History
A 1-year-old 18-kg (39.6-lb) sexually intact male Pygmy goat with tetralogy of Fallot (ToF) was presented for castration, scrotal ablation, and penile amputation at New Bolton Center, School of Veterinary Medicine, University of Pennsylvania. Six months earlier, ToF had been diagnosed in this goat during diagnostic evaluations and treatment for fever and ocular discharge attributed to a mild respiratory infection. On presurgical physical examination for castration, the goat was bright, alert, and responsive and had pink and moist mucous membranes with a capillary refill time of approximately 2 seconds (reference range, ≤ 2 seconds). Peripheral pulses were strong, synchronous, and regular, and the heart rate (HR) was 88 beats/min (reference range, 70 to 150 beats/min). Cardiac auscultation revealed a harsh grade 4/6 holosystolic crescendo-decrescendo heart murmur with the point of maximum intensity over the pulmonic valve and a softer band-shaped, grade 3/6 pansystolic murmur with the point of maximum intensity over the tricuspid valve area. Pulmonary auscultation revealed no abnormalities in the caudodorsal lung fields but harsh respiratory sounds in the cranioventral fields bilaterally. Preoperative thoracic radiography and echocardiography revealed no signs of congestive heart failure and no substantial changes in heart size or pressure-flow dynamics from findings 6 months earlier, when ToF was diagnosed. Results of a CBC and serum biochemical analyses and plasma fibrinogen concentration were within reference limits. Additionally, the Hct and serum total protein concentration were 30.8% (reference range, 22% to 38%) and 7.5 g/dL (reference range, 6.1 to 7.5 g/dL); however, the serum concentration of cardiac troponin I was high (1.41 ng/mL; reference limit, ≤ 0.07 ng/mL).
We discussed extralabel drug use and the associated recommended meat withdrawal intervalsa for the medications that we had planned to administer. The owners conveyed that they regarded the goat as a pet and would not allow the goat to enter the food chain during the overall timespan discussed. Food and water were withheld approximately 18 and 8 hours, respectively, before surgery. Florfenicol (40 mg/kg [18.2 mg/lb], SC) was administered the night before surgery.
Question
How would you induce and maintain anesthesia for this goat with ToF?
Answer
In patients with ToF, anesthetic goals should include avoidance of drugs or techniques that decrease systemic vascular resistance (SVR) or increase pulmonary vascular resistance (PVR) because these changes could worsen the right-to-left shunting of blood across the ventricular septal defect (VSD). Additionally, with a right-to-left intracardiac shunt and decreased pulmonary blood flow, inhalational induction takes longer than IV induction.1 Ketamine was used because it does not decrease SVR and has a minimal impact on PVR, thereby affecting the magnitude of the right-to-left shunting of blood across a VSD to a lesser degree than would other anesthetic agents.
On the day of surgery, a 16-gauge over-the-needle polyurethane catheter was aseptically placed in the right jugular vein for drug and fluid administration, and the goat was sedated with midazolam (0.5 mg/kg [0.23 mg/lb], IV). The skin over the left auricular artery was desensitized with 0.2 mL of 2% lidocaine hydrochloride gel. The artery was then aseptically catheterized with a 24-gauge over-the-needle polyurethane catheter for blood sampling, serial arterial blood gas analyses, and monitoring direct blood pressure with a transducer placed at the level of the right atrium.b The anesthetic event was performed in a facility near sea level. Initial arterial blood gas analyses were performed at time 0 (T0) and repeated approximately every 15 to 20 minutes throughout the anesthetic event (T1 through T5; Table 1). Echocardiographic assessments of blood flow velocities through the VSD and stenotic pulmonary valve were performed and repeated intraoperatively.
Perioperative patient variables for a 1-year-old 18-kg (39.6-lb) male Pygmy goat with tetralogy of Fallot undergoing general anesthesia for castration, scrotal ablation, and penile amputation, showing results for the initial assessments before induction of anesthesia with ketamine (T0), then subsequent assessments performed approximately every 15 to 20 minutes (T1 through T5) throughout the anesthetic event, for which a total IV anesthesia maintenance protocol with ketamine and midazolam was used.
Variable | Reference range | T0* | T1† | T2† | T3† | T4† | T5* |
---|---|---|---|---|---|---|---|
Patient position | — | Sternal | Sternal | Dorsal recumbency | Dorsal recumbency | Sternal | Standing |
Heart rate (beats/min) | 70–95 | 85 | 130 | 110 | 110 | 130 | 120 |
Respiratory rate (breaths/min) | 15–30 | 16 | 10 | 18 | 31 | 41 | 18 |
Direct blood pressures | |||||||
SAP (mm Hg) | 80–120 | 120 | 150 | 130 | 133 | 160 | — |
MAP (mm Hg) | 75–100 | 103 | 130 | 115 | 112 | 138 | — |
DAP (mm Hg) | 60–80 | 93 | 120 | 105 | 100 | 120 | — |
Fio2 (%) | 0.21 | 0.80 | 0.96 | 0.97 | 0.97 | 0.21 | |
ETco2 (mm Hg) | 30–50 | — | 30 | 35 | 28 | 35 | — |
Blood gases | |||||||
Paco2 (mm Hg) | 30–50 | 34.8 | 34.9 | 39.7 | 31.8 | 35.9 | 36.7 |
Pao2 (mm Hg) | 70–700 | 84.2 | 307 | 541.5 | 557.4 | 488.9 | 75.0 |
Sao2 (%) | 90–100 | 97.0 | 99.8 | 99.9 | 99.9 | 99.9 | 95.2 |
Spo2 (%) | 90–100 | 95.0 | 99 | 100 | 100 | 99 | — |
Echocardiography | |||||||
VSD vel max (m/s) | — | 2.2 | 2.4 | — | — | 2.7 | — |
PS vel max (m/s) | — | 4.0 | 4.2 | — | — | 4.6 | — |
The goat was spontaneously breathing room air when assessments were made at T0 and T5.
The goat was spontaneously breathing 100% O2 delivered through a circle rebreathing circuit attached to an anesthetic machine when assessments were made at T1 through T4.
— = Not applicable or not measured. DAP = Diastolic arterial blood pressure. ETco2 = End-tidal partial pressure of CO2. Fio2 = Fraction of inspired O2. MAP = Mean arterial blood pressure. Paco2 = Arterial partial pressure of CO2. Pao2 = Arterial partial pressure of O2. PS vel max = Maximum velocity of blood flow through the stenotic pulmonary valve. Sao2 = Arterial O2 saturation of hemoglobin. SAP = Systolic arterial blood pressure. Spo2 = Pulse oximetry measured O2 saturation of hemoglobin. VSD vel max = Maximum velocity of blood flow through the VSD.
Butorphanol (0.1 mg/kg [0.045 mg/lb], IV) was administered, and the goat was preoxygenated for 5 minutes with 100% O2 delivered by face mask. During preoxygenation, the goat was equipped for monitoring heart rhythm and rate (base-apex ECG), direct blood pressure, and O2 saturation of hemoglobin (pulse oximetry) with a multiparameter monitorc throughout the anesthetic event, with values recorded every 5 minutes.
General anesthesia was induced with ketamine (3.5 mg/kg [1.6 mg/lb], IV). Intubation with a 6.5-mm-internal-diameter cuffed endotracheal tube was achieved with the goat in sternal recumbency and confirmed with capnography. The end-tidal partial pressure of CO2 and inspired fraction of O2 were monitored and recorded every 5 minutes. The free end of the endotracheal tube was connected to a circle rebreathing circuit attached to an anesthetic machine, and the goat was allowed to breathe 100% O2 spontaneously. Esophageal temperature was monitored continuously and recorded every 15 minutes. Anesthesia was maintained with a constant rate infusion of ketamine (5 mg/kg/h [2.3 mg/lb/h], IV) and midazolam (0.5 mg/kg/h, IV).
The goat was positioned in dorsal recumbency; the hair over the scrotal, penile, and preputial areas was clipped; and the skin was aseptically prepared. Local anesthesia was provided with bilateral intratesticular injections of 2% mepivacaine solution (2.5 mg/kg [1.1 mg/lb], total dose divided equally). The goat also received flunixin meglumine (2.2 mg/kg [1 mg/lb], IV) and pantoprazole (1.1 mg/kg [0.5 mg/lb], IV) prior to the surgery.
Closed castration, scrotal ablation, and prophylactic penile amputation were performed as planned. Ten minutes before the end of the surgery, the infusions of ketamine and midazolam were stopped. The duration of surgery was 40 minutes, and the duration of anesthesia was 70 minutes.
The goat was positioned in sternal recumbency, and flumazenil (0.03 mg/kg [0.01 mg/lb], IV) was administered. Soon afterward, the goat began to swallow and chew and was subsequently extubated. Recovery was uneventful, and once the goat had fully recovered, morphine (0.1 mg/kg, IM) was administered for postoperative analgesia.
Overall, the goat's cardiopulmonary parameters remained stable after induction of general anesthesia (Table 1). In addition, results of echocardiographic measurements performed during anesthesia indicated slight increases in the blood flow velocities for the left-to-right shunt through the VSD (2.2 m/s at T0 vs 2.7 m/s at T4) and the stenotic pulmonary valve (4.0 m/s at T0 vs 4.6 m/s at T4).
The day after surgery, the goat's serum concentration of cardiac troponin I was 0.53 ng/mL, which was lower than on admission (1.41 ng/mL). Additionally, the goat ate, drank, urinated, and defecated well and had no swelling at the surgical site. A second dose of florfenicol was administered, and analgesic treatment was changed from flunixin meglumine to meloxicam (2 mg/kg [0.9 mg/lb], PO, once and then 1 mg/kg [0.45 mg/lb], PO, 48 hours later). The goat remained bright, showed no signs of discomfort throughout hospitalization, and was discharged 3 days after surgery. The owners were instructed to monitor the goat for any changes in attitude, appetite, urination, or defecation; keep the animal confined to a small area for the following 2 weeks while healing; continue to monitor for any exercise intolerance, coughing, weakness, or increased respiratory rate or effort potentially suggestive of progressing cardiac disease; and not allow the goat to enter the food chain for at least 72 days because of the medications administered.
Discussion
Tetralogy of Fallot is a congenital malformation of the heart that consists of pulmonary stenosis, right ventricular hypertrophy (secondary to right ventricular outflow tract obstruction [RVOTO] caused by pulmonary stenosis), VSD, and an overriding (dextropositioned) aorta.1,2,3,d Although VSD seems to be the most common cardiac defect in cattle and sheep,4 information is scarce regarding the frequency of various types of congenital cardiovascular defects in goats.3 Clinical signs are the consequence of chronic hypoxemia and may or may not be present depending on the magnitude of the right-to-left shunting.5 For instance, animals with ToF may have a palpable thrill over the thorax, appear cyanotic or clinically normal, or have a history of poor growth or exercise intolerance, alone or in combination.2,3,4,5
Cyanosis develops when absolute deoxyhemoglobin concentrations approach 3 to 5 g/dL,6 which occurs in patients with total hemoglobin concentrations within reference limits when the arterial O2 saturation of hemoglobin (Sao2) approaches 70% to 80% (reference range, 93% to 100% in humans) and the PaO2 is 50 mm Hg (reference range, 80 to 100 mm Hg in humans).7 Cyanosis can be difficult to discern in anemic patients until their Sao2 is very low (eg, < 50%),6 whereas cyanosis becomes evident at higher Sao2 (eg, 80% to 85%) in polycythemic patients.7 Polycythemia secondary to chronic O2 deficiency can be present in patients with ToF and may lead to thrombosis and coagulation abnormalities.1 The goat of the present report had a PCV of 30.8%, a Pao2 of 84.2 mm Hg, and Sao2 of 97% and was classified as having acyanotic ToF without polycythemia. These findings were consistent with minimal right-to-left shunting of blood through the VSD.
Longer-term outcomes are poorly described in large animal species, likely because of a tendency to euthanize animals at the time of diagnosis. Given that the degree of pulmonic stenosis generally determines the clinical course of ToF in other species,5 we suspect that large animals with mild to moderate RVOTO and large VSDs would likely develop left-sided congestive heart failure, whereas those with severe RVOTO would likely have clinical complications of right-to-left shunting and chronic cyanosis.4,5,6
Anesthetic goals in patients with ToF are to maintain or increase SVR, minimize changes in PVR, provide mild cardiac depression that avoids increasing the right-to-left shunt, and prevent hypercyanotic episodes.2,8,9 Because of decreased pulmonary blood flow and right-to-left intracardiac shunting of blood in patients with ToF, inhalational induction is prolonged, whereas IV induction is faster in these patients.1 In affected babies and children, ketamine is the induction agent of choice because it increases or maintains the SVR.10,11 Further, a meta-analysis that included results from 7 studies and 139 human patients shows that the use of ketamine did not alter SVR or PVR.12 Ketamine can produce cardiovascular stimulation by central activation of the sympathetic nervous system; however, the use of propofol infusion and high concentrations of inhalant anesthetic agents can cause systemic vasodilation and a decrease in blood pressure, which may result in lower Sao2 in patients with right-to-left intracardiac shunting or could change the direction of the shunt flow through the VSD to one that is left to right.13
In the goat of the present report, butorphanol and midazolam were administered to reduce O2 consumption and myocardial work.14 The induction of anesthesia with ketamine, followed by an infusion of midazolam and ketamine, resulted in sympathetic stimulation, evidenced by the increases in HR and systolic, mean, and diastolic arterial pressures from T0 (85 beats/min and 120, 103, and 93 mm Hg, respectively) to T1 (130 beats/min and 150, 130, and 120 mm Hg, respectively; Table 1). Furthermore, the increase in the velocity of the left-to-right shunt across the VSD measured with continuous-wave Doppler echocardiography at T0 (2.2 m/s) versus T1 (2.4 m/s) was consistent with a greater increase in SVR, compared with changes in PVR.
Controlled ventilation is often necessary to prevent hypoxemia, hypercapnia, and acidosis, as such could increase PVR.14 However, the goat of the present report was allowed to breathe 100% O2 supplied through a circle rebreathing circuit attached to an anesthetic machine, and the animal's O2 saturation of hemoglobin as measured with pulse oximetry remained within reference limits.
Ketamine-based anesthesia can lead to prolonged recovery periods, emergence delirium, and nonpurposeful movements, particularly when used in a total IV anesthesia maintenance protocol. These effects vary by species, with small ruminants affected less than other domestic species, such as cats.15 In contrast, propofol is associated with short recovery durations. In the goat of the present report, we kept the dosage of ketamine low by adding midazolam to the anesthetic maintenance protocol and providing multimodal analgesia. Further, shortly after reversal with flumazenil administration, the goat was extubated; approximately 10 minutes after extubation, the goat was standing and ambulatory, as was expected following the relatively short duration of anesthesia.
More investigation into the effects of ketamine in goats with ToF is warranted. Anesthesiologists need to understand the physiologic alterations in patients with ToF and the pharmacological effects of anesthetics for successful perioperative management in affected patients. Findings for the goat of the present report provided a practical reminder of the intricacies involved in determining an anesthetic protocol to avoid substantial alterations in the hemodynamic status in a patient with a complex congenital heart defect.
Footnotes
Food Animal Residue Avoidance Databank [database online]. Available at: farad.org. Accessed Jan 13, 2021.
IBP Disposable Pressure Transducer, Baxter Healthcare Corp, Deerfield, Ill.
Datex-Ohmeda S/5, GE Healthcare, Helsinki, Finland.
McKenna SLB, Barkema HW, McClure JT, et al. Tetralogy of Fallot in a 2-year-old Holstein heifer (abstr). Can Vet J 2003;44:312–313.
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