4. 3 Steps to Own the Oxygen!
① Preoxygenation
• Why, how, and for how long?
② Apneic oxygenation
• Why and how?
• Positioning, ventilations and maneuvers
• Paralytic agents
③ Putting it into practice
• Risk stratification and logistics
31. Preoxygenation period
• Head up positioning
• Ear-to-sternal notch position
• Place nasal cannula
• High flow oxygen: 15 L/min via NRB
• Preoxygenate for 3 minutes with tidal breaths, or
8 maximal inspirations/ expirations
• If hypoxic, consider:
– CPAP (e.g. 5-15 cmH20 to achieve SO2 >98%)
– BVM with PEEP valve
32. Apneic period
• Push sedative and roc
• Commence 15 L/min oxygen via nasal cannula
• Remove face mask
• Maintain airway with jaw thrust
• If hypoxic, consider:
– CPAP
– BVM (6 breaths/ min) with PEEP valve
Aim is to intubate avoid desats and aspirationSO2 <70% -> dysrhytmia, hemodynamic compromise, brain injury, deathSafe apnea time is time to 90% sats…Why do we have an apnoea time?RSI is modification of the anesthetic RS induction approach to patients presumed to be at risk of aspiration… same in ED: full stomachs, body habitus, SICK
Once sats drop into the low 90s you’re on the slippery slide.. And its steep!Critical desats can occur witihin moments<1 minute in a well patient breathing room air!
450 mL O2 in lungs at room air, 3L at FiO2 100% - + about 1L in the bloodstreamOxygen consumption 3mL/min or 250-300 mL/imn in an adult
Anesthesia circuit delivering 100% oxygen would be ideal!
Standard NRBs are loose fitting face masks without 1-way valves covering all of the ports – Fi)2 60-70% max at 15 L/minNot sufficient for complete denitrogenationIncrease to ~90% with 30+L – keep opening the valve (only calibrated to 15)BVM only useful if giving +ve pressure ventilations or good inspirations with tight seal (use 2 hands) to open the 1-way valvesMost are close to room air only if lacking ideal 1-way valves
Ideal is 90% end tidal oxygen level (rarely performed in ED, widely performed in OT)Ask patient to exhale fully first!8 breaths = 60 seconds in cooperative patientsMost critically ill patients cannot take vital capacity breathsTimes assume FiO2 >90%
Use CPAP or BVM with PEEP valve if unable to adequately preoxygenate without PPVIf unable to preox >95% then shunt physiology exists = alveoli perfused but not ventilated e.g. collapse, pneumona, APOMay be overcome by postive pressure ->recruits alveoliEvidence from 6 small RCTsNo gastric distention or CV compromise – likely negligible effects <25 cmH2O)
Supine is not ideal – posterior lungs more prone to collapse, difficult to take full breaths
Repeated studies show that head up prolongs safe apnea times by about 90 seconds – about 50 seconds in the obeseUse reverse Trendelenbergifcan’t bend at the waist e.g. spinal immobilisation(head elevation also gives a betterr view of the cords duriglaryngoscopy)
1 min at room air vs 8 mni if high fio220 seconds to 85% if critically ill – metabolic demand, anemia, low CO, volume depletion
Benumof et al – physiological modellingAssumes completedenitrogenation and 80% o2 content of lungsIgnores pulse ox lag – petip 30-60 sec behind centralSudy by Mort et al – preox less effective in the critically ill – less than 20% have >50 mmHg rise
250 mL/min O2 uptake - <20 mL/min CO2 released (buffering + solubility + Hb affinityNet flow of gas from nasopharynx to alveoliIn optimal conditon PaO2 100 can be maintained for 100 mintues! (hypercapnia leads to acidosis and death eventually)Used routinely for brain death exams in ICUNASAL PRONGSNear FiO2 100% achieved when apneic – 15L/min, air not entrained, nasoharynx acts as a reservoirMain limitiation of high flow is discomfort and dessication – not so impt in apneicpatietns!Mouth opening does seem to matter experiemntallyFacemaks tend to vent air and get in the way – BVM only useful if PPV providedNasal prongs can be applied in advance and left on during intubation attemptsIn practice extends safe apnea time by about 1-2 minCan continue CPAP until moment of intubation – improves apneic oxygenation, prevents absorption atelectasis and prevents recurrence of factors that lead to shunt physiologySome people like to insert an LMA (Braude)
Need to avoid laryngoscopy until adequate realtion - good views and obtunded gag reflexBenefits – ventilation + oxygenationApnea PCO@ increases 8-16 mmHg in first min then 3/min – only impt if metabolic acidosis, tox (NCB, salicylate) or raised ICPAvoid >25 cmH20 – slow 6-8/min or >>> gastric distention and dynamic hyperinflation
Apneic oxygenation requires airway patency to be maintainedUse adjuncts – esp if OSA etcEAM at same level as sternal notch = best view of cords for DLMay need to build rampCan’t do it if c-spine precuationsCP – shown in some studies to decrease gas entry into stomach but…No reall evidence of efficacy in preventing meaningful harmMRI shows oftn pushes esophagus to side, laryngotracheal compression results, worsens DL view, people get in the way, impairs BVM and increase pressuresAlways performed incorrectly
Rocuronium appears to have longer safe apnoea times than suxamethonium. For instance, in ASA grade 1 or 2 patients with BMIs of 25 to 30 receiving general anesthetic, patients who were given rocuronium took 40 seconds longer to fall to an SO2 of 93% when apneic than those who received suxamethonium. This might be a result of increased oxygen consumption from the fasciculations caused by depolarising neuromuscular blockade.
