TECHNICIAN UPDATE

Tachycardia Under Anesthesia— A Reaction to Potassium G Penicillin

Joyce Guthrie, RVT

On June 3, 2019, Barbie, a 15-month-old Quarter Horse mare was referred to the University of Missouri Veterinary Health Center, (MU-VHC), with a history of an unresolved colic.

The mare had a history of colic approximately 2 months prior, which had been treated and resolved on the farm by the referring veterinarian with the nonsteroidal anti-inflammatory, flunuxin meglamine and hand walking.

The current episode began on June 1, 2019 with signs of inappetence and rolling on the ground indicative of abdominal pain. As with the prior episode, Barbie had been treated by the referring veterinarian using flunixin meglumine and hand walking. Although there was some improvement and restoration of appetite, further signs of abdominal pain (including rolling on the ground) were evident by the next day. The mare was further treated with IV fluids and flunixin meglumine.

Additional diagnostic tests undertaken by the referring veterinarian on the morning of June 3 included abdominocentesis, complete blood count (CBC) and abdominal ultrasonography. The peritoneal fluid appeared clear and yellow, a normal finding. Identified abnormalities included mild leukocytosis and abnormal small intestine. The mare was subsequently referred to MU-VHC for emergency treatment of colic.

Upon arrival at MU-VHC, the mare exhibited ongoing signs of pain, and she tried to lie down and roll in the parking lot. The mare presented in adequate physical condition (BCS 4/9), weight 261 kg, rectal temperature 99.6° F (99.5° F–101.5° F), heart rate 64 bpm (28–44 bpm), respiratory rate 24 brpm (11–24 brpm). Ongoing signs of abdominal pain included continual attempts to collapse and roll. To facilitate further diagnostic testing, xylazine hydrochloride was administered on an as-needed basis over the course of the next 90 minutes. The mare’s mucous membranes were dark pink and moist, but the capillary refill time was <2 sec (a normal finding). Other abnormalities noted on physical examination included decreased borborygmi throughout all 4 quadrants and the presence of tympany in the dorsal aspect of the right flank.

Following aseptic catheter placement in the left jugular vein, additional diagnostic tests included routine bloodwork (CBC, plasma biochemical profile, blood lactate and serum amyloid-A [SAA] concentrations), per rectum-abdominal palpation, passage of a nasogastric tube (for evaluation of reflux), and abdominal ultrasonography. Significant findings included absence of reflux, cloudy appearance to peritoneal fluid with neutrophilia 41,440/uL (<5,000/uL) 82% non-degenerated neutrophils and 18% macrophages, which sometimes displayed leukophagia. This was indicative of some degree of peritonitis and significant inflammation. Laboratory test results included an elevated SAA concentration (308 ug/mL, reference range <20 ug/mL) and normal lactate concentrations in both blood and peritoneal fluid. Ultrasonography revealed some dilated loops of small intestine in inappropriate locations, but with peristalsis present. There appeared to be a consolidated area of overlapping intestines or a small intestinal loop that was within the cecal lumen— this was not a repeatable finding—indicative of intussusception or severe inflammatory bowel disease. On abdominal palpation-per rectum, there was a small colon impaction that felt very hard and firm, and distended loops of small intestine. It was noted that the “impaction” was unusually hard and large. A foreign body was a potential differential diagnosis. Due to the intermittent colic, the age of the mare, peritoneal fluid analysis, findings on ultrasonography and abdominal palpation, the differentials were intussusception, small colon impaction—possibly a foreign body—and gastrointestinal disease leading to adhesions. were differential diagnosis. The findings were discussed with the owner, who agreed to an exploratory celiotomy.

Anesthesia

Barbie was walked to the anesthesia induction area where her mouth was rinsed and her feet cleaned. She was moved into the induction stall and sedated using xylazine hydrochloride 1.1 mg/kg (280 mg, IV). Following sedation, she was induced for general anesthesia with midazolam 0.033 mg/kg (8.5 mg, IV) and ketamine hydrochloride 2.5 mg/kg (700 mg, IV). The ketamine dosage was increased (normal dose 2.2 mg/ kg) due to a reduction in the midazolam dose (normal dose 0.066). Midazolam causes muscle relaxation with resultant respiratory depression. To avoid this effect, the midazolam dose was reduced by one-half. The mare was then intubated with a 22 mm endotracheal (ET) tube and mechanically lifted into the sur gery suite where she was placed in dorsal recumbency. An ET tube cuff was inflated and used while the horse was connected to the anesthesia machine. For the first 2 hours of the procedure she was maintained using 2% isoflurane in 100% oxygen at a rate of 5 L/min. Subsequently, isoflurane administration was reduced to 1.5% for the remaining 2 hours of the procedure. Throughout the anesthetic period, the mare’s eyes were protected from dehydration by topical administration of an ophthalmic lubricant. Warmed lactated Ringer’s solution was administered to maintain euvolemia (20 mL/kg/hr for the first 2 hours and 10 mL/kg/hr for the remainder of the procedure). Routine, intra-procedural monitoring included electrocardiography, employment of an arterial catheter (aseptically placed in the facial artery) for invasive blood pressure monitoring and arterial blood gas analysis. Blood gas analysis on healthy horses, or when normal values are resultant, would be routinely monitored at hourly intervals. If results warrant, they may be monitored more frequently. Other parameters that were monitored while under anesthesia; heart rate, pulse quality, respiratory rate and character, eye position, palpebral reflex, nystagmus, mucous membrane color and capillary refill. An anesthesiologist directly observed treatment form designed for these parameters was employed.

During placement of the arterial catheter it was evident that the pulse was weak (difficult to palpate). This could have been due to vasodilation and hypotension caused by the pre-anesthesia and induction drugs administered, and the addition of isoflurane. Therefore, ephedrine (25 mg, IV) was administered 5 minutes after placement of the patient on the surgery table. Ephedrine is a vasopressor that stimulates the alpha-1 adrenergic receptors causing vasoconstriction and a resultant increase in blood pressure. Ephedrine is administered as a bolus, which can be accomplished quickly and easily. Ephedrine’s effects generally persist for 20–30 minutes, by which time a 2% lidocaine hydrochloride constant rate infusion (CRI) had been started and the isoflurane administration rate had been reduced. Dobutamine hydrochloride is another vasopressor that might have been considered at this time, but it is administered as a CRI and must be given to effect. Dobutamine hydrochloride is a positive inotrope and stimulates the beta-1 adrenergic agonist receptors to increase the strength of the myocardial contraction. Dobutamine hydrochloride takes longer to adjust to the proper dosage for the given situation, ephedrine was deemed the better choice. Following placement of the arterial catheter the mean arterial pressure (MAP) was 79 mmHg, and quickly declined to 65 mmHg over the next 15 minutes. Without establishing and maintaining adequate blood pressure (MAP 70-75 mmHg) proper tissue and organ perfusion could not be maintained. A dobutamine hydrochloride CRI (1 mg/mL, 1.5 drops/3 sec, IV) was administered and adjusted to as needed to maintain the MAP between 70–75 mmHg. Concurrently a 2% lidocaine hydrochloride CRI was initiated with a loading dose (340 mg, 17 mL, IV, followed by 784 mg, 39 mL/hr IV). Lidocaine hydrochloride promotes balanced anesthesia by providing visceral analgesia, anti-inflammatory properties and acts as an anti-arrhythmic, thus allowing rate of administration of the isoflurane to be reduced-assisting with maintenance of normal blood pressure.

Results of arterial blood gas analysis undertaken within 15 minutes of the start of the surgical procedure were within acceptable limits. Results of subsequent blood gas analysis 50 minutes later was also within acceptable limits, although there was evidence of incipient arterial hypercarbia (62.8 mmHg, normal range is 35–45 mmHg). Satisfactory arterial oxygen tension (PaO2 351.2 mmHg-on 100% oxygen, respiratory rate 8 brpm) indicated that the mare was oxygenating well. Therefore, adjustments to the anesthetic protocol were not made at that time. Unacceptable respiratory acidosis/hypercarbia was identified one hour later. (PaCO2 76.3 mmHg, pH 7.174, HCO3 28.4 mmol/L, B.E. –1.7 mmol/L, PaO2 374.1 mmHg). Therefore, mechanical ventilation was initiated at 6 brpm and tidal volume (Vt) at 3 L. Follow-up evaluation 30 minutes later indicated that acidosis had improved (PaCO2 47.8 mmHg, pH 7.28, HCO3 22.4 mmol/L, B.E. 4.2 mmol/L PaO2 422.9 mmHg).

Throughout the anesthetic period, Barbie’s heart rate fluctuated consistently between 40 and 50 bpm. The surgical diagnosis was a linear foreign body in the lumen of the small intestine. Pieces of a rope-like material were maneuvered into the transverse and proximal small colon to be exteriorized through several enterotomy sites. Although some minor spikes in HR and blood pressure coincided with some of the surgical manipulations, the HR never exceeded 58 bpm. Two hours into the surgical procedure the opioid butorphanol tartrate (6 mg, 0.6 mL, IV) was administered for pain in response to one such spike and the HR remained below 50 bpm until the incident reported as follows:

IV Flunixin meglumine and penicillin G potassium were requested to be administered by the surgeon. Flunixin meglumine was administered first and the IV line was flushed using heparinized saline. Penicillin G potassium (K-pen) was administered slowly (over 5 minutes). The HR increased suddenly from 49 bpm to 80 bpm during K-pen administration. Over the next 5 minutes HR continued to increase to 100 bpm. This time point coincided with the closure of the abdominal incision. It seemed unlikely for the tachycardia to be associated with surgical manipulation. I will note here that on the initial biochemistry panel the mare’s potassium was 3.3 mEq/L (2.7–4.4 mEq/L). Potassium was also normal (3.42 mmol/L) on the initial blood gas evaluation and continued to remain within nor mal parameters, actually decreasing, (2.86 mmol/L) on subsequent blood gas analysis. The mare was also on LRS, which contains potassium, for the duration of the anesthetic period. An HYPP-positive horse would have been prone to an episode due to stress brought on by colic and or anesthesia, resulting in an increase in potassium. Therefore, I ruled out HYPP as a cause for the tachycardia. There was no nystagmus, no spike in blood pressure, no movement, and no sign that the mare was waking up prematurely from anesthesia. Two treatments with ketamine hydrochloride (100 mg, 1 mL, IV) were therefore administered (5 minutes apart) as a precautionary measure (to prevent premature arousal). The issue was still unresolved and the heart rate continued to rise, now 115 bpm. I consulted with a board-certified veterinary anesthesiologist, discussing the possibility that the tachycardia was the result of iatrogenic hyperkalemia due to the IV administration of the K-pen. It was noted the most common side effect of potassium administration is lowering of the HR (relative bradycardia). Tachycardia had been documented previously on cases here at MU-VHC. (I have observed tachycardia associated with K-pen administration on 2 cases with awake standing horses, in hospital within the last 5 years.) The anesthesiologist advised giving 0.05 mL/0.5 mg phenylephrine IV. This is an alpha1 adrenergic-agonist and a vasopressor. The reason for giving this vasopressor was to cause vasoconstriction and reflex bradycardia—the HR should decrease. At this time the HR was 120 bpm. Following consultation, phenylephrine (0.05 mL/0.5 mg IV) was administered 4.5 hours from the beginning of anesthesia. Within seconds the HR began to decline and decreased to 60 bpm. 10 minutes later the HR had increased to 110 bpm. Therefore, a second dose of phenylephrine (0.05 mL/0.5 mg IV) was administered. The HR decreased quickly to 60 bpm.

Figure 1: The day she went home.

Photos courtesy of the owner.

Figure 2: Two months post-op

Figure 3: 1 year post-op

Uneventful post-anesthetic recovery was facilitated by the use of a demand valve. The mare was extubated 30 minutes after having been placed in the recovery stall. Prior to extubation, it was necessary to spray the vasoconstrictive agent oxymetazoline (5 mL), into both nasal passages due to nasal passage congestion. The mare’s anesthetic recovery was in other respects, routine and uneventful.

Post-op

Following return to a stall, treatment of the mare included administration of 2% lidocaine hydrochloride (as a CRI, dosed as above), polymyxin B (5000 IU/kg, IV, BID), heparin (50 IU/kg, SQ, TID), metronidazole (15 mg/kg PO, BID), flunixin meglumine (1.1 mg/kg, IV, BID). Fluid administration was continued (LRS 1 L/hr, IV). Broad spectrum antibiotic treatment was afforded with-ceftiofur sodium (2.2 mg/kg, IV, BID) and gentamicin sulfate (6.6 mg/kg, IV, SID) for the first portion of her post-op recovery. After the incident with tachycardia under anesthesia, it was decided that K-pen was not the antibiotic for Barbie! Barbie was in the hospital for 24 days before being released to go home. Her post-recovery was relatively uneventful, especially considering that she had consumed what appeared to be a rope halter and lead rope, which had to be removed in sections from several enterotomy sites. Although the prognosis for a successful outcome was initially rather unfavorable, a year later, she has been started under saddle and is doing very well. MeV

About the Author Joyce Guthrie RVT, is an Equine Anesthesia and Surgery Technician at the University of Missouri College of Veterinary Medicine, Veterinary Health Center, in Columbia, Mo.

For more information:

Rowland S. Blood Pressure Management in Equine Anesthesia. In Veterinary Technician. Vetlearn.com 2013 Mudge M. Intravenous Lidocaine for Controlling Pain and Inflammation, Article, AAEP Convention Dec. 1–5, 2007, Orlando, FL. Reprinted in The Horse March 23, 2008.