pressure ( SAP ) should be maintained above 90 mmHg [ 5 ] . If the blood pressure drops below this , the patient ' s depth of anaesthesia should be assessed , and isoflurane reduced if appropriate .
Hypoventilation is a common concern in anaesthetised patients and is identified by hypercapnia with an endtidal carbon dioxide ( EtCO 2
) level over 45 mmHg . The level can be reduced by first assessing the depth of anaesthesia and reducing the isoflurane ( if appropriate ), and applying intermittent positive-pressure ventilation ( IPPV ), either manually or mechanically , if warranted .
Permissive hypercapnia ( of EtCO 2 as high as 60 mmHg ) can be considered beneficial in a healthy patient [ 6 ] , and ventilating a patient can cause acute lung injury , so care must be taken with IPPV . Inflating the chest to a high pressure can also compress blood vessels and reduce venous return to the right atrium , which can cause hypotension . It is advisable to keep peak inspiratory pressure ( PIP ) at 10 – 15 cmH 2
O , but it can go as high as 20 cmH 2
O ( measured using a circuit manometer ). A blood gas assessment can be used to decide if intervention is required . It is worth noting that carbon dioxide ( CO 2
) measured in an arterial blood gas sample can often be higher than EtCO 2
[ 7 ]
.
Heat loss is a frequent occurrence during anaesthesia , as the body ' s ability to regulate temperature is impaired . Anaesthetic agents inhibit natural thermoregulatory mechanisms , preventing shivering , movement and vasoconstriction , which are essential for heat production and retention . Additionally , many commonly used drugs cause vasodilation , further exacerbating heat loss and counteracting the body ' s response to hypothermia . Furthermore , preparing patients for surgery cools them before the surgery starts , and surgery often requires a body cavity to be opened , which lowers the body temperature . What is more , inhaled gases are cooling and intravenous fluids are often cold .
gaseous exchange in the lungs . Monitoring can be done invasively by taking arterial blood samples and measuring PaO 2 on the blood gas analysis machine , or it can be estimated using FiO 2
. When a patient is on 100 % oxygen at sea level , the
PaO 2 is expected to exceed 500 mmHg and can reach as high as 663 mmHg [ 10 ] . This value represents the efficiency of oxygen uptake in the lungs . When on 21 % oxygen ( or room air ) the PaO 2 is 105 mmHg at sea level .
If a patient has a PaO 2 of 500 mmHg on 100 % oxygen and the FiO 2 is reduced to 50 %, the PaO 2 would be expected to decrease proportionally to approximately 250 mmHg due to the change in oxygen concentration .
A PaO 2 measurement lower than expected means there is a decrease in the PaO 2
: FiO 2 ratio . Reduced oxygenation can eventually lead to a PaO 2 of < 60 mmHg , which would indicate that the patient is hypoxic .
Equipment
An anaesthetic machine with an isoflurane vaporiser was set up with a rebreathing system using a double soda lime canister and a coaxial breathing circuit with a 6-litre reservoir breathing bag . A pressure leak test was performed to 30 cmH 2
O and no leaks were observed . The system was connected to an active scavenging system ( Figure 1 ).
Active warming using temperature-controlled heating blankets or air-warming devices can be effective in warming a patient . Using warmed fluids or fluid warmers may also help , and a rebreathing circuit would help to conserve heat . Heat and moisture exchangers ( HMEs ) can be used on the breathing circuit to reduce loss by mimicking the physiological upper airway . A coaxial circuit can also be an improvement over the standard circle circuit [ 8 ] .
Temperature affects minimum alveolar concentration ( MAC ) so , as the patient cools , less inhalant gas may be required to maintain surgical anaesthesia . Hypothermia also prolongs recovery times , so normothermia is preferable . If a patient wakes shivering , this can increase
O 2 consumption [ 9 ] , which is another consideration for compromised patients .
Hypoxia can occur during anaesthesia and can be identified by monitoring PaO 2
, which reflects the
Figure 1 . The equipment prepared for the anaesthesia of the donkey in this case .
18 Veterinary Nursing Journal