Introduction: Judging the correctness of the fit of an uncuffed tube in the trachea is an imprecise process that may rely on detecting a just-audible leak (1). An optimal fit would allow adequate ventilation whilst minimising trauma to the subglottis. If ventilation is prolonged, the fit of the tube in the trachea might vary. We believe that the leak of gas around an uncuffed tube may be evaluated to give an indication of the 'goodness' of fit.
Methods: In the laboratory we developed a technique for quantifying the leak around the tracheal (TT) tube in ventilated children from continuous measurements of airway pressure and inspiratory and expiratory flow acquired from a ventilator. We obtained ethical approval to study 20 children who were ventilated on our PICU after undergoing cardiac surgery on cardiopulmonary bypass. Recordings were made for sequences of 100 breaths at intervals of approximately 4 h during the time that the child was ventilated. Audible leak was graded as 'none', 'minute', 'moderate' or 'large' by the anaesthetist responsible for each patient. We calculated the leak conductance (in l.s.(-1) cmH2O(-1)) for each recording and plotted it against time for each patient. We calculated the average leak conductance from the normalised area under this curve to account for irregular sampling, and used this quantity (2) to compare patients who were grouped according to grade of audible leak.
Results: A total of 286 recordings were collected from 20 patients (age range 5-1356 days, mean age 193 days, weight range 3.0-12.4 kg, mean weight 5.5 kg). Eleven recordings were rejected due to failure or artefact, and two patients were excluded due to insufficient data (two or fewer recordings). Five patients were graded with moderate or large audible leak and the mean leak conductance for this group (mean 360 l.s (-1) cmH(2)O(-1)) significantly exceeded the leak conductance for the remaining 13 patients with no leak or minute leak (mean 125 l.s (-1) cmH(2)O(-1))) with P < 0.05. Discussion: We have demonstrated that the physical characteristics of a leak around an uncuffed ET tube can be quantified (3) and that, in this small study, serial measurement of leak conductance is consistent with qualitative audible assessment of leak.
Conclusion: Leak conductance may provide an objective method for analysing the leak around an uncuffed TT tube and thereby the fit of the tube in the subglottis. We believe that this technique, if adapted, may be useful in the clinical setting.
References
1 Fine GF, Borland LM. The future of the cuffed endotracheal tube. Pediatr Anesth 2004; 14: 38-42.
2 Matthews JNS et al. Analysis of serial measurements in medical research. BMJ 1990; 300: 230-235.
3 Smith JH et al. Leak resistance around uncuffed endotracheal tubes. Pediatr Anesth 2005; 15: 805-806.
Murphy TW, Reay CA, Sims AJ, Smith JH. Serial measurement of leak conductance around uncuffed endotracheal tubes in children after cardiopulmonary bypass. Paediatr Anaesth. 2006 Dec;16(12):1300-1.
Dr. Andrew J. Sims, Freeman Unit, Newcastle upon Tyne. Tel: +44 (0)191-223-1370.
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