Effects of Ampicillin/Sulbactam and Enrofloxacin on the Blood Pressure of Isoflurane Anesthetized Dogs

Introduction

Surgical prophylaxis is one of the most common reasons antibiotics are administered in veterinary and human medicine.¹ The prevention of surgical site infection has been a focus in human medicine to decrease the patient morbidity and mortality as well as health care costs. Not every surgical procedure requires antimicrobial prophylaxis, and antibiotics, like all drugs, do have documented and potential side effects that can adversely affect patient well being.^2–6

The most common types of bacteria found at the surgical site of clean surgical incisions are Staphylococcus spp. and Escherichia coli.^4,7 Cephalosporins are commonly chosen for surgical prophylaxis because they are bactericidal, effective against most gram-positive organisms, and have few side effects.⁸ Other classes of antibiotics may be chosen, such as penicillins and fluoroquinolones, when there is a high index of suspicion of contamination with either gram-negative or anaerobic bacteria such as in hepatobiliary or lower gastrointestinal tract surgery.^9,10

Ampicillin Na, a synthetic aminopenicillin that is effective against gram-positive and some gram-negative bacteria, is a penicillin commonly used for surgical prophylaxis. One limitation of ampicillin Na is that it can be inactivated by β-lactamase-producing bacteria. Sulbactam Na is a β-lactamase inhibitor that is added to ampicillin Na to extend its spectrum against β-lactamase-producing bacteria. The combination of ampicillin Na/sulbactam Na^a extends the spectrum to include a wider spectrum of gram-positive, gram-negative, and some anaerobic bacteria. This antibiotic is chosen when more than just skin contamination is present or expected. Reported side effects of penicillins include urticaria, edema, bronchospasm, thrombophlebitis, and anaphylaxis.¹¹

Enrofloxacin^b is a commonly used fluoroquinolone in veterinary medicine. It is effective against gram-negative bacteria, Mycoplasma, and some gram-positive bacteria. Enrofloxacin is sometimes used in combination with other medications, such as cephalosporins or penicillins, to extend the gram-negative antibacterial coverage. Side effects of fluoroquinolones that have been reported include seizures, vomiting, anorexia, diarrhea, chondrotoxicity (young growing dogs), hypotension, bradycardia, and nephropathies.^6,12

In anesthetized horses, ampicillin has been shown to cause a decrease in arterial blood pressure (BP).³ Morgan et al. (2000) evaluated Na cefazolin, Na cefoxitin, and Na ampicillin to determine their effects on the BP of dogs anesthetized with halothane.⁴ None of the antibiotics in that study caused any significant changes in BP or the heart rate (HR). Although not statistically significant, there was a trend in the ampicillin group toward a decrease in the BP in the first minute after IV bolus administration. Marbofloxacin, as well as other fluoroquinolones, has been shown to cause a significant decrease in arterial BP in dogs.^6,12 That effect was considered to be secondary to histamine release.⁶ To the authors’ knowledge, enrofloxacin has not been evaluated for its effect on the BP of healthy anesthetized dogs.

Other antibiotics have also been shown to have an effect on the BP of awake and anesthetized animals. In 1975, Adams examined the cardiovascular depressant effects of several aminoglycoside antibiotics including neomycin, streptomycin, kanamycin, and gentamycin. That study found that those antibiotics caused myocardial depression and inhibition of the arterial smooth muscle contraction via decreased availability of Ca, leading to hypotension.² The antibiotics that cause those side effects are not commonly used for prophylaxis but may be indicated in patients with established infections based on culture and sensitivity. The user should be aware of reported side effects.

The previously noted studies prompted the authors of this study to speculate about the effects of other commonly used perioperative antibiotics, such as ampicillin Na/sulbactam Na and enrofloxacin, on BP. Typically in the authors’ hospitals, those antibiotics are given slowly, usually with a syringe pump, over 20–30 min. In previous studies, the antibiotics were administered as a bolus.^4,12

The current recommendations for prophylactic antibiotic administration are IV administration 30–60 min prior to the skin incision and then q 90–120 min until skin closure.^13,14 If those guidelines are not followed, the antibiotics are then considered therapeutic and not prophylactic. Concerns about the potential hypotensive effects of antibiotics may lead to reluctance to administer those medications to anesthetized patients. That can be detrimental to the outcome of the surgery or devastating to the patient if a surgical infection occurs because of the lack of appropriate administration of prophylactic antibiotics.

The objective of this study was to determine the effects of IV ampicillin Na/sulbactam Na and enrofloxacin, given over 30 min, on the arterial BP of healthy dogs anesthetized with isoflurane and fentanyl. The null hypothesis was that ampicillin Na/sulbactam Na and enrofloxacin will have no effect on the arterial BP of healthy anesthetized dogs compared with dogs receiving saline.

Materials and Methods

This study was approved by the Oklahoma State University Institutional Animal Care and Use Committee. Eight 4 yr old castrated male beagles were included in this study. All dogs were healthy based on physical examination, serum biochemical analysis, and complete blood count. This study was designed as a blinded, prospective, randomized, crossover study in which each dog received saline^c, enrofloxacin, and ampicillin Na/sulbactam Na. A wash out period of at least 7 days was used between each study treatment (antibiotic or saline) administration and anesthetic episode.¹⁵ Food was withheld for 12 hr prior to anesthesia in all dogs.

Study treatment administration was randomized so each dog would receive each treatment only once. Researcher A was blinded to the treatment being administered. Three pieces of paper, representing saline, enrofloxacin, or ampicillin Na/sulbactam Na were placed in a separate bag for each dog. Each time the dogs were anesthetized, one piece of paper was chosen out of the bag to determine which treatment would be administered to the dog. Each treatment was diluted to the same total volume to prevent introduction of bias from the volume of the solution. The paper was drawn by an assistant who also loaded the treatment in the syringe and placed it in the syringe pump. Researcher A gathered all data and was unaware of the contents of each syringe. All treatments were given via a syringe pump^d set to administer the determined amount over 30 min, according to manufacturer guidelines.^16,17 The amount of dilution needed to obtain an equal total volume was within either manufacturer specifications or reported dilutions appropriate for each drug.^16,17

After cephalic IV catheter placement, each dog was administered 3 μg/kg of fentanyl^e IV. Anesthesia was induced with IV propofol^f to effect, and the dogs were orotracheally intubated with a size 7 cuffed endotracheal tube. The dogs were initially administered 1% isoflurane^g in O₂ via endotracheal tube and mechanically ventilated to maintain an end-tidal CO₂ (ET_CO2) of 35–45 mm Hg. The dogs were also administered a fentanyl continuous rate infusion at 10–20 µg/kg/hr. Additional propofol was available for rescue IV administration if the dogs became light under anesthesia. The isoflurane was adjusted to achieve a stable end-tidal isoflurane (ET_iso) level that also did not cause hypotension. An adequate anesthetic depth was determined by a lack of response to nonnoxious stimuli such as palpebral reflex and manipulation of the limbs. Either the right or left dorsal pedal artery, caudal saphenous artery, or the intermediate auricular artery was catheterized with a 22 gauge or 24 gauge polyethylene catheter^h. The systolic arterial pressure (SAP), mean arterial pressure (MAP), and diastolic arterial pressure (DAP) were continuously monitored with a fluid-filled pressure transducerⁱ connected to an anesthetic monitor^j, which also monitored the lead II electrocardiogram. A rectal thermometer probe was also connected to that device. The pressure transducer was zeroed before each anesthetic episode at approximately the level of the right atrium. The ET_iso, ET_CO2, and respiratory rate were measured on a gas analyzer^k. The gas analyzer was calibrated prior to every anesthetic episode. The O₂ saturation was measured with a pulse oximeter^l continuously throughout the procedure. Sterile 0.9% saline solution was administered at 10 mL/kg/hr during anesthesia. A patient warmer^m was placed under each patient, and a warm towel blanket was placed over each patient to maintain the rectal temperature between 36.7°C and 38.9°C. If the rectal temperature exceeded 38.9°C, the patient warmer was turned off. If the temperature remained elevated, the warm towel was removed. If the rectal temperature decreased below 36.7°C, the towel was replaced with a new warm towel directly from the towel warmerⁿ, which was kept at 66.1°C.

The isoflurane was maintained at a level that provided adequate anesthetic depth with a MAP > 60 mm Hg for at least 20 min to achieve a stable plane of anesthesia. Baseline readings of all parameters were taken for 20 min prior to the administration of any treatment. Each treatment (antibiotic or saline) was administered IV via a syringe pump over 30 min.

The enrofloxacin (10 mg/kg) was diluted with 2 × the volume of 0.9% sterile saline (0.88 mL/kg of 0.9% sterile saline).¹⁶ The ampicillin Na/sulbactam Na (22 mg/kg) was diluted to 30 mg/mL with 50 mL of 0.9% saline (20 mg ampicillin Na/10 mg sulbactam Na) and then further diluted with 0.9% saline to a volume equal to the diluted enrofloxacin (0.59 mL/kg 0.9% sterile saline). An equal volume of 0.9% sterile saline (1.32 mL/kg) was administered to the saline control group. The anesthetic fluid rate was decreased during the treatment administration to keep the total fluid volume at 10 mL/kg/hr.

All parameters, except body temperature, were measured q 1 min for 20 min prior to administration, during the infusion over 30 min, and for 30 min after the infusion was ended. Body temperature was measured q 5 min. If dogs had a MAP < 60 mm Hg for 15 min, they were considered hypotensive and the study was ended in accordance with the Institutional Animal Care and Use Committee approved protocol. The BP was monitored with the arterial line until those dogs were no longer considered hypotensive (i.e., MAP > 60 mm Hg for 5 min). The study was also ended if the HR was < 40 beats/min or if there were any cardiac arrhythmias. If bradycardia occurred, glycopyrrolate^o (0.005 mg/kg) was administered IV. The HR was monitored until it was > 100 beats/min and there were no cardiac arrhythmias. IV crystalloid boluses of 10–20 mL/kg were given to treat the hypotension (if needed) once the inhalant anesthesia was discontinued. The arterial line was removed once the patient was extubated, and the cephalic catheter was removed once the dog was ambulatory without assistance.

Results

The mean body weight of the dogs was 9.39 kg ± 0.77 kg. The propofol induction dose ranged from 3.6 mg/kg to 8.7 mg/kg (mean, 6.1 mg/kg). There were a total of 19 anesthetic episodes included in the study. Five anesthetic episodes were excluded from the study because of hypotension prior to either antibiotic or saline administration, bradycardia, or cardiac arrhythmias. Of those five episodes, three were in the control group, one in the enrofloxacin group, and one in the ampicillin Na/sulbactam Na group. A total of five anesthetic episodes from the control group, seven from the enrofloxacin group, and seven from the ampicillin Na/sulbactam Na group were included in the statistical analysis. Three dogs received additional propofol after induction to maintain anesthesia (range, 1.1 mg/kg–2.4 mg/kg; mean, 1.5 mg/kg). One dog in the enrofloxacin group became hypotensive for > 15 min, and the study was ended. That dog received additional propofol (1.1 mg/kg) to maintain anesthesia, but the hypotension was present prior to the administration of the additional propofol.

In the control group, the SAP ranged from 83 mm Hg to 175 mm Hg, with a mean of 125.7 mm Hg. The SAP in the enrofloxacin group ranged from 90 mm Hg to 151 mm Hg, with a mean of 121.9 mm Hg. The SAP in the ampicillin Na/sulbactam Na group ranged from 78 mm Hg to 163 mm Hg, with a mean of 125.2 mm Hg. There was no significant difference in the SAP among the three groups (Figure 1). In the control group, the DAP ranged from 36 mm Hg to 78 mm Hg, with a mean of 53.6 mm Hg. The DAP in the enrofloxacin group ranged from 31 mm Hg to 76 mm Hg, with a mean of 47.9 mm Hg. The DAP in the ampicillin Na/sulbactam Na group ranged from 30 mm Hg to 75 mm Hg, with a mean of 52.2 mm Hg. At time 45, the DAP was significantly higher in the control group than the enrofloxacin group (P = 0.039; Figure 2). The ampicillin Na/sulbactam Na group was not significantly different than either of the other two groups at time 45. In the control group, the MAP ranged from 42 mm Hg to 102 mm Hg, with a mean of 125.7 mm Hg. In the enrofloxacin group, the MAP ranged from 35 mm Hg to 77 mm Hg, with a mean of 66.3 mm Hg. In the ampicillin Na/sulbactam Na group, the MAP ranged from 50 mm Hg to 103 mm Hg, with a mean of 71.4 mm Hg. At time 25, (P = 0.029), 35 (P = 0.012), 45 (P = 0.0068), the MAP in the control group was significantly higher compared with both the enrofloxacin and the ampicillin Na/sulbactam Na groups. At time 55, the MAP of the control group was significantly higher compared with the enrofloxacin group (P = 0.0077) but was not significantly different from the ampicillin Na/sulbactam Na group (Figure 3). There was no significant difference in the SAP and DAP of the three groups over time (P values in the control group, 0.12 and 0.077; enrofloxacin, 0.65 and 0.98; and ampicillin Na/sulbactam Na, 0.19 and 0.63) as shown in Figures 1, 2. There was no significant change in the MAP of the enrofloxacin and ampicillin Na/sulbactam Na groups over time (P = 0.99 and 0.15, respectively). The control MAP significantly increased over time (P = 0.01; Figure 3).

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There was no statistically significant change over time in the HR of the control and ampicillin Na/sulbactam Na groups (P = 0.055 and 0.18, respectively). There was significant variability in the enrofloxacin group’s HR over time (P = 0.0006). At time 35 (P = 0.033) and 55 (P = 0.038), the HR was significantly lower in the control group compared with the enrofloxacin group but not significantly different from the ampicillin Na/sulbactam Na group (Figure 4). One dog had ventricular escape arrhythmias secondary to bradycardia.

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The rectal body temperature was maintained between 36.7°C and 38.9°C. There was no significant difference in the ET_iso among the three groups. The mean ET_iso of the three groups combined was 0.84%. There was a significant increase in the ET_iso of all three groups over time (P value in control, 0.0076; enrofloxacin, 0.022; and ampicillin Na/sulbactam Na, 0.0003) as shown in Figure 5. The respiratory rate was controlled by mechanical ventilation at a level that maintained the ET_CO2 between 35 mm Hg and 45 mm Hg. The O₂ saturation remained above 95% during the study period.

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Discussion

Under normal circumstances, the arterial BP is tightly regulated by rapid, intermediate, and long-term mechanisms.¹⁸ Anesthesia can cause hypotension, even in normal healthy patients. In 2007, Redondo et al. evaluated 1,281 cases of anesthetized dogs and found that the average invasive MAP in those anesthetized patients was 80 mm Hg ± 22 mm Hg, the average invasive SAP was 109 mm Hg ± 26 mm Hg, and the average invasive DAP was 66 mm Hg ± 22 mm Hg. Either a MAP of < 60 mm Hg or a SAP of < 80 mm Hg is considered hypotension.¹⁹ Hypotension can be caused by a combination of the medications administered and the methods used to maintain appropriate anesthetic depth. Isoflurane is a commonly used inhalant anesthetic in veterinary medicine. It causes a dose-dependent cardiovascular depression mainly by causing a decrease in systemic vascular resistance.²⁰ Propofol is also a very commonly used induction agent that can also have cardiovascular side effects. It has negative ionotropic effects that can decrease cardiac output as well as decrease sinus nodal activity, which decreases the HR and BP.¹⁵ Positive-pressure ventilation under anesthesia can also cause hypotension by impeding the venous return to the heart because of increased intrathoracic pressure causing compression of the vasculature.²¹ Certain classes of antibiotics have also been reported to cause hypotension in anesthetized animals and humans.^2,5

Methods that are used to combat anesthetic-induced hypotension include decreasing the concentration of inhalant required, administering medications to maintain HR (and therefore cardiac output), administering IV fluid boluses, and administering ionotropic agents. Classes of medications that can be used to decrease the inhalant anesthetic requirement include sedative tranquilizers and opioids. Sedative tranquilizers include medications such as acepromazine, which has been shown to decrease the inhalant anesthetic requirement by 48% and benzodiazepines, which can decrease the inhalant requirement up to 20%.^22,23 Anticholinergics are often used as part of the premedication prior to anesthesia to help maintain the HR and, therefore, the cardiac output during anesthesia.²⁴ Fentanyl is a short-acting μ-agonist opioid that is commonly used in anesthetized patients for its potent analgesic effects and for its sparing effects on the minimum alveolar concentration. Fentanyl is 100 × more potent than morphine and is very short acting so it can be easily titrated as a continuous rate infusion, depending on either the patient or clinical situation.²⁵ Fentanyl can also have up to a 66% sparing effect on the minimum alveolar concentration with isoflurane, if administered at high doses during anesthesia. Fentanyl also increases parasympathetic tone, which causes a vagally mediated bradycardia. That bradycardia is often counteracted by anticholinergics given either after induction or as a premedication. Intuitively, one might think that because fentanyl decreases the inhalant anesthetic requirement, the BP would increase because isoflurane causes a dose-dependent hypotension.²⁰ That increase does not occur due to the vagally mediated bradycardia caused by the fentanyl.²⁵ In this study, the authors chose not to give any anticholinergic medications to counteract the bradycardia because they did not want to cause any increase in BP that may have masked any hypotension caused by the antibiotics. Further, a rescue protocol was in place to prevent severe bradycardia and other dangerous dysrhythmias.

The anesthetic protocol used in this study was chosen to eliminate the influence of drugs, other than isoflurane and propofol, on the BP of the dogs. Propofol is such a short-acting drug that any hypotension it may cause during induction did not affect the BP measurements during the study period. The authors did not want to mask any hypotension that may have been caused by the antibiotics by using medications such as anticholinergics or ionotropic agents. In this way, the study authors could evaluate the effect of only the antibiotics on the BP without interference from other drugs. In this study, each treatment group’s BPs did not significantly change over time; however, the MAP of the treatment and control groups did statistically differ at certain time points. The differences were due to an increase in MAP in the control group rather than a decrease in the corresponding time points in the enrofloxacin group. One possible explanation for the lack of increase in the MAP in the treatment groups is a type II statistical error because there were two less anesthetic episodes in the control group than the treatment groups. Another possibility is that the antibiotics prevented the increase in BP that was seen in the control group. In previous studies, the antibiotics were administered as a bolus.^4,5,12 In Morgan et al. (2000), there was a trend toward a significant decrease in MAP in the ampicillin group.⁴ BP measurements were only taken for 10 min after the bolus administration. If the BPs were measured over a longer period of time, as in this study, a significant difference may have been seen. In Chanoit et al. (2005), a significant decrease in MAP was seen after bolus administration of marbofloxacin at high doses.¹² In the current study, the authors administered the antibiotics at clinically published doses over 30 min, which may have decreased any hypotensive effects that may have been caused by the antibiotic administration.^16,17 This blunted hypotensive effect may explain the lack of increase in the MAP in the treatment groups that was seen in the control group. All groups maintained MAPs above the cut off for hypotension (60 mm Hg) through all time points, and that blunted hypotensive effect should not be clinically significant. While there was no statistically significant change in the HR in the control group over time, there was a trend toward decreasing over time. The decrease in HR in the control group at time points 35 and 55 may have been due to the increase in BP, causing a decrease in the HR via the baroreceptor reflex. The increase in BP causes stretching of the vessel walls that contain the baroreceptors. That causes stimulation of nerves that transmit to the medulla, causing inhibition of the vasoconstrictor system and excitation of the vagal center. In turn, vasodilation of veins and arterioles in systemic circulation occurs as well as a decrease in HR and cardiac output.¹⁸

The increase in the ET_iso of the three groups over time was statistically significant. That may have been due to inadequate time under anesthesia to allow an even plane of anesthetic depth. That said, the comparison among the groups did not show any significant difference in the ET_iso of the three groups; therefore, any change in BP due to an increase in isoflurane concentration would have been the same among the three groups. That would also have caused a decrease in the BP over time, as opposed to an increase in BP as seen in the control group.

The exclusion of the five anesthetic episodes from statistical analysis was due to hypotension (n = 3), bradycardia (n = 1), and cardiac arrhythmias (n = 1). The episodes excluded for hypotension were ended prior to the administration of any medication and were therefore not related to antibiotic administration. The episodes excluded for bradycardia and cardiac arrhythmias were the same dog on separate occasions. The ventricular escape arrhythmias were secondary to bradycardia. On each occasion, the dog received one of the antibiotics. Bradycardia has been reported as a side effect of different fluoroquinolones, but usually at higher than clinically used doses.⁶ Bradycardia has not been reported as a side effect of ampicillin or any of its derivatives. Given that the bradycardia occurred in the same dog each time, it is more likely an effect of the vagally mediated bradycardia caused by either the administration of fentanyl or an idiosyncratic reaction unique to that dog.²⁵

Ampicillin Na/sulbactam Na and enrofloxacin are occasionally used for prophylaxis in healthy anesthetized patients. They are more commonly used in either patients with underlying disease or patients that may have contamination or an infection when culture and sensitivity testing results are not available, such as septic peritonitis. Those patients are often not hemodynamically stable at the time of anesthesia. This study did not evaluate the effects of the antibiotics on unhealthy patients. One limitation of this study was that no conclusion could be made as to the effect of the antibiotics on the BP of unhealthy patients. In such a study, it would be difficult to control variables and provide standardization.

Conclusion

Antibiotics are commonly used in healthy and unhealthy patients for perioperative prophylaxis. The delay of administration can increase the risk of surgical site infections that could potentially have devastating consequences, depending on the surgical site. This study showed that enrofloxacin and ampicillin Na/sulbactam Na do not cause hypotension in healthy, anesthetized dogs when given over 30 min and at recommended doses. Further studies would be required to determine the effects of those antibiotics on hemodynamically unstable patients. This study serves as a baseline for comparison in further studies.