To the Editor:—
The variable ventilation pattern described by Boker et al. 1was certainly thought provoking. Their finding of improved lung function was ascribed to the modulation in tidal volume (using an approximate threefold magnitude change from smallest to largest) despite a constant minute ventilation. They also asserted that they were not just instituting a “sigh” breath, because previous studies showed that “sighs” did not improve lung function.
It seems to me that they were not giving “sigh” breaths but inadvertently using positive end-expiratory pressure. The variable-ventilation computer they used would change the tidal volume to one of 376 prescribed settings, sequentially breath by breath, but by definition, there would be no change in minute ventilation. To do this, the inspiratory and expiratory times of each breath would have to be adjusted to be minute ventilation neutral.
The largest tidal volume used was 14.6 ml/kg and the smallest was 6.4 ml/kg, but the mean was kept at 10 ml/kg. If we take the example of giving a supramaximal breath (14.6 ml/kg) immediately followed by the smallest breath (6.4 ml/kg), we can see that the respiratory rate would have to change from 6.8 breaths/min to 15.6 breaths/min. The total time per breath would decrease from 8.8 s to 3.84 s. The supramaximal expiratory time (assuming an inspiratory:expiratory ratio of 1:2, as stated in the article) should be 5.8 s. However, because the algorithm must shift to the next breath quickly because of the time restraints of the new respiratory rate (15.6 breaths/min), it may only allow 2.56 s of expiratory time (inspiratory:expiratory ratio of 1:2; breath duration of 3.84 s).
Limiting the expiratory time by over 50%, after giving a breath volume 146% above the mean, does not allow complete emptying of the alveoli before the next breath, especially in those alveoli with slow time constants. This “stacking” will cause an intrinsic increased end-expiratory pressure or auto–positive end-expiratory pressure. If this is the case, the improvement in oxygenation seen in the study would be due to recruitment and less atelectasis secondary to positive end-expiratory pressure, not to the variability in tidal volume.2The example I used is the most extreme variation in consecutive breaths, but similar changes in the shortening of expiratory time would occur any time the respiratory rate of the subsequent breath was higher than the previous. These changes may even result in an inverse inspiratory:expiratory ratio.
The authors do not mention end-expiratory pressures in their results but may be able to shed some light on this possibility.
Carolinas Medical Center, Charlotte, North Carolina. email@example.com