Reptiles Flashcards
Reptile Thermoregulation
Ectotherms - derive body heat from surrounding environment, use behaviors (basking/burrowing) to regulate
Preferred optimal Temperature Zone
During anesthesia - should be maintained at high end of POTZ to ensure optimal metabolic function
○ 20-25℃ for most aquatic/temperate
○ 25-35℃ for tropical
Oxygen Consumption
Oxygen consumption rates range from zero to values similar to resting mammals
Function of species, temperature
Metabolic Rates:
○ Varanids/Lacertid lizards > boid snakes/chelonians
○ Surface dwelling > Burrowing
○ Insectivores > Herbivores
○ Higher rates may have more rapid metabolism, excretion (inconclusive)
Non-Crocodilian Heart - Anatomy
Three chambers; two separate atria, single continuous ventricle
Ventricles: divided by Muskelleiste or Muscular ridge
○ Originating from ventral ventricular wall, runs from ventricular apex to base
○ Divides ventricle into two anatomically defined but connected chambers: Cavum Pulmonale (RV) and Cavum Dorsale (LV)
Cavum Dorsale sometimes further divided into Cavum Venosum, Cavum Arteriosum
PM Cells in Cheloians
cardiac muscle constituting sinus venosum acts also as pacemaker of heart
Electrical activity is detectable on the electrocardiogram by the presence of a “SV wave” preceding atrial depolarization
Blood Flow Through Heart Non-Crocodilian Reptile Heart
Shunting Normally Present
During ventricular systole: muscular ridge presses against dorsal wall of ventricle, separates Cavum pulmonale from Cavum dorsale
Crocodilian Heart
Two divided atria, two ventricles
More similar to mammals but with adaptations to aquatic lifestyle
○ Subpulmonary Conus in pulmonary outflow tract of RV
○ Aortic anastomosis that connects two aortic arches
Foramen of Panizza
Foreman of Panizza
Small window located between interventricular septum at confluence of left right aortic arches
■ Acts as a pressure valve allowing blood to flow between venous, arterial systems
■ Flow from high pressure to low pressure leads to venous admixture
Direction of Shunting through Foreman of Panizza During Respiration
LV pressure greater allowing small amount of oxygenated blood to flow through FofP into venous blood supply
Direction of Shunting through FoP during Submersion
Held air in lungs restricts pulmonary capillary blood flow → Pulmonary hypertension → Increased RV and Pulmonary Arterial pressure ⇛ R-to-L shunt through PofF
● Deox blood diverted from lungs through left aortic arch to stomach and liver (↓ O2 sensitive)
● Oxy blood diverted to heart and brain
Combo of blood shunting and anaerobic metabolism may allow submersion for 5-6 hours!
Direction of Net Shunt in Both Non-Crococdilian, Crocodilian Hearts
Direction of NET shunt determines whether systemic or pulmonary circulation receives majority of CO
Size, direction controlled by pressure differences in the pulmonary and systemic circulations
Controlled by cholinergic, adrenergic factors that regulate Vascular Resistance
Which reptilian species has a particular large intracardiac shunt?
Turtles, which have poorly developed ventricular separation and similar pulmonary and systemic arterial blood pressures
Effect of Shunting on Anesthesia
○ Sudden changes in levels, directions = sudden/unexpected awakening
○ Implications for monitoring of airway gas monitoring and pulse oximetry (SpO2 useless)
Function of Shunting
- Serves to stabilize O2 content of blood during respiratory pauses
- R-to-L shunt facilitates heating by increased systemic blood flow
- R-to L shunt directs blood away from lungs during breath holds
Reptilian Blood Pressure
Systemic pressures vary greatly by species - inability of reptiles to regulate homeostasis independent of temperature and environment
○ Reported MAP ranges: SIGNIFICANTLY LOWER THAN MAMMALS
■ Chelonians: 15-30 mmHg
■ Varanids (lizards): 60-80 mmHg
■ Green Iguana: 40-50 mmHg
■ Snakes - dependent on “gravitational stress”
● Arboreal > Aquatic
Allometric relationship reported: as body mass increases, so does MAP
Pulmonary System of Reptilians
■ Lower O2 consumption due to decreased metabolic rate
■ Lungs of non-crocodilian species are suspended freely in pleuroperitoneal cavity
Sac-like with varying degree of partitioning
○ Smaller respiratory surface area relative to lung volume
○ Highly aerobic species - numerous septae, invaginations to increase gas exchange surface area
Lungs of Non-Crocodilian Reptiles
–Suspended free in pleuroperitoneal cavity - no diaphragm
–Highly aerobic species
–Chelonians, lizards: paired lungs
–Snakes: functional RIGHT LUNG, tracheal lung (unknown significance)
Trachea of Reptiles
● Complete tracheal rings in chelonians and proximal bifurcation
● Snakes possess a “tracheal lung” of unknown significance
Lizards, snakes = incomplete rings
Crocodiles = complete rings
What are the functional units of the reptilian lung?
Ediculi and Faveoli (analogous to mammalian alveoli)
NO TRUE ALVEOLI
Role of the Glottis During Respiration in Non Croc Reptiles
Glottis closed during most of respiratory cycle, opening only during inspiration and expiration
Movement of Respiration in Non-Croc Reptiles
Lack of diaphragm - rely on thoracic musculature for respiration
Inspiration and expiration are ACTIVE
Muscles for respiration same as those for locomotion: resp and locomotion cannot occur simultaneously
Respiration in Chelonians
expansion of thoracic cavity not possible due to attachment of lungs to carapace dorsally and abdominal viscera ventrally
○ Inspiration: enlarging visceral cavity
○ Expiration: forcing viscera up against lungs to drive air out
■ Contraction of posterior abdominal muscles and pectoral girdle muscles
Non Croc Reptiles Control of Resp
Interaction between central system generating pattern of respiration and afferent chemoreceptor input most likely
CO2 partial pressure and pH important for stimulation
■ O2 tension may play role in normal ventilation, variable responses to inspired CO and O2
“Episodic Breathers”
■ Bursts of activity followed by a pause of varying duration
Pulmonary Vascular Perfusion intermittent - changes are in concert with rate and rhythm
○ Ambient temperature may have variable effects on RR, TV and MV
Crocodilian Pulmonary Anatomy
-Lungs = well developed
- Muscular flap on nostrils, waterproof valve during submersion
-Gular Folds
-Glottic Valve
Clinical Significance of Muscular Flap on Nostrils in Crocodilians
○ Nostrils have muscular flap that acts as waterproof valve during submersion
■ Obtunded by immobilizing agents (risk of drowning)
Gular Folds in Crocodilians
■ Protrude from floor of the mouth
■ Respiratory valve when elongated soft palate presses against it
Glottic Valve in Crocodilians
closes to hold air in lungs
■ Opens to allow passive exhalation with elastic recoil of IC muscles and membranes
Respiratory M in Crocodilians
Intercostals, two transverse membranes (postpulmonary and posthepatic)
● Transverse membranes fibrous with a muscular component
● Act as “diaphragm”
Expansion of intercostals and caudal pull of membranes create negative pressure around lungs’
Direction of Airflow Through Crocodilian Lungs
Unidirectional airflow in alligators - pass through parabronchi (similar to birds)
Renal System in Repiles
■ Reptiles cannot produce urine more concentrated than plasma
● Excrete nitrogenous waste as Uric Acid (uricotelic)
● Some turtles and crocodilians excrete Urea
Uric Acid in Reptiles
produced by liver, very insoluble in water, excreted as a semi-solid
● Urine in renal tubule = very dilute to allow uric acid to remain in solution
● Dilute urine empties into Urodeum, then bladder or large intestine (via Coprodeum
○ Water reabsorbed in coprodeum and Uric acid precipitates
■ Allows excretion of nitrogenous waste with little water loss
Cloaca in Reptiles
3 distinct parts (degree of sep varies)
○ Coprodeum: most craniad; continuation of terminal colon
○ Urodeum: connections from urogenital system and bladder
○ Proctodeum: most caudal; “Vent”
Bladders in Reptiles
-Snakes: no bladder
- can be used for water storage
● Reptilian urine is not a good indicator of renal function*
Reptiles: Salt-Excreting Glands
allow for excretion of high concentrations of Na, K, Cl
● Reptiles that live in arid environments can tolerate marked fluctuations in total body water
● Plasma osmolality can rise to extremely high levels compared to other species
Renal Portal System in Reptiles
Blood from hind legs pass through kidneys before systemic circulation
● Effect on PK of drugs likely of little significance in healthy animals (much debate and studies are conflicting)
○ Best to avoid injection of nephrotoxic drugs in hind-limbs
Hepatic System in Reptiles
Lower metabolic capacity
● Sensitive to temperature changes
● Likely accounts for prolonged drug effects (ex. antibiotics)
Prolonged recoveries seen with drugs requiring extensive hepatic metabolism
Similar in structure, function to vertebrates
Considerations for Anesthesia of Reptiles
–Maintain at preferred body temp throughout ax, recovery
–Perform procedures early in the day
–Fasting generally not indicated
–Caution with species that can automize (drop) tails eg Geckos
–Chelonians: successful use of inhalant techniques requires intubation, controlled ventilation DT breath holding
Monitoring in Reptilian Species
–Doppler - HR, rhythm (can also use SPo2 for HR)
– ECG, temperature, +/- BP
–IBP not practical
Pulse Ox: Reflectance probe in cloaca or esophagus may be more accurate
With pulse ox, ETCO2 - more important to watch trends over time
Depth Monitoring in Reptiles
HR, RR not indicative of depth
M relaxation, loss of reflexes - righting reflex in lizards, snakes; head withdrawal reflex in turtles
Pain, Analgesia - what has been identified
● Spinothalamic, paleospinothalamic tracts identified
● Brain structures necessary for experiencing pain (neocortex) present with connections to brainstem and dorsal thalamus in midbrain
● Presence of opioid receptors, endogenous opioids
● Presence of nociceptive-related peptides: glutamate, Substance-P
Tonic Immobility: antipredator behavior in reptiles?
reptiles appear to “ignore” painful stimuli in presence of perceived threats
What pain structure has not been identified in reptiles?
Primarily nociceptive neurons (c-modal fibers) not yet identified in reptiles (no studies)
○ High threshold A-δ mechanical nociceptors and C-fiber mechanical nociceptors have been ID’d in one species of snake
Locoregional Anesthesia
Published local anesthetic doses for reptiles not published
○ Should not exceed TD’s published for mammals
Intrathecal administration of analgesics, LA reported for chelonians, bearded dragons
○ Coccygeal of clinical interest for procedures involving bladder, genitals, hind limbs
Mandibular NBs for crocodiles - intra PO/extra PO approaches
Correct Placement for Intrathecal Techniques in Chelonians, Bearded Dragons
- Chelonians: most proximal intervertebral space of coccygeal vertebrate identified via palpation of dorsal midline on tail - correct placement = CSF, NO BLOOD
- Bearded Dragons: similar technique, accurate placement cannot be confirmed by aspiration of CSF
Corum et al 2019 (VAA)
–Tolfenamic acid in red eared sliders: absence of AEs, favorable PK profile = potential for likely effective use in turtles
Larouche et al 2019 (VAA)
Ball Pythons: MAC reduction of iso by midaz IM = 57%, N2O = 17%
Chen et al 2020 (VAA)
Garter snakes: alfaxalone 30mgkg +/- dexmed 0.05mg/kg or 0.1mgkg intracoelomic –> alfax alone = reliable sedation; dexmed + alfax = variable durations of immobilization with low HR
Extended period of duration of immobilization with alfax + dexmed in snakes that lost righting reflex
Rockwell et al 2021 (VAA)
alfax 15mgkg SQ provides adequate anesthesia for brief procedures or intubation
Additional analgesia required for painful procedures
Mones et al 2021
Blind perineurial injection for brachia plexus block in eastern box turtles: 0.1, 0.2, 0.3mL methylene blue dye successful in 29/30 injections
Generally C5-T1
Ferreira et al 2021
Bupivacaine 1mg/kg recommended for neuraxial anesthesia in bearded dragons; 2mg/kg = extensive cranial spread, apnea, motor blockade of thoracic limbs
Immobilization of Crocodiles
Gallamine Triethiodide
Succinylcholine NOT recommended
Gallamine Triethiodide
short acting, nondepolarizing NMB
■ Produces flaccid paralysis
■ May cause open mouth due to muscle relaxation: “Flaxidil reaction”
■ Recovery 12-24 hours without reversal (neostigmine)
Drug of choice to immobilize crocodilian species in South Africa
Succinylcholine in Crocodile
NOT recommended
Depolarization before muscle relaxation ⇛ hyperkalemia, lactic acidemia, extreme muscle pain, phallus prolapse in males
Used successfully with diazepam prior (dec stress, dose); Gallamine preferred
Reptilian Recovery
● Prolonged compared to birds and mammals
● Maintain body temperature
● CMV: wean by manually ventilating every 1–5 minutes using an Ambu-bag or similar to allow buildup of CO2
Since hyperoxia may downregulate respiration in reptiles (Glass & Johansen 1976), and possibly promote R-L shunting, recovery in room air (21% oxygen) as opposed to 100% oxygen may be used.
How to Hasten Recovery in Reptiles
IM Epinephrine may increase HR and decrease R-to-L shunting, hastening recovery (Gatson et al. 2017)
Drug Access - Snakes
Intracoelomic
○ Coccygeal vein (ventral midline of tail)
○ Jugular vein (requires cut down)
○ Palatine vein (larger snakes)
○ Heart (emergency situations only)
Lizards - IV
○ Coccygeal vein
○ Ventral abdominal vein
○ Cephalic vein (cut down)
○ Jugular vein (at level of tympanum; cut down may be needed; close to lymphatic sinus - contamination may occur)
Lizards - Other Drug Access Points
Intraosseous: distal femur, proximal tibia, proximal humerus
○ IM injections:
Tricep muscle; avoid cranial surface due to risk of radial nerve damage
Chelonians: Drug Access
○ Dorsal coccygeal vein (midline dorsal to vertebrae)
○ Dorsal cervical sinus
○ Occipital venous sinus
○ Subcarpacal/Subvertebral Sinus
○ Jugular veins (cut down)
○ Intraosseous: Plastron (may end up intracoelomic), humerus, femur
