Plus-Hex CLINICAL
Possible explanations for this are that the FGF is inadequate ( mainly for non-rebreathing systems ), there is a fault in the equipment , there is excessive apparatus dead space or the soda lime within a rebreathing system is exhausted [ 3 , 5 , 6 , 8 , 13 ] . The anaesthetist can use these parameters to monitor whether a lower-thancalculated FGF is suitable for a specific patient .
For example , the previously mentioned 12 kg dog is anaesthetised and the breathing system is a Lack . The calculated starting FGF is 3,600 ml / min . When capnography is used , the anaesthetist can experiment with using an FGF of 3,200 ml / min and watch the capnogram , ETCO 2 and FiCO 2 for indications that this
FGF is inadequate .
If the lower FGF shows no changes in the monitoring , then a further reduction could be applied , with further monitoring of the capnography to assess that change , and so on , to ascertain the individual patient ' s requirements [ 12 ] . When rebreathing is identified , the anaesthetist then knows the limit of the FGF for that patient and can return it to a level that ensures there is no rebreathing [ 14 ] .
Potential complications
The patient Reflecting on the patient factors that ETCO 2 is dependent on ( metabolism , circulation / perfusion and ventilation ), the anaesthetist can think about whether a specific patient may have complications that could affect capnography monitoring .
Metabolism may affect capnography when the patient has :
• Hyperthermia / hypothermia
• Seizures
• A thyroid storm
• A light / deep anaesthetic plane .
Circulation / perfusion may affect capnography when there is :
• A ventilation / perfusion ( V / Q ) mismatch
• Lung pathology
• The presence of alveolar dead space
• Hypotension
Ventilation may affect capnography when there is :
• A V / Q mismatch
• Lung atelectasis
• Positioning that affects adequate ventilation
• Reduced functional residual capacity
• Reduced thoracic compliance
• A deep anaesthetic plane .
A patient may have a combination of these attributes , so thorough clinical examination , history taking and pre-anaesthetic blood testing may be needed to prepare such a patient for anaesthesia [ 2 , 4 , 6 , 13 , 15 ] .
Comprehensive monitoring and recording of patient parameters ( e . g . temperature , blood pressure , anaesthetic plane , dead space identification ) will enable many complications to be ruled out almost immediately or addressed speedily , so more thought can be given to the less readily monitored potential complications .
The equipment
Minimum metabolic oxygen requirements for patients seen in small animal practice are 10 ml / kg / min . This may need to be supplied in excess , as some flowmeters are calibrated to be accurate at a minimum flow rate of 100 ml / min , while others may be at 250 ml / min . In addition , with flow rates below 250 – 500 ml / min vaporiser output may be inaccurate [ 3 , 12 ] .
Breathing system and anaesthetic machine checks should be performed before any anaesthesia , to ensure there are no detectable leaks or breakages . Checklists can be useful in preventing error .
With any breathing system , if FGF provision is kept to a minimum but a change in the patient ' s plane of anaesthesia is observed , the FGF may need to be momentarily increased to ensure the changed percentage of inhalational agent can be delivered to the patient quickly . In this case , the FGF should be returned to its previous level once the desired response in anaesthetic plane is seen [ 12 , 13 ] .
The reliability of ETCO 2 levels can be brought into question where FGF rates are high and cause dilution of the sample , and when the sampling volume for the capnograph is high and air is allowed to diffuse through the equipment , or it interferes with patient ventilation [ 2 , 4 , 5 ] .
• Reduced cardiac output .
Volume 38 ( 4 ) • August 2023
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