the ETT was placed, the cuff was inflated to 27 cmH 2
O using a cuff inflator device( AG Cuffill [ 21 ]). Inflating the ETT cuff is essential to seal the trachea; however, over-inflating the cuff can damage the respiratory tract tissues [ 21, 22 ]. The recommended cuff pressure is 20 – 30 cmH 2
O [ 21, 22 ].
The intubation was smooth and uneventful, and once the airway was secured a heat and moisture exchanger( HME) filter was attached to the end of the ETT. A HME filter is a device that is fitted between the ETT and the anaesthetic circuit; it ensures that the gases inspired and expired by the patient are moist [ 23 ]. This helps prevent the mucous membrane and cilia within the airway losing moisture and becoming damaged by the dry gases produced by the anaesthetic machine. The HME will also warm the air passing through the ETT to the patient, helping to maintain normothermia [ 23 ].
Maintenance of anaesthesia
The patient was maintained on a circle circuit with a 2 litre bag. This is a rebreathing circuit that contains soda lime, which is a carbon dioxide( CO 2
) absorbent. The expired gases from a patient contain a high concentration of CO 2 and a low concentration of O 2
. When these gases pass through the soda lime, the CO 2 is absorbed, leaving a small concentration of O 2 in the gas remaining in the circuit. A circle circuit is designed to enable the O 2 to be recirculated and reused by the patient. The reused gases will also contain moisture and heat from the patient, which helps to maintain the health of the trachea as well as the patient ' s temperature [ 24 ].
The circle circuit also enables the use of low-flow anaesthesia. This is a technique whereby the fresh gas flow is less than the alveolar ventilation, meaning that at least 50 % of the exhaled gas is returned to the patient after the CO 2 has been removed [ 25 ]. There are many benefits of using low-flow anaesthesia, such as reduced environmental pollution, reduced anaesthetic gas consumption, and preservation of the temperature and humidity of the inspired gases. The sustainability protocol of the veterinary hospital in this case states that low-flow anaesthesia should be used whenever possible, so the patient was maintained at 1 l / min fresh gas flow throughout the anaesthetic.
A combination of O 2 and, where possible, medical air was used throughout the anaesthetic. Medical air is a purified gas whose main components are nitrogen( N 2
), O 2 and argon, all of which are components of environmental air. O 2 is vital to the body and is easily absorbed during gaseous exchange within the alveoli; however, the use of 100 % O 2 to ventilate a patient can cause atelectasis, which is the collapse of part or all of the lung, meaning gaseous exchange cannot occur within this section of collapsed lung [ 26 ]. Atelectasis is not always life-threatening, as the unaffected parts of the lungs will continue to inflate and deflate; however, it can be a cause for concern if it develops into pneumonia. The risk of atelectasis can also be increased by accidental bronchial intubation when using a long ETT that causes obstruction of a lung lobe orifice, or by the patient being in the same position for a long period of time [ 27, 28 ], both of which are known to commonly occur in anaesthetic procedures [ 28 ]. N 2 is an important component of air, which is not easily absorbed during gaseous exchange, so it maintains pressure within the alveoli, allowing the lungs to stay inflated [ 29 ] and reducing the risk of atelectasis.
The inhalational anaesthetic agent isoflurane was used to maintain anaesthesia in this patient. Isoflurane induces muscle relaxation and reduces pain sensitivity by altering tissue excitability [ 30 ]. This is achieved by inhibiting the neurotransmitter-gated ion channels within the central nervous system and the spinal cord. It is important that the concentration of isoflurane used is titrated correctly, as the side effects associated with this agent include hypotension, vasodilation, depression of the hypothalamus and respiratory depression, all of which can cause anaesthetic-related complications. The patient should be closely monitored throughout the anaesthetic for any signs of these side effects [ 30, 31 ].
Mechanical ventilation was available but not required in this case, as the patient maintained adequate ventilation and oxygenation [ 32 ]. Mechanical ventilation is typically indicated in the event of hypercapnia or hypoxaemia, which can result from anaesthetic- and analgesic-induced hypoventilation [ 32, 33 ]. By regulating the respiratory rate, mechanical ventilation supports
CO 2 removal, O 2 delivery and a stable anaesthetic depth when needed [ 32 ].
Monitoring of anaesthesia
Throughout the general anaesthetic, the patient ' s vital signs were continuously monitored and were recorded on an anaesthetic chart every 5 minutes, in line with the hospital ' s general anaesthetic protocol. Monitoring included electrocardiography( ECG), pulse oximetry, capnography, and non-invasive blood pressure( BP) and temperature measurement.
Under Schedule 3 of the Veterinary Surgeons Act 1966, it is the responsibility of the VS to maintain the anaesthesia, but a VN can assist them by acting as their hands, for example, altering the percentage of anaesthetic agent [ 18 ]. As the patient in this case was assigned ASA grade I, the assisting VN monitored the anaesthetic independently during the surgery and was instructed to alert the anaesthetist to any concerns. The hospital ' s protocol states that the anaesthetist can leave a VN to monitor the anaesthetic independently if they
52 Veterinary Nursing Journal