Equipment and safety

Question 1

What’s a variable performance device?

Answer 1

An oxygen delivery device where the inspired oxygen concentration is dependent on the pts peak inspiratory flow rate, geometry of device, pts ventilation & whether nose or mouth breathing. Useful for recovery but not when precise [] of O2 needed. Deliver @ flows 2-15L/min, reservoir usually small (mask or nasopharynx).
Nasal prongs, Hudson mask, tracheostomy mask, Ayres T piece (Mapleson F)

Question 2

Are greater O2 concentrations delivered with low or high rates of ventilation with nasal prongs?

Answer 2

low (= greater fractional contribution of the fixed flow of O2 to inspired gas mixture)

Question 3

What’s a fixed performance device?

Answer 3

FiO2 constant despite changes in resp rate, peak insp flow rate, tidal volume
No random entrainment of room air to unpredictably alter FiO2
Feed connector has holes allowing controlled entrainment of atmospheric air into the oxygen stream by jet mixing
Eg. venturi mask, high flow nasal O2, anaes FM, mapleson A-E, BMV

Question 4

Contraindications for NHF?

Answer 4

epistaxis
base of skull fracture
surgery to the nose or upper aero-digestive tract
nasal obstruction; e.g. nasal fracture, tenacious secretions, tumour

Question 5

evidence for HFNO?

Answer 5

The precise role of high flow nasal cannulae in the management and prevention of hypoxia is controversial

Failure of HFNC might cause delayed intubation and worse clinical outcomes in patients with respiratory failure (Kang et al, 2015)
The FLORALI study, a small multicenter, open-label trial found that in patients with acute hypoxaemic respiratory failure and without hypercapnia, treatment with high-flow nasal oxygen, standard face mask oxygen, or non-invasive ventilation did not result in a significantly different intubation rates. There was a significant difference in favour of high-flow nasal oxygen in 90 day mortality (Frat et al, 2015; FLORALI study)
Preoxygenation and apnoeic oxygenation
Compared to HFFM (high flow face mask), HFNC as a preoxygenation device did not reduce the lowest level of desaturation in an RCT (Vour’ch et al, 2015 – PREOXYFLOW trial)
A case series of 25 patients with difficult airways undergoing general anaesthesia for hypopharyngeal or laryngotracheal surgery had mean apnoea times of 14 minutes without desaturation (i.e. SaO2 >90%) (Pateal et al, 2015; THRIVE study)

Question 6

What’s the approx FiO2 with nasal cannula @ different flow rates?

Answer 6

1  24%
2  28%
3  32%
4  36%
5  40%
6  44%

Question 7

What’s the approx FiO2 with Hudson mask @ different flow rates?

Answer 7

6  40%
7  50%
8  60%
FiO2 limit ≈ 60%

Question 8

:) which muscles can be used to assess TOF?

Answer 8

adductor pollicis (ulnar nerve)- ideal since readily accessible, results aren’t confounded by direct muscle stimulation
anterior compartment of leg (common peroneal nerve)
plantar muscles of feet (posterior tibial nerve)- place -ve just post to med malleolus & +ve just prox to it, monitor accelerometer with PF of great toe
facial nerve (facial expression)- place +ve just ant to tragus, -ve just above it, measure acceleromyographic (quant) at orbiculares oculi @ eyelid or corrugated supercilious at eyebrow

Question 9

:) how peripheral nerve stimulators set up? what’s the train of four? what is double burst stimuli? what’s tetany & the post tetanic count? what is PTC useful for? caveat to using PTC? What to give if want to reverse & PTC 0? 1-2? TOF 2? TOF 4 ratio <0.9? what’s the single twitch?

Answer 9

Surface electrodes placed on skin over course of a peripheral nerve to avoid direct stimulation of the muscle being monitored- negative electrode distally (1cm from wrist crease), positive 4-5cm proximally to ensure most effective stimulation.

4x successive identical supramaximal (60-80mA to ensure all composite n fibres depolarised) 2Hz 0.1ms stimuli delivered to the motor nerve of a peripheral muscle (eg. ulnar nerve for adductor pollicis).
accelerometer/EMG/kinemyography measures the motor response- displayed & if 1 motor response (“twitch”) detected, strength is 10% of baseline values. Once 2nd/3rd/4th, first twitch recovery is 20, 35 & 45% of baseline values. Once the 4th twitch returned, can Ax TOF ratio. If amplitude of 4th is 50% of 1st, TOFR is 0.5.

DBS= mini-tetanic sequence of 3x short bursts of high-freq (50Hz) tetanic stimuli, followed by a 2nd series 750ms later. Compare the ratio of 2nd muscle response to first, as with TOF. When there is full recovery of neuromuscular function, 2x equal muscle contractions occur. Developed because subjectively (if absence of quantitative monitor) we can more accurately assess fade with 2 vs 4 stimuli.

If stimuli applied @ a frequency of >30Hz for 5 seconds, the resulting motor twitches become fused into a sustained muscle contraction & the response may be larger than a single stimulus since elastic forces don’t need to be overcome for each twitch & the titanic stimulus mobilises pre-synaptic ACh into NMJ.

For PTC, a tetanic stimulation (50Hz) is applied for 5 seconds, then stimuli of 1Hz are started 3 seconds later over 20 seconds. Only useful for deep block (TOFC zero, >95% receptor blockade by NDNMBD). Useful for surgery such as retinal, where movement or coughing could have devastating effects so want deep block (PTC very low). The number of twitches inversely relates to depth of block, approximates time to recovery (eg. PTC of 1 indicates 30 mins), TOF generally returns with a PTC of 9. Should wait at least 3 mins after PTC before TOF monitoring since subsequent TOF may be increased (underestimate block)- 6 mins before a further tetanic.
PTC 0, wait or give sugammadex 16mg/kg (only if roc or veg)
PTC 1-2 wait or sugammadex 4mg/kg
adductor pollicis TOF 2 wait or give sugammadex 2mg/kg
don’t give neostigmine unless spontaneous recovery of TOF of 4 & allow 10 mins at least for full effect. Iif add pollicis TOFR <0.9, give neostigmine 0.05mg/kg of <0.4, 0.02mg/kg if >0.4 but < 0.9 or sugammadex 2mg/kg
glycol is 0.2mg per 1mg neo IV as similar onset of action

single twitch requires a baseline twitch pre-paralysis to be useful. one supra maximal stimulus is delivered every 10 seconds (0.1Hz for 0.2ms). No reduction in twitch height until 75% of receptors occupied. when single twitch completely abolished, intubating conditions optimal. fastest means of assessment but used to determine potency of NMBAs vs clinically. Height is reduced for both ND & DMAs (no fade)

Question 10

:) why don’t we monitor NM function on paralysed limb?

Answer 10

upregulation of ACh receptors so pt resistant to NDNMBAs & there’ll be exaggeration of TOFR in paretic limb so underestimation of systemic NMB

Question 11

:) do you get fade on TOFC with depolarising block?

Answer 11

no, the decline in amplitude is similar unless phase II block- fade in response to TOF or tetanus & post-tetanic potentiation suggests phase II block.

Question 12

:) pros/cons of electromyography vs kinemyography vs acceleromyography? what’s the traditional gold standard?

Answer 12

electro don’t require unrestricted movement of stimulated muscle, not subject to reverse fade & are in agreement with mechanomyography (>AMG) & have better precision to monitor degree of recovery & don’t require baseline calibration prior to NMBA, but they are effected by OT electrical interference- AMG or KMG aren’t. they also require expensive manufacturer-specific electrodes.

mechanomyography (now n/a) was gold standard for objective monitoring, since EMG can be used when limbs restricted & high agreement w mechanomyography, considered new gold standard.