1995 OPEN FORUM Abstracts
INSPIRED VERSUS TRACHEAL [NO].
Dean Hess, PhD, RRT, Robert M. Kacmarek, PhD, RRT, William E. Hurford, MD. Departments of Respiratory Care and Anesthesia, Massachusetts General Hospital and Harvard Medical School, Boston MA.
Several sites have been reported for analysis of [NO] in therapeutic delivery systems. Although [NO] is most commonly measured in the inspiratory limb of the ventilator, others have measured [NO] in the trachea. We have found that exhaled [NO] is 25 - 75% of inhaled [NO] in patients with ARDS. We designed a lung model study to determine whether tracheal [NO] is less than inspired [NO], and if it is affected by I:E ratio and expired [NO]. METHOD: A Puritan-Bennett 7200 ventilator was set to deliver a tidal volume of 0.75 L, rate 12 breaths/min, FIO_2 0.60, and I:E ratio of 1:3, 1:2, 1:1, or 2:1. Nitric oxide (785 ppm in N_2) was diluted with N_2 (Bird Air/O_2 Blender, Palm Springs CA) and added to the high pressure air inlet of the ventilator to produce an inspired [NO] of 20 ppm. A dual-chambered test lung (Michigan Instruments, Grand Rapids, MI) was configured so that the exhaled gas was precisely controlled to 0 ppm NO (N_2), 7 ppm NO in N_2, or 14 ppm NO in N_2. The test lung configuration was similar to that previously used to separately measure inspired and expired gases during calorimeter validation.¹ NO was measured by chemiluminescence (Eco Physics CLD 700AL; calibrated with 80 ppm NO and used with the slow digital filter). Gas was alternately sampled from the inspiratory ventilator circuit, expiratory ventilator circuit, and from a 14 gauge catheter (Becton-Dickinson, Sandy, UT) placed into the model trachea. Because the [NO] measured in the trachea fluctuated during the respiratory cycle, tracheal [NO] was recorded as the average of the highest and the lowest values. The sample rate of the analyzer was 660 mL/min.
Results: The tracheal [NO] was less than the inspired [NO] (20 ppm) for all measurements. The difference between inspired [NO] and tracheal [NO] became less at longer inspiratory times and higher expired [NO]. There was a significant difference between tracheal [NO] for the I:E ratios (P = 0.008) and expiratory [NO] concentrations (Pɘ.001). As shown in the Figure, the tracheal [NO] may be considerably less than the inspired [NO] of 20 ppm. Unlike tracheal [NO], [NO] in the inspiratory limb of the ventilator circuit is not affected by changes in expiratory [NO] or ventilatory pattern.
SEE ORIGINAL GRAPH
Conclusions: To compare the results of studies of inhaled [NO], it is important that the inhaled [NO] is measured in a similar manner. We urge investigators to monitor and report inhaled [NO] from the inspiratory limb of the ventilator circuit, because this is the only true measure of the inhaled dose. (supported in part by the Puritan-Bennett Corporation)
1. Ritz R, Cunningham J. Indirect calorimetry. In: Kacmarek RM, Hess D, Stoller JM. Monitoring in respiratory care. Mosby - Year Book, Chicago, 1993.