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Department of Anesthesiology, Aachen University Hospital, Aachen, Germany
To the Editor:
With great interest we have read the article by Sprung et al. (1), and we would like to comment on some methodological considerations. The authors have stated that with constant PEEP neither oxygenation nor lung mechanics improved by doubling of tidal volume or respiratory rate. Morbidly obese patients exhibited a significant higher alveolar-arterial oxygen difference (A-aDO2) than normal-weight patients, but did not react differently to interventions. The authors attributed this to a probably decreased functional residual capacity in morbidly obese patients and increased shunt fraction.
Even though neither venous admixture nor shunt were determined, we agree that the main mechanism by which A-aDO2 was increased was most likely due to increased shunt fraction, which could have been estimated from a shunt-nomogram (2), because atelectasis formation does occur with induction of anesthesia per se (3). Whether the applied intervention could be effective in opening of atelectatic lung regions can be viewed from a theoretical point of view. The main determinants of recruitment are inspiratory opening pressure and prevention of expiratory collapse by PEEP, which has been elegantly demonstrated by intravital microscopy (4) and CT studies (5). To open up atelectasis the mean airway pressure has to be increased, but by doubling respiratory rate without changing I:E ratio the opposite happens. Unfortunately, no data on this issue are supplied in the paper. On the other hand, an increase in tidal volume was shown to result in tidal recruitment, but if the PEEP will not be increased, cyclic opening and closure of collaptic alveoli will reoccur (4) and was associated with the release of inflammatory mediators (6). Thus both interventions are prone to failure in this setting.
This situation is further aggravated by an increased intra-abdominal pressure during laparoscopy. In healthy subjects, this does not necessarily lead to ventilation-perfusion mismatch (7), but there is a further cephalad displacement of the diaphragm, a decreased chest wall compliance and the need to increase transpulmonary pressure to counterbalance basal atelectasis formation. The authors have impressively shown the decrease in respiratory system compliance, but it would be helpful to know whether lung compliance changed and if it was also unresponsive to body positioning. This could have been done easily by measuring esophageal pressures.
The authors have concluded that they could not observe significant improvements because ventilation with "PEEP 5 cm H2O was near to ideal and further...alveolar recruitment could not be obtained with larger tidal volume." To support this view they cite a study by Horton et al. (8), who had observed a variable decrease in PaO2 with increased PEEP. This was however associated with a fall in cardiac output caused by exerted alveolar pressure of hyperinflated lung and cannot be compared with the effects of pressure on opening atelectasis.
The fact that an elevated A-aDO2 is highly associated with increased venous admixture caused by atelectasis formation clearly leads us to the conclusion, that there is a need for a recruitment strategy in ventilating these patients. However, on the basis of recently published work as cited above this should probably focus more on transpulmonary opening pressures and PEEP rather than on inspiratory volumes without taking into account that tidal recruitment might be lost during expiration below closing pressures.
References
Mayo Clinic University of Maryland
In Response:
We agree with some of the comments made by Henzler et al. regarding our recent article (1). The title of their letter clearly summarizes the future direction of respiratory management for patients with increased A-aO2 gradients. We routinely use a lung recruitment strategy in combination with PEEP to improve intraoperative oxygenation. However, for years, large tidal volumes and/or PEEP have been advocated for improving oxygenation. It was the intent of our study to test that hypothesis (1).
It is very important for anesthesiologists to be familiar with all the different maneuvers for increasing PaO2 and their effectiveness: large tidal volumes, PEEP, and alveolar recruitment maneuvers. The physiologic background of the alveolar recruitment strategy lies in the fact that the initial pressure needed to open collapsed alveoli is quite high (2,3). This alveolar opening pressure may not be achieved with either isolated PEEP or large tidal volumes. In order to prevent the reoccurrence of atelectasis, the recruitment maneuver must be followed by enough PEEP to stabilize the newly opened units (2,3). Rothen et al. (3) showed that peak inspiratory pressures of at least 40 cm H2O were needed to fully reverse anesthesia-induced collapse of healthy lungs in normal-weight patients and these opening pressures are even higher in morbidly obese patients.
We disagree with Henzler et al.’s suggestion that lung compliance could be easily measured by inserting an esophageal balloon and measuring esophageal pressures. Accurate measurement of transpulmonary pressure in supine, anesthetized, morbidly obese patients during pneumoperitoneum and with a nasogastric tube in place may be inaccurate and difficult to interpret (4). Furthermore, Fahey et al. (5) demonstrated that during laparoscopic surgery the changes in respiratory system compliance were almost entirely due to changes in lung compliance and not to changes in chest wall compliance.
Henzler et al. also suggest that the ineffectiveness of PEEP in improving oxygenation may be attributed to a decrease in cardiac output caused by hyperinflation (6). In our study, cardiac output and other hemodynamic parameters were not influenced by either high inflation pressures or pneumoperitoneum (7), therefore, it appears that the low lung compliance in our patients prevented transmission of high inspiratory pressures to intrapleural space. The stable hemodynamics further supports that lung rather than the chest wall compliance was primarily affected (5).
In conclusion, we agree that the lung recruitment strategy with careful monitoring of hemodynamics is an effective method to improve the oxygenation during any type of surgery, not only in the morbidly obese during laparoscopy.
References
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