Respiratory System in Veterinary Nursing: PDF Overview

Revision Quick refreshers on key topics The respiratory system Melanie Bailey , RVN, Cert AVN (Anaesthesia) Willows e V terinary Referral Centre, Solihull Melanie qualied in 2006 and has worked in a variety of rst opinion and referral hospitals. In January 2024 she joined the nursin...

Revision Quick refreshers on key topics The respiratory system Melanie Bailey , RVN, Cert AVN (Anaesthesia) Willows e V terinary Referral Centre, Solihull Melanie qualied in 2006 and has worked in a variety of rst opinion and referral hospitals. In January 2024 she joined the nursing team at Willows Veterinary Referral Centre. Her special interests are anaesthesia, pain management, and emergency and critical care. She is currently studying a masters in advanced veterinary nursing at Harper Adams. Melanie is married to an arable farmer; they live on a farm in a W rwickshire and have two young sons. According to her husband they have far too many animals, whereas she believes there is always room for one more! Introduction Respiration is referred to in terms of Nasopharynx external and internal respiration. External respiration is the gaseous exchange Frontal sinus Pharynx between the air and the blood, which Nasal cavity occurs within the lungs. Internal respiration is the exchange of gases between the Turbinates blood and the tissues, which happens all over the body. Hard palate Veterinary textbooks generally refer to the process of external respiration – the transport of oxygen (O2) into an animal's lungs and transport of carbon dioxide (CO2) out of the animal's lungs. Anatomy of the respiratory system Soft palate Tongue The respiratory system consists of the Oropharynx nose, pharynx, larynx, trachea, bronchi, Epiglottis bronchioles and alveoli. The part of the Larynx system spanning from the nose to the Trachea larynx is referred to as the upper respiratory Oesophagus tract (Figure 1) and the part from the trachea to the alveoli is referred to as the Figure 1. The upper respiratory tract. Adapted from Colville. lower respiratory tract (Figure 2). 54 Veterinary Nursing Journal  Align-justify STUDENT The bronchi divide as they enter a lung lobe, where Epiglottis they continue to divide into smaller bronchioles until Larynx they reach their terminal bronchiole. This subdivision of the bronchi is called the bronchial tree. As the bronchi divide, the cartilage becomes sparser and eventually Thyroid cartilage disappears altogether. Cricoid cartilage The area from the bronchi to the alveoli looks like Trachea upside-down trees or broccoli. At the end of each Left bronchus terminal bronchiole is an alveolar duct, which terminates at an alveolus. Alveoli are vascularised air sacs where the Bronchial tree gaseous exchange in the lungs takes place. Left lung Gaseous exchange is a process by which oxygen enters the blood and carbon dioxide leaves the blood. Molecules move by diffusion down a gradient, from a high concentration to a lower concentration. Oxygen diffuses from the alveoli into the blood, and carbon dioxide diffuses out of the blood through the alveoli into the lungs to be breathed out (Figure 3). Right lung Pleural covering Alveolus Alveolus Alveolar duct Alveolar sac Air in and out Figure 2. The lower respiratory tract. Adapted from Colville. The respiratory system starts at the nose, which is divided into left and right chambers, partly filled with nasal turbinates. The turbinates are lined with a ciliated mucous membrane that moistens and warms the air entering the respiratory tract. The membrane also filters and traps particles in the air entering the nasal cavity, Blood vessel (capillaries) preventing the particles from moving further into the respiratory tract. Figure 3. Gaseous exchange. Adapted from Colville. The pharynx is the area at the back of the mouth, often referred to as the throat. The nose, mouth, eustachian tubes from the middle ear, oesophagus and larynx all connect to this area. The pharynx divides caudally The lungs are two spongy organs in the thorax, one to the oesophagus (as part of the digestive tract) and either side of the mediastinum. They are connected ventrally to the larynx. to the trachea by the left and right bronchi. Both are divided into separate lobes. The right lung has four The larynx is a rigid hollow structure made of cartilage, lobes: the cranial lobe, middle lobe, accessory lobe and which forms the opening to the lower respiratory tract. caudal lobe. The left lung is divided into three lobes: the The opening to the larynx is called the epiglottis, and cranial lobe, middle lobe and caudal lobe. The lobes are it is this structure that is visualised and gently pushed divided by fissures, which help to increase the surface down like a drawbridge when placing an endotracheal area and maximise gaseous exchange, and can enable tube during intubation. The larynx is also known as the the body to ‘wall off’ a problem area of the lung, if voice box; it contains structures that enable the animal necessary, and divert blood to healthy regions. to make sounds. Each lobe has its own arterial and venous supply. The The trachea is a tube made of incomplete (C-shaped) pulmonary artery branches many times as it follows the rings of hyaline cartilage. Between the cartilage rings bronchioles, before becoming pulmonary capillaries near are bands of fibrous connective tissue and smooth the alveoli. Once the blood has been reoxygenated, it muscle lined by ciliated epithelium. The trachea enters moves from the alveoli via capillaries to the pulmonary the thorax at the thoracic inlet and branches into the veins, before returning to the heart to be circulated left and right bronchi, just above the heart. around the body (Figure 4, page 56). Volume 39 (3) June 2024 55  As mentioned earlier, carbon dioxide is a by-product of cellular respiration. The level of carbon dioxide in the body is kept within a narrow range, as part of the acid–base balance mechanism. This important homeostatic mechanism ensures the body is not too acidic or too alkaline, with the optimum pH being 7.4. When monitoring end-tidal carbon dioxide (ETCO2) with capnography during anaesthesia, or when looking at a patient's blood gases, it is important to be aware of the optimum range and the signs of abnormal levels, to help maintain the correct pH for the body to function. Control of respiration Respiratory centres in the medulla oblongata of the brain control respiration. Within these respiratory centres are individual controls for inspiration, expiration and the Figure 4. The blood supply of the alveoli. Adapted from Colville. constant monitoring of the pH of the blood. The body automatically alters the rate and depth of breathing depending on information provided by these centres. You The lungs are covered in a thin membrane called the can consciously control your breathing, but not for long, pleural lining; this acts to lubricate the lungs to reduce as the automatic system will override this, preventing you friction during breathing. It also helps to adhere the from consciously suffocating yourself. lungs to the thoracic wall, which aids ventilation. Each alveolus is also lined with a surfactant, which helps to There are also mechanical stretch receptors, which set prevent the alveoli from collapsing as air moves in and the volume of inflation and deflation in the lungs. Preset out of them. points during inspiration and expiration send nerve impulses between the lungs and brain, triggering the next breath in or out. How do animals breathe? Chemical receptors in the carotid arteries and aorta When compared with atmospheric pressure, the pressure constantly monitor the pH of the blood and the levels in the thorax is negative, which means it acts like a of oxygen and carbon dioxide. If changes are detected vacuum. This holds the lungs open against the animal's outside the normal range, a signal is sent to the medulla, chest wall. When animals breathe in, air is drawn into the which alters the respiratory rate and depth as required. lungs by the movement of the diaphragm and intercostal muscles. The diaphragm is dome-shaped when it is In cellular respiration, if an animal is unable to obtain relaxed, so when it contracts and flattens, the space in oxygen effectively, ADP is still converted to ATP, but lactic the thoracic cavity increases. The intercostal muscles (the acid is produced as a by-product of this process. As a muscles between the ribs) contract, which pulls the ribs result, lactate levels increase, because of poor perfusion up and forwards and draws air down into the lungs. or shock, for example. When animals breathe out, the opposite happens: the diaphragm and intercostal muscles relax, compressing the Metabolic acidosis is one of the most common acid–base lungs and pushing the air out again. The Herring–Breuer disorders. The body tries to correct this by increasing the reflex signals when the lungs are full and prevents them respiratory rate to expel carbon dioxide. The medulla from over-inflating. oblongata will detect the rise in the pH of the blood and signal to the respiratory system to speed up the respiratory rate, to get rid of the excess carbon dioxide. Cellular respiration Conversely, if carbon dioxide levels decrease, the medulla will slow down respiration to retain carbon dioxide. In addition to internal and external respiration, there is a third type of respiration: cellular respiration, in When weaning a patient off ventilation, ETCO2 is which the mitochondria in cells use glucose as a fuel, allowed to rise slightly, which will encourage the patient combined with oxygen and adenosine diphosphate to start breathing for themselves. Patients with a low (ADP), to produce adenosine triphosphate (ATP), carbon ETCO2 will be slow to breathe for themselves, until the dioxide and water. All aerobic organisms require oxygen level of carbon dioxide rises. To take over a patient's for cellular respiration. ATP is used for a number of breathing when putting it on to a ventilator, a slightly essential cellular processes that cannot occur without higher respiratory rate is set, so ETCO2 levels lower and oxygen; without oxygen, cells begin to die within the patient's drive to breathe is reduced, allowing the minutes and death of the animal occurs quickly. ventilator to take over. 56 Veterinary Nursing Journal  Align-justify STUDENT If a drop in the level of oxygen is detected in the body, Table 1. Respiratory rates. the respiratory centre will increase the rate and depth of respiration. However, if oxygen levels continue to drop Species Breaths per minute and the patient becomes hypoxic, or remains below that critical level, the respiratory centre can become Dog 10–30 (small dogs will have a higher rate) depressed and unable to effectively signal that more oxygen is needed. Cat 20–30 Rabbit 30–60 An increase in respiratory rate can decrease the carbon dioxide level in the blood, which is detected by chemoreceptors that will trigger a reduction in the respiratory rate to increase carbon dioxide levels. An Supplemental oxygen unwanted side effect of an animal breathing 100% oxygen instead of room air is that these chemoreceptors There are many methods of delivering supplemental will register the change in the oxygen level and will oxygen to a patient, including flow-by oxygen, nasal depress respiration to allow the carbon dioxide level cannulas, face masks, oxygen collars, oxygen tents/ to return to normal, in some cases totally inhibiting kennels and intubation. The least stressful method respiration. Carbon dioxide and hydrogen ion (H+) should be adopted wherever possible. concentrations have a greater impact on respiratory drive than the requirement for oxygen. Oxygen can have a drying effect on the respiratory tract, which can lead to respiratory infections. The drying effect is less likely to cause a problem in short-term Nursing considerations oxygen delivery, but longer-term oxygen supplementation should be humidified via a water canister filled with When veterinary nurses assess patients, their respiratory sterile saline or water. rate, depth and effort are observed, often numerous times a day as part of routine observations. It is In small animals, such as cats and smaller dogs, the use preferable to assess respiration while the patient is of oxygen tents or kennels can help minimise stress. It settled in its kennel or crate, before the door is opened. is important to remember that every time the tent or This is not always possible when compromised patients kennel is opened, oxygen will disperse into the room. are rushed through from the consultation room, but we should assess respiration before attempting any nursing Oxygen tents and kennels can quickly become hot; if care. the unit is not temperature controlled, regularly monitor the temperature inside with a room thermometer and When handling patients with respiratory compromise, use ice packs to cool it, as necessary. stress should be minimised as these patients are at high risk of a potentially fatal decompensation. A slow and When using a circle circuit to deliver oxygen, the system steady approach to handling is advised, so as not to should be flushed to ensure there is no residual volatile cause their respiratory rate to increase further. agent present. Consideration must also be given to where the oxygen enters the circuit: before or after the The normal ranges for the respiratory rates of dogs, adjustable pressure-limiting (APL) valve. The APL valve cats and rabbits are shown in Table 1. Questions to may need to be closed, to prevent oxygen being lost consider when assessing a patient's respiration include: down the scavenging hose before reaching the patient. It is important to reopen the APL valve afterwards. Is the patient ventilating effectively? Are there signs of increased respiratory effort? In practice I have witnessed the oxygen supply being turned up and the end of an anaesthetic circuit waved Is the patient's chest moving as expected? in the general direction of a patient; this will not work. A well-fitting mask should be used, ideally, but in cases Is the patient breathing with its mouth open? where a mask causes the patient excessive stress, steady (Remember that panting is not a true indicator of flow-by oxygen is better than nothing. respiratory rate.) A glossary of terms relating to respiration, which are not Does the patient's position indicate respiratory covered in this article but which you may find useful, distress? (An animal in respiratory distress will often can be found in Table 2 on page 58. adopt a position that aids breathing: neck extended, elbows out, breathing through its mouth and it may also show signs of abdominal effort.) Is the patient eating and drinking normally? (An animal in respiratory distress may not eat or drink, as doing so can further compromise its breathing.) Volume 39 (3) June 2024 57  Table 2. Glossary of useful respiratory terms not covered in this article. Term Definition/comments Tidal volume (TV) Tidal volume refers to the volume of air breathed in and out during one breath. It is calculated as 10–15 ml/kg. Minute volume or Minute volume (or minute ventilation) refers to the volume breathed in and out during minute ventilation 1 min. This is calculated as tidal volume multiplied by respiratory rate. Functional residual The FRC refers to the air left in the lungs between breaths, after the animal has capacity (FRC) breathed out. As there is always some air in contact with the alveoli, this air allows gaseous exchange to continue during expiration. It also holds the lungs open due to the percentage of nitrogen in room air. Rabbits and small animals have a very small FRC. This is why, when intubating rabbits, they can quickly turn blue (a sign of hypoxia) and preoxygenation is important. Dead space Dead space refers to the volume of the respiratory tract that is not involved in gaseous exchange. You may also be familiar with this term in relation to anaesthesia. Anatomical dead space is the region from the nose to the terminal bronchioles. Mechanical dead space is the additional volume in the equipment beyond the tip of the patient's nose, such as an endotracheal tube that is excessively long before it meets the Y piece on the anaesthetic circuit. Ventilation/perfusion V/Q is used to indicate the concentration of both oxygen and carbon dioxide in the ratio or V/Q ratio bloodstream. A V/Q mismatch could indicate that oxygen is not getting into the bloodstream or carbon dioxide is not able to get out. This could be because the alveoli receive the oxygen but there is no blood flow to them, or there is blood flow without oxygen. This can be caused by an obstructed airway, blood clots or contusions to the lungs. A pulmonary shunt is where blood passes from the venous to the arterial circulation without taking part in gaseous exchange. REFERENCES 1 Colville TP, Bassert JM. The respiratory system. In: Clinical Anatomy and Physiology for Veterinary Technicians. 3rd ed. St. Louis: Elsevier Health Sciences; 2015. p.361–377. 2 Fraser M, Girling S. Anatomy and physiology. In: Cooper B, Mullineaux E, Turner L. (eds.) BSAVA Textbook of Veterinary Nursing. 6th ed. Gloucester: BSAVA Publications; 2020. p.52–120. 3 Fraser M. Physiology relevant to anaesthesia. In: Welsh E. (ed.) Anaesthesia for Veterinary Nurses. Chichester: John Wiley & Sons; 2013. p.19–27. Available from: https://doi. org/10.1002/9780470752302.ch2. 4 Cooper B, Mullineaux E, Turner L. Appendix 2. In: Cooper B, Mullineaux E, Turner L. (eds.) BSAVA Textbook of Veterinary Nursing. 6th ed. Gloucester: BSAVA Publications; 2020. p.910. 5 Lynch A. Providing supplemental oxygen to patients. Today's Veterinary Practice. 2018;8(4), 44–50. Available from: https:// todaysveterinarypractice.com/respiratory-medicine/providing- supplemental-oxygen-to-patients/ [Accessed 9 April 2024]. 58 Veterinary Nursing Journal