Plus-Hex CLINICAL
Figure 4 . A normal elbow connector ( left ) and a low-dead-space capnography attachment ( right ).
All ETTs used in small animal anaesthesia have a universal 15 mm male connector on the proximal end , to which a breathing system is attached . A side-stream or mainstream capnograph attachment can be connected between the ETT and breathing system . Low deadspace side-stream capnograph adapters are available for 2 – 5 mm ETTs , reducing apparatus dead space , as shown in Figure 4 . Some ETTs have a separate lumen channel which allows the suctioning of secretions or fluid sitting above the cuff .
Red rubber ETTs
Red rubber ETTs are easy to place , as the cuff lies flat against the ETT , but it is a low-volume / high-pressure ( LV / HP ) cuff , so overzealous inflation can compress the internal diameter of the ETT [ 2 ] . The pilot balloon does not self-seal , so it may spontaneously deflate during a procedure . Red rubber ETTs are non-transparent , so blockages in the tube cannot be seen ; they also kink easily , they can irritate the mucosa and they do not mould to the shape of the trachea . They can be autoclaved and reused , but , over time , they may harden .
PVC ETTs
PVC ETTs are transparent and have self-sealing pilot balloons and cuffs , which are typically high-volume / low-pressure ( HV / LP ). They are easy to place , due to their rigidity before they warm , and soften to the tracheal shape , but they are still difficult to kink in the airway . They can also be reinforced with a metal coil ( armoured ETTs ), as shown in Figure 5 , to eliminate the possibility of kinking during maximal neck flexion , which may occur during dental procedures , ophthalmic procedures and cisternal sampling of cerebrospinal fluid ( CSF ). The armoured ETTs are not suitable for patients undergoing magnetic resonance imaging ( MRI ), which typically precedes CSF sampling . Armoured ETTs have a thick wall to accommodate the metal coil , which reduces the internal diameter , potentially increasing airway resistance . PVC ETTs cannot be autoclaved , but they are inexpensive so they are replaced after single or multiple uses .
Figure 5 . Armoured ETT ( left ) and a flexed normal PVC ETT ( right ).
Silicone ETTs
Silicone ETTs are versatile , as they can be autoclaved , reused and repaired . They are available in armoured form , they mould to the tracheal shape and the pilot balloon self-seals . However , they are expensive and have the less desirable LV / HP cuff . Their soft material means a stylet may be required to add rigidity for placement .
Regardless of type , if the ETT becomes kinked , you may see a capnography waveform with a prolonged phase II and lack of plateau , commonly known as a shark fin trace ( Figure 6 ). The prolonged phase is caused by an increase in resistance during exhalation . This trace may also be seen in patients with a mucus plug in the ETT , an upper airway blockage or cats with asthma .
Figure 6 . A shark fin trace on a capnograph .
Laryngeal mask airways and supraglottic airway devices *
Laryngeal mask airways ( LMAs ) and SGADs were designed for use in human medicine . Docsinnovent Ltd introduced a veterinary-specific SGAD to the market , called v-gel ( Figure 7 , overleaf ), which is now available in original and advanced models .
A benefit of LMAs and SGADs is that they can be placed without the use of a laryngoscope . They make contact with the soft tissue around the laryngeal opening instead of the laryngeal and tracheal mucosa directly . As this device does not enter the trachea , it reduces the risk of trauma and is less likely to induce laryngospasm than conventional ETTs . Capnography should be used to confirm correct positioning . Care must be taken after placement to ensure there is no rotation of the SGAD while changing patient recumbency .
Volume 38 ( 3 ) • June 2023
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