Causes of respiratory failure.

There are many causes of respiratory failure in veterinary patients. Assessment of oxygenation is imperative for the diagnosis and monitoring of these patients. Oxygen therapy should be instituted when hypoxemia is diagnosed to prevent tissue hypoxia, end-organ damage, and death. Methods of administering oxygen include commercial oxygen cages, mask oxygen, nasal cannulation (for dogs), and intubation. Mechanical ventilation is an option in many referral hospitals for patients who are severely hypoxemic and are not responding to inspired oxygen concentrations achieved with other methods of oxygen administration. One rule of thumb used to assess need for mechanical ventilation is a PaO2 of less than 50 mm Hg despite aggressive oxygen therapy, or a PaCO2 of greater than 50 mm Hg despite treatment for causes of hypoventilation. A mechanical ventilator has the ability to vary the FiO2 by increments of one, from 21% to 100% (0.21-1) oxygen in inspired gas. Positive end-expiratory pressure (PEEP) is also available on most ventilators. PEEP allows the alveoli to remain open on expiration, allowing gas exchange to occur in both inspiration and expiration. PEEP also helps diseased alveoli to inflate, increasing the available surface area for gas exchange and improving arterial blood oxygen tension. Because patients requiring mechanical ventilation have severe respiratory failure that did not respond to conventional oxygen therapy, the prognosis is guarded for most of these patients unless ventilation is instituted due to primary hypoventilation and lung parenchyma is normal. Hypoxemia caused by respiratory failure is a common problem in small animal veterinary patients. Assessment of blood oxygenation and continual monitoring of respiratory rate and effort are essential in management of these patients. Oxygen therapy should be instituted if hypoxemia is diagnosed. The prognosis depends on the underlying disease process and response to treatment with an enriched oxygen environment.     

Application of Airway Pressure Therapy in Veterinary Critical Care: Part I: Respiratory Mechanics and Hypoxemia

Over the past several decades, recognition of acute respiratory failure as the cause of death in patients suffering from various clinical conditions has prompted aggressiv investigation into the area of respiratory physiology and supportive respiratory care. With the evolution of emergency medicine and critical care services in both human and veterinary medicine, many patients previously considered unsalvageable due to the severity of their underlying disease are now being resuscitated and successfully supported, creating a new population of critically ill patients. Where only a decade ago these patients would have succumbed to their underlying disease, they now survive long enough to manifest the complications of shock and tissue injury in the form of acute respiratory failure. Investigation into the pathophysiology and treatment of this acute respiratory distress syndrom (ARDS) has facilitated increased clinical application of respiratory theerapy and machanical ventilation.¹ The purpose of this paper is to provide a basic review of respiratory mechanics and the pathophysiology of hypoxemia as they relate to airway pressure therapy in veterinary patients and to review the use of airway pressure therapy in veterinary patients This paper is divided into two parts; part I reviews respiratory mechanics and hypoxemia as they apply to respiratory therapy, while part II deals specifically with airway pressure therapy andits use in clinical cases.     

Helium‐oxygen gas‐carrier mixture (heliox): a review of physics and potential applications in veterinary medicine

Background – To review the physics of helium with regard to airway physiology, as well as known human and potential veterinary applications of administration of inhaled helium‐oxygen gas‐carrier mixture (heliox). Data Sources – Human and veterinary studies. Human Data Synthesis – Helium‐oxygen mixtures have been used in human medicine for over 70 years as an adjunct therapy in various upper and lower respiratory disorders. Helium's low density promotes laminar flow through partially obstructed airways, resulting in a decreased work of breathing. Veterinary Data Synthesis – Little to no evidence‐based medicine exists to support or oppose the use of heliox in veterinary species. However, domestic animal species and humans share several common pathophysiologic aspects of various obstructive airway disorders. Thus, veterinary patients may also ultimately and significantly benefit from this novel therapy. Conclusion – Prospective studies are needed in veterinary medicine to determine the utility of heliox in clinical scenarios.     

An overview of positive pressure ventilation

Objective: The purpose of this article is to give the reader a brief overview of the indications for positive pressure ventilation (PPV). Data sources: Current human and veterinary literature. Summary: There are numerous indications for PPV in veterinary medicine. These include both pulmonary parenchymal disease and diseases that affect ventilation. When choosing PPV, a clinician must have a comprehensive understanding of the different ventilation mode options available and the physiologic effects of ventilation on the patient. Conclusions: PPV is becoming more widely used in veterinary medicine and is improving the survival of animals with hypoxemic and hypercapnic respiratory failure.     

Strategies for mechanical ventilation

With the advancement of veterinary critical care medicine, an increasing number of veterinary patients are being supported with positive-pressure ventilation. Animals with potentially reversible ventilatory failure (PaCO2 > 60 mmHg) caused by neuromuscular disease or pulmonary parenchymal disease or with pulmonary parenchymal disease causing hypoxemia (PaO2 < 60) despite supplemental oxygen are candidates for ventilatory support. The equation of motion for the respiratory system is defined and is used to describe the potential interactions between the patient and the ventilator. Commonly used modes of ventilation are described in terms of control and phase variables. The intent of this report is to aid clinicians in choosing an optimal ventilatory strategy for each patient that will best achieve the desired physiologic goals with minimal detrimental side effects.     

Developments in management of the newborn foal in respiratory distress 2: Treatment

New developments in therapy for foals in respiratory distress are discussed. Therapy is based on preservation of the foal's life by maintenance of a patent airway, resuscitation with fluids and warmth, provision of humidified oxygen to raise the fractional concentration of inspired oxygen sufficient to avoid hypoxia and provision of ventilatory support when hypercapnia becomes critical. Ventilatory support described includes assisted and controlled ventilation, positive end expiratory pressure, continuous positive airway pressure and intermittent mandatory ventilation. The aims of these techniques are discussed together with their associated indications, disadvantages and complications. Secondary therapy includes coupage, airway hygiene, drug therapy and stress management. Knowledge of equine neonatology is limited in comparison with human neonatology. More information in basic physiology and pharmacology relating to equine neonatology is needed and the efficacy of various modes of therapy must be evaluated.     

Role of Lung Surfactant in Respiratory Disease: Current Knowledge in Large Animal Medicine

Lung surfactant is produced by type II alveolar cells as a mixture of phospholipids, surfactant proteins, and neutral lipids. Surfactant lowers alveolar surface tension and is crucial for the prevention of alveolar collapse. In addition, surfactant contributes to smaller airway patency and improves mucociliary clearance. Surfactant-specific proteins are part of the innate immune defense mechanisms of the lung. Lung surfactant alterations have been described in a number of respiratory diseases. Surfactant deficiency (quantitative deficit of surfactant) in premature animals causes neonatal respiratory distress syndrome. Surfactant dysfunction (qualitative changes in surfactant) has been implicated in the pathophysiology of acute respiratory distress syndrome and asthma. Analysis of surfactant from amniotic fluid allows assessment of fetal lung maturity (FLM) in the human fetus and exogenous surfactant replacement therapy is part of the standard care in premature human infants. In contrast to human medicine, use and success of FLM testing or surfactant replacement therapy remain limited in veterinary medicine. Lung surfactant has been studied in large animal models of human disease. However, only a few reports exist on lung surfactant alterations in naturally occurring respiratory disease in large animals. This article gives a general review on the role of lung surfactant in respiratory disease followed by an overview of our current knowledge on surfactant in large animal veterinary medicine.     

Suggested Strategies for Ventilatory Management of Veterinary Patients with Acute Respiratory Distress Syndrome

Objective: To review the current recommendations and guidelines for mechanical ventilation in humans and in animals with acute respiratory distress syndrome.
Human data synthesis: Acute respiratory distress syndrome (ARDS) in humans in defined as an acute onset of bilateral, diffuse infiltrates on thoracic radiographs that are not the result of heart disease and a significant oxygenation impairment. These patients require mechanical ventilation. Research has shown that further pulmonary damage can occur as a result of mechanical ventilation. Various alveolar recruitment maneuvers and a low tidal volume with increased positive end expiratory pressure (PEEP) have been associated with an increased survival.
Veterinary dat synthesis: Two veterinary reports have characterized ARDS in dogs using human criteria. There are no prospective veterinary studies using recruitment that ventilator-induced lung injury (VILI) occurs in dogs, sheep, and rats.
Conclusion: Recruitment maneuvers in conjunction with low tidal volumes and PEEP keep the alveoli open for gas exchange and decrease VILI. Prospective veterinary research in needed to determine if these maneuvers and recommendation can be applied to veterinary patients.     

Dynamic prediction of fluid responsiveness during positive pressure ventilation: a review of the physiology underlying heart–lung interactions and a critical interpretation

ObjectiveCardiovascular responses to hypovolemia and hypotension are depressed during general anesthesia. A considerable number of anesthetized and critically ill animals may not benefit hemodynamically from a fluid bolus; therefore, it is important to have measures for accurate prediction of fluid responsiveness. Static measures of preload, such as central venous pressure, do not provide accurate prediction of fluid responsiveness, whereas dynamic measures of cardiovascular function, obtained during positive pressure ventilation, are highly predictive. This review describes key physiological concepts behind heart–lung interactions during positive pressure ventilation, factors that can modify this relationship and provides the basis for a rational interpretation of the information obtained from dynamic measurements, with a focus on pulse pressure variation (PPV).Database usedPubMed. Search items used were: heart–lung interaction, positive pressure ventilation, pulse pressure variation, dynamic index of fluid therapy, goal-directed hemodynamic therapy, dogs, cats, pigs, horses and rabbits.ConclusionsThe veterinary literature suggests that targeting specific PPV thresholds should guide fluid therapy in lieu of conventional assessments. Understanding the physiology of heart–lung interactions during intermittent positive pressure ventilation provides a rational basis for interpreting the literature on dynamic indices of fluid responsiveness, including PPV. Clinical trials are needed to evaluate whether goal-directed fluid therapy based on PPV results in improved outcomes in veterinary patient populations.     

Application of Airway Pressure Therapy in Veterinary Critical Care: Part II: Airway Pressure Therapy

As the specialties of emergency medicine and critical care have grown and evolved in both human and veterinary medicine, so has the need for more advanced care of patients with primary lung disease. Treatment of acute respiratory failure has been the focus of several articles in the human medical literature of the past few years.^1,8 This paper deals with airway pressure therapy and its application in cases of acute respiratory failure in veterinary medicine. The reader is referred to part I of this paper for a reveiw of respiratory mechanics and hypoxemia as they apply to respiratory therapy.     

Endotracheal stenting therapy in dogs with tracheal collapse

Tracheal collapse in dogs is a common respiratory disorder, typically presenting with a history of chronic cough, increasing respiratory difficulties, and episodes of dyspnoea. Medical treatment is the therapy of choice and surgical repair is considered when patients do not respond well. Minimally invasive endotracheal stenting is a promising new therapy under investigation, but there remain significant challenges to overcome potential complications. The purpose of this article is to provide a comparative overview of intra-luminal stenting of the trachea in human and veterinary medicine. The currently available stents and their potential clinical application to the veterinary patient will be discussed.     

Experimental drug therapy for respiratory disorders in dogs and cats.

Experimental therapy in veterinary medicine is based on empiric reasoning. If a particular therapy is labeled experimental, it means that its effectiveness has not been demonstrated scientifically. Empiric therapy is experimental and is based on experience, not on scientific proof. The purpose of this article is to suggest the use of specific experimental drug therapies for certain respiratory disorders in dogs and cats.     

An update on cardiovascular adrenergic receptor physiology and potential pharmacological applications in veterinary critical care

Objective: To review the recent human and veterinary literature on current adrenergic receptor physiology/pathophysiology and potential applications in veterinary critical care. Data sources: Human and veterinary clinical studies, reviews, texts, and recent research in receptor molecular biology. Human data synthesis: Recent development of molecular cloning and other biological research techniques has advanced the field of adrenergic physiology. The past decade of research has made available new knowledge of adrenergic receptor subtypes as well as their locations and functions. Many of the diagnostic compounds used in biochemical research to distinguish between α‐ and β‐receptor subtypes may emerge as important additions to the arsenal of cardiovascular pharmaceuticals. Veterinary data synthesis: Veterinary adrenoceptor research is typically directed at investigating the effects of commercially available medications. Such studies demonstrate important species differences in addition to potential side effects and new indications for therapy. Many of the human molecular biology studies are performed on animal species, which can have direct application to veterinary medicine. Conclusions: Proper cardiovascular responses are essential to maintaining tissue perfusion and cellular homeostasis. α‐ and β‐adrenergic receptors play a vital role not only in the pathophysiology but also in the therapy of diseases involving the cardiovascular system. For adrenergic pharmacotherapy to be successfully used, a thorough understanding of the mechanisms underlying adrenoceptor physiology is necessary. Recent research has illuminated various subsets within the α‐ and β‐receptor classifications. Awareness of currently available and emerging adrenoceptor subtype‐specific treatment options allows precise pharmacologic targeting of disease processes in critical illness.     

Cardiopulmonary resuscitation

Cardiopulmonary resuscitation (CPR) is a technique used in both human and veterinary medicine. Although a number of innovative adaptations to CPR have been researched, the mainstay of CPR remains intubation, adequate ventilation, chest compressions, and basic drug therapy. The purpose of this article is to review the techniques and drugs commonly used in both closed chest and open chest CPR.     

A hundred years of importation: The first animal quarantine station in North America; Lévis, Québec, 1876-1982

Quarantine, as a means of preventing disease importation, has been used for people and animals since the mid-19th century in Canada. The first animal quarantine facility in North America was established at Lévis, Québec in 1876. This quarantine station existed at Lévis until 1982 when it was closed and the function moved to Mirabel, Québec, near the International Airport. Veterinarians were in charge during the life of the Lévis Quarantine Station and some were also in charge of the Port of Quebec or a nearby District Office prior to the 1950's. In 1884 and 1886 the value of such a facility was illustrated in preventing the entry into Canada of contagious bovine pleuropneumonia and a vesicular disease. It was described in 1933 as “undoubtedly our most important quarantine station” and a year's operating costs as “trifling in comparison to losses which could occur if a foreign plague invaded this country”. This facility's history also illustrated the close veterinary and human medical cooperation during the early days of organized veterinary medicine in Canada. The station was an example for the establishment of other such facilities in North America.     

Hyperbaric oxygen therapy. Part 1: history and principles

Objective – Review the historical development and physiologic principles of hyperbaric oxygen therapy (HBOT) based on human and veterinary experimental literature and current equipment in use. Data Sources – Review of basic physiologic concepts. Data from human and veterinary journals were reviewed through Pubmed and Veterinary Information Network database searches as well as reference searches on several articles covering hyperbaric therapy in clinically applicable situations. Human Data Synthesis – HBOT has been gaining acceptance as an adjunctive treatment in human medicine. The understanding of the physiology and application of hyperbaric therapy is increasing through ongoing research and greater access to hyperbaric equipment. Veterinary Data Synthesis – Several animal models have been utilized to examine the effects of HBOT. Most models utilize dogs and rats but pigs, cats, and other species have been studied. Conclusions – Hyperbaric therapy utilizes several physiologic principles of how gases respond under pressure and more specifically of how oxygen responds under pressure. The increase in concentration of oxygen in solution, based on its solubility under pressure, increases the diffusion gradient for its delivery deeper into tissues, which is the premise of HBOT. Ultimately the increases in dissolved oxygen generated by hyperbaric therapy have several physiologic effects that can alter tissue responses to disease and injury. As this technology becomes more available to clinical practice, HBOT should be considered as a therapeutic option.     

Rationale for hyperbaric oxygen therapy in traumatic injury and wound care in small animal veterinary practice

Hyperbaric oxygen therapy is in wide use in human medicine around the world. Although hyperbaric oxygen therapy is available for veterinary use, it is still significantly underutilised. The physical principles, gas laws and physiologic mechanisms by which hyperbaric oxygen therapy is therapeutic, especially in traumatic injuries and complicated wound care, are discussed. Then, considerations are offered for the implementation of hyperbaric oxygen therapy in veterinary practices. Finally, a review of clinical indications for veterinary practices, including a presentation of select literature, is provided. Applying hyperbaric oxygen therapy in an earlier and more consistent manner could improve short- and long-term outcomes in complicated wounds. The authors also hope this information may stimulate interest in the design of future, prospective studies for the various clinical situations described.     

The anatomy and physiology of the avian endocrine system.

The endocrine system of birds is comparable to that of mammals, although there are many unique aspects to consider when studying the anatomy, physiology, and biochemistry. Avian endocrinology is a field of veterinary medicine that is unfamiliar to many practitioners; however, it is important to have a comprehensive understanding when evaluating companion birds in clinical practice. This article covers the anatomy and physiology of the normal avian, and readers are referred to other articles for a more detailed explanation of altered physiology and pathology.     

Treatment of respiratory distress in a prematurely born foal

A foal born 3 weeks prematurely was treated for respiratory distress, using a combination of oxygen therapy and mechanical ventilatory assistance. Clinical response and arterial blood gas tensions were monitored regularly. Continuous positive-airway pressure and intermittent positive-pressure ventilation administered via a nasotracheal tube were effective in improving arterial oxygenation and ventilatory function.