Oxygen therapy is one of the most sought medical cares in emergency health services for various cardiopulmonary disorders like dyspnoea, shock, sepsis, SIRs, head trauma and so on. It is a life-saving therapy for a patient struggling for breathing and also for cardiovascular resuscitation. With the advancement of time and development of newer techniques, it has been quite easier for the veterinarians to treat patients suffering from respiratory challenges related to cardiovascular resuscitation, albeit limited to small animals at present. The primary technique by which oxygen therapy is instituted to one patient includes invasive and non-invasive method. This article reviews the recent updates on the oxygen therapy in veterinary patients.
Oxygen is one of the most essential and indispensable component for every living entity on the earth without which life would not have been same as it is now. The profound importance of oxygen can be judged based on the complex processes and pathways in which it is associated, starting from the gross food chain to the microcellular level in the body. It is one of the three basic nutrients inevitable for the very existence of the flora and fauna on the planet earth, the other two being light and water. Nature has maintained a delicate balance between all the necessary elements for proper functioning of the body. However, this natural homeostasis is sometimes interrupted by many conditions which lead to chaotic situations. And to embattle those adverse conditions, modern science has put some interventions that strengthen the life saving measures in human and animal. Oxygen therapy is one of such intervention which is very much essential during many clinical operations.
Oxygen therapy denotes the delivery of high concentrations of oxygen into the respiratory system to increase the oxygen levels in the blood so that more oxygen reaches at tissues level (Morgan, 2011). It is a key component in respiratory care. The body constantly takes oxygen and releases carbon dioxide and when this process is inadequate, oxygen levels in the blood falls and the patient may need supplemental oxygen. The purpose is to increase oxygen saturation in tissues where the saturation levels are too low due to illness or injury (Macintire et al., 2005).
Physiology of Respiration
Ventilation: Oxygen uptake occurs when it is extracted from the environment during respiration, followed by oxygen movement to the lungs and in and out from the alveoli (Dzal et al., 2015).
Diffusion: The pressure gradient between alveolar air and pulmonary capillary blood results in oxygen diffusion across the respiratory membrane (Boron and Boulpaep, 2008).
Delivery: Oxygen delivery occurs via the cardiovascular system, when oxygen bound with haemoglobin, and carbon dioxide is eliminated from the body (Pittman, 2011).
Metabolism: Metabolism occurs when oxygen bound with haemoglobin reaches tissue capillaries. Oxygen diffuses down a gradient into the mitochondria of interstitial cells (Leach and Treacher, 1992).
Regulation of Ventilation: The respiratory centres presents in animal brain which is responsible for respiration includes cerebral cortex, pons and medulla oblongata. Medulla also has chemoreceptor to regulate carbon dioxide levels (Coates and Nattie, 1984).
Indications of Oxygen Therapy
This occurs when inadequate haemoglobin is available to transport oxygen. Conditions like haemorrhages, chronic tick infestation, blood protozoan diseases, epistaxis, nutritional deficiency etc. results anaemic hypoxia (Radostits et al., 1997).
Stagnant hypoxia is caused by low blood flow, inadequate oxygen delivery to the tissues, dehydration, haemorrhage, low cardiac output and CHF (Radostits et al., 1997).
When to Institute Oxygen Supplementation
Clinical signs like nasal flaring, pallor of mucus membrane, cyanosis, panting, irregular chest wall movements etc., require immediate supplementation of oxygen (Lee and Drobatz, 2004; Macintire et al., 2005). Quantitatively, oxygen should be provided to any patient with saturation of oxygen (SaO2) or pulse oximetry reading (SpO2) of <93% or with an arterial partial pressure of oxygen (PaO2) of <80 mm Hg (Mazzaferro, 2015)
Use of Pulse Oximetry for Detection of Level of Oxygen Deficiency
Pulse oximetry is the most non-invasive and stress-free method of monitoring oxygen delivery to the tissues (Fig. 1). It calculates the percent of oxygen saturation of haemoglobin in arterial blood using spectrophotometry (Ford and Mazzaferro, 2006). The probe passes light through the tissues at two different wavelengths: a red and infrared light absorption. Oxygenated haemoglobin absorbs more infrared light and allows more red lights to pass through. Deoxygenated haemoglobin absorbs more red lights and allows more infrared light to pass through. The difference in light absorption is calculated and the final figure is displayed as a percentage (PO2%) (King and Clarke, 2010; Hendricks and King, 1993).
Fig. 1: Use of pulse oximeter in a dog (Figure source: https://minivetguide.com)
Methods of Oxygen Therapy
There are 2 methods of oxygen therapy used in small animals.
Non-Invasive Methods of Oxygen Therapy
Non-invasive ventilation (NIV) refers to the administration of ventilator support without using an invasive artificial airway (endotracheal tube or tracheostomy tube). This method includes-
This method is the simplest among all non-invasive methods. Here, oxygen pipe placed adjacent to, or within 2cm of a patient’s nostril or mouth. In this method there is no need of humidifier (overall fractional oxygen concentration being delivered to the lungs < 35). A flow rate of 2L to 3L/minute was maintained, which provides a FiO2 of 25 to 40 per cent (Loukopoulos and Reynolds, 1997)
Face mask of variable sizes are widely available. It can be comfortably placed over the patient’s muzzle (Fig. 2). A tight-fitting seal is required to allow for optimal oxygen delivery. While using a face mask, attention should be made to ensure that there is a gap between the patient’s nares and the oxygen delivery point and patient’s nares should not be squashed to the end of the mask. In this method, flow rate of 2L/minute to 5L/minute can provide a FiO2 of up to 50 to 60 % (Boyle, 2012). If a loose-fitting mask is used, a much higher flow rate will be required.
Tight-fitting masks may results rebreathing of carbon dioxide and an increase in temperature and humidity of intake air.
Fig. 2: Use of face mask for oxygen supplementation in a cat (*Source: Richmond, 2010)
Oxygen Hood /Oxygen Collar
Oxygen hood /oxygen collar produces a very effective oxygen delivery method, using low flow rates. Commercial oxygen hoods are available in markets. It can be made easily by using a rigid Elizabethan collar and plastic wrap. An oxygen tube must be attached two inches from the base of the collar. The plastic wrap is placed across the front of the collar, covering two-thirds of the front and secured to the sides. Opening acts as a vent which releases excess oxygen and exhaled carbon dioxide. As oxygen is heavier than air, it remains in the lower two-thirds of the collar, acting as a reservoir (Ford and Mazzaferro, 2006). Here flow rates of 0.5L/minute to 1L/minute should maintained, will deliver a FiO2 of 30 to 40 %. Heat and humidity within the hood/collar should be monitored. Also ophthalmic ointments are applied on a regular basis to prevent drying (Loukopoulos and Reynolds, 1996).
Oxygen Incubator/Oxygen Cage
This provides an excellent means of supplying oxygen to smaller patients within a stress-free environment. Medical pediatric incubators and commercially available portable oxygen cages/tents can be used in veterinary practice (Fig. 3). A flow rate- 2 to 10 L/minute (depending on the size of the cage) should be maintained. It also has some disadvantages like the patients can rapidly become hyperthermic, there is rapid increment of humidity within a short time limiting its use for short periods and it restrict immediate access to the patient (Ford and Mazzaferro, 2006).
Fig. 3: Figure showing oxygen chamber (*Source: www.elanaspantry.com)
Invasive Methods of Oxygen Therapy
This technique is only possible when the animal is unconscious or under anaesthesia. It includes a) nasal oxygen prongs, b) nasal oxygen catheters and c) trans-tracheal oxygenation.
Nasal Oxygen Prongs
Nasal prongs are widely used in human medicine. Size of prongs varies from new borne, pediatrics to adults. A prong is placed at each nare and aligned with the opening of the nostril (Fig. 4). Prongs can slip out easily, which may require securing with a suture or surgical staples. Here the flow rates ranges from 3L/minute to 6L/minute (Mazzaferro EM, 2009).
Fig. 4: Nasal oxygen prongs in dog (*Source: Richmond, 2010)
Nasal Oxygen Catheters
Application of nasal catheter for oxygen delivery is the best method among all the invasive method as it is inexpensive, technically easy to place and also well tolerated by the patient (Fig. 5). Oxygen catheters are available in various diameters and lengths (Boyle, 2012). Flow rates of 50ml/kg/minute to 150ml/kg/minute should be maintained (can provide a FiO2 of 30 to 70 per cent) (Dunphy et al., 2002).
Fig. 5: Bilateral placement of nasal oxygen catheters (*Source: Richmond, 2010)
This technique is used in patients intolerant to nasal oxygen delivery like patients suffering from upper airway obstruction. Catheters made up of silicones are placed surgically between the fourth and fifth tracheal cartilaginous rings and the end of the catheter can then be attached to the oxygen source. Here the flow rate is 50ml/kg/minute, which provides 40 to 60 % FiO2 (Mann et al., 1992).
Oxygen toxicity occurs when oxygen administered in excessive amounts over a prolong period of time (Mensack and Murtaugh, 1999). It causes injuries like alveolar damage and decrease pulmonary function, which is fatal to animals. To avoid pulmonary oxygen toxicity, small animals should not receive a FiO2 of more than 60 per cent for longer than 24 to 72 hours.
Success of oxygen therapy varies from patients to patients and depends mostly on the severity of the disease. As tolerance of each method varies from patient to patient, determining the most suitable method can be a challenging task, which needs experience and skilled personnel (Camps-Palau et al., 2000). While delivering oxygen to the patients care should be taken that oxygen administration should be stress free and if the animal becomes anxious or frightened, and starts to struggle, an alternative method should be initiated (Tseng and Waddell, 2000). Choosing the suitable method along with the flow rate of oxygen and careful observation of patient is the key of successful oxygen therapy.