Upper airway collapsibility is known to increase under anesthesia. This study assessed how this increase in collapsibility evolves during slow Propofol induction and how it relates to anesthesia-induced changes in upper airway muscle activity and conscious state.
Nine healthy volunteers were studied. Anesthesia was induced with Propofol in a step-wise manner (effect-site concentration steps of 0.5 microg x ml(-1) from 0 to 3 microg x ml(-1) and thereafter to 4 microg x ml(-1) and 6 microg x ml(-1) [target-controlled infusion]). Airway patency was maintained with continuous positive airway pressure. Pharyngeal collapsibility was assessed at each concentration by measuring critical pressure. Intramuscular genioglossus electromyogram and anesthetic depth (bispectral index score) were monitored throughout. Loss of consciousness was defined as failure to respond to loud verbal command.
Loss of consciousness occurred at varying Propofol effect-site concentrations between 1.5 and 4.0 microg x ml(-1). Initially genioglossus electromyographic activity was sustained with increases in Propofol concentration, increasing in some individuals. At or approaching loss of consciousness, it decreased, often abruptly, to minimal values with an accompanying increase in critical pressure. In most subjects, bispectral index score decreased alinearly with increasing Propofol concentration with greatest rate of change coinciding with loss of consciousness.
Slow stepwise induction of Propofol anesthesia is associated with an alinear increase in upper airway collapsibility. Disproportionate decreases in genioglossus electromyogram activity and increases in pharyngeal critical closing pressure were observed proximate to loss of consciousness, suggesting that particular vulnerability exists after transition from conscious to unconscious sedation. Such changes may have parallels with upper airway behavior at sleep onset.
"Once an individual loses consciousness, particularly in the supine position, the upper airway is inevitably obstructed [8,21]. The mechanisms of UAO in patients with OSA or during GA are multifactorial. "
[Show abstract][Hide abstract] ABSTRACT: Upper airway obstruction (UAO) is a major problem in unconscious subjects making full face mask ventilation difficult. The mechanism of UAO in unconscious subjects shares many similarities with that of obstructive sleep apnea (OSA), especially the hypotonic upper airway seen during rapid eye movement sleep. Continuous positive airway pressure (CPAP) via nasal mask is more effective at maintaining airway patency than a full face mask in OSA patients. We hypothesized that CPAP via nasal mask and ventilation (nCPAP) would be more effective than full face mask CPAP and ventilation (FmCPAP) for unconsciousness subjects and we tested our hypothesis during induction of general anesthesia for elective surgery.
A total of 73 adult subjects requiring general anesthesia were randomly assigned into four groups: nCPAP P0, nCPAP P5, FmCPAP P0, and FmCPAP P5, where P0 and P5 represent positive end expiratory pressure (PEEP) 0 and 5 cmH2O applied prior to induction. After apnea, ventilation was initiated with pressure control ventilation at a peak inspiratory pressure over PEEP (PIP/PEEP) of 20/0, then 20/5 and finally 20/10 cmH2O each applied for 1 min. At each pressure setting, expired tidal volume (Vte) was calculated using a plethysmograph device.
The rate of effective tidal volume (Vte > estimated anatomical dead space) was higher (87.9% vs.21.9%; P < 0.01) and the median Vte was larger (6.9 ml/kg vs. 0 ml/kg; P < 0.01) with nCPAP than that with FmCPAP. Application of CPAP prior to induction of general anesthesia did not affect Vte in both approaches (nCPAP pre- vs. post; 7.9 ml/kg vs. 5.8 ml/kg, P = 0.07) (FmCPAP pre- vs. post; 0 ml/kg vs. 0 ml/kg, P = 0.11).
nCPAP produced more effective tidal volume than FmCPAP in unconscious subjects.Trial registration: Clinical trials.gov, Identifier: NCT01524614.
"The soft palate is quite mobile and previous studies using lateral neck X-rays have shown that the distance between the soft palate and the posterior pharyngeal wall decreases during anaesthesia, to zero in some cases  . Several previous studies have documented airway collapse at the level of the nasopharynx during sleep  and anaesthesia  ; however these studies were done in subjects during spontaneous ventilation and the collapse occurred only when nasal pressure was suddenly reduced to low, even sub-atmospheric levels. Hence, the findings are not directly relevant to the current situation which involves positive pressure ventilation. "
[Show abstract][Hide abstract] ABSTRACT: OSA patients present unique challenges in the peri-operative period. They routinely require more monitoring, oxygen therapy, unplanned ICU admissions, longer hospital stays, and have more adverse events than healthy counter-parts. Some data suggest that perioperative CPAP use is associated with reduced morbidity and mortality of patients with OSA, and yet its application remains inconsistent. This review aims to summarize existing literature on the peri-operative use of CPAP, identify barriers to its implementation, and begin defining an algorithm for the practical application of peri-operative CPAP.
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