Choosing Tidal Volume for Ventilation

Abstract

Lee and associates1Lee PC Helsmoortel CM Cohn SM Fink MF Are low tidal volumes safe?.Chest. 1990; 97: 430-434Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar describe a study in which they compared two groups of patients ventilated with either high or low tidal volumes. While the data presented are interesting, there are several unanswered questions which might be clarified by the reporting of additional data. The paper does not specify how the tidal volumes used were determined. This is important as there are often disparities between the set and delivered tidal volumes on adult volume ventilators.2McDonald PM Mechanical ventilation.in: Spearman CB Sheldon RL Egan's fundamentals of respiratory therapy. 4th ed. CV Mosby Company, St. Louis1982: 519Google Scholar The most common cause of this disparity results from tidal volume not delivered to the patient because of compressible volume loss within the ventilator-circuit system. The amount of volume lost in this manner varies between makes of mechanical ventilators and between different types of ventilator tubing circuits.2McDonald PM Mechanical ventilation.in: Spearman CB Sheldon RL Egan's fundamentals of respiratory therapy. 4th ed. CV Mosby Company, St. Louis1982: 519Google Scholar Unless the investigators accounted for compressible volume loss when determining the tidal volumes used in the study, the result could be that at least four groups of patients are being compared. In this paper, the majority of patients were ventilated either with a Bear 1, which does not compensate for compressible volume loss, or with a Puritan-Bennett 7200a, which does.3Puritan-Bennett 7200A operating manualGoogle Scholar Typical ventilator-circuit compliance for the Bear 1 is about 5 ml/cm H2O, though this may vary from 4 to 7 ml/cm H2O with individual types of tubing. In a 70 kg patient, the study design would dictate that the patient receive either 840 ml or 420 ml. Using the mean of the airway pressure changes presented in the paper (29.6 and 20.0 cm H2O for each group), one can calculate an average compressible volume loss of 148 and 100 ml for each group. These losses would result in a delivered tidal volume of 9.9 ml/kg and 4.6 ml/kg if the hypothetical 70 kg patient were ventilated by the Bear 1, compared to 12 ml/kg and 6 ml/kg if the patient were ventilated by the Puritan-Bennett 7200a. As a result, this study would still be comparing high and low tidal volume groups; however, the actual volume disparity would be greater with higher peak airway pressures as in patients with reduced lung/chest compliance or increased airway resistance. To prevent this effect from confusing the study results, data comparing the 4 groups should be presented if compressible volume loss was not taken into account with the original study design. It therefore would be valuable to see data comparing the findings of mean peak pressures and mean PEEP separated by ventilator used and by tidal volume groups to see if the differences between the groups is enlarged or narrowed if ventilator used is controlled. It is not clear how the rate of infection was determined. For example, was incidence of infection during the time of ventilation measured, was incidence for the time period of the study measured, or was rate of infection per unit of time measured? The question is whether the increased number of infections in the high tidal volume group related to the higher tidal volume used, merely a result of the patient's being mechanically ventilated longer thus having greater exposure to nosocomial infection, or perhaps, a result of a longer time in the study for culture results to become positive? A further explanation of how infection rate was determined would be helpful. Finally, it was not stated in the methods section whether the practitioners making the decision to extubate patients were blinded to the tidal volume group in which their patients were members. If they were not blinded then it is, of course, possible that their knowledge of group membership could have influenced their decision as to when to extubate, making the data less reliable. Admittedly, blinding the practitioners to the tidal volumes used would be difficult to accomplish. It might have been better if a strict criteria were used (and published) to determine the time of extubation. Choosing Tidal Volume for VentilationCHESTVol. 98Issue 5PreviewWe appreciate the opportunity to respond to Dr. Elton's letter. Full-Text PDF