2016 FALL: Arterial Blood Gas | Xylitol Toxicity | Elbow Dysplasia | Current Heartworm Topics | Adderal Toxicity | Continuing Education
Arterial Blood Gas – Why Don’t We Do Them More Often?
Myth 1: It’s too hard to obtain the sample.
Arterial blood gases may seem hard to acquire but actually are no harder in medium to large dogs than sampling a vein. Because the artery has a thicker muscular wall, it is can be slightly more difficult to pierce, but by using your finger to anchor the artery, you can make it easier. The most common place to try is the dorsal pedal artery on the medial aspect of the metatarsus. Another common location is the femoral artery or the lingual artery if under anesthesia. The femoral artery must be held off manually for 5 minutes after sampling. When you have a large dog under anesthesia, use this time to practice feeling an artery and even trying to obtain a sample in more controlled circumstances.
Myth 2: It’s too hard to handle the sample.
Arterial samples are handled the same way as venous with a few exceptions. Do not agitate an arterial sample as it can falsely elevate your oxygen content. You do not need to purchase a specific arterial sampling syringe, although those are nice. Aspirate heparin into a tuberculin then evacuate it; this will nicely receive the sample and prevent clot formation. The needle can be pierced into the rubber stopper of a lab tube to prevent further oxygenation. You have about 10 minutes to analyze the sample at room temperature, but if you need longer, the sample can be on ice for 30 minutes without degradation.
Myth 3: I need a special machine to analyze it.
I-STAT cartridges and some other in-house blood analyzers are designed to analyze oxygen and carbon dioxide concentrations. Check with the manufacturer if you are not sure, but any machine that measures pH and bicarbonate is likely to be able to analyze PaO2 and PaCO2.
Why bother if I have a pulse oximeter? Pulse oximeters are wonderful machines but have limitations. Sometimes they have difficulty picking up a signal in animals with pigmented mucosa, icterus or if the patient is not perfusing well. Measuring saturation of oxygen (SaO2) with a pulse oximeter only evaluates oxygenation and not ventilation. This can lead to false assumptions that animals with normal oxygenation do not have lung disease. Many patients can hyperventilate to tolerable SaO2 values but may collapse due to exhaustion if they have to keep hyperventilating.
|PaO2 mm Hg||SaO2%|
Myth 4: It’s too hard to interpret arterial blood gases.
With minimal practice, interpreting the basics of oxygenation and ventilation values is not difficult, i.e. determining that the patient is within normal limits or not. The two main values of importance are PaO2 (oxygenation) and PaCO2 (ventilation). The normal value for PaO2 on room air is 80-100 mmHg and for PaCO2, 35-45 mmHg. Most of us will want to perform an arterial blood gas to determine if a patient is hypoxic and needing supplemental oxygen. That would be verified by an arterial blood gas in the following two scenarios:
Scenario 1: The PaO2 is less than 80 mmHg. If you want to verify this finding with a pulse oximeter, you would see a SaO2 of less than 95%.
Scenario 2: The PaO2 is in the 80s but the PaCO2 is less than 30 mmHg. Finding this result indicates that the patient is hyperventilating significantly to normalize their oxygen level.
Both scenarios document a need for oxygen supplementation.
More information about arterial blood gas interpretation can be found in many books and on VIN. You can also take labs locally and at national conferences to practice sampling and interpretation Any one of our three critical care specialists at BluePearl in Georgia are also available to help in any way. Please let us know if you would like more information or if we can help if you have a difficult respiratory case.
Xylitol Toxicity: Not Just Gum
The number of dogs presenting for xylitol ingestion and toxicity has increased dramatically over the past few years. Xylitol is a crystalline sugar alcohol used as a sugar substitute sweetener in many products including sugar-free gum, candy, chewable vitamins, nutritional supplements and baked goods. It is available in a granulated form for baking. Based on demonstration of anti-cariogenic properties, xylitol is added to toothpastes and other oral hygiene products. In a retrospective evaluation of 192 cases of xylitol ingestion, 96% of the dogs presented for ingestion of sugar-free gum.1
Xylitol has a wide margin of safety in people but is extremely toxic to dogs. Compared to humans, dogs experience a rapid and severe increase in blood insulin resulting in profound hypoglycemia which can last up to 24 hours. Similar effects are seen in cows, goats and rabbits. Cats and ferrets have not shown toxic effects. Ingestion of large amounts of xylitol has resulted in liver failure in dogs.
Diagnosis of xylitol toxicity is based on history of ingestion, symptoms and bloodwork. Common presenting clinical signs include vomiting, lethargy and weakness. Diarrhea, collapse and seizures may be seen. Hypoglycemia has been reported within 30 minutes of ingestion but can occur up to 12 hours post-ingestion.
Treatment recommendations are based on the amount of xylitol ingested (see table below). There is no known antidote and a narrow window for safe decontamination. Xylitol is rapidly absorbed with peak plasma concentrations at 30 minutes. Emesis is recommended in asymptomatic dogs. Activated charcoal is not likely to be beneficial as charcoal does not bind to alcohol-type compounds.
Calculating the amount of xylitol contained in products can be difficult. Products that list xylitol as the first ingredient tend to be the most toxic. The amount of xylitol in gum can range from 0.9 mg to 1000 mg/piece. While some gum products specify the xylitol content on the label, many manufactures consider this to be proprietary information. The ASPCA Animal Poison Control Center (APCC) recommends that if xylitol is the first sugar alcohol in the ingredient list, then the estimated dose should be based on the total amount of sugar alcohols. If not the first ingredient, xylitol should be estimated to be 0.3gram/piece of gum. Granulated (baking) xylitol contains 190 grams/cup. Gabapentin liquid contains 300 mg xylitol/ml and could reach toxic levels at higher doses.
The ASPCA APCC recommends that dogs ingesting 50-100 mg/kg should receive decontamination and monitoring.2 However, hypoglycemia has been reported in a dog ingesting an estimated 30 mg/kg xylitol dose.1 Dogs ingesting >100 mg/kg are at increased risk for hypoglycemia and should be treated more aggressively. Ingestion of higher doses increases the risk of liver failure and coagulopathy. All dogs reported developing xylitol-induced liver failure ingested > 500 mg/kg; however, it is not clear at this time whether the effect is dose-related or idiosyncratic. In a case report of dogs developing acute liver failure subsequent to large dose xylitol exposure, six of the eight dogs did not develop hypoglycemia prior to the onset of liver failure. 3
The prognosis is very good for dogs treated promptly and for dogs with uncomplicated hypoglycemia. In a 2015 retrospective study reporting 192 cases of xylitol ingestion, 15.6 % developed hypoglycemia, and 21.9 % of dogs with serum biochemistry panels performed developed mildly increased ALT or total bilirubin. No dogs developed clinical signs or biochemistry values consistent with liver failure. All dogs survived. Mild increases in liver enzymes usually resolve within a few days with supportive care.
Severe or progressive increases in liver enzyme activities (>1000), hyperbilirubinemia and coagulopathy, carry a more guarded prognosis. Mortality rate of 70-80% is reported when acute liver failure develops. Hyperphosphatemia appears to be a poor prognostic indicator.
|Ingested Dose||Treatment recommendation|
|50-100 mg/kg||Induce vomiting if asymptomatic, observe|
|100-500 mg/kg||Induce vomiting if asymptomatic. Obtain baseline glucose, potassium, phosphorus, biliruibin and liver enzymes. Monitor blood glucose every 1-2 hours for at least 12 hours and other tests every 24 hours for at least 72 hours. If hypoglycemia develops, 1-2 ml/kg bolus of 25% dextrose and continue fluids supplemented with 2.5-5% dextrose to maintain normal glucose. Continue until glucose concentration can be maintained without supplemental dextrose.|
|>500 mg/kg||As above, except administer supplemental dextrose despite normal BG. Add liver protectants (N-acetylcysteine or Denamarin)|
We would like to thank our colleague from BluePearl in Washington, Linda Barton, DVM, DACVECC, for allowing us to use this article for Companion.
Please don’t hesitate to give our criticalists, Drs. Datte or Monnig, a call if you have questions concerning the treatment of xylitol poisoning.
1. Dunayer EK. New findings on the effects of xylitol ingestion in dogs. Veterinary Medicine 2006; 791-797.
2. DuHadway MR, Sharp CR, Meyes KE, et al. Retrospective evaluation of xylitol ingestion in dogs: 192 cases (2007-2012) JVECCS 25(5) 646-654.
3. Dunayer EK. Gwaltney-Brant SM. Acute hepatic failure and coagulopathy associated with xylitol ingestion in eight dogs. Journal American Veterinary Medical Association 2006; 229, 1113-1117.
Elbow dysplasia includes the following conditions: fragmented medial coronoid process (FCP), osteochondritis dissecans (OCD), elbow incongruity, ununited anconeal process (UAP) and ununited medial epicondyle. FCP is a small piece of bone on the inner side of the joint, which has broken off the ulna. This fragment of bone irritates the joint and abrades the cartilage of the adjacent humerus (similar to a pebble in your shoe that is irritating your foot). UAP is a condition in which the anconeal process fails to fuse with the body of the ulna during the growth phase. Normally this bony process fuses to the ulna bone by 20 weeks of age. OCD is a condition in which a piece of cartilage becomes partially or fully detached from the surface of the humerus. Elbow incongruity is a poorly fitting joint, which results in excessive wear of the cartilage within the joint. The ununited medial epicondyle is a piece of bone, which has separated from the medial side of the humerus bone. This is the site where a group of forelimb muscles attaches. If this region fails to develop into bone within the normal timing of development, the muscles may pull this part off the humerus, resulting in a loose piece of bone that rubs on the inner side of the elbow and on the ulnar nerve.
Clinical signs include acute or chronic forelimb lameness that worsens after exercise. Large and giant breed dogs are commonly affected, with both elbows involved in 35% of affected dogs. Signs often become apparent at 5-12 months of age.
Physical examination findings may reveal varying degrees of lameness and pain with manipulation of the elbow joint, (specifically with medial palpation, flexion and supination). Craniocaudal, 90° lateral and flexed lateral radiographs of the elbow joint are recommended to evaluate UAP, ununited medial epicondyle and sometimes OCD, as well as to document changes consistent with arthritis. CT scan and arthroscopic surgery of the elbow joint are usually the best diagnostic tools used to diagnose FCP, OCD and incongruity of the elbow joint.
Arthroscopic surgery is the recommended treatment for elbow conditions. The joint is examined with a very thin telescope to confirm the diagnosis. If an OCD cartilage flap is present, it is removed from the joint. The OCD bed is trimmed at the edges, and small holes are made in the OCD bed so that healing will occur faster. The area of the medial coronoid is examined with the arthroscope. If fragmented, it is removed with a combination of an arthroscopic shaver and small instruments. If an UAP is present, the bony fragment is removed via a 2 cm incision made on the side of the joint. If an ununited medial epicondyle is determined to be the problem, the joint is examined with the arthroscope, as some of epicondyle will be removed through a separate 3 cm incision. Elbow incongruity is not treated surgically if it is mild. If this problem is significant, the ulna bone is cut just below the elbow joint to allow the joint to fit better.
Arthroscopic elbow surgery is currently one of the best methods available to both diagnose and treat elbow conditions. Although all dogs that have elbow dysplasia develop arthritis, surgical intervention with arthroscopy can help decrease pain within the joint. Continued medical management including weight control, fatty acid supplementation, nutraceuticals, intermittent use of NSAIDs and physical rehabilitation may be indicated to manage arthritis long term.
A member of the surgery team is on-call 24/7 to provide consultations to our emergency doctors and to perform emergency surgeries when on call. Our specialists are available for questions and consultations on surgical conditions Monday through Friday.
As the Worm Turns: Current Heartworm Disease Topics
The American Heartworm Society (AHS) was founded in 1974 by a group of veterinarians and scientists with the mission of leading the veterinary profession and public in understanding heartworm disease (HWD). The AHS participates in funded heartworm (HW) research and provides easily accessible expert guidelines detailing current information on effective procedures for the diagnosis, treatment and prevention of heartworm disease. The AHS is now considered the leading expert source on HWD, a topic on which our knowledge is constantly expanding. Renewed emphasis on the importance of heartworm prevention and new information about heartworm resistance, testing and treatment prompted the AHS to revise its guidelines for dogs and cats in 2014. This article will discuss some of the topics of concern associated with the AHS guideline changes.
What about reports of heartworm resistance to the available macrocyclic lactones preventatives?
Resistance, or lack of efficacy (LOE), of a HW preventative is defined as a dog testing HW positive despite appropriate dosing and consistent monthly administration of the preventative. The majority of cases of suspected resistance to HW preventatives can be explained by owner compliance issues; either failure to administer an adequate dose or failure to administer the preventative on time. This fact has been confirmed by retrospective epidemiologic studies that evaluated dogs receiving HW preventatives with subsequent infection, including a review of the owners’ purchase pattern of the preventatives. An animal can become infected with HW by missing or delaying administration of just one dose of preventative, especially in endemic areas. If a dog on preventative therapy becomes positive for HW, it is important to question owners about appropriate administration of the preventative as well as review their preventative purchase record.
A few microfilariae strains have been identified that tolerate high doses of the macrolide preventatives. Six resistant strains have been identified with research in the past 10 years. When the L3 larvae of these microfilariae were injected into dogs they matured to adult HWs despite appropriate administration of preventative doses of the macrocyclic lactones. Resistance of these specific microfilariae strains was found with all tested products: ivermectin, milbemycin, selamectin and moxidectin injectable. The majority of resistant isolates have been identified from the Mississippi River Valley region where the majority of LOE has been reported. The extent and degree of spread of these isolates to other regions is not clearly documented. There is some concern for these isolates traveling northward, even to Canada, with the transport of rescued dogs following Hurricane Katrina. The mechanisms for microfilarial resistance are not understood and are the subject of ongoing genetic research.
Other possible reasons for LOE of HW preventative therapy include failure of attempted administration (dog spits pill out), failure of absorption of the active ingredient (GI malabsorption), biologic variation in dog’s drug metabolism, and the dog’s immune system parasite response. Failure of owner compliance remains the most consistent factor in preventative failure. The currently available preventatives are highly effective when administered per manufacturers’ recommendations with year round administration. To minimize risk of preventative failure, the veterinary practice team must educate clients regarding implications of HW infection, risk of HW in their area, and ensure that their pets are receiving appropriate preventative dosing.
What are Wolbachia and how are they involved in HWD?
Wolbachia are symbiotic gram-negative intracellular bacteria related to other rickettsial organisms. They are found in the reproductive tract of adult HW and in all stages of microfilariae. Wolbachia are necessary for all stages of dirofilariae to mature, thrive, reproduce and maintain infectivity. The pathogenesis of HWD is suspected partially related to vascular immune response to a surface protein produced by Wolbachia resulting in pulmonary and renal vasculitis. Wolbachia, like other rickettsial organisms, are susceptible to doxycycline. Use of doxycycline in conjunction with adulticide therapy is recommended to remove or reduce the number of Wolbachia from HW adults and microfilariae. Potential benefits of Wolbachia removal include reduced ability of HW adults to reproduce, reduced infectivity of microfilariae if ingested by mosquitos, inability of adults and microfilariae to thrive leading to deterioration and death, and reduction of host lung and kidney pathology by reducing reaction to Wolbachia surface protein and adult worm death. Higher dosing of doxycycline is now recommended for HWD treatment at 10 mg/kg q 12 hours for 30 days with ivermectin prior to initiation of immiticide therapy.
What about the slow-kill protocol?
Monthly treatment of dogs with preventative doses of ivermectin was found in studies to shorten life span of adult HW to two to three years instead of three to seven years. Ivermectin is the only macrocyclic lactone that has shown efficacy as an adulticide. Studies with milbemycin, selamectin and injectable moxidectin have not shown effect on adult worm survival. Subsequent studies found that doxycycline added to ivermectin therapy may shorten adult HW survival time to 9 to 24 months. These studies were the basis of the slow-kill protocol. While initially considered an alternative therapy to immiticide management, other studies have shown that dogs managed with slow-kill therapy continue to develop severe pulmonary pathology that may progress to pulmonary hypertension and heart failure. Damage to the pulmonary vasculature occurs due to pneumonitis and pulmonary vasculitis caused by the host immune response to the persistent presence of live and dying adult worms. Another concern for the slow-kill protocol is possible development of ivermectin resistance by adult HW and microfilariae that may be produced during the initial months of therapy. This could result in ivermectin resistant strains of microfilariae that may infect feeding mosquitos.
The AHS no longer recommends use of the slow-kill protocol due to risk to the patient and concern for development of ivermectin resistance. Slow-kill protocol should only be considered in patients with other significant medical illness where immiticide is contraindicated or for clients with financial hardship.
A visit to the American Heartworm Society website can provide more detail about these and other topics related to HWD.
We would like to thank our colleague from BluePearl in Illinois, Susan E. Yohn, DVM, DACVIM, DABVP, for allowing us to use this article for Companion.
Adderal (Dextroamphetamine) Toxicity
Adderal is an amphetamine that causes release of endogenous catecholamines leading to central nervous system and cardiovascular stimulation. Examples of other amphetamines include Ritalin® (methylphenidate), methamphetamine, Dexedrine® (dextroamphetamine) and Vyvanse® (lisdexamfetamine). Amphetamine toxicity in dogs has become common due to increased availability with use for treatment of attention deficit disorder.
Clinical Signs and Physical Examination
Clinical signs from toxicity develop rapidly and are typically seen 2- 4 hours after exposure (up to 8 hours with extended release). Adderal toxicity will cause restlessness, hyperactivity, panting, shaking, seizures, ataxia, head bobbing, circling, weakness and collapse. With severe cases sudden death is possible. On physical examination we can also diagnosis tachypnea, bilateral mydriasis, hyperthermia, cardiac arrhythmias and tachycardia. Clinical signs can last between 12 and 48 hours. Clinical signs tend to be longer with extended release formulas.
If no clinical signs are present with recent exposure emesis induction should be performed and activated charcoal administered. Once clinical signs develop agitation should be managed with acepromazine. Diazepam should be avoided as it can worsen dysphoria and central nervous system signs. Serotonin syndrome can be treated with cyproheptadine (oral or rectal). With severe tachycardia propranolol can be considered if the blood pressure is normal. If severe tachycardia and hypertension are present esmolol should be considered. Hospitalization with supportive care including fluid diuresis, monitoring vitals, minimizing sensory stimuli and thermoregulation is commonly needed.
Many patients with minimal clinical signs or aggressive care will make a full recovery. A poor prognosis is present with severe hyperthermia, head bobbing, severe tachycardia and seizures. Complications can include disseminated intravascular coagulation and rhabdomylosis leading to renal failure.
Our 24/7 emergency and critical care service is open year round and staffed by highly trained doctors and technicians, and our ER doctors have access to specialists 24 hours a day, 7 days a week.
I am excited to announce we will be hosting a daylong continuing education event at the Music City Center in Nashville on Sunday, October 30 offering 5 CE hours to veterinarians, technicians and assistants. We will cover a broad spectrum of practical and cutting-edge clinical topics for the primary care veterinarian, with a separate program for veterinary technicians and assistants. Light snacks, lunch and a parking voucher are included with registration. There is no charge to attend, but a $20 donation is greatly appreciated. All donations are non-refundable and are donated in their entirety to Snooty Giggles Dog Rescue. Click to see the agenda or register online. We hope you will join us!
We enjoy partnering with you and appreciate your support through your referrals and attendance at our CE events. We always strive to improve upon how we can best serve you, your clients and patients, so we welcome your feedback.
Kevin Au, BVMS, MS, DACVS-SA
For the most current information about BluePearl CE, visit our online calendar.