamine is normally used in combination with other injectable agents such as xylazine or diazepam. A reason to avoid ketamine in certain breeds is that it can induce malignant hyperthermia but careful selection of vendors should avoid this risk.
Diazepam or midazolam
These drugs are quite useful for pigs as they produce relaxation, unlike the effect in many other domesticated species. Diazepam and midazolam produce useful sedation of relatively short duration (20-30 minutes) for non-painful procedures such as catheter placement or wound dressing. Midazolam is water-soluble so tends to be absorbed more quickly and have a shorter duration. Diazepam is not water-soluble and is painful on injection. Because of their muscle relaxant and anti-anxiety properties, these agents are often combined with ketamine to decrease the undesirable side effects of ketamine.
In general, the phenothiazine tranquilizers, including acepromazine, produce inconsistent effects in the pig.
Pigs are quite resistant to the tranquilizing, sedative, and muscle relaxant effects of xylazine and this is not a suitable drug to use alone. It is better used in combination with other agents such as ketamine. It can induce salivation and bradycardia with reduced cardiac output.
F. Intravenous Catheter Placement
Once the pig is sedated, an IV catheter is placed for fluid and drug administration. The auricular vein on the lateral pinna is the most easily accessible peripheral vein for placing a 20-gauge catheter. A rubber band secured around the base of the ear with a hemostat may improve the success of catheter insertion by distending the vein. The hemostat is released once the catheter is in the vein. Skin glue should be applied to secure the catheter in place. When the pig is positioned for surgery the pinna may bend in such a way that the catheter becomes kinked if the tip is close to the junction of the pinna with the head. To prevent kinking a large roll of gauze bandage can be placed on the medial side of the pinna and the pinna wrapped around the gauze roll. The catheter can then be secured with tape that completely surrounds the pinna and is taped to the table surface. A wooden tongue depressor could be used instead of a gauze roll.
Pigs of the Yucatan breed with short ears and darkly pigmented skin present a challenge for peripheral catheter placement. The medial surface of the forelimbs or hind limbs is a more suitable location and in some cases may require a cutdown.
The medial saphenous vein is an ideal site if a large bore catheter is required. It is more efficient to make a hole in the skin first by using a larger bore needle than that of the catheter to be used. The catheter can then be easily inserted through this hole for insertion into the vein. The catheter should be fixed in place with skin glue.
Either intravenous or inhalation induction can be done in pigs. For pigs with cardiac compromise pre-oxygenation should be done first. A conical face mask is ideally suited to the facial structure of the pig and pigs do not resent being masked. The pig is pre-oxygenated (oxygen flow 3-5 liters/minute) for 3-5 minutes and then anesthesia induced by inhalation or intravenous injection.
For inhalation induction isoflurane or sevoflurane is gradually introduced. Once the mask is removed to allow intubation the pig recovers consciousness fairly quickly if it has not been heavily sedated and this limits the time available for intubation. Other disadvantages of inhalant induction are the inherent risk of vomiting or aspiration of saliva when the airway is unsecured as well as pollution of the environment with inhalant vapor. Pregnant personnel should not be involved in this method of induction.
A common drug protocol for intravenous induction of healthy pigs is ketamine added to diazepam or midazolam. The drugs can be mixed in the same syringe and given to effect. It is preferable to have extra drug drawn up because intubation can be a frustrating process. An anticholinergic drug as part of premedication will limit the salivation normally caused by ketamine.
Propofol is not usually suitable because apnea may occur, especially with rapid injection. However experienced personnel often become adept at intubation and in this case a short period of apnea may not be of concern. A useful feature of pigs is that they can be manually ventilated fairly satisfactorily by placing a tight fitting mask over the snout and squeezing the rebreathing bag on the anesthesia machine. This means that if apnea has occurred after propofol the pig can be oxygenated in between attempts at intubation. This depends somewhat on the size of the pig but for a weight range of 50-90 kg this method is usually successful. The choice to use a muscle relaxant should depend on the skill of the operator at the task of intubation because of the time that might be required, and the ability of the operator to manually ventilate via the snout as just described. Usually it is the anatomy of the oropharynx and larynx that creates most of the difficulty with intubation rather than laryngospasm.
Propofol would not be the best choice for a pig with moderate to severe cardiac compromise and for these animals etomidate and midazolam are a better choice for induction. A disadvantage of etomidate is the cost of the drug and in humans who are septic or critically ill adrenocortical suppression (6,7) is a distinct disadvantage. Depending on the duration and extent of the procedure to be done and the status of the animal at the time, steroid cover may be indicated if etomidate is chosen (Table 7).
Table 7. Steroids, antibiotics, and ancillary drugs for pigs.
The pig airway is similar to that of ruminants in that the left apical bronchus branches from the trachea several centimeters above the tracheal bifurcation and if an excessively long endotracheal tube is used it could potentially block that lobe from ventilation. However this is unlikely complication because of the length of the neck compared to the length of most endotracheal tubes that are commercially available.
Pigs can be difficult to intubate because of certain features of their laryngeal anatomy and the long distance from snout to larynx (Figs. 1a, 1b). There is a middle ventricle (diverticulum) in the floor of the larynx near the base of the epiglottis, as well as lateral ventricles that may catch the tip of the endotracheal tube as it is being passed through the glottis. The path for the endotracheal tube is almost a Z shape because of the sharp angles between the sections of the larynx and the junction of larynx and trachea.
Figure 1a. Saggital view of a pig skull showing the location of the epiglottis. Photograph by Andrew Cunningham.
Figure 1b. Close view of epiglottis at the base of the tongue.
The epiglottis of the pig has a rounded shape at its edges (Figs. 2a, 2b) that makes it easy for the endotracheal tube to slip over it and enter the esophagus.
Figure 2a. Rounded edges of the epiglottis of a pig.
Figure 2b. Shape of the glottis structures that allow the endotracheal tube to slip into the esophagus instead of the trachea.
Another feature of pigs that makes intubation difficult is the linear thickening along the center of the dorsum of the tongue (Figs. 3a, 3b) that obstructs a clear view of the glottis and often causes the laryngoscope blade to slip to the side. The endotracheal tube then tends to deviate laterally so that the tip catches on the tissue fold lateral to the epiglottis. If resistance is felt during intubation the tube should be withdrawn and redirected to avoid causing inflammation or hemorrhage in that area.
Figure 3a. Thickened ridge along the dorsal surface of the tongue.
Figure 3b. Close view of dorsal thickening of the tongue obstructing the view of the glottis.
The pig is easily intubated in the sternal position, although some people prefer the pig to be dorsal. The jaws are held open and the tongue is withdrawn. The epiglottis is often trapped by the soft palate and therefore not visible. It can be released by applying pressure at the base of the tongue with the laryngoscope blade. If this is not successful the endotracheal tube can be rotated so that the tip pushes the palate upwards (dorsally) as the tube advances and then the tube is rotated and directed ventrally. The larynx is desensitized with lidocaine and a cuffed endotracheal tube that is liberally lubricated is inserted through the glottis. A technique that the author has found to be useful is to slowly rotate the tube by using a gentle spiral motion as it is slowly advanced through the glottis. This tends to avoid the ventricles that can snag the tip. Once in the trachea the cuff of the tube is inflated and the tube secured to the upper or lower jaw. Capnography should be used to ensure correct placement.
Protection of the eyes with eye lubricant is essential to avoid any possibility of a corneal scratch. Ideally the hind limbs should be supported by padding on the lateral sides (between the pig and the table) to avoid excessive stretch of the ligaments surrounding the hip joints Abduction can be a source of morbidity after recovery such as pain and stiffness of the hind limbs.
I. Maintenance and Monitoring
Anesthesia for maintenance and monitoring
Isoflurane or sevoflurane in oxygen using a semi-closed circle absorber circuit are most commonly used for maintenance of anesthesia. These agents have no analgesic properties and can initiate malignant hyperthermia in susceptible animals. Isoflurane is the most commonly used agent with a vaporizer setting at 1.5% to 2.5% and a flow rate of oxygen at 15-20 mL/kg/minute. The vaporizer setting can be decreased soon after analgesic infusions have commenced. Similarly, sevoflurane is initially provided at 2.5% to 3.5% and decreased when analgesics are added. Halothane is no longer available commercially in the United States. It is the inhalant with the greatest propensity to sensitize the myocardium to arrhythmias and is not a good choice for cardiothoracic surgery.
Nitrous oxide is sometimes used with oxygen as the carrier gases for pigs but the minimum alveolar concentration (MAC) value for nitrous oxide in pigs (277±188) is significantly greater than the value for man (104±19) and this indicates that nitrous oxide has minimal anesthetic effect in a pig.
The tidal volume should be set at 10-15 ml/kg with a peak inspiratory pressure not exceeding 25 cm H2O. Respiratory rate should be set at 16-20 breaths per minute.
These drugs provide only relaxation and no analgesia or anesthesia. Suitable nerves for monitoring neuromuscular blockade are the peroneal, ulnar, and facial nerves. For cardiovascular surgery the facial nerve is probably most suitable as movement in response to nerve stimulation offers the least distraction to the surgical team.
During maintenance of anesthesia of a paralyzed pig it is imperative to monitor signs of anesthetic depth to ensure that the animal is not conscious and paralyzed. Because eye reflexes are lost the best method is to monitor heart rate and blood pressure for any spikes with surgical stimulation or any other unexpected increases. Lacrimation is not a reliable indicator of awareness.
Analgesics should be included in the anesthetic protocol and in the author’s experience constant rate infusions of fentanyl, lidocaine and ketamine provide excellent analgesia with minimal effect on cardiovascular parameters. These can be infused via a syringe pump or added to the balanced electrolyte solution used for fluid therapy during surgery. If the latter option is chosen a separate electrolyte solution without added analgesics should be used for any fluid bolus that might be required to avoid an overdose of the analgesic drugs. Apart from the analgesic properties of these drugs there is a reduction of MAC that allows a lower percentage of inhalant to be used, thereby helping to maintain acceptable blood pressure.
Before anesthesia it is useful to create a computer chart of the dose range for each drug based on the animal’s body weight so that changes to the infusion rate can be made easily without having to make a calculation each time. Once the body weight is input the formula will calculate the flow needed for each drug at each dose rate for that particular animal.
Intercostal nerve blocks are beneficial and can be performed before surgery and repeated postoperatively to manage pain caused by a chest tube. Pig skin is quite thick and firm over the chest so it may be difficult to determine the correct depth for the needle. Posterior to each rib are the artery, vein, and nerve in this order. Bupivacaine at 1.5 mg/kg as a total dose can be divided into equal volumes to block two intercostal nerves on either side of the intercostal space where the incision is to be made (lateral chest wall) or where the chest tube exits. This is not as practical for a sternotomy because the length of the incision crosses many nerves and the dose of bupivacaine required for all the nerves would be excessive.
Epidural injection of analgesics can be done in pigs but there are some considerations that must be recognized. This technique is more suited to surgery on the abdomen or hindquarters of the animal and it is unclear to what extent epidural analgesia is helpful for thoracic surgery. The most effective drugs to use would be local anesthetics but the resultant paralysis creates much more work for animal care attendants and there is a risk of urinary infection. Cardiovascular collapse is also a risk if the injection is inadvertently done into the subarachnoid space. The most commonly used drugs for epidural injection in pigs are the alpha-2 agonists (xylazine, etc.) that have adverse effects on the cardiovascular system and therefore are not suitable for cardiovascular research. Opioids have not been investigated thoroughly for pig epidural injection.
Heart rate is normally in the range of 80-100 beats per minute and is somewhat responsive to changes in depth of anesthesia. A significant increase is a good indication of awareness in a paralyzed animal. Steadily increasing heart rate may indicate increasing hypercarbia if other signs of anesthesia are unchanged and could also be a response to blood loss.
The pulse can be palpated on the common digital artery just above the dew claws, the femoral artery, or the auricular artery on the lateral pinna in Yorkshires. The auricular artery is less reliable for easy palpation in Yucatans.
If arrhythmias such as premature ventricular contractions develop during anesthesia first ascertain whether the cause could be something simple that is relatively easy to reverse. For example painful stimuli, hypercarbia, excessive depth of anesthesia, or hypoxemia can contribute to arrhythmias. With cardiac surgery one could expect arrhythmias to develop due to direct stimulation of the myocardium and in this case 2% lidocaine is useful as a bolus at 2 mg/kg IV or sometimes sprinkled lightly directly onto the heart. An infusion may be necessary for ongoing treatment of arrhythmia at a rate of 75-85 mg/kg/minute.
Arterial Blood Pressure
This varies with depth of anesthesia. A mean value of 90-100 mm Hg is normal and hypotension during anesthesia means that anesthesia is too deep, or the pig is hypothermic or hypovolemic. Blood pressure can be measured indirectly by a Doppler ultrasonic probe and cuff or by oscillometry using a cuff around a limb. For indirect measurement the Doppler crystal can be placed posterior to the feet above the common digital arteries, just above the dew claws, and the cuff placed around the limb a few centimeters above this (Fig. 4). The width of the cuff should be as close as possible to 40% of the circumference of the limb.
Figure 4. Position of Doppler probe and cuff on a hindlimb for indirect measurement of arterial blood pressure.
For direct measurement a peripheral artery can be catheterized and connected via a transducer to an amplifier for a digital reading of systolic, diastolic and mean blood pressures. The auricular artery on the dorsal spine of the pinna (Fig. 5) is easily accessible and can be quickly catheterized with a 20 gauge catheter. The common digital arteries just dorsal to the dew claws are useful pulse points but very difficult to catheterize because of the proximity of the hard dew claws and the sharp angle that has to be negotiated for threading the catheter once the vessel is penetrated. The femoral and carotid arteries are other pulse points but both are fairly deep and require a cut-down for catheterization. These locations require very firm pressure for several minutes after removal of the catheter.
Figure 5. Pinna of a pig showing artery and vein locations.
Inotropes and pressors for cardiovascular support
Cardiac output and contractility can be supported if necessary by infusions of inotropes and/or pressors. Dobutamine, a beta agonist, will improve cardiac output primarily by increasing contractility. Ephedrine, an alpha and beta sympathomimetic drug, will increase contractility and systemic vascular resistance but there may be some tachyphylaxis with repeated use. Dopamine can be used in pigs but tachycardia at higher doses can limit its usefulness for treating hypotension.
The gold standard for PaO2 measurement is an arterial blood gas sample that can be obtained easily from a catheter inserted into the auricular artery. A guide to oxygenation can be obtained from a pulse oximeter that is useful for continuous calculation of hemoglobin oxygen saturation. This can be applied before or immediately after induction. The lip, tongue, or pinnae are the most commonly used sites but the tail is also suitable (Fig. 6). In dark skinned pigs the tongue and lip are the most useful sites.
Figure 6. Pulse oximeter probe on tail.
Arterial blood gas
For most domesticated species arterial blood gas values are similar. Pigs are omnivores and their base excess/deficit status is close to that of humans. Typical values for a pig being ventilated with isoflurane and an inspired oxygen fraction (FIO2) of 0.98 would be PaCO2 of 36-40 mm Hg and PaO2 in the range 550-650 mm Hg. Some V/Q mismatch may exist but is not usually extensive in pigs as compared to ruminants.
This is a valuable tool at induction for ensuring correct placement of the endotracheal tube and also during anesthesia for monitoring ventilation. Capnography can warn of various problems related to the animal or the equipment, such as venous air embolism, pulmonary embolism, bronchospasm, hypothermia, a significant decrease in cardiac output, rebreathing, inspiratory or expiratory obstruction, a leak in the circuit or cuff, or a malfunctioning exhalation valve.
For surgery of long duration a balanced electrolyte solution such as Ringer’s lactate is preferable to sodium chloride. The flow rate of IV fluids is generally 10ml/kg/hour but may need to be adjusted to allow for blood loss. A pressure bag should be available for rapid fluid replacement if indicated but this cannot be achieved via an auricular catheter because of the catheter size limitation. A large bore catheter in a vessel such as the medial saphenous vein or jugular vein would be required for a rapid flow rate. Changes in central venous pressure (CVP) will indicate the need for fluid replacement. If blood loss is significant a colloid solution should be used in conjunction with the crystalloid solution to retain oncotic pressure in the vasculature. This is usually done once the value of total plasma protein is approximately 4g/dL or lower. Hydroxyethylstarch solutions (e.g. Hetastarch® or Voluven®) can be administered at 5ml/kg as a bolus with repeated doses as needed up to a maximum of 20ml/kg/day (Hetastarch®) or 50 ml/kg/day (Voluven®). Great care should be taken not to exceed an appropriate volume and it is wise to measure the colloid oncotic pressure at the end of the procedure. A disadvantage of colloid solutions is the risk of allergic reaction and the effect on coagulation parameters if used in large quantities, although Voluven is less likely to create these problems (11).
Blood groups, blood volume, and blood transfusion
In cardiovascular research with long-term studies there is sometimes the need for blood transfusion during or immediately after surgery to maintain optimum health of the animals. For acute hemorrhage during surgery auto-transfusion is the optimum method of maintaining tissue perfusion and oxygenation. Normal blood volume of a pig is 5-6% of body weight so for the average pig of 50 kg with a blood volume of 2.5-3.0 liters a 10% loss is 250-300 milliliters.
There are sixteen blood group systems described in pigs, EAA, EAB, EAC, EAD, EAE, EAF, EAG, EAH, EAI, EAJ, EAK, EAL, EAM, EAN, EAO, and EAP (12). Research pigs are young and healthy when purchased and would not have received a blood transfusion so it is generally safe to transfuse whole blood without matching beforehand. If heart transplant is the subject of research much greater care should be taken with blood group compatibility because of the risk of adverse reactions. In pigs these reactions include bleeding, disseminated intravascular coagulation, and progressive hypotension (13,14).
Erythrocytes of pigs are more fragile than those of humans (15). Reports of survival time of erythrocytes in pig plasma vary depending on the age of the animals and the method used (16). A generally accepted life span is 86 ± 11.5 days (14). Bleeding time in pigs is reported as 3.4 ± 1.9 min and partial thromboplastin time (PTT) as 26 ± 5 sec (17).
Central venous pressure (CVP)
This is easy to measure via a Swan Ganz catheter or a regular jugular catheter and the value should be 5-10 cm H2O. The trend in values can guide the rate of fluid replacement.
For prolonged procedures a Foley catheter in the urinary bladder allows measurement of urine volume that should be 1 mL/kg/hour. Values for urine volume, CVP and arterial blood pressure will guide decisions on fluid therapy during the procedure.
Hematology and chemistry
Values for PCV, TP, blood glucose and lactate should be obtained periodically, e.g. every two hours. Glucose is required for muscle contraction and brain function and hypoglycemia will contribute to hypotension, decreased cardiac contractility, and a prolonged anesthetic recovery. A dextrose infusion can be instituted to correct hypoglycemia. The usual concentration is 2.5% but 5% may be required in severe cases. The osmolality of the solution depends on the solvent and the solution with the closest match to the osmolality of extracellular fluid (280) is 2.5% dextrose in 0.45% saline. Increased lactate indicates insufficient tissue perfusion which results in muscle pain that will affect the quality of anesthetic recovery.
The substantial fat layer of pigs means that they usually retain body heat reasonably well in the early stages of anesthesia although there is always some loss of heat. Heat loss increases after the chest is opened and is also promoted by the cold dry gas provided for ventilation. A decreasing body temperature means that less anesthetic agent is required and recovery can be prolonged. A fluid warmer for the intravenous fluid line to the pig can afford some protection but will not make a significant impact on the loss of body heat.
The pig has a small eye which is recessed and difficult to assess depending on the position of the pig on the surgery table. With a light plane of anesthesia the eye is central and there is a strong palpebral reflex. A surgical plane of anesthesia is characterized by ventral rotation of the eye with loss of the palpebral reflex. As anesthesia deepens further the eye will return to a central position. A dilated pupil in a spontaneously breathing pig indicates excessive anesthetic depth. With muscle relaxant use the pupil will be dilated, the globe will be centrally located, and the palpebral reflex is lost.
Use of a wound soaker (diffusion) catheter
Towards the end of surgery when closure is beginning, a wound soaker (diffusion) catheter should be interposed in the tissue layers to allow infusion of local anesthetic post-operatively. This is a good method of controlling postoperative pain and allows for a faster return to normal activity and appetite. There are different types commercially available in various sizes and local anesthetic can be infused for two to three days either by syringe pump or by the balloon incorporated in the catheter design. If lidocaine is used as a constant rate infusion the dose to use is 2 mg/kg/hour with a flow rate set at 5ml/hour. To accomplish this in an adult pig lidocaine commercially available as a 2% solution should be diluted to 1.5%. Remove 187 ml from a 250 ml bag of normal saline and replace with 187 ml of 2% lidocaine. The resultant concentration of lidocaine will be 1.5%. In the author’s experience this provides very good local analgesia without causing seroma formation. The skin of the chest wall is quite thick and the flow rate of local anesthetic could be increased to 6ml/hour if analgesia does not appear to be sufficient. An alternative to a lidocaine infusion is bupivacaine injected into the catheter every 6 hours (Table 6).
This is the part of anesthesia where problems may develop that can jeopardize the entire day’s work. Often it is due to inattention to the animal once it has recovered because of other tasks required at the end of the day. Losing an animal at this stage is demoralizing and expensive and can cause questions to be raised by the institution’s Animal Care and Use Committee. For best results it is advisable to assign staff for the express purpose of monitoring the animal for several hours after the end of surgery. The attending veterinarian is an important resource for all investigators and should be consulted if any doubt arises about animal care, especially for the postoperative period.
Once the procedure is complete the inhalant should be turned off and oxygen flow maintained for at least five minutes. The pharynx should be suctioned to remove any residual saliva because of the risk of laryngospasm. The pig must be kept in a warm environment until fully recovered. Extubation should not be attempted until signs of chewing or swallowing are present and the pig is able to stay in sternal recumbency. Care must be taken not to extubate too late however, as laryngospasm may occur. The other problem is damage to the cuff by sharp molars during extubation. Close observation during this period is also necessary in order to take immediate corrective measures if respiratory obstruction or laryngospasm should occur. Vomiting may also occur post-operatively. If the pig becomes agitated a low dose of diazepam or midazolam is very effective for sedation and relaxation without causing respiratory depression.
Two important causes of morbidity and mortality after cardiovascular surgery are insufficient reversal of neuromuscular blockade, and residual pneumothorax. Ensuring that neuromuscular blockade is fully reversed is a first step. If the pig is unable to lift its head after extubation residual neuromuscular block should be suspected and evaluated and additional antagonist can be administered if indicated. Other causes of weakness are hypokalemia, hypocalcemia, hypoglycemia, hypothermia, and moderate to severe anemia. These conditions are easy to assess and treat.
Evacuation of the chest is equally important and the animal’s ventilatory status should be very closely monitored in the early recovery phase. In some cases continuous suction may be required for a time. For certain cases and where practicable the arterial catheter should remain in place for several hours with regular flushing to allow blood gas sampling to continue. The intravenous catheter in the auricular vein must also be secured carefully as pigs tend to shake their ears if they feel a foreign object is attached. It must be securely fastened to the skin (skin glue or suture) and sealed with a luer lock cap. A pulse oximeter can give early warning of hypoxemia and is easily attached to the pinna. An unreliable reading can also result from peripheral vasoconstriction caused by hypothermia, pain, or anxiety.
Postoperative oxygen is best supplied via face mask at 2-3 L/min. Pigs completely resent any interference with their nares and insertion of a Foley catheter into the nasal passages is not practical nor is it a rewarding experience for personnel.
A third possible cause of morbidity and mortality is awareness during a long and painful surgical procedure and this indicates the need to provide humane care for the animal during surgery with careful monitoring by personnel who are not distracted by other tasks. Intolerable stress to the animal can be the cause of early postoperative death.
Basic values to monitor are hematocrit, plasma protein, lactate, blood glucose, body temperature, and if indicated colloid oncotic pressure. A complete chemistry and electrolyte panel and an arterial blood gas sample should be run if the pig does not appear to be recovering well. Always ensure that struggling is not caused by residual pneumothorax. Lung sounds should be monitored for wheezing that could indicate bronchoconstriction caused by aspiration. The total volume of intravenous fluids administered should be calculated. A good indication of satisfactory recovery of a pig is an interest in food.
Postoperative analgesia is very important for a smooth recovery. The use of a diffusion catheter for local anesthetic infiltration along the incision site is an excellent method of controlling postoperative pain and the catheter can remain in place for 3-4 days. Intercostal nerve blocks can be repeated to block nerves on either side of the dermatome where the chest tube exits the chest. If the pig appears to be uncomfortable buprenorphine can be injected IV and the ketamine, lidocaine, and fentanyl infusions can be continued at reduced rates for approximately 12 hours postoperatively (Table 6). Apart from the humane aspect of providing analgesia, it is extremely important to recognize that a comfortable pig will recover more quickly and be a more reliable research subject than a struggling pig that is in pain.
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