On behalf of my co-authors I would like to thank Drs. Yokata and Sari for their correspondence and for sharing our interest in the pathophysiology of one-lung ventilation (OLV) as it applies to intraoperative management of patients during thoracic anesthesia. Their letter highlights three issues that arise out of our study of the interrelation of positive end-expiratory pressure (PEEP) and lung compliance with oxygenation during one-lung anesthesia:
(1) Would it be more useful to use the inflection point derived from the expiratory limb of the pressure-volume (PV) curve of the ventilated lung, rather than the inspiratory lower inflection point (LIP), as a surrogate marker for the functional residual capacity (FRC) and the optimal endexpiratory lung volume? This is a possibility. As they mention, the expiratory inflection point has been demonstrated to be a useful guide for ventilatory management of patients with ARDS in the intensive care unit. Unfortunately, our experimental protocol did not allow for measurements on the expiratory portion of the PV curve. Although there are important differences in the respiratory mechanics between ARDS patients and those having intraoperative one-lung anesthesia, this would be a worthwhile question to study.
(2) The level of auto-PEEP does not correlate with Pao2during one-lung ventilation (OLV). This is correct. A comparison of the PV curves of two patients with low levels of auto-PEEP in figure 2 and figure 3 of our manuscript demonstrates this point. 1The patient in figure 2 had an identifiable LIP, the application of PEEP raised the end-expiratory pressure closer to the LIP level and the patient had an improvement of Pao2with PEEP. The patient in figure 3 did not have a measurable LIP. Presumably this patient and others with similar PV patterns do not get down to the level of their FRC at end-expiration and are not helped by applied PEEP. So it is not merely the presence of auto-PEEP but also the underlying lung mechanics that determine whether a patient will benefit from PEEP during OLV.
(3) Gravity and position may not be an important determinant of blood flow redistribution during OLV. The authors quote the study of Mure et al ., 2which was performed in closed-chest dogs to back up their point. While it is correct that we now appreciate that anatomic factors have a major contribution to the distribution of pulmonary blood flow, it is not clear how relevant this is to clinical OLV in the open-chest human. In fact, a recent report has confirmed the importance of operative position on oxygenation during OLV. A study by Watanabe et al ., 5demonstrated that there was a significant difference in oxygenation during OLV dependent on the patients position, with the mean Pao2in the lateral position exceeding that in the supine position by greater than 100 mmHg. Since the publication of our manuscript another study has been reported which validates our findings. Fujiwara et al . 3applied PEEP to the ventilated lung or continuous positive airway pressure (CPAP) to the nonventilated lung in a series of patients during OLV. They found that PEEP and CPAP were equivalent therapies and that both significantly increased Pao2during OLV. This is very different from the previous findings of Capan et al ., 4who showed that CPAP clearly increased mean Pao2during OLV while PEEP decreased mean Pao2. The difference between these two studies is in the patient populations. Capan studied patients with moderate or severe COPD having thoracotomies for lung cancer surgery. Fujiwara studied patients having OLV for esophageal surgery who are more likely to have normal pulmonary function. This supports our thesis that patients with normal or supra-normal lung elastic recoil (e.g . restrictive lung mechanics: pulmonary fibrosis, obesity) are the patients most likely to reach an end-expiratory lung volume below their FRC, and thus are the most likely to benefit from PEEP to the ventilated lung during OLV.