Pharmacodynamics of propofol and alfaxalone in rattlesnakes (Crotalus durissus).

To characterise the effect of two common induction agents, propofol and alfaxalone, on mean arterial blood pressure (MAP) and heart rate (HR), we equipped 19 adult South American rattlesnakes (Crotalus durissus) with an indwelling arterial catheter approximately 24 h prior to recording of baseline resting values. Then, seven snakes received alfaxalone (15 mg kg-1) intravascularly (IV) through the catheter, while groups two and three (both n = 6) received propofol (15 mg kg-1 IV). The first two groups were not handled, while the group 3 was manually restrained for 2 min for a mock injection of 0.2 ml saline into the ventral tail vein. Baseline HR was similar in all groups and handling caused a significant tachycardia (p = 0.031) in group three. When given IV to undisturbed animals, both propofol and alfaxalone induced a significant increase in HR (p = 0.0022 and p = 0.0045, respectively) lasting approximately 30 min, but with values only significantly exceeding baseline for the first 5 min for propofol and the first 10 min with alfaxalone. Handling caused a significant increase in MAP (p = 0.0313). Propofol did not affect MAP (p = 0.1064), while alfaxalone caused a marked hypertension (although only significant at 2 min; p = 0.031). Manual restraint significantly increases both HR and MAP, which may lead to a masking of true cardiovascular effects of anaesthetic agents.

CARDIOVASCULAR EFFECTS OF ALFAXALONE AND PROPOFOL IN THE BULLFROG, LITHOBATES CATESBEIANUS.

Alfaxalone is becoming a popular anesthetic for nonmammalian vertebrates, but the physiological effects of its administration remain largely unknown in these taxa. Therefore, the cardiovascular responses to a clinically relevant dose of alfaxalone (10 mg/kg) are reported in the bullfrog ( Lithobates catesbeianus), following intramuscular (IM) and intravascular (IV) administration (via a femoral artery catheter) and compared with an IV dose of propofol, another parenteral GABA (γ-aminobutyric acid) agonist in common veterinary use as an induction agent. Heart rate (HR) and mean arterial blood pressure (MAP) (assessed by direct measurement from the catheter) are reported from under undisturbed conditions to assess both the direct effects of the drugs and the interaction with the stress of handling associated with IM injection of alfaxalone where IM administration is possible. Alfaxalone caused HR to increase significantly for over 45 min in both groups from a baseline of approximately 30 beats/min. This was significantly different from the lack of significant HR response on the IV administration of propofol. MAP increased in the peri-injection period with both routes of administration for alfaxalone but after IV use decreased significantly from 10 min following administration. Propofol did not affect blood pressure after 5 min from injection. Assessment of immobilization following intramuscular injection of alfaxalone in a pilot study was in accordance with the literature, as it provided no antinociception as a sole agent but did produce sedation and loss of righting reflex.

When local anesthesia becomes universal: Pronounced systemic effects of subcutaneous lidocaine in bullfrogs (Lithobates catesbeianus).

Sodium channel blockers are commonly injected local anesthetics but are also routinely used for general immersion anesthesia in fish and amphibians. Here we report the effects of subcutaneous injection of lidocaine (5 or 50mgkg-1) in the hind limb of bullfrogs (Lithobates catesbeianus) on reflexes, gular respiration and heart rate (handled group, n=10) or blood pressure and heart rate via an arterial catheter (catheterized group n=6). 5mgkg-1 lidocaine did not cause loss of reflexes or change in heart rate in the handled group, but was associated with a reduction in gular respiratory rate (from 99±7 to 81±17breathsmin-1). 50mgkg-1 lidocaine caused a further reduction in respiratory rate to 59±15breathsmin-1, and led to a progressive loss of righting reflex (10/10 loss by 40min), palpebral reflex (9/10 loss at 70min), and contralateral toe pinch withdrawal (9/10 loss at 70min). Reflexes were regained over 4h. Systemic sedative effects were not coupled to local anti-nociception, as a forceps pinch test at the site of injection provoked movement at the height of the systemic effect (tested at 81±4min). Amphibians are routinely subject to general anesthesia via exposure to sodium channel blockers such as MS222 or benzocaine, however caution should be exercised when using local injectable lidocaine in amphibians, as it appears to dose-dependently cause sedation, without necessarily preventing local nociception for the duration of systemic effects.

Assessing the influence of mechanical ventilation on blood gases and blood pressure in rattlesnakes.

OBJECTIVE: To characterize the impact of mechanical positive pressure ventilation on heart rate (HR), arterial blood pressure, blood gases, lactate, glucose, sodium, potassium and calcium concentrations in rattlesnakes during anesthesia and the subsequent recovery period. STUDY DESIGN: Prospective, randomized trial. ANIMALS: Twenty one fasted adult South American rattlesnakes (Crotalus durissus terrificus). METHODS: Snakes were anesthetized with propofol (15 mg kg(-1)) intravenously, endotracheally intubated and assigned to one of four ventilation regimens: Spontaneous ventilation, or mechanical ventilation at a tidal volume of 30 mL kg(-1) at 1 breath every 90 seconds, 5 breaths minute(-1), or 15 breaths minute(-1). Arterial blood was collected from indwelling catheters at 30, 40, and 60 minutes and 2, 6, and 24 hours following induction of anesthesia and analyzed for pH, PaO2, PaCO2, and selected variables. Mean arterial blood pressure (MAP) and HR were recorded at 30, 40, 60 minutes and 24 hours. RESULTS: Spontaneous ventilation and 1 breath every 90 seconds resulted in a mild hypercapnia (PaCO2 22.4 ± 4.3 mmHg [3.0 ± 0.6 kPa] and 24.5 ± 1.6 mmHg [3.3 ± 0.2 kPa], respectively), 5 breaths minute(-1) resulted in normocapnia (14.2 ± 2.7 mmHg [1.9 ± 0.4 kPa]), while 15 breaths minute(-1) caused marked hypocapnia (8.2 ± 2.5 mmHg [1.1 ± 0.3 kPa]). Following recovery, blood gases of the four groups were similar from 2 hours. Anesthesia, independent of ventilation was associated with significantly elevated glucose, lactate and potassium concentrations compared to values at 24 hours (p < 0.0001). MAP increased significantly with increasing ventilation frequency (p < 0.001). HR did not vary among regimens. CONCLUSIONS AND CLINICAL RELEVANCE: Mechanical ventilation had a profound impact on blood gases and blood pressure. The results support the use of mechanical ventilation with a frequency of 1-2 breaths minute(-1) at a tidal volume of 30 mL kg(-1) during anesthesia in fasted snakes.

Ultrasound imaging of the anterior section of the eye of five different snake species.

BACKGROUND: Nineteen clinically normal snakes: six ball pythons (Python regius), six Burmese pythons (Python bivittatus), one Children's python (Antaresia childreni), four Amazon tree boas (Corallus hortulanus), and two Malagasy ground boas (Acrantophis madagascariensis) were subjected to ultrasound imaging with 21 MHz (ball python) and 50 MHz (ball python, Burmese python, Children's python, Amazon tree boa, Malagasy ground boa) transducers in order to measure the different structures of the anterior segment in clinically normal snake eyes with the aim to review baseline values for clinically important ophthalmic structures. The ultrasonographic measurements included horizontal spectacle diameter, spectacle thickness, depth of sub-spectacular space and corneal thickness. For comparative purposes, a formalin-fixed head of a Burmese python was subjected to micro computed tomography. RESULTS: In all snakes, the spectacle was thinner than the cornea. There was significant difference in spectacle diameter, and spectacle and corneal thickness between the Amazon tree boa and the Burmese and ball pythons. There was no difference in the depth of the sub-spectacular space. The results obtained in the Burmese python with the 50 MHz transducer were similar to the results obtained with micro computed tomography. Images acquired with the 21 MHz transducer included artifacts which may be misinterpreted as ocular structures. CONCLUSIONS: Our measurements of the structures in the anterior segment of the eye can serve as orientative values for snakes examined for ocular diseases. In addition, we demonstrated that using a high frequency transducer minimizes the risk of misinterpreting artifacts as ocular structures.