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The avian respiratory system: implications for anaesthesia

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Philosophical Transactions B
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John Ludders
John Ludders
John Ludders
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Abstract

Anaesthesia is not a natural state for any animal, including birds. The unique anatomic and physiological attributes of the class Aves that have made it possible for birds to inhabit every continent on this planet and to live in a variety of environments, some considered challenging if not inhospitable to mammals, pose challenges to their anaesthetic management. Indeed, it is more challenging than the anaesthetic management of mammals, a reality substantiated by the fact that the risk of anaesthesia-related death of birds is up to 20 times higher than for dogs and cats. This article highlights those anatomic (respiratory system, renal–portal system), physiological (gas exchange, respiratory control mechanisms in respiratory brainstem and peripheral chemoreceptor areas, including intrapulmonary chemoreceptors) and pharmacological attributes (pharmacokinetics and pharmacodynamics) that make anaesthetic management, both inhalant and injectable anaesthesia, of birds challenging, and how those challenges are managed. This article is part of the theme issue ‘The biology of the avian respiratory system’.
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The avian respiratory system: implications
foranaesthesia
John Ludders
Clinical Sciences, Cornell University College of Veterinary Medicine, Ithaca, NY 14853, USA
JL,0000-0001-7511-6890
Anaesthesia is not a natural state for any animal, including birds. The
unique anatomic and physiological attributes of the class Aves that have
made it possible for birds to inhabit every continent on this planet and
to live in a variety of environments, some considered challenging if not
inhospitable to mammals, pose challenges to their anaesthetic management.
Indeed, it is more challenging than the anaesthetic management of
mammals, a reality substantiated by the fact that the risk of anaesthesia-
related death of birds is up to 20 times higher than for dogs and cats. This
article highlights those anatomic (respiratory system, renal–portal system),
physiological (gas exchange, respiratory control mechanisms in respiratory
brainstem and peripheral chemoreceptor areas, including intrapulmonary
chemoreceptors) and pharmacological attributes (pharmacokinetics and
pharmacodynamics) that make anaesthetic management, both inhalant and
injectable anaesthesia, of birds challenging, and how those challenges are
managed.
This article is part of the theme issue ‘The biology of the avian
respiratory system’.
1. Introduction
The unique anatomic and physiological attributes of the class Aves have
made it possible for birds to inhabit every continent on this planet and live
in a variety of environments, some of which are considered challenging if
not inhospitable to mammals. It is plausible that the highly efficient avian
respiratory system is largely responsible for this success. The fossil record
strongly suggests that the microstructure of the preserved lung presumably
had considerable functional utility, and suggests that the general structural
design of the avian lung has been conserved for a very long time [1]. It is
possible that the avian lung enabled birds to attain powered flight, with at
least two significant evolutionary consequences: (1) wide dispersal across this
planet, which led to adaptive radiation, resulting in the class Aves being the
most species-rich of the amniote clades, and may have contributed to (2) the
survival of their descendants through the Cretaceous–Palaeogene extinction
event [1].
Anaesthesia is not a natural state for any animal, including birds, and
the attributes that enable birds to survive in their natural environments pose
challenges to their anaesthetic management. The fact that the avian pulmo-
nary system differs significantly from that of mammals led investigators in
the early to mid 1900s to conclude that the nature of the avian respiratory
system rendered the use of inhalant anaesthetics, such as ether and halothane,
unusable for anaesthesia of birds [2]. That view was based on the mistaken
belief that avian air sacs retained inhalant anaesthetics, or that inhalant
anaesthetics were concentrated in the air sacs and lungs, thus producing
death by anaesthetic overdose [2,3]. In the 1960s, subsequent studies of ether
[3] and halothane [2,4] demonstrated that volatile anaesthetics can be used to
reliably produce anaesthesia in birds. However, the tremendous diversity in
© 2025 The Author(s). Published by the Royal Society. All rights reserved.
Review
Cite this article: Ludders J. 2025 The avian
respiratory system: implications for anaesthesia.
Phil. Trans. R. Soc. B 380: 20230439.
https://doi.org/10.1098/rstb.2023.0439
Received: 15 April 2024
Accepted: 23 September 2024
One contribution of 18 to a theme issue ‘The
biology of the avian respiratory system’.
Subject Areas:
physiology
Keywords:
analgesics, anaesthesia, anaesthetics, birds,
minimum anaesthetic concentration,
pharmacokinetics
Author for correspondence:
John Ludders
e-mail: jwl1@cornell.edu
Biology of the avian respiratory system: development, evolutionary morphology, function and clinical considerations
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The respiratory biology of birds has been of interest to researchers for centuries, particularly owing to its dramatically heterogeneous structure, unusual ability for non-ventilatory structures to invade nearly all parts of the body (including the skeleton) in many taxa, and its exceptional efficiency under high-altitude hypoxia. Advances in imaging, experimental and developmental techniques, as well as recent palaeontological specimens have facilitated new discoveries, analyses and progress in the field. Comprehensively, this theme issue shows the origin of the modern avian respiratory system, current controversies and how the evolution of respiratory structures in birds has impacted their biology from the molecular, to the cellular, to the phylogenetic level. This collection of articles addresses progress the field has made, gaps in our knowledge and where the field needs to go, with a primary focus on adult and embryonic form and function but also touching on vocalization and clinical aspects of avian respiratory biology. This article is part of the theme issue ‘The biology of the avian respiratory system’.
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Anaesthetic mortality in cats: A worldwide analysis and risk assessment
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Background Patient safety is essential in small animal anaesthesia. This study aimed to assess anaesthesia‐related deaths in cats worldwide, identify risk and protective factors and provide insights for clinical practice. Methods A prospective multicentre cohort study of 14,962 cats from 198 veterinary centres across different countries was conducted. Data on anaesthesia‐related deaths, from premedication up to 48 hours postextubation, were collected. Logistic regression was used to analyse patient demographics, American Society of Anesthesiologists (ASA) classification, procedure type and anaesthetic drugs. Results The anaesthesia‐related mortality was 0.63%, with 74.5% of deaths occurring postoperatively. Cats with cachexia, a higher ASA status or who underwent abdominal, orthopaedic/neurosurgical or thoracic procedures exhibited elevated mortality. Mechanical ventilation use was associated with increased mortality. Mortality odds were reduced by the use of alpha2‐agonist sedatives, pure opioids in premedication and locoregional techniques. Limitations Limitations include non‐randomised sampling, potential biases, unquantified response rates, subjective death cause classification and limited variable analysis. Conclusions Anaesthetic mortality in cats is significant, predominantly postoperative. Risk factors include cachexia, higher ASA status, specific procedures and mechanical ventilation. Protective factors include alpha2‐agonist sedatives, pure opioids and locoregional techniques. These findings can help improve anaesthesia safety and outcomes. However, further research is required to improve protocols, enhance data quality and minimise risks.
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Complexity of avian evolution revealed by family-level genomes
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  • Apr 2024
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Despite tremendous efforts in the past decades, relationships among main avian lineages remain heavily debated without a clear resolution. Discrepancies have been attributed to diversity of species sampled, phylogenetic method and the choice of genomic regions1–3. Here we address these issues by analysing the genomes of 363 bird species⁴ (218 taxonomic families, 92% of total). Using intergenic regions and coalescent methods, we present a well-supported tree but also a marked degree of discordance. The tree confirms that Neoaves experienced rapid radiation at or near the Cretaceous–Palaeogene boundary. Sufficient loci rather than extensive taxon sampling were more effective in resolving difficult nodes. Remaining recalcitrant nodes involve species that are a challenge to model due to either extreme DNA composition, variable substitution rates, incomplete lineage sorting or complex evolutionary events such as ancient hybridization. Assessment of the effects of different genomic partitions showed high heterogeneity across the genome. We discovered sharp increases in effective population size, substitution rates and relative brain size following the Cretaceous–Palaeogene extinction event, supporting the hypothesis that emerging ecological opportunities catalysed the diversification of modern birds. The resulting phylogenetic estimate offers fresh insights into the rapid radiation of modern birds and provides a taxon-rich backbone tree for future comparative studies.
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Minimum anesthetic concentration of isoflurane and sevoflurane and cardiorespiratory effects of varying inspired oxygen fractions in Magellanic penguins (Spheniscus magellanicus)
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  • Mar 2024
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The aim of this study was to determine the minimum anesthetic concentration of isoflurane (MACISO) and sevoflurane (MACSEVO) and evaluate the cardiorespiratory changes induced by varying fractions of inspired oxygen (FiO2) in Magellanic penguins (Spheniscus magellanicus). Twenty adult penguins (3.53 ± 0.44 kg) of undetermined sex were used. Both MACISO (n = 9) and MACSEVO (n = 13) were established using an up-and-down design. Next, twelve mechanically ventilated penguins were maintained at 1 MACISO or 1 MACSEVO (n = 6 per group) with the FiO2 initially set at 1.0. Three FiO2 values (0.6, 0.4 and 0.2) were then held constant during anesthesia for 20 minutes each. Arterial blood samples were collected for gas analysis after the 20-minute period for each FiO2. Mean ± SD MACISO was 1.93 ± 0.10% and MACSEVO was 3.53 ± 0.13%. Other than heart rate at 0.6 FiO2 (86 ± 11 beats/minute in MACISO and 132 ± 37 beats/minute in MACSEVO; p = 0.041), no significant cardiorespiratory differences were detected between groups. In both groups, decreasing the FiO2 produced increased pH values and reduced partial pressures of carbon dioxide and bicarbonate. Partial pressures of oxygen (PaO2) gradually lowered from 1.0 FiO2 through 0.2 FiO2, though hypoxemia (PaO2 < 80 mmHg) occurred only with the latter FiO2. The MACISO and the MACSEVO for the Magellanic penguin fell within the upper range of reported avian MAC estimates. To prevent hypoxemia in healthy, mechanically ventilated, either isoflurane- or sevoflurane-anesthetized Magellanic penguins, a minimum FiO2 of 0.4 should be used.
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Anaesthetic mortality in dogs: A worldwide analysis and risk assessment
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Article
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Central Chemoreceptors: Locations and Functions
Article
  • Jan 2012
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A region of suppressed recombination misleads neoavian phylogenomics
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  • P NATL ACAD SCI USA
Genomes are typically mosaics of regions with different evolutionary histories. When speciation events are closely spaced in time, recombination makes the regions sharing the same history small, and the evolutionary history changes rapidly as we move along the genome. When examining rapid radiations such as the early diversification of Neoaves 66 Mya, typically no consistent history is observed across segments exceeding kilobases of the genome. Here, we report an exception. We found that a 21-Mb region in avian genomes, mapped to chicken chromosome 4, shows an extremely strong and discordance-free signal for a history different from that of the inferred species tree. Such a strong discordance-free signal, indicative of suppressed recombination across many millions of base pairs, is not observed elsewhere in the genome for any deep avian relationships. Although long regions with suppressed recombination have been documented in recently diverged species, our results pertain to relationships dating circa 65 Mya. We provide evidence that this strong signal may be due to an ancient rearrangement that blocked recombination and remained polymorphic for several million years prior to fixation. We show that the presence of this region has misled previous phylogenomic efforts with lower taxon sampling, showing the interplay between taxon and locus sampling. We predict that similar ancient rearrangements may confound phylogenetic analyses in other clades, pointing to a need for new analytical models that incorporate the possibility of such events.
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Outcomes and Complications Associated With Caudal Thoracic and Abdominal Air Sac Cannulation in 68 Birds
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Air sac cannulation is used both as an emergency procedure in avian patients with severe upper respiratory compromise, as well as a means of routine ventilation for surgery of the head and neck. The objective of this retrospective study was to describe and quantify the complications associated with air sac cannulation in birds. Medical records were retrieved for all patients that underwent caudal thoracic or abdominal air sac cannulation at a single center between August 2004 and October 2020. Patient signalment, indication for air sac cannulation, location of air sac cannula (ASC) placement, occurrence and category of complications encountered, and survival data were recorded. Eighty-four ASCs were placed in 68 birds across 6 orders; 95.2% (80/84) of cases survived general anesthesia for initial ASC placement. The side and position of ASC placement were known in 33.3% (28/84) and 21.4% (18/84) of cases, respectively. Survival to ASC removal was known in 91.3% (73/80) of cases; 43 (58.9%) of these 73 cases survived to ASC removal. Complications were observed in 32.5% (26/80) of cases, and 11.5% (3/26) of cases died as a direct result of the complication. The most common reported ASC complication was loss of patency in 23.8% (19/80) of cases. Increased likelihoods for complications were seen in cases where exercise intolerance (P = 0.04) or abnormal respiratory sounds (P = 0.04) were reported at presentation. Increased likelihoods for survival to ASC removal were seen with intercostal placements (P = 0.049) and peri-interventional antibiotic therapy (P = 0.005). Decreased likelihood for survival to ASC removal was seen in cases where voice change was reported at presentation (P = 0.02). This study demonstrates a moderate risk of ASC complication, with a guarded overall prognosis for survival to ASC removal.
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The Modulation by Anesthetics and Analgesics of Respiratory Rhythm in the Nervous System
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  • Aug 2023
  • CURR NEUROPHARMACOL
Rhythmic eupneic breathing in mammals depends on the coordinated activities of the neural system that sends cranial and spinal motor outputs to respiratory muscles. These outputs modulate lung ventilation and adjust respiratory airflow, which depends on the upper airway patency and venti- latory musculature. Anesthetics are widely used in clinical practice worldwide. In addition to clinically necessary pharmacological effects, respiratory depression is a critical side effect induced by most gen- eral anesthetics. Therefore, understanding how general anesthetics modulate the respiratory system is important for the development of safer general anesthetics. Currently used volatile anesthetics and most intravenous anesthetics induce inhibitory effects on respiratory outputs. Various general anes- thetics produce differential effects on respiratory characteristics, including the respiratory rate, tidalvolume, airway resistance, and ventilatory response. At the cellular and molecular levels, the mechanisms underlying anesthetic-induced breathing depression mainly include modulation of synaptictransmission of ligand-gated ionotropic receptors (e.g., γ-aminobutyric acid, N-methyl-D-aspartate,and nicotinic acetylcholine receptors) and ion channels (e.g., voltage-gated sodium, calcium, and po-tassium channels, two-pore domain potassium channels, and sodium leak channels), which affect neu-ronal firing in brainstem respiratory and peripheral chemoreceptor areas. The present review compre-hensively summarizes the modulation of the respiratory system by clinically used general anesthetics,including the effects at the molecular, cellular, anatomic, and behavioral levels. Specifically, analgesics, such as opioids, which cause respiratory depression and the "opioid crisis", are discussed. Finally, underlying strategies of respiratory stimulation that target general anesthetics and/or analgesics aresummarized.
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Mortality outcomes based on ASA grade in avian patients undergoing general anesthesia
Article
  • Apr 2022
  • J EXOT PET MED
Background The American Society of Anesthesiologists physical status classification system is commonly used in all fields of veterinary medicine, with higher grades correlated with increased mortality in non-avian companion animals, but little evidence is available for avian species. This study aims to investigate whether prospective ASA grade is a reliable predictor of mortality in avian species undergoing general anesthesia. Methods Anesthetic records for avian patients undergoing inhalational isoflurane anesthesia were retrospectively examined to assess mortality outcomes during and up to 48 hours after cessation of anesthesia. Prospective ASA grade assigned at the time of anesthesia was used to categorize anesthetized patients. Results A total of 1820 anesthetic records were included over a 3-year period. A total of 81 patients (4.45%) died during anesthesia or within 48 hours of cessation of anesthesia. Patients assigned an ASA grade of 1 had a mortality rate of 0% (95% confidence interval [CI] 0.0–0.2), and patients assigned an ASA grade of 2 had a mortality rate of 0.6% (CI 0.2–1.3%). Patients assigned ASA grades 3, 4, and 5 had mortality rates of 5.9% (CI 4.3–8.0), 18.8% (CI 13.4–25.7) and 50.0% (CI 29.9–70.1) respectively. Patients assigned an ASA grade of 3 were found to have 11.5 times the odds of a mortality event (95% CI 5.0–31.8) compared to ASA grades 1 and 2. A further increase in odds of 40.0 times (CI 16.9–113.8) was identified in patients assigned an ASA grade of 4, and 185.2 (CI 57.6–668.1) identified in patients assigned an ASA grade of 5. Age, sex and weight were shown to have no statistically significant impact on odds of death. Investigation into timing of death showed that the majority of patients died following cessation of general anesthesia (81.48%), with the highest mortality rate occurring between 0 and 3 hours postgeneral anesthesia. Conclusions and clinical relevance This study indicates that patients assigned ASA grades of 3 or greater are at higher odds of death compared to those assigned ASA grades 1 or 2. The odds of death increased with increasing ASA grades. The highest mortality rate was identified within 0–3 hours of cessation of general anesthesia.
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