A level of pressure support ventilation (PSV) was calculated for each patient using the formula: minimal pressure support (PSmin)=PIFR X R, where PIFR is the peak spontaneous inspiratory flow rate and R is the total resistance of the respiratory system. Although the formula for PSmin has not been validated, it serves as a useful starting point to standardize the initial pressure support (PS). The PSmin has been defined as that level of PS which is necessary to simulate spontaneous breathing. At PSmin, it is assumed that the PS is just adequate to overcome the resistance of the ETT and the ventilator circuit. Peak spontaneous inspiratory flow rate was measured during two spontaneous breaths while the patient was receiving ventilator assistance (constant positive airway pressure, 0 cm H2O) and the average value used.
In a recently completed study, we showed that the work of breathing (Wi) was significantly increased in the immediate postextubation (EXTO) period as compared with the Wi while breathing spontaneously through an endotracheal tube (ETT). This was an unexpected result, since it is generally believed that when inspiratory flow rates are comparable, the added resistance of an ETT increases Wi. Both the increased length and the smaller internal diameter of an ETT (50 to 64 mm2 for 8- and 9-mm tubes, respectively) as compared with the normal glottis are thought to be responsible for the increased ventilatory load. The goal of this study was to examine if the increased Wi in the postextubation period could be caused by changes in the trachea or glottis because previous studies have documented that endotracheal intubation causes upper airway damage.
Materials and Methods
Eight patients, identified as eligible for wearing by their primary physicians, were studied during weaning (Table 1). There were five men and three women. The mean age of the patients was 71 (range, 23 to 91) and the mean time of intubation was 5.5 days (range, 1 to 14). Three of the patients had congestive heart failure; two had COPD and pneumonia, and one of the patients was intubated because of status epilepticus. All patients were ventilated by means of a Puritan Bennett 7200A Ventilator. At the time of weaning, all patients were alert and cooperative. The experimental protocol was approved by the institutional review board of the hospital, and all patients gave their informed consent prior to participation in the study.
Table 1—Patient Demographics*
|No. Ventilator Days||PSmin, cm H20||Diagnosis*|
*CHF=congestive heart failure.
Recently we showed that work of breathing was higher in the immediate period after extubation as compared with spontaneous breathing through an endotracheal tube. In this study, we evaluated the glottis and trachea as potential sites of increased airway resistance after extubation. We measured breathing pattern, work of breathing, and pressure time product in eight patients during weaning from mechanical ventilation. We acquired data during pressure support ventilation and spontaneous breathing via the ventilator, with the endotracheal tube in place, and after extubation. During bronchoscopy at the time of extubation, we examined the trachea and measured the cross-sectional area of the glottis.