To the Editor:
We read with great interest the review paper by Walsh et al. published recently in Anesthesiology, which concluded that utilizing low tidal volume settings in non–acute respiratory distress syndrome (ARDS) intensive care unit (ICU) or thoracic surgery patients did not change clinical outcomes. Although the Acute Respiratory Distress Syndrome Network (ARDSNET) trial published in 2000 cemented the use of low tidal volume (6 ml/kg predicted body weight) in ICU ARDS patients, the ensuing two decades saw a multitude of trials in non-ARDS ICU patients and surgical patients with contradictory findings. While many were observational or retrospective trials with their inherent biases, even more recent randomized clinical trials seemed to suffer from the same contradictory results. Are we therefore to conclude that tidal volume size does not matter in non-ARDS patients? Is it possible that this murky answer stems from asking the wrong questions? Rather than continuing down this same path, maybe a shift in research strategy and direction is warranted. Perhaps, something is rotten in the state of our dogma.
Ever since modern electronic positive pressure ventilators appeared in critical care, clinicians have used predicted body weight calculators to determine appropriate tidal volume. The predicted body weight formula utilizes two variables to derive its value: (1) height and (2) sex. Its use ensured that taller patients and men received larger tidal volumes. While population-based studies of anatomy corroborated these trends, astute observers realized the limitations of such formulas at predicting the anatomy of an individual. There is a reason that retail clothing stores have fitting rooms rather than formula-based calculators: without individualized measurements, the probability of a perfect fit is low. Clinicians continue to use predicted body weight formulas to select tidal volume, but their reliance on patient height, determined mainly by femur length, ensures that tidal volumes are matched to the size of the leg, not the lung.
Our group studied the relationship between predicted body weight and total lung capacity in thoracic surgery patients, utilizing both traditional data from pulmonary function tests and novel computed tomography–based image processing. Using the Vitrea software package (V6.4, Vital Images, USA), we generated each patient’s three-dimensional lung image and calculated the precise volume of air contained at end inspiration. Surprisingly, we found that lung volumes in patients with similar predicted body weight varied by as much as 100%, raising serious concerns about relying on predicted body weight formulas alone to select protective tidal volumes in these patients. Such a wide distribution of lung volumes in similarly sized patients may explain the contradictory findings seen in lung protection studies conducted outside of the ARDS patient set. Surgical patients and non-ARDS ICU patients do not share the same baby lung pathophysiology seen in classical ARDS patients, and therefore may not benefit from highly restricted tidal volumes. In fact, thoracic surgery patients have highly variable pulmonary anatomy, compliance, and function depending on their disease process; expecting a simple predicted body weight–based formula to select the best tidal volume for all of them may be the root of the problem. These patients may benefit from personalized tidal volumes derived from their own lung imaging and spirometry data. In our study, we generated a new tidal volume formula based on data analysis: vital capacity (ml)/8. This simple formula was then compared to the standard lung protective formula predicted body weight × 7; in the entire patient cohort, the calculated mean volumes differed by only 4%. However, in a subgroup of patients with low lung compliance, our formula significantly reduced tidal volume in every patient, in some by as much as 60%. This study was exploratory in nature, and large human trials would be warranted before revising clinical care pathways. Nevertheless, it demonstrated that lung-centric variables coupled with new technology can be utilized to better match ventilator settings to individual patients’ pulmonary anatomy and physiology. We humbly recommend that moving forward, researchers should buck the dogma, concentrate on the lungs, and question rote formulas that focus elsewhere and ignore the target organ.
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