/// QUARTERLY BEAT / DECEMBER 2022
QUARTERLY BEAT / DECEMBER 2022 ///
that get good air flow versus those that get good perfusion. However, in some cases due to inflammatory products (e.g., pulmonary edema, pneumonia) areas of the lungs receive blood flow but no ventilation, while others receive ventilation but no blood flow. This mismatching results in the development of low blood oxygen. V-Q mismatch will respond to oxygen supplementation. The final area that may result in hypoxemia is diffusion impairment. Typically, during transition phase through the capillary bed, the oxygen diffuses within the first third of the length of the capillary. However, if the alveolar-capillary membrane is very thickened, there may be diffusion impairment. While an understanding of potential causes of respiratory distress are mandatory, the first steps in the clinical evaluation of a patient presenting with respiratory distress are to provide a supplemental source of oxygen and to obtain a brief history from the client. All emergency facilities should have a form of supplemental oxygen available. Supplemental oxygen may be provided via a variety of options, including flow-by, face mask, nasal oxygen, e-collar and cellophane wrap (“oxygen hood”), oxygen cage and intubation with intermittent positive pressure ventilation (IPPV). Flow-by oxygen is provided by holding oxygen tubing near the mouth and nostrils of the affected patient. The flow rate is usually set at 100ml/kg/minute or greater. Flow-by oxygen is an easy and rapid solution; however, the actual increase over room air’s 21% oxygen content may be minimal, particularly with an anxious or uncooperative pet. Oxygen may also be provided with a face mask, with the oxygen tubing attached to a cone that is placed over the nose and mouth of the patient. The FiO2 with a face mask is also variable, although with very weak animals a high percentage may be reached. Both flow-by and face mask oxygen may require veterinary personnel to hold both the pet and the oxygen supply. Nasal oxygen involves placement of flexible catheter into the nasal passages and insufflation of humidified oxygen. Nasal oxygen is particularly useful in pets that are not either panting or open-mouth breathing. Nasal oxygen is commonly placed after patient stabilization, rather than urgently in the emergency setting. A home-made oxygen hood may be created with an Elizabethan Collar and cellophane wrap or may be commercially purchased (Jorgenson Laboratories). An oxygen cage is also frequently used to provide supplemental oxygen. Oxygen cages are commonly well-tolerated by both cats and dogs and are also capable of reaching high concentration of oxygen; however, if the cage door is open for patient manipulation, the FiO2 will rapidly fall. Finally, intubation and IPPV is the best option for providing high levels of supplemental oxygen, removing respiratory fatigue, and eliminating patient fear and anxiety.
During the initial stabilization of the pet, a history should be obtained from the owner. In some cases, the precipitating cause of the respiratory distress is straight-forward, such as with traumatic injuries, while in other cases, the onset may be more insidious. Animals with pre-existing medical conditions, such as cardiac disease, neoplasia or megaesophagus may also be predisposed to the development of respiratory distress. Owners should be questioned as to past medical conditions, history of routine veterinary care, including heartworm prophylaxis, and finally, the progression of the signs of respiratory distress should be described. Specifically, distress may be acute in onset, or more progressive. In cats in particular, the development of respiratory distress may be preceded by anorexia, lethargy or abnormal behavior. Respiratory distress may be further characterized by the location of the lesion or the underlying pathophysiological condition. Often, localization of the lesion can help to guide the clinician to the most likely cause. Specifically, respiratory distress may be localized to upper airway, lower airway, parenchymal or pleural space disease. Common pathophysiological causes for respiratory distress include anatomical abnormalities, airway collapse, pulmonary edema of cardiac and non-cardiac causes, infection, inflammatory and trauma. For the emergency clinician, the most appropriate first step is to localize the lesion and then to review specific differentials based upon signalment, history and other physical examination findings. Upper airway diseases may be appreciated by loud stridorous breathing, with an increased inspiratory time. Many dogs are hyperthermic on initial presentation due to decreased ability to cool. The upper airways represent the primary source of resistance to airflow. Upper airway obstructions can be either dynamic or fixed. Dynamic obstructions are characterized by the paradoxical movement of tissues into the lumen of airway during inspiratory inspiration. Common dynamic obstructions include laryngeal paralysis and tracheal collapse, while fixed obstruction include extraluminal obstructions such as neoplasia or cellulitis and intraluminal obstructions such as laryngeal tumors or nasopharyngeal polyps. Both dynamic and fixed severe upper airway obstructions will also result in the development of airway mucosal edema and possibly everted laryngeal saccules due to irritation from the increased air flow rates through a narrow lumen. Emergently, upper airway obstruction should be suspected in a dog with loud, noisy breathing. Therapy for a suspected dynamic obstruction should include sedation and supplemental oxygen. Sedation is beneficial in the dynamic obstruction in reducing the anxiety associated with inspiration because with increased inspiratory efforts there is a resulting paradoxical decline in airway diameter. Low
WEBINAR HIGHLIGHTS
Approach to the Small Animal Respiratory Emergency
ELIZABETH ROZANSKI , DVM, DACVIM, DACVECC Tufts University, North Grafton, MA
In this VETgirl webinar “Approach the Small Animal Respiratory Emergency,” Dr. Elizabeth Rozanski, DVM, DACVIM, DACVECC discusses approach to the small animal respiratory emergency! In these proceedings, she reviews the importance of “pattern recognition” to classifying respiratory distress and more! Missed the webinar? Check it out HERE !
Respiratory distress of any origin represents a true emergency as rapid treatment is warranted to identify the underlying cause, to limit the sensation of difficulty breathing and to provide diagnostic and therapeutic information for clients of affected animals. Clearly, there are a variety of potential underlying reasons for the development of respiratory distress. For the clinician in emergency practice, success in patient management revolves around developing a knowledge base of potential causes of respiratory distress and “pattern recognition” of common emergent problems affecting dogs and cats. The goals of this article are to review the function of the respiratory system, to describe pathophysiological causes for hypoxemia, to illustrate various methods for classifying respiratory distress, to highlight common emergency conditions resulting in respiratory distress and to provide guidelines for emergent management. As a review, the primary goal of the respiratory system is to promote gas exchange. Air enters the upper respiratory system via either the nose or the mouth. The air is warmed and humidified and then passes down the respiratory tree. Larger particles of debris are filtered out. The trachea branches into successive generation of smaller and smaller airways. Gas exchange occurs at the level of the alveoli. While the flow of air (Ventilation-V) is obviously essential, the other aspect of gas exchange involves the delivery of blood (perfusion-Q) to the level of the alveoli. Blood makes its way to the capillaries adjacent to the alveoli by way of the pulmonary vasculature. The blood leaves the right heart via the pulmonary outflow tract and then flows through the pulmonary arteries. The arteries branch into smaller and smaller vessels until the level of the capillary. Red cells go through the capillaries in single file and during their time
in contact with the alveoli, the oxygen diffuses out of the alveoli and binds to the hemoglobin as the CO2 (waste gas) will diffuse off the hemoglobin molecule into the alveoli for expiration. In normal animals, the capillary endothelium is very impermeable to larger molecules such as albumin. In animals that develop hypoxemia there are five possible broad causes. Each disease observed clinically will fit into one of more of the following categories. These causes include a low inspired oxygen concentration, hypoventilation, shunt, V-Q mismatch and diffusion impairment. Normal oxygen concentration in room air is 21%, thus a low inspired oxygen concentration (Termed FiO2) is very uncommon. However, this may occur at higher altitudes or due to an anesthesia machine dysfunction. Low FiO2 may be corrected by giving a higher concentration of oxygen. Hypoventilation results from absent or ineffective (low) tidal volumes due to causes such as drug therapy (opioids), or loss of central respiratory drive. Hypoventilation is treated by either mechanical ventilation or by reversing the cause that triggered the hypoventilation. Importantly, hypoventilation may not be appreciated as respiratory distress as respiratory attempts are limited. Shunt is a term that refers to the complete by-passing of the lungs by the unoxygenated blood. The classic example of a shunt is the right to left shunt that accompanies the cardiac defect of Tetralogy of Fallot (right ventricular hypertrophy, ventricular septal defect, pulmonary stenosis and over-riding aorta). In these animals, the severity of the PS results in the shunting of the un-oxygenated blood into the left ventricle. Pure shunts will not respond to oxygen supplementation. V-Q mismatch means that there is poor coordination of the areas of the lung that are getting perfused versus those that are getting ventilated. Normally, there is a good match between areas
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