Article Notes
Toner notes the rapid adoption of high-flow nasal oxygen for apnoeic oxygenation, particularly in the context of competing alternatives that have not enjoyed the same popularity.
Specifically, it is highlighted that there is a lack of high-quality RCTs confirming the ability of Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) to acceptably clear CO2 with prolonged periods of apnoea. RCT results are awaited.
Daniel Jolley
What is automatic tube compensation? Glad you asked!
"Automatic tube compensation (ATC) is a new option to compensate for the non-linearly flow-dependent pressure drop across an endotracheal or tracheostomy tube (ETT) during inspiration and expiration. ATC is based on a closed-loop working principle. ATC is not a true ventilatory mode but rather a new option which can be combined with all conventional ventilatory modes."
Daniel Jolley
Take me back to the First Part
This study confirmed the well-known observation of the ventral ventilation shift under positive pressure ventilation, and quantified the contribution from the endotrachial tube itself, versus from muscle relaxation and IPPV.
This ventral shift under IPPV has also been shown to occur during pressure support ventilation with an LMA, when compared with spontaneous breathing under GA (Radke 2012).
Using electrical impedance tomography Lumb et al. confirmed this ventral shift in supine IPPV subjects, and demonstrated that this is primarily due to IPPV rather than the ETT itself, – although they found tube presence contributed to ~16% of the change.
"The generally accepted physiological explanation ... is that of greater cephalad movement of the diaphragm in dependent vs. non‐dependent lung regions during anaesthesia, resulting in changes in regional lung compliance."
"...regional ventilation with positive pressure ventilation during anaesthesia, even with no tracheal tube in place, is grossly different when compared with spontaneous ventilation, with greater ventilation of the left lung and ventral regions of both lungs. These effects are exacerbated by ventilation through a tracheal tube, leading to a greater degree of inhomogeneity of overall ventilation compared with when awake.
Take-home message
The authors note that while anaesthetists understand the detrimental effect of inadvertent endobronchial intubation, simply having the ETT tip close to the carina also worsens V/Q mismatch and is not as well appreciated. In these situations, tube withdrawal and/or 90o rotation may improve V/Q match.
Although this may be clinical insignificant for most patients, it should be considered when needing to improve gas exchange, particularly in critical care patients.
Daniel Jolley
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
An editorial on pandemic information overload?
Yep. 😉
But setting aside the irony of adding 2,000 more words on COVID-19 information overload, Kearsley & Duffy neatly explore the challenge:
"Since the outbreak of this pandemic, our e-mail inboxes, social media feeds and even general news outlets have become saturated with new guidelines, revisions of guidelines, new protocols and updated protocols, all subject to constant amendments."
What's the thesis?
The authors' argument is that too much information in the era of COVID may be a bad thing: the marginal benefit of 'more' may be overwhelmed by the negative cost.
They acknowledge the tension between the pragmatic and perfect when it comes to information sources in the face of a rapidly advancing disease – and in particular the recurrent waves of shifting clinical guidelines.
Kearsley & Duffy mention the important role of rapid research, worryingly tempered by a surge in volume, but fall in quality, along with mainstream promotion of non-peer reviewed and pre-print investigations. They note how information technology in the pandemic climate exploits our biases: confirmation, anchoring, and novelty.
At an individual level they discuss the risk of pandemic 'alert fatigue', the growth of social media and excessive information sharing making quality assessment difficult, and the negative effect of both on well-being.
The take-home
We each have significant personal responsibility to consider the consequences when sharing information, especially if incomplete or risk of misunderstanding when stripped of context.
"As we learn to live with this virus it is important for us to be cognisant that we are all at risk of error; we need to work to reduce information overload and focus on unifying our approach to both information dissemination and presentation. We must go back to basics and apply the well-practiced human factors principles of good teamwork, communication and leadership.
...
We need to avoid a situation where a crisis is overmanaged and underlead; “Ipsa scientia potestas est" or 'knowledge itself is power' – from what COVID-19 is teaching us however, can too much knowledge be a bad thing?"
Daniel Jolley
Though these findings are in stark contrast to Kheterpal’s (2020) massive 45,000 subject matched cohort study that did show sugammadex reduced postop respiratory complications.
Daniel Jolley
LMA and Caesarean – why should I care?
There is a small attitude change underway in the use of supraglottic airway devices (SGA) in obstetric anaesthesia. While there is already an appreciation of their role in obstetric airway rescue, we now see a shift in some countries to use an SGA as the primary airway choice for Caesarean section under general anaesthesia.
Anaesthesiologists need to be aware of this attitudinal shift, and importantly appreciate the inherent compromises and uncertainties driving it.
In this editorial, Metodiev & Mushambi review changing attitudes toward obstetric airway preference, the realities of maternal aspiration risk, and several large studies suggesting acceptable safety when using a SGA for Caesarean GA.
The tension between airway and aspiration
It is well accepted that regional anaesthesia for Caesarean section is overwhelmingly the best choice, driven first by the historical experience of maternal general anaesthesia risk. The very features that underline this safety improvement are also those in tension when considering endotracheal intubation or SGA: risk of failed intubation versus aspiration.
Studies showing safety
Several retrospective, prospective and randomised studies totalling more than 8,000 patients have concluded that in these populations, SGA use (mainly 2nd generation devices, such as ProSeal™ or LMA Supreme™) was not associated with any greater risk of aspiration. This includes both the single largest study investigating 3,000 women (Halaseah 2010), and two RCTs (Yao 2019 & Li 2017), none of which identified any cases of aspiration (although there was a single regurgitation).
So on the surface, SGA use appears arguably safe, particularly with careful patient selection. Among the studies, generally obese patients and those with reflux were excluded, muscle relaxants were frequently used, an orogastric tube was inserted, and cricoid pressure was used at least for some periods of airway intervention.
And yet we do know from NAP4 (2011) that aspiration is a real danger, accounting for 50% of anaesthesia-related deaths.
Is gastric ultrasound the answer?
No. Next question... 😉
While gastric ultrasound shows some utility in quantifying residual gastric volume, it is 1. Not possible to equate this to aspiration risk in pregnant patients, 2. Technically difficult in the pregnant patient.
They conclude that...
"...there is insufficient evidence to recommend universal or selective replacement of tracheal tubes with SGA devices during general anaesthesia for Caesarean delivery. Aspiration remains the main concern."
Be smart
And before you get too excited by the lack of observed aspiration in these large studies, as Metodiev & Mushambi note, many of the studied populations were Asian and Middle Eastern, having different diets and obesity prevalence than Europe, Oceania and North America.
Daniel Jolley