Are Low Tidal Volumes Safe? DISCUSSION
The data presented herein support the idea that routine use of low Vt is safe in SICU patients. Furthermore, our results suggest that the incidence of pulmonary infections may be decreased by using low Vt, although this somewhat surprising observation needs to be confirmed in a larger study designed to specifically address this issue.
Questioning the necessity for moderate to large Vt (10 to 15 ml/kg) is somewhat heretical. Why were our results different from those obtained by Bendixen et al? Important differences between our methods and those used in the older study may account for the divergent conclusions. The patients studied by Bendixen et al were paralyzed and fully anesthetized, whereas our patients were awake, and typically, breathing spontaneously with the ventilator running in SIMV mode. Muscle relaxants decrease functional residual capacity and promote microatelectasis. Perhaps the most important difference between the two studies was the use of PEEP In the study by Bendixen et al, patients received zero end-expiratory pressure, whereas our patients routinely received a minimum of 5 cm H20 PEEP. Cheney demonstrated that the use of PEEP with low Vt (5 cm H20) in dogs can result in improvement of oxygenation equal to that of increasing VT.
РаО2/FIo2 ratios were significantly higher among patients in the high Vt arm. Although statistically significant, the clinical significance of this is small. Thus, on average, for FIo2 = 0.4, the mean Pa02 for group 1 (12 ml/kg) was 118 mm Hg vs 104 mm Hg in group 2. A decrement in arterial oxygenation of this magnitude with Pa02 in this range is clinically irrelevant. However, in patients with profound hypoxemia, a 14 mm Hg change in Pa02 would be important and thus, our results cannot be extrapolated to the care of patients with large intrapulmonary shunts and severe arterial desaturation.
We can only speculate as to why the duration of SICU stay, length of intubation, or incidence of infectious complications tended to be lower with the use of lower Vt. Each of these parameters is determined by a multitude of factors. Lower airway pressures associated with low Vt may result in less microscopic barotrauma, resulting in lower overall morbidity. There are obvious inherent difficulties in confirming this hypothesis in a human study.
Our study suffers from several limitations. Our sample sizes were small. In addition, our patient population contained a limited number of patients with severe respiratory insufficiency due to chronic obstructive pulmonary disease, adult respiratory distress syndrome, or pneumonia. It would be reasonable to question whether our findings “favoring” the use of low Vt would persist in a larger study or in sicker respiratory patients. We did not attempt to assess our patients for segmental atelectasis. The frequency of abnormal initial chest roentgenograms in our patient population due to contusion, infection, and pulmonary edema would have made the determination of segmental atelectasis after a period of mechanical ventilation impractical and inaccurate.
We also did not gather static or dynamic compliance data in our study. The numerous variables involved, including varying levels of relaxation and sedation in a given patient from hour to hour, rate of spontaneous breathing, state of hydration, and different ventilator circuits and tubing used, could not be well controlled and would preclude any meaningful conclusions.
In conclusion, the present data indicate that the routine use of low Vt is safe in a selected population of surgical patients that excludes victims of head trauma and patients recovering from neurosurgical or cardiac surgical procedures. The results cannot be extrapolated to a recommendation for employing low Vt in caring for patients with severe lung injury and profound arterial hypoxemia. Nonetheless, the trend toward decreased morbidity associated with the use of low Vt is intriguing and warrants further study.