Article Notes
Cough: why care?
Although often minor, common post-operative complications have by definition a broad impact on the perioperative experience. Some common complications, such as coughing on extubation, can also have significant surgical consequences such as for neurosurgical or ophthalmic procedures.
Both coughing on extubation (reported incidence 15-94%) and post-operative sore throat (21-72%) are very common among surgical patients.
What did they do?
Yang and team performed a high-quality meta-analysis of RCTs investigating the effect of intravenous lidocaine/lignocaine on coughing at extubation. Many of these trials also looked at further secondary effects, such as post-operative sore throat. They included 16 trials, totalling 1,516 subjects. Although the trials demonstrated significant heterogeneity, subgroup analyses still confirmed the study's findings.
And they found...
There was significant reduction in cough RR 0.64 (0.48-0.86 & NNT=5), and post-operative sore throat RR 0.46 (0.32-0.67), though no difference in laryngospasm, adverse events or time to extubation with modern volatile agents.
Analysing various lidocaine timings (pre-operative vs intra-operative) and dose ranges (low <1.5mg/kg or high >1.5 mg/kg) yielded no evidence of clear advantage. Nonetheless the findings are consistent with previous reviews, such as from Clivio et al. (2019) showing lidocaine 1.5 mg/kg reduced cough (RR: 0.44; 0.33–0.58), and that the effect is probably dose responsive.
Ok, but how does lidocaine work?
The mechanism of action reducing cough is not understood, although several possibilities have been proposed, including...
"...the suppression of airway sensory C fibres, the reduction of neural discharge of peripheral nerve fibres, and the selective depression of pain transmission in the spinal cord."
Bottom-line
Peri-operative intravenous lignocaine effectively reduces coughing on extubation and reduces post-operative sore throat, without any increase in adverse events.
Daniel Jolley
In this review, Karmali & Rose challenge the dogma surrounding endotracheal tube sizing for adult anaesthesia, traditionally sizing based on sex.
What did they cover?
They explored both the functional consequences (good and bad) of ETT size, as well as airway trauma.
Noting that an ETT ≥ 6.0mm ID will accomodate most intraluminal devices, and in fact at these smaller sizes fibreoptic intubation or passage through an LMA is easier, however smaller tubes are more readily obstructed and deformed.
Ventilation through smaller ETTs
While smaller tubes may require slightly higher inspiratory pressures, these are generally not clinically significant with modern ventilators, and importantly do not translate to higher intra-tracheal or alveolar pressures experienced by the patient.
Similarly, expiratory gas flow is not significantly effected by a small ETT (6.0 mm) for most patients even at high minute ventilations (although use cautiously in patients with chronic airway limitation). Significant gas trapping at normal MV will start to occur with ETT < 5.0 mm.
Size and airway trauma?
While the internal diameter (ID) is important for anaesthesia conduct, it is the external diameter that matters for airway trauma (a standard 8.0 mm ID ETT has a 10.5 mm ED!).
They note while there is wide individual variation in tracheal dimensions, the trachea is narrowest at the subglottis – and thus adequate visualisation of the glottis at time of intubation is an incomplete indicator of the tube size suitability for the subglottis.
Not only do some adult women have an airway size at the lower-limit of acceptability for traditional 7.0-8.0 mm ETTs, but there is also correlation between ETT size and airway trauma, hoarseness and sore throat. A large ETT can result in mucosal ischaemia and ulceration after as little as 2 hours.
They conclude...
"Instead of opting for ‘the largest tube that the larynx will comfortably accommodate’, we perhaps should consider using the smallest tube which permits the safe conduct of anaesthesia."
For routine anaesthesia of ASA 1 & 2 patients, an ETT sized 6.0-7.0 mm is probably the best balance between ventilation needs and airway trauma.
Be smart
But remember, many of the concerns for tracheal tube trauma are based upon critical care experience, not anaesthesia. While a smaller tube is very likely beneficial for most elective adult patients, most benefit will simply be reduction in post-operative sore throat and hoarseness.
Daniel Jolley
While the reliability of this simple formula is interesting, the authors note the wide variability in nasal tubes from different manufacturers, particularly in length and guide markings for the same-sized tube. Thus although interesting, there is questionable utility in this formula.
Correct ETT depth is probably better determined clinically: visually observing the cuff pass the laryngeal inlet and cords, and auscultation to exclude endobronchial intubation. At best, a predictive-depth formula is a useful sanity-check.
Daniel Jolley