History

A 5-year-old 49.7-kg (109.3-lb) castrated male Boxer was referred to the surgical oncology service of the Matthew J. Ryan Veterinary Hospital at the University of Pennsylvania for further evaluation and treatment of a right-sided pulmonary mass discovered by the referring veterinarian on thoracic radiographs. The dog had a 3-month history of a nonproductive cough that had been treated with prednisone at an initial dosage of 1 mg/kg/d (0.45 mg/lb/d), PO, with the dosage having been gradually tapered over the previous month. There were no abnormal findings during a complete preanesthetic evaluation at the time of admission, including a physical examination with cardiopulmonary auscultation, and results of a CBC and serum biochemical analyses were within reference limits. The patient was premedicated with butorphanol (0.2 mg/kg [0.09 mg/lb], IM), and general anesthesia was induced with propofol (3 mg/kg [1.36 mg/lb] to effect, IV). An endotracheal tube was placed and anesthesia was maintained with isoflurane in oxygen. The patient underwent CT for further evaluation of the pulmonary mass, which was noted to occupy the perihilar region of the right cranial lung lobe, with involvement of the right caudal lung lobe. Multiple CT-guided tissue fine-needle aspirates of the mass were obtained for cytologic evaluation. The patient recovered from the procedure without apparent complications. Results of cytologic examination of the fine-needle aspirates were suggestive of a diagnosis of pulmonary carcinoma. Therefore, the patient was scheduled for elective right cranial, middle, and caudal lung lobectomies via right lateral thoracotomy.

On the day of surgery, the dog was panting and appeared anxious, with a heart rate of 104 beats/min and a rectal temperature of 38.5°C (101.3°F). After the dog was premedicated with oxymorphone (0.2 mg/kg, IM), the right cephalic vein was catheterized with an 18-gauge, 1.16-in over-the-needle polyurethane catheter.^a A balanced electrolyte solution^b was administered IV throughout at a rate of 5 mL/kg/h (2.3 mL/lb/h). Supplemental oxygen was provided via face mask for 5 minutes prior to induction of general anesthesia with propofol (2 mg/kg [0.9 mg/lb], IV) and midazolam (0.2 mg/kg, IV). The patient was then orotracheally intubated with an 11-mm-internal-diameter cuffed Murphy endotracheal tube and connected to a rebreathing circuit. Anesthesia was maintained with delivery of isoflurane in oxygen. After endotracheal intubation, a 20-gauge, 1.16-in over-the-needle polyurethane catheter^a was placed in the left dorsal pedal artery and connected to a transducer calibrated (zero reference point) at the level of the right atrium. Pulse rate, respiratory rate, arterial oxygen saturation (Spo₂; measured by means of pulse oximetry), indirect blood pressure (measured by means of an oscillometric technique), end-tidal partial pressure of carbon dioxide (measured by means of capnography), and end-tidal isoflurane concentration were monitored continuously with a multiparameter monitor^c and recorded every 5 minutes. Heart rate and rhythm were also monitored continuously with a lead II ECG. A second IV catheter^a was placed in the right lateral saphenous vein.

The skin over the right thoracic region was clipped and prepared for surgery with standard aseptic technique, and paravertebral nerve blocks were performed from the third to the seventh thoracic paravertebral spaces as previously described.¹ Briefly, thoracic paravertebral injections were performed with a 22-gauge, 3.13-in insulated echogenic needle^d that was attached to a prefilled extension set connected to a nerve stimulator.^e A fixed frequency of 2 Hz and pulse width of 0.1 ms were used. Initial stimulating current was 1 to 1.5 mA for verification of needle placement, with the current decreased to 0.5 mA after the correct injection sites were confirmed. After stimulation at 0.5 mA and a negative blood aspiration test, a solution of 0.5% bupivacaine and dexmedetomidine (0.5 μg/mL) was injected at each paravertebral space (0.06 mL/kg [0.027 mL/lb]; total volume, 3 mL/site). The patient also received cefazolin (22 mg/kg [10 mg/lb], IV) prior to surgery, with additional doses administered every 2 hours throughout the procedure.

The patient was positioned in left lateral recumbency and pressure-controlled, volume-limited, continuous positive-pressure ventilation was initiated. Peak inspiratory pressure was 10 cm H₂O, tidal volume was approximately 10 mL/kg (4.5 mL/lb), and end-expiratory pressure was 4 cm H₂O. A standard right lateral thoracotomy was performed at the fifth intercostal space. Exploration of the right hemithorax revealed an irregular soft tissue mass involving the entire right cranial lung lobe and extending toward the hilus. The mass also appeared to involve the right caudal lung lobe, where it was evident as an irregular ovoid shape near the hilus with approximate dimensions of 5 × 4 × 4 cm (2 × 1.6 × 1.6 inches). The right middle and accessory lung lobes appeared grossly normal. Because the mass involved the right cranial and right caudal lung lobes, the vascular supply and bronchus of the right middle lung lobe could not be salvaged. Therefore, right pneumonectomy was elected, with sparing of the accessory lung lobe. Upon application of a thoracoabdominal stapler^f to the hilus of the right cranial, middle, and caudal lung lobes in preparation for resection, the Spo₂ decreased to 77% from a prior value of 98%, and the heart rate increased to 170 beats/min from a prior value of 105 beats/min. The direct arterial blood pressure remained within reference limits despite pulse deficits (mean arterial pressure, 100 to 110 mm Hg). A recruitment maneuver was performed, which consisted of applying continuous positive airway pressure of 20 cm H₂O for 15 to 20 seconds once, and was effective in returning the Spo₂ to 98% within seconds after completion. An arterial blood gas sample was obtained for evaluation at this time and indicated moderate respiratory acidosis (pH, 7.26; Paco₂, 58.6 mm Hg; Hco₃, −26.5 g/dL), a Pao₂ of 384.8 mm Hg, a lower-than-expected ratio of Pao₂ to fraction of inspired oxygen (approx 385), and electrolyte concentrations within reference limits. The Spo₂ of 98% reflected adequate oxygenation and the heart rate decreased to 150 beats/min; however, an arrhythmia was evident on the ECG (Figure 1). A bolus dose of fentanyl (0.003 mg/kg [0.001 mg/lb], IV) was administered in case increased nociception was the cause; however, there was no change in the heart rate or rhythm. Therefore, to exclude the possibility that the arrhythmia was a compensatory sympathetic response to a sudden decrease in blood volume following pneumonectomy, a crystalloid fluid bolus (5 mL/kg) was administered over 15 minutes, which produced minimal change in heart rate or rhythm. Within 10 minutes after the arrhythmia was first observed, the rhythm converted to sinus tachycardia; the reason for this conversion was not readily apparent. Hypotension was not evident at any time (mean arterial pressure, 75 to 95 mm Hg).

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Lead II ECG (A) and direct arterial blood pressure (B) tracings from a 5-year-old castrated male Boxer undergoing an elective right lateral thoracotomy for lobectomy of the right cranial, middle, and caudal right lung lobes for treatment of suspected pulmonary carcinoma. Tracings were obtained intraoperatively immediately after application of a thoracoabdominal stapler to clamp the hilus of the affected lung lobes. Paroxysmal supraventricular tachycardia is evident with sinus beats followed by runs of normally conducted QRS complexes without P waves and irregular pulses. Direct blood pressure was measured by means of a catheter placed in the left dorsal pedal artery and connected to a transducer calibrated (zero reference point) at the level of the right atrium (60 mm Hg = 5 mm). Paper speed = 50 mm/s; 1 mV = 10 mm.

Citation: Journal of the American Veterinary Medical Association 251, 5; 10.2460/javma.251.5.515

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A thoracostomy tube was placed at the eighth intercostal space. With the surgery completed, the thoracotomy incision was closed routinely; however, upon closure of the thoracic cavity, aspiration of the thoracostomy tube revealed a lack of negative pressure, and an odor of anesthetic gas was evident. The thoracotomy incision was reopened, and an approximately 1.5-cm-long laceration of the accessory lung lobe was discovered. An attempt to suture the laceration with 7-0 silk was unsuccessful; therefore, an accessory lung lobectomy was subsequently performed with a thoracoabdominal stapler.^f On repeated closure of the thoracotomy incision, negative pressure was achieved with aspiration of the thoracostomy tube. From the first attempt at closure to completion of the surgery (80 minutes), the patient's heart rate varied between 130 and 160 beats/min. Administration of 3 additional bolus doses of fentanyl (0.003 mg/kg, IV) produced minimal decreases in heart rate. Arterial pressure remained within reference limits (mean arterial pressure, 80 to 90 mm Hg). At the end of surgery, as the team moved the patient out of the operating room to the recovery area, the dog's heart rate ranged from 130 to 150 beats/min with sinus tachycardia. Six minutes after isoflurane administration was discontinued, the dog began swallowing and chewing, and it was subsequently extubated. However, as the endotracheal tube was removed, the dog demonstrated signs of emergence delirium and was immediately administered acepromazine (0.01 mg/kg [0.0045 mg/lb], IV) and calmed with gentle restraint. Additionally, in this immediate postextubation period, the dog again developed tachycardia (heart rate, 193 beats/min), and an arrhythmia was again evident (Figure 2).

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Postoperative lead II ECG tracing of the dog in Figure 1 obtained immediately after the patient was extubated. Severe paroxysmal supraventricular tachycardia is evident; the ventricular rate of depolarization is 193 beats/min. Positive P waves are not evident or are fused with the preceding T wave. Paper speed = 25 mm/s; 1 mV = 10 mm.

Citation: Journal of the American Veterinary Medical Association 251, 5; 10.2460/javma.251.5.515

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Question

What type of arrhythmia occurred in this patient in the perioperative period (Figures 1 and 2)? What was the most likely cause and how should this patient be treated??

Answer

This dog developed paroxysmal supraventricular tachycardia. First, it developed sinus rhythm with paroxysmal supraventricular tachycardia (Figure 1); this was followed by severe supraventricular tachycardia (Figure 2). It was not possible to discern whether the arrhythmia originated from the sinus node or from an ectopic focus. We speculated that the likely cause was a loss of vagal tone induced by the pneumonectomy versus hypovolemia, pain, inadequate depth of anesthesia, or a combination of these factors. Immediately following extubation, further interventions were performed to address the arrhythmia. First, 2 doses of esmolol (0.2 mg/kg, IV) were administered, which converted the arrhythmia to sinus tachycardia (heart rate, 165 beats/min). Subsequently, a 6% hetastarch in saline (0.9% NaCl) solution bolus (10 mL/kg [4.5 mL/lb], IV) was administered, and a fentanyl infusion was initiated (loading dose, 0.005 mg/kg [0.002 mg/lb], followed by an infusion rate of 0.006 mg/kg/h [0.003 mg/lb/h], IV). These treatments decreased the heart rate to approximately 150 beats/min. Because the patient remained tachycardic, additional attempts to further decrease the heart rate included administration of bolus doses of diltiazem (0.125 mg/kg [0.057 mg/lb], IV) and lidocaine (2 mg/kg, IV), neither of which appeared to be effective. An abbreviated right-sided thoracic ultrasound examination elicited a moderate to severe pain response when pressure was applied to the axillary region, but did not reveal any pericardial or pleural fluid. No signs of pain were observed with palpation of the thoracostomy tube insertion site or the thoracotomy incision. A second arterial blood gas analysis was performed and showed persistent respiratory acidosis and improved oxygenation (pH, 7.25; Paco₂, 60.9 mm Hg; Hco₃, −25.9 g/dL; Pao₂, 319.8 mm Hg), with the patient receiving flow-by oxygen via face mask. At this time, the dog was transferred to the intensive care unit for continued postoperative care. The dog was monitored with a continuous lead-II ECG and intermittent measurement of arterial oxygen saturation (by means of pulse oximetry) in addition to intermittent physical examinations, observation for signs of accumulation of fluid or air within the pleural cavity via the thoracostomy tube, and measurement of indirect blood pressure (by an oscillometric method) every 6 hours. An initial dose of dexmedetomidine (0.001 mg/kg [0.00045 mg/lb], IV) was administered, which produced a temporary second-degree atrioventricular block with a ventricular rhythm of 70 to 75 beats/min that resolved within 25 minutes. The patient then received dexmedetomidine (0.001 mg/kg/h, IV) and fentanyl (0.006 mg/kg/h, IV) constant rate infusions for analgesia.

Overnight, the dog continued to exhibit paroxysmal supraventricular tachycardia intermittently at intervals of 3 to 8 hours despite the dexmedetomidine and fentanyl infusions. Because blood pressure was adequately maintained during these episodes of postoperative supraventricular tachycardia, ECG monitoring was discontinued because of an apparent lack of clinically important consequences associated with the arrhythmia. Additionally, after a consultation with the cardiology service, no further antiarrhythmic treatments were recommended. Subsequent further diagnostic evaluation including with a 24-hour Holter monitor and echocardiogram were recommended by the cardiology service, but declined by the owner. Over the following 24 hours, the dog was weaned off the fentanyl and dexmedetomidine infusions and switched to oral codeine (3.0 mg/kg [1.4 mg/lb], q 8 h) administration. The dog was discharged home 4 days after surgery. At a 2-week follow-up appointment for suture removal, the postoperative supraventricular tachycardia had not yet resolved (heart rate, 190 to 200 beats/min), but the owner reported that the dog appeared comfortable at home.

Discussion

Although scarcely reported in the veterinary literature, intra- and postoperative arrhythmias are commonly observed in human patients undergoing thoracic surgery. In 1943, Bailey and Betts² published the first case series of arrhythmias following pulmonary resection in 9 human patients. The reported^2–9 incidence of postoperative arrhythmias has ranged between 9% and 29% in the human literature, with postoperative supraventricular tachycardia and atrial fibrillation being the most common arrhythmias reported. A recent study¹⁰ evaluated pre- and postoperative changes in electrical activity, cardiac dimensions, and function of the right heart in 20 dogs undergoing pulmonary resections. Despite the fact that all dogs had a normal sinus rhythm or first-degree atrioventricular block prior to surgery, within 30 days after surgery, postoperative supraventricular tachycardia and atrial fibrillation occurred in 15 dogs that underwent pneumonectomy and in 5 dogs that underwent lobectomy.

The pathogenesis of postsurgical supraventricular arrhythmias is not well understood. It has been suggested that the cardiac autonomic nerves are damaged by surgical manipulation in some patients, affecting sympathetic activity.¹¹ This may be particularly relevant for dogs undergoing right-sided pneumonectomy, because the right vagus nerve influences the pacemaker activity of the sinoatrial node and atrioventricular nodal conductance.¹¹ Furthermore, a prospective study in human patients noted ECG markers of autonomic dysregulation following esophagectomy and right-sided pulmonary resections, both procedures that require a right lateral thoracotomy approach.¹² Additionally, postoperative pain may further increase sympathetic stimulation, potentially increasing the likelihood for arrhythmias.¹³ Although conflicting evidence exists, this theory is supported by a report¹⁴ suggesting that thoracic epidural anesthesia with bupivacaine decreases the incidence of postoperative supraventricular tachycardia.

Multiple studies^15–19 have been published analyzing potential risk factors for postoperative supraventricular tachycardia in patients undergoing thoracic surgery. The most definitive risk factor is advanced age.^5,15 In an experimental study²⁰ in 15 dogs undergoing pneumonectomy, 7 of 8 of the older animals versus 0 of 7 of the younger animals had episodes of postoperative supraventricular tachycardia. Additionally, at necropsy, histologic examination indicated mild to moderate myocarditis and epicarditis in half (4/8) of the older dogs.²⁰ Male sex, chronic obstructive pulmonary disease, congestive heart failure, thoracotomy surgical approach, and surgical complications have been reported as clinical predictors of postoperative supraventricular arrhythmias.^13,16,21 More extensive pulmonary resections are associated with a higher incidence of arrhythmias,^{6,17,18,22,23} particularly for right-sided procedures.^15,24

For the patient of the present report, we suggest it is highly unlikely that any preoperative examination could have predicted the arrhythmia that was observed. To date, preoperative pulmonary evaluations, cardiac function tests, and diagnostic imaging do not identify human patients at increased risk of postoperative supraventricular tachycardia after pulmonary lobectomy.^5,6,18,25 However, a recent study¹⁹ suggested that preoperative brain natriuretic peptide concentration > 30 ng/mL may be a predictor of risk for postoperative atrial fibrillation in human patients undergoing thoracic surgery. This association has not been evaluated in veterinary patients.

The 2014 American Association for Thoracic Surgery guidelines²⁶ for the prevention and management of perioperative atrial fibrillation and flutter outline treatment strategies depending on hemodynamic stability of the patient. Goals for all patients include discontinuing adrenergic inotropic support (eg, dopamine) when possible and optimizing fluid and electrolyte status. Factors that increase sympathetic drive, such as pneumothorax, hemorrhage, and pain should be appropriately addressed. In hemodynamically stable patients, medications that control heart rate (eg, diltiazem, digoxin, esmolol, metoprolol, and amiodarone) can be added.^10,26 Patients with indicators of hemodynamic instability, such as hypotension, systolic dysfunction, or congestive heart failure, require intensive monitoring and potentially multiagent cardiac treatment.²⁶ The 2014 guidelines²⁶ also recommend prophylactic administration of diltiazem or amiodarone to patients considered at risk for postoperative supraventricular tachycardia, such as those undergoing pneumonectomy.

The present report provides a reminder to clinicians involved in the care of veterinary patients undergoing pneumonectomies or segmentectomies that postoperative supraventricular tachycardia is an important potential complication of these procedures. Measurement of preoperative brain natriuretic peptide concentration may help predict which patients will develop postoperative supraventricular tachycardia, but prospective studies are necessary to investigate this. Meanwhile, premedication with diltiazem or amiodarone should be considered in patients at increased risk of developing postoperative supraventricular tachycardia, including larger-breed dogs, older patients (eg, > 8 years), patients with preexisting chronic lower airway disease, and those undergoing right-sided procedures. Additionally, dogs with preexisting arrhythmias may be at a higher risk because of underlying cardiac disease, although this also requires investigation. As the veterinary profession progresses and more complex surgical procedures are performed, it is likely that such complications will become increasingly apparent and relevant to our case management, emphasizing the importance of thorough perioperative preparation and monitoring.