By Lisa A. Miller, DVM
The topic of whether or not to use sedation and/or anesthesia for diagnostic orthopedic radiographs recently came up on the RR-folk email discussion list. This article will discuss anesthesia and the drugs used for its maintenance in general, the potential reasons for using an anesthetic protocol for our pets when undergoing diagnostic and/or emergency radiographs for various conditions, as well as the use of anesthesia in obtaining OFA view and Penn-Hip diagnostic radiographs.
Anesthetic drugs may be grouped into categories by their mode of action and/or their purpose in an anesthetic protocol. There are those used as premedicants for general anesthesia, those used to induce general anesthesia, those used to maintain anesthesia, and those used for pain relief alone (analgesics). Drugs are selected based upon the health status of the patient, expected duration of the procedure, and the degree of pain anticipated. An important concept to keep in mind is that simple, one-drug anesthesia is not necessarily safer, and may actually be more detrimental, than combination-drug anesthesia. There are no anesthetic protocols that are safe and effective in all circumstances; however, your veterinarian will decide which protocols may be appropriate for your dog based on the above. Premedicating the patient, pre-oxygenating prior to induction, and maintaining an intravenous catheter and administering IV fluids during general anesthesia can improve the relative safety of any drug or drug combination.
Premedicants are used to reduce anxiety, provide pre-emptive pain relief, reduce the amount of anesthetic induction and maintenance anesthesia required, produce some muscle relaxation, and help provide “smooth” inductions and recoveries. Drugs with analgesic effects, such as opioids and drugs with sedative effects (including phenothiazine tranquilizers) are often included in a premedication scenario. However, premedicants may include agents that have no analgesic or sedative qualities at all, such as atropine, but that are used for other purposes, such as their cardiovascular protective effects. These drugs are usually administered as a subcutaneous (under the skin) or intramuscular injection around 15 to 20 minutes prior to anesthetic induction in non-emergency procedures.
Sedative and anti-anxiety drugs include medications like acepromazine, chlorpromazine, and valium or midazolam. Some of these may be administered orally or injectably and serve to calm and relax dogs that may be anxious, hyperexcitable, in respiratory distress due to an airway obstruction, or frightened. Acepromazine is a tranquilizer that alone or combined with opioids provides excellent sedation. It has no analgesic properties, but has an excellent anti-anxiety effect. These drugs are used in much lower doses for minimal sedation than when they are used alone or in combination for anesthetic induction.
Anesthetic induction agents are designed to rapidly induce unconsciousness that is free from excitement and struggling, and which will allow the placement of an endotracheal tube, if necessary, for maintenance of general gas anesthesia. An “ideal” anesthetic induction or protocol for all situations does not exist. Induction drugs are chosen with the intent of providing the smoothest induction possible with the least amount of undesired effects.
Non-opioid intravenous anesthetics (such as thiopental sodium, ketamine, Telazol, diazepam and midazolam, propofol, and etomidate) produce sedation, amnesia and deep sleep. The other components of anesthesia (loss of sensation, reflex and motor function) often require the addition of opioid analgesics or inhalant anesthetics. A few of the more popular non-opiod IV anesthetics are:
Propofol. Propofol is a non-narcotic, non-barbiturate and rapid-acting intravenous anesthetic. The rapid, smooth recovery from anesthesia is probably Propofol’s most important attribute. It is occasionally administered as continuous IV infusions (for maintenance of anesthesia or for refractory seizure-cases) or repeated boluses. Even when administered in this fashion, however, the recoveries are generally rapid and uneventful. Thus, propofol may be an excellent alternative to inhalation anesthesia. It is an excellent anesthetic for many outpatient procedures, such as small mass removals, dental prophylaxis, radiographs, etc.
Ketamine. Ketamine is one of the most commonly used anesthetic induction drugs in veterinary medicine. It is usually used in combination with an alpha-2 agonists (such as xylazine) or a benzodiazepine (such as valium/diazepam) to improve anesthetic induction. It is also known to induce analgesia for superficial pain (e.g., skin incisions) and has been used to prevent wind-up of the pain pathways in the spinal cord.
Etomidate. Etomidate is used infrequently in veterinary medicine. It is a short-acting sedative-hypnotic that causes minimal cardiopulmonary depression, and as such, it is often an excellent choice of anesthetic induction drug in the compromised dog.
Alpha-2-agonist drugs, xylazine and medetomidine (Domitor), provide excellent sedation, analgesia and muscle relaxation. Domitor is more potent and provides a longer duration of analgesia; however it should be used only in young, healthy animals. Its advantage is that it is reversible and may administered as an intravenous or intramuscular injection.
If our dogs are undergoing general anesthesia, they are usually maintained on inhalant, gas anesthetics, unless a constant rate infusion of an injectable anesthetic, like propofol above, is used. The two most common inhalants used today are isoflurane and sevoflurane.
Radiographs are often taken for several reasons. Some of the most common scenarios include radiographs to evaluate orthopedic injuries or pain, to evaluate the lungs and heart in states of disease or distress, in an emergency situation for triage, or to evaluate other potential abnormalities. Effective radiology techniques are designed to 1) obtain high-quality, diagnostic radiographs, 2) limit exposure of the patient and staff to excessive x-ray beam exposure, 3) be as pain-free and stress-free as possible for the patient, especially in a critical or painful animal.
Radiographs are often taken in an emergency situation in order to provide the veterinarian with a succinct piece of information to help him or her diagnostically, as well as in formulating an emergency therapeutic plan, and providing you with prognostic information. This is a different scenario than those listed below, one in which quick radiographs are taken, with as little stress caused as possible to our dogs. Positioning is often determined by the injury to the dog, the patient’s overall stability, and the purpose for the radiograph. Often no sedation to minimal sedation is used and the patient is literally “on and off” the table. Radiographs in cases of bloat (GDV), vehicular trauma, difficulty whelping, fresh fractures or gunshots, and others are included in this category.
Not surprisingly, when our dogs have fractures, sprains, strains or joint swelling, they are often extremely painful. To obtain a diagnostic-quality film, especially when evaluating for pre-surgical films, or for submission to a radiologist for evaluation, sedation/analgesia is an essential and valuable tool. Positioning of the affected joint or limb is crucial to evaluate bones and soft tissues for abnormalities, especially when dealing with areas where subtle changes may exist. Radiographs in these cases are not only used to tell IF an abnormality exists, but often to plan a surgical repair. Good positioning is achieved when a patient is relaxed and still, and when a veterinary staff is able to use techniques such as foam padding, soft-tape, positioning-boxes and other “tools.” We cannot ask our dogs to “hold still and turn your paw slightly counter-clockwise,” and as such, we must position THEM. If they are in pain, or if a joint is swollen, positioning for a diagnosis may certainly be uncomfortable. Analgesics are helpful in providing pain-relief and in relaxing the patient for these procedures, making it easier on everyone involved, including our dogs!
All dogs should of course be approached appropriately and every effort should be made to calmly interact with them. Struggling with a frightened or painful animal, or forcefully holding them down is stressful to the patient and staff, will take longer to take the radiographs, and achieves nothing, in my opinion. Many times, poor radiographs will have to be repeated, causing repeated stress to the patient, in addition to costing you more for additional films, when good-quality initial films would have been better for all involved.
Often, your veterinarian may take chest or abdominal radiographs to evaluate for evidence of heart or lung disease, the presence of metastatic cancer, gastrointestinal abnormalities, urinary tract disease or other problems. Positioning for evaluation in these cases is often just as important as in orthopedic films. A dog’s legs must be moved forward or back, out of the way of the area being evaluated. Additionally, on radiographs of the chest, films taken during inspiration versus expiration may look dramatically different, and motion during the “shot” will interfere with interpretation. Often mild sedation is helpful to relax the patient, and in some cases, such as pancreatitis, analgesics may help a painful pet to be more comfortable.
The OFA radiographic view and PennHIP radiographic views are different, both in what they are specifically evaluating for and in the positioning required to achieve a quality radiograph. Without discussing the pros and cons of the two methods of evaluation themselves, their differences from the standpoint of this article are as follows:
If we are evaluating hip joint laxity and/or the likelihood of the joint to luxate or subluxate, then LACK of sedation or anesthesia may affect a regular OFA view in that we may be falsely making the hip joint “look better” via the surrounding musculature being tensed, etc. Conversely, you cannot make a “good hip” look bad with anesthesia. For the best and most accurate assessment, the dog should be under general anesthesia.
Sedation helps us achieve a better quality film in some regards, and thus may help to better diagnose subtle changes that may be present radiographically in regards to evidence of degenerative joint disease (DJD) in the joint on an OFA view.
Taking a near-perfect OFA radiograph is a challenge. With an anesthetized dog in a positioning device (such as a foam-padded V-tray), it is easy to make positioning corrections and approach that perfect view. With an awake and/or restrained dog, you are forced to restart the process each time you make an exposure. Errors in positioning can either overstate or underestimate the true status of the hip joint.
As far as the PennHIP view is concerned, the following is quoted directly from the website (www.pennhip.org):
“To obtain diagnostic quality radiographs, the musculature around the hip joint must be completely relaxed. For the comfort and safety of the animal, this requires either heavy sedation or general anesthesia. The selection of the individual sedation/anesthesia protocol is left to your PennHIP veterinarian's discretion, so long as the dog is sufficiently sedated to obtain a diagnostic quality radiograph (and, of course, so long as the drugs and dosages used are safe).”
The author wishes to acknowledge Dr. William H. Adams, DVM, DACVR and Dr. Chris Dassler, DVM, DACVS for their assistance in consultation for this article.