Cardiopulmonary values are often determined in ill and anesthetized green iguanas (Iguana iguana) to assess cardiovascular and respiratory performance; however, values in healthy green iguanas are not available for comparison, making clinical interpretation difficult. In addition, anesthesia monitoring techniques such as arterial and venous blood gas analyses, pulse oximetry, and direct arterial blood pressure measurements commonly used in human and companion mammal medicine have not been validated for use in reptiles.1–3 Therefore, application and interpretation of invasive and noninvasive monitoring techniques is in its infancy, which makes diagnosis of cardiopulmonary disease and monitoring of anesthesia challenging.
Determination of arterial blood gas values provides information on pulmonary function, whereas venous samples are used for metabolic assessments.4 However, in most reptiles, catheterization of a peripheral artery for blood pressure and blood gas measurements is difficult, leading to noninvasive monitoring techniques such as pulse oximetry being more commonly used. Although studies5,6 have been conducted to determine the accuracy of pulse oximetry in anesthetized reptiles, little cardiovascular research has been undertaken involving conscious reptiles.
Association of blood gas values with physiologic status in reptiles is particularly difficult because of the unique aspects of their respiratory anatomy and physiology, which differ from those of mammals and birds.7 For example, reptiles have the unique ability to tolerate varying degrees of hypoxia, are capable of converting to anaerobic metabolism, and possess intrapulmonary shunts, which bypass gas exchange in the lungs. Large intrapulmonary shunts reduce the efficiency of gas exchange and consequently cause a reduction in Pao2.7 Healthy cardiovascular performance is also closely linked to thermoregulation in reptiles.8 Studies9 of cardiopulmonary physiology in snakes have demonstrated a positive correlation between habitat and arterial blood pressures. Additionally, aquatic, terrestrial, and arboreal snakes have different cardiovascular responses to gravity, with arboreal species having higher arterial blood pressures and more effective regulation than aquatic species.9–12 To the authors' knowledge, no reports exist of similar adaptations in terrestrial versus arboreal species of lizards.
Information on arterial blood pressure in conscious green iguanas, a strictly arboreal species, is restricted to the effects of posture and hemorrhage.13 In reptiles, low Pao2 is the primary drive for an increase in respiratory rate.7 The baroreceptor reflex plays a vital role in the moment-to-moment control of blood pressure and HR in other animal species.14–16 In reptiles, a baroreceptor reflex in response to hemorrhage and body tilting has been demonstrated, with snakes serving as the primary example in research, yet important natural history–specific differences exist in snakes.17 For example, aquatic snakes are less effective at maintaining blood pressure than terrestrial and arboreal snakes. The sensitivity (gain) of the baroreceptor reflex, when expressed as a percentage change in HR per unit pressure change, is approximately the same in reptiles, amphibians, and mammals.11,13,14 In addition, the ultrastructural appearance of the baroreceptors of lizards is similar to that of mammals.17
Green iguanas are diurnal and semiarboreal. To understand the importance of circadian rhythms in lower vertebrates, arterial and venous blood gas values need to be determined at various points during a 24-hour period. Reference values from healthy conscious animals are also needed to better understand iguanid cardiopulmonary physiology and interpret findings in sick or anesthetized iguanas. The objectives of the study reported here were to establish reference ranges for cardiopulmonary variables in conscious, healthy green iguanas breathing room air, to investigate the effects of 100% inspired oxygen on arterial blood gas values, and to characterize the baroreceptor reflex and control of blood pressure in this species.
Mean arterial blood pressure recorded at the midpoint of the heart rate range
Diastolic arterial blood pressure
Mean gain (or sensitivity) of baroreceptor reflex
Mean arterial blood pressure
Arterial oxygen saturation
Systolic arterial blood pressure
Torbugesic, 10 mg/mL, Fort Dodge Animal Health, Overland Park, Kan.
Rapinovet, 10 mg/mL, Schering Plough, Kenilworth, NJ.
Isoflurane, Abbott Laboratories, Abbott Park, Ill.
Small animal ventilator (VT-5000), BAS Vetronics, Bioanalytical Systems Inc, West Lafayette, Ind.
Hospira, Lake Forest, Ill.
Perma-Hand Silk Suture, Ethicon Inc, Cornelia, Ga.
BPE-T50, Instech Laboratories Inc, Plymouth Meeting, Pa.
PDS II, Ethicon Inc, Cornelia, Ga.
Becton-Dickinson Precision Glide needles, Franklin Lakes, NJ.
Becton-Dickinson Venflon Injection Port, Franklin Lakes, NJ.
SurgiVet 3 Parameter Advisor Vital Signs Monitor, Smiths Medical North America, Waukesha, Wis.
V3301 Surgivet Pulse Oximeter, Smiths Medical North America, Waukesha, Wis.
I-Stat analyzer and EG4+ cartridges, Abbot Point of Care, Princeton, NJ.
DPT-400 Deltran IV disposable pressure transducer, Columbia Medical Inc, Redmond, Ore.
100-μL 700 Series Hamilton Glass Syringe, Harvard Apparatus, Holliston, Mass.
Sigma-Aldrich Corp, St Louis, Mo.
Stata, version 10, Stata Corp, College Station, Tex.
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Lewis SJ, Whalen EJ, Beltz TG, et al. Effects of chronic lesions of the anteroventral third ventricle region on baroreceptor reflex function in conscious rats. Brain Res 1999; 835:330–333.
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Ak A, Ogun CO, Bayir A, et al. Prediction of arterial blood gas values from venous blood gas values in patients with acute exacerbation of chronic obstructive pulmonary disease. Tohoku J Exp Med 2006; 210:285–290.
Chu YC, Cen CZ, Lee CH, et al. Prediction of arterial blood gas values from venous blood gas values in patients with acute respiratory failure receiving mechanical ventilation. J Formos Med Assoc 2003; 102:539–543.
Hess JC, Grimm KA, Benson GJ, et al. The use of vascular access ports in nonanesthetized green iguanas (Iguana iguana) to collect baseline arterial blood gas parameters. Vet Anesth Analg 2005; 32:18.