Plus-Hex CLINICAL
Introduction
Veterinary nurses often have to make considerations for patient anaesthesia before discussing the case with the leading veterinary surgeon . These include decisions on the requirements for an appropriate breathing system , fresh gas flow ( FGF ) provision and patient monitoring [ 1 ] . Many practices now have access to multi-parameter monitors capable of measuring end-tidal carbon dioxide ( ETCO 2
) levels with an associated capnogram and the fraction of inspired carbon dioxide ( FiCO 2
). Capnography provides an easy-to-apply , non-invasive , real-time method of monitoring that enables the anaesthetist to assess patient ventilation , systemic metabolism , cardiac output and circulation / perfusion [ 2 , 3 ] .
ETCO 2 is the result of expired gases from the alveoli of the patient . Under normal circumstances , ETCO 2 is said to underestimate the arterial partial pressure of carbon dioxide ( PaCO 2
) by 2 – 5 mmHg , which is deemed to be clinically insignificant . Therefore , ETCO 2 is approximately equal to PaCO 2 and , by extrapolation , approximately equal to alveolar carbon dioxide ( CO 2
), making ETCO 2 a reliable and accurate method of monitoring ventilation [ 2 , 4 – 6 ] . ETCO 2 is dependent on three patient factors : production of CO 2 in the tissues
( metabolism ); movement of CO 2 from tissues to the lungs ( circulation / perfusion ); and movement of CO 2 from the blood vessels into the alveoli and then out of the body ( ventilation ).
Most capnographs in clinical monitoring use infrared absorption spectroscopy to determine the concentration of CO 2 in the expired air according to the Beer – Lambert law [ 2 , 3 , 6 , 7 ] . When calculations are made for the FGF requirements of the patient , the result is the number of litres deemed to be required to provide that patient with their oxygen requirements and to allow the chosen breathing system to function as intended . However , this number can sometimes be an overestimation of the actual FGF required by a specific patient . A general rule is that FGF should exceed the minute volume of the patient to prevent rebreathing , but there is no benefit to providing an overflow of fresh gases , as the additional gas is wasted through the scavenging system , along with any unused inhalational agent that has not been involved in gaseous exchange [ 8 ] . With this in mind , the FGF can be titrated down to the actual patient requirement , using capnography as a monitoring guide [ 9 – 11 ] .
Learning outcomes
• Understand what inspired CO 2 means for the patient .
• Recognise the capnogram that informs you of the inspired CO 2
.
• Understand the benefits of reducing FGF provision to patient requirements .
• Recognise the impact of waste anaesthetic gases on the environment .
Fresh gas flow calculations and breathing systems
FGF calculations are generally part of the usual working day for veterinary nurses . The main body of the basic calculation can be found in Figure 1 . Breathing system choice then becomes a factor to obtain the final calculation for a patient ' s requirements , and the circuit factor ( CF ) of the chosen breathing system must be used ( Table 2 , overleaf ) [ 12 ] .
The function of the CF is to ensure that sufficient FGF is being used to facilitate expulsion of the exhaled gases [ 12 , 13 ] . It is also a guide to the efficiency of the system . Non-rebreathing systems must have a CF applied to the FGF calculation , while rebreathing systems do not need this as there is a CO 2 absorbent as part of the system . This is usually soda lime , which undergoes a chemical change that absorbs CO 2 and generates water , heat and a pH change [ 3 ] . Once the CO 2 has been absorbed the other exhaled gases are free to be rebreathed [ 8 ] .
Figure 1 . Basic fresh gas flow calculation method .
Volume 38 ( 4 ) • August 2023
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