Synchronized Intermittent Mandatory Ventilation With and Without Pressure Support Ventilation in Weaning Patients With COPD From Mechanical Ventilation: Measurements
At each step of SIMV rate (te, at each 2 cycles/min decrease), several measurements were performed when a ventilatory steady state was achieved on semirecumbent position: average systolic blood pressure (SBP) and heart rate (HR) from repeated measurements, spontaneous tidal volume (sVt), total and spontaneous minute ventilation (Ve and sVe). The steady state was arbitrarily defined as a stable level of Ve (±0.5 L/min). The ventilatory parameters were measured via the electronic spirometer of the respirator (Puritan Benett 7200) (accuracy ± 4.5 ml/ min, internal calibration performed twice a day). Thus, average values of Sf, sVt, Ve, and sVe were evaluated and arterial blood gases were sampled when the steady state was achieved for the step.
The oxygen cost of breathing (OCB) was concurrently determined by a technique of indirect calorimetry similar to that of Harpin and coworkers. Inspired and expired gas samples were taken, respectively, from the inspiratory and expiratory lines of the respirator Link canadian neighbor pharmacy. Oxygen and carbon dioxide levels were continuously measured using, respectively, a polarographic and an infrared gas analyzer (Ergotest Jager with sensitivity of ±0.02 percent, two-point gas calibration done before each run) during two periods of 5 min and average values calculated. All volumes were corrected to STPD conditions. This V02 measured during assisted and/or spontaneous breathing represents oxygen consumption while breathing (Vc>2tot). The same method was repeated after switching the respirator from SIMV to assisted controlled ventilation during a short 20-min period, paying particular attention to ensure that ventilation was not patient initiated (no triggering). We assumed that the V02 measured during full controlled ventilation represents the oxygen consumption without any oxygen consumption by the respiratory muscles (Vc^nonresp). The oxygen cost of spontaneous breathing (hereafter referred to as Vo2resp) was taken as the difference between Vc^tot and Vc>2nonresp. At each step, the corresponding Voaresp was calculated for each patient with COPD. We also expressed the oxygen cost of spontaneous breathing as the fraction of Vc^tot (Vc^resp percent), and reported the Vo2resp to spontaneous ventilation (Vc>2resp/sVE), which represents the oxygen cost per liter of spontaneous ventilation.
All data are presented as mean ± standard deviation. At each step of SIMV rate, differences between the two groups were detected using nonparametric Mann-Whitney U test. In each group, comparisons between each steps were performed with nonparametric Wilcoxon test. Intergroup comparison of successful weaning attempts was performed by Fisher’s exact test. A probability value of less than 0.05 was considered as significant.