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TECHNICIAN UPDATE
Unresponsive Tachycardia in an Equine Patient During Surgical Correction of a Small Intestinal Volvulus
By Lori Dressel CVT, BSP, VTS (AVTAA) Lg Animal, and Jane Quandt DVM, MS, DACVAA, DACVECC
24-year-old, 340-kg gelding pony was admitted to the University of Georgia Veterinary Teaching Hospital for evaluation of small intestinal colic. According to the owner, the patient became lethargic and depressed for 12 hours before the referring veterinarian was called. The patient was referred because the pony was nonresponsive to treatment and had previous colic surgery, an exploratory celiotomy 3 years prior for a strangulating lipoma. At that time 8 feet of necrotic bowel were removed, and a jejunocecostomy was performed.
Clinical Examination
Upon arrival at the teaching hospital, the patient was dull, lethargic and depressed. He had a body condition score of 5/9, heart rate of 40 bpm, respiration rate of 20 bpm, and a rectal temperature of 99.3° F. The mucus membranes were hyperemic and moist with a prolonged capillary refill time of 3 seconds. No abnormalities were heard on auscultation of the heart and lungs. Borborygmi were auscultated in all abdominal quadrants. The blood work showed a PCV of 22% and TS of 6.0 g/dL (range PCV 26–45%, TS 4.6-6.9 g/dL). Abnormal CBC findings were a fibrinogen of 500 mg/dL (range 100-400), and lymphopenia of 1.118x10 ^3/µL (range 5.7-11.7x10 ^3/µL). There was no net reflux; the rectal exam was normal, and the abdominal ultrasonography results showed a thickened small intestine. The patient was placed on IV fluids and taken to a stall for observation.
Clinical Findings
Over the next few days the patient seemed to gradually improve until the evening of the 5th night. The patient required several doses of IV xylazine and butorphanol due to uncontrollable pain with violent rolling. The patient underwent an exploratory celiotomy that evening. The surgical diagnosiswas a distended small intestine and colon displacement. Biopsies were taken of the intestine and histopathologically diagnosed as lymphocytic plasmacytic enteritis with eosinophils.
Following surgery, the patient was maintained on IV fluids, IV lidocaine CRI, IV anti-inflammatories such as flunixin meglumine, oral gastrointestinal protectant omeprazole and sucralfate, IV dimethyl sulfoxide (DMSO), and IV antibiotics gentamicin sulfate and penicillin G potassium. There were intermittent bouts of tachycardia, fever, and tachypnea.
The patient was passing feces, and food was slowly being reintroduced. The patient had a lymphopenia of 1.001x10 ̂3, and fibrinogen of 600 mg/dL. The pony seemed to be recovering until 7 days later, when over a 15-hour period, he became more painful.
The horse is sedated and awaiting transfer to the induction stall. While the sedation takes effect, the patient is cleaned and clipped, and his mouth is rinsed.
Images courtesy of Lori Dressel
The patient is in right lateral with his head extended. A bite block was placed in between the front teeth and a 26mm endotracheal tube was placed before the patient is hoisted and placed in the OR in dorsal recumbency.
Images courtesy of Lori Dressel
The patient received several doses of IV xylazine, detomidine hydrochloride, and butorphanol for pain control. The patient produced 6 L of gastric reflux overnight, a nasogastric tube was placed, and fluids were increased to keep up with the fluid loss. Surgery was recommended, the patient was given 22,000 IU/ kg IV penicillin G potassium, 6.6 g/kg IV gentamicin sulfate, and 1.1 mg/kg IV flunixin meglumine preoperatively.
Clinical Procedure
At this point, the anesthesia technician was called in to manage the anesthesia for the surgical celiotomy. The author was in charge of the anesthesia and analgesia, Pre-medication was 0.29 mg/kg IV xylazine and induction behind a swing door with 0.08 mg/kg diazepam and 1.73 mg/kg ketamine given IV. A 26 mm endotracheal tube was introduced to the trachea with the pony in lateral recumbency. The cuff was inflated to 20 cm H2O to prevent inhalant leaking from around the endotracheal tube. Anesthesia was maintained with isoflurane in 100% oxygen delivered at a concentration of 1% to 3%.
Monitoring consisted of ECG, inhalant gas analysis, capnography, pulse oximetry, direct blood pressure, blood gas analysis, body temperature, pulse rate, and respiratory rate. Blood pressure was monitored via an arterial catheter placed in the facial artery and measured with a pressure transducer. Intermittent positive pressure ventilation was initiated at 2 breaths per minute, 4 L tidal volume and a peak inspiratory pressure (PIP) of 20 cmH20. Plasma-Lyte, was administered at 10 mL/kg/hr IV and a baseline blood gas analysis was preformed.
The initial MAP was 55 mmHg and IV dobutamine was started at 2 µg/kg/min or to effect. The baseline blood gas revealed a mild metabolic and respiratory acidosis (Table 1) and an elevated lactate of 4.9 mmol/L. The mechanical ventilator rate was increased to 8 bpm with a tidal volume of 5 L and a PIP of 23 cmH20. It was observed that the patient’s blood pressure decreased during the inspiratory phase of ventilation.
Mechanical ventilation causes positive pressure in the thorax which decreases blood pressure due to decreased venous return. This hypotension will be exacerbated by hypovolemia. Once MAP was above 70 mmHg an IV lidocaine bolus of 1.3 mg/kg was given over a 5-minute period and a CRI of 0.05 mg/kg/min was started.
Recovery was important, as the horse had spontaneous reflux. And endotracheal tube was taped in with two nasopharyngeal tubes placed in each nostril. Once standing, the endotracheal tube was removed, but the horse will still have an airway with the nasopharyngeal tubes.
Once placed in dorsal recumbency, the patient is put on inhalant, oxygen and a ventilator. Also, all measuring parameters are placed and recorded every 5 mins.
The patient was given 0.02 mg/kg of butorphanol IV for analgesia. The patient’s HR started at 30 bpm for the first 40–45 minutes, then the HR began to increase. Surgery found a non-strangulating volvulus of the small intestine. The HR continued to progressively increase, and when it reached 65 bpm, the inhalant was switched to sevoflurane delivered at a concentration of 2% to 4%. Two liters of IV heta- starch were given to support the cardiovascular system and treat volume depletion.
A second blood gas was performed, which revealed that the metabolic acidosis (Table 1) was still present, lactate was 4.1 and the calcium had decreased from 1.5 to 1.299 mmol/L. The sevoflurane was increased due to the high HR. Dobutamine was discontinued at 1 hour and 30 minutes into the procedure due to a HR of 75 bpm. An additional 0.02 mg/kg of IV butorphanol was given for analgesia. The HR continued to increase to 85 bpm at 2 hours post-induction. Lidocaine and all other drugs had been discontinued except for the heta-starch and inhalant.
Surgery had progressed to closing of the linea alba. At this time, the author called Dr. Jane Quandt—the on-call anesthesiologist—due to the HR, as it could not be lowered. It was suggested to try an alpha-2 agonist drug.
Distended small intestine seen with small intestinal issues
Xylazine at 25 mg was administered for the analgesic and bradycardic effect. The HR decreased to 55–60 but the blood pressure dropped to 60 MAP and dobutamine was re-instituted. The HR began to increase again after 15-20 minutes. The hetastarch infusion was complete, and fluids were discontinued after a total of 15 L. The respiratory rate was decreased to 5 bpm to wean the patient off the ventilator. Since there had been no reflux throughout the surgery, the nasogastric tube was removed, and was found to be patent.
Upon removal of the nasogastric tube, 4 to 6 L of spontaneous reflux came from both nostrils and the mouth of the patient. The HR subsequently dropped to 45-50 bpm.
The patient began trembling, 2 hours and 45 minutes post induction, so the following drugs were administered IV to provide sedation and analgesia: 0.38 mg/kg ketamine, 0.17mg/kg xylazine and 0.02 mg/kg butorphanol. The patient began spontaneous respirations, was hoisted into the recovery stall and placed in left lateral recumbency. Another 1 to 1.5 L of reflux came from the mouth and nostrils while the pony was being hoisted to the recovery stall. While the patient was still in the hoist with the head down, the mouth was rinsed to remove any remaining reflux.
In recovery, the patient was given 0.17 mg/kg of xylazine IV. A nasopharyngeal tube was placed to maintain an open airway, as there was nasal edema present. Extubation was delayed until the patient was actively swallowing to prevent aspiration of fluid into trachea.
The patient’s recovery was assisted via head and tail ropes. Recovery was excellent: the patient was held down for the first several attempts and then allowed to stand on the first strong attempt. The pony walked back to the stall 60 minutes after discontinuation of the inhalant.
Teaching Points
Cardiac output is determined by stroke volume (SV) and heart rate (HR) (CO= HR x SV). The heart rate affects the cardiac output, CO, which affects blood pressure. In horses, the largest change in cardiac output is usually attributable to HR change. The normal resting heart rate of a horse is 24–40 bpm and 220–240 bpm in an exercising horse.
Inhalant anesthesia also causes changes in the cardiovascular system. The factors that affect the cardiovascular system during anesthesia are:
• anesthetic drug doses,
• changes of PaC02,
• mechanical ventilation,
• noxious stimulation/pain,
• procedure duration,
• drugs and inhalant used
• vascular fluid volumes.
All of which can potentially affect an anesthetic episode in horses undergoing surgery for GI lesions. The horse in this report had a heart rate 50–55 bpm prior to surgery, which is considered tachycardic for the equine patient.
The anesthetic drug doses that this patient was given for pre-medication and induction were titrated to effect due to his compromised condition. The premedication given was xylazine which is an alpha-2 agonist. Xylazine, which produces excellent sedation and analgesia, can produce increased blood pressures, followed by decreases in BP, HR, CO, and second degree atroventricular block and ileus.
The inhalant anesthetic isoflurane was used. The vaporizer setting was kept at 1% to 3% and the end tidal gas never exceeded 1% at the beginning of surgery. A lidocaine CRI was added to the protocol to improve analgesia, improve gastrointestinal motility, provide antiinflammatory effects and provide MAC-sparing effects.
As the procedure progressed and the HR continued to increase, isoflurane was discontinued, and sevoflurane was started. Although isoflurane and sevoflurane have similar cardiovascular effects, the change in inhalants was done to determine if this may have minimized the continued increase of the HR. No change in HR was subsequently observed.
Oxygenation is impaired by respiratory-depressant drugs, recumbency and positioning in a normal patient. Gas exchange, whether it is oxygen or carbon dioxide, is even more compromised by the horse’s size and weight. When a compromised patient is placed in dorsal recumbency for a colic procedure, gas exchange is generally impaired. This is due to compression of the dependent lung lobes by the GI tract pushing on the diaphragm, causing the blood flow to the compressed lobes to be less oxygenated due to ventilation/perfusion mismatch.
Procedures lasting longer than 61 minutes have a decreased complication and death risk compared with those lasting longer than 241 minutes. The anesthesia time for this patient was 152 minutes, which may have increased risk.
The patient’s high heart rate was not a complication in any of the previous surgeries. The unresponsive tachycardia throughout this surgical procedure was hypothesized to be due to a sympathetic response caused by stomach distention that resolved by spontaneous gastric reflux and relief of gastric distention.
Gastric distention should be considered ruled out for tachycardia, in anesthetized patients with small intestinal disease, if no other cause of tachycardia is found.
About the Authors:
Lori Dressel graduated in 1998 from St. Petersburg College with a degree in Veterinary Technology. From there, she completed a 6-month internship at Littleton Equine Clinic. She then joined the technical staff of the large animal and neonatal Intensive Care unit at The University of Pennsylvania for 3 years. She earned her Bachelors in Equine Business Management from Cazenovia College. For the following years, Lori held a position as a large animal anesthesia technician at the University of Georgia until 2013. In September of 2011, Lori earned her specialty in Large Animal Anesthesia and analgesia from the Academy of Veterinary Technician Anesthetists (AVTAA). As of 2015 she has been on medical leave.
Jane Quandt, DVM, MS, DACVAA, DACVECC, graduated from Iowa State University College of Veterinary Medicine in 1987. After doing small animal practice for 1 year, she did anesthesia residency. She completed an anesthesia residency and received a Masters in anesthesia at the University of Minnesota and became boarded in anesthesia in 1993. To improve her ability to manage critical cases she did a second residency in small animal emergency and critical care and became boarded in small animal emergency and critical care in 2007. She was on faculty at the College of Veterinary Medicine at the University of Minnesota for 10 years. She joined the faculty at The University of Georgia College of Veterinary Medicine in 2011 and is currently a tenured full professor in comparative anesthesia. She has published several journal articles and book chapters and has presented at national and international conferences on topics related to anesthesia and analgesia in both small and large animal species. Dr. Quandt was awarded the Carl Norden-Pfizer Distinguished Veterinary Teacher Award and the Zoetis Distinguished Veterinary Teacher Award.
