Perilli, Valter MD; Sollazzi, Liliana MD; Bozza, Patrizia MD; Modesti, Cristina MD; Chierichini, Angelo MD; Tacchino, Roberto Maria MD; Ranieri, Raffaela MD
Amajor problem in anesthesia for morbidly obese patients is the adequacy of pulmonary ventilation (1,2). Anesthesia adversely affects respiratory function, leads to a smaller functional residual capacity (FRC), and promotes airway closure and atelectasis (2–4). In obese patients, FRC markedly decreases with possible hypoxemia in the perioperative period (5–9). Although many studies have been performed to determine the optimal ventilatory settings and posture in these patients, the question has not been resolved (2). In particular, there are few reports that deal with changes in respiratory mechanics and gas exchange in obese patients placed in reverse Trendelenburg position during general anesthesia. Furthermore, Buchwald (10) claimed that the use of a fixed-support retractor system and reverse Trendelenburg position is extremely useful in obese patients undergoing surgery of the upper abdomen.
Therefore, the aim of this study was to evaluate the effects of reverse Trendelenburg posture (RTP) on gas exchange variables and respiratory mechanics in obese patients undergoing abdominal surgery.
After institutional approval, informed consent was obtained. We studied 15 otherwise healthy nonsmoking morbidly obese patients undergoing biliopancreatic diversion.
A standardized anesthetic regimen was used; after 5 min of breathing oxygen, anesthesia was induced with thiopental (3–5 mg/kg) and tracheal intubation was facilitated by succinylcholine (1 mg/kg IV). Anesthesia was maintained with incremental doses of fentanyl (up to 5 μg/kg), isoflurane, and N2O; neuromuscular blockade was induced with atracurium (0.5 mg/kg plus 0.5 mg · kg−1 · h−1).
Patients’ lungs were mechanically ventilated (Servo C; Siemens Elema AB, Berlin Germany) with constant inspiratory flow, aiming at an ETco2 of 30 mm Hg; respiratory rate was 12 ± 2 breaths/min, tidal volume (TV) ranged from 6 to 8 mL per kg of body weight and was kept essentially constant throughout the surgical procedure with the fraction of inspired oxygen (Fio2) of 50%. No positive end-expiratory pressure was used and airway plateau pressure was maintained at a level of ≤35 cm H2O.
In addition to the usual multiparametric monitoring, direct arterial pressure monitoring (radial artery catheter inserted after the induction) was used. The average time of the surgical procedure was 131 ± 36 min, and fluid administration consisted of 2–3 L lactated Ringer’s solution (8–10 mL · kg−1 · h−1).
Respiratory mechanics and blood gases were examined at the following standard times: 1) after tracheal intubation, 2) after laparotomy, 3) after positioning of subcostal retractors, and 4) with retractors in RTP (head up 30°). The time interval between each phase was never <20 min.
In these sets of measurements, end-expiratory and end-inspiratory airway occlusions have been obtained by using end-expiratory and end-inspiratory hold buttons of the Servo C for end-expiratory and end-inspiratory occlusion, respectively. The occlusion at the end of expiration provides measurement of intrinsic positive end-expiratory pressure. Specifically developed software provided online time-related trends of airway pressures during inspiratory and expiratory block. This system was based on a personal computer equipped with a 12-bit analog-to-digital converter (20 samples/s per channel) and connected with a Servo C37 pin analogic plug by using a short shielded cable. The use of this software allows elaboration of data in real time.
The measurement of respiratory mechanics was repeated for a wide range of TVs (6–8 different TVs for each patient) to obtain a volume/pressure curve for each patient in all phases. The different TVs were assessed by changing respiratory frequency on the ventilator, and after each measurement baseline ventilation was resumed. TVs ranged from 150 mL to 1200 mL and respiratory rates ranged from 6 to 30 breaths/min. During these maneuvers, Spo2 never decreased to <91%, and no muscular twitch was elicitable (Microstim Wellcome). No patient showed evidence of barotrauma on radiographs taken after surgery, and there were no pulmonary complications before hospital discharge.
The total respiratory system compliance (Ctot) was conventionally obtained by dividing the deflation volume by the difference between the plateau pressure measured during end-inspiratory airway occlusion (breath holding for 2–4 s) and end-expiratory pressure. In each phase before respiratory mechanics measurements, arterial blood samples were taken to evaluate pulmonary gas exchange. Alveolar-arterial oxygen difference [P(A-a)o2] was obtained from the following equation:MATH where PB is the actual barometric pressure, PH2O is the water vapor tension at 37°C, and RQ the respiratory quotient assumed to be 0.8.
A commercial software package (Statgraphics; SGS, Rockville, MD) was used for analysis of the data. Values were expressed as mean and sd unless otherwise stated; ranges are shown in Table 1. Statistical analysis was performed as analysis of variance followed by post hoc comparisons of means (Tukey) when the F values indicated significant differences among groups. Regression analysis was used to determine relationships (Pearson’s correlation coefficients) between variables. P < 0.05 was accepted as statistically significant.
Anthropometric and spirometric data are in Table 2; respiratory data are summarized in Table 1.
A wide P(A−a)o2 occurred in all patients throughout the operation; the higher values coincided with the application of subcostal retractors. At this time (Phase 3), P(A−a)o2 showed a significant increase and Pao2 decreased. When the patients were placed in RTP, oxygenation improved with a return toward baseline values; P(A−a)o2 was ameliorated in 80% of the patients (12/15). As for respiratory mechanics, with the same ventilatory setting, average Ctot was higher and airway pressures were significantly lower in Phase 4 than in the other phases. Volume-pressure relationships were obtained in all phases for each patient; individual lines of regression and pooled data (with fitted lines) are shown in Figures 1 and 2. Correlations between airway pressures and TVs were found highly significant for both pooled and individual patients (Table 3). The slopes of Phases 1, 2, and 3 are significantly lower than that of Phase 4 (P < 0.01). Gas exchange variables showed a weak correlation with compliance [P(A−a)o2 vs Ctot r = −0.32, P < 0.03], whereas changes of P(A−a)o2 were more closely correlated with variations of Ctot (r = −0.45, P < 0.05). Stepwise regression analysis showed that Fio2 and Ctot accounted for 44% of change in Pao2. Other than diastolic blood pressure, cardiovascular variables did not change significantly (Table 4).
Measurement of respiratory mechanics can be useful for examining patients whose lungs are mechanically ventilated, and some techniques are suitable to evaluate anesthetized patients; we used one of these techniques, i.e., the rapid occlusion during constant flow inflation.
As previously mentioned, the maintenance of an adequate pulmonary ventilation and oxygenation may still be a major problem in anesthetized obese patients, because anesthesia significantly affects respiratory function. The decrease of FRC is one of the main side effects of anesthesia on respiratory function, and this change is particularly marked in morbidly obese patients (2,8,9,11–13). Pelosi et al. (14) demonstrated that the reduction of FRC is closely related to body mass index.
A cranial shift of the diaphragm has been identified as an important factor causing decrease of FRC in obese patients undergoing general anesthesia (8,14); the loss of tone of this muscle may determine the reduction in lung volume because of unopposed intra-abdominal pressure (8,14). However, also atelectasis seems related to a number of interacting factors that include the shape of chest wall structures, volume, and distribution of blood (14). In clinical practice, some morbidly obese patients do not tolerate the supine posture, and it may even be fatal to them (2,7,15).
Large TVs (15–20 mL/ideal body weight) are often recommended for these patients to move tidal ventilation higher than the closing volume and consequently increase arterial oxygen tension (2,5,7,16). This traditional approach to mechanical ventilation intends to aggressively recruit and ventilate atelectatic lung units, but may risk overdistention of the normal lung units. Thus, large TVs may cause a decrease in Paco2, respiratory alkalosis, cardiovascular impairment, and excessive stretch of nondependent lung regions (2,16,17). Furthermore Bardoczky et al. (16) demonstrated that very large TVs do not improve oxygenation in obese patients.
However, even the use of positive end-expiratory pressure to increase FRC and improve oxygenation in obese patients is questionable (2,5). Although the use of positive end-expiratory pressure is of proven value for improving oxygenation in many situations involving respiratory failure, its role in anesthetized patients is controversial. In normal subjects, positive end-expiratory pressure can reduce the atelectasis but not necessarily the shunt (18); however, recently Pelosi et al. (19) claimed that positive end-expiratory pressure can improve oxygenation and respiratory mechanics in obese patients. Nevertheless, a possible detrimental effect of positive end-expiratory pressure on the oxygenation of obese patients has been described by Salem (20).
In this study performed during upper abdomen surgery, we used the RTP to counteract the weight of the abdominal contents and the effects of the retractors on the diaphragm. A wide P(A−a)o2 was observed in all of our patients in every phase, with a further significant increase during Phase 3; this worsening seems related to the application of subcostal retractors, which may cause a further decrease of FRC and a more limited movement of the diaphragm during mechanical ventilation. In Phase 4, with the patients placed in RTP without removing the retractors, oxygenation indexes improved, and P(A−a)o2 returned toward the baseline values.
As for respiratory mechanics, RTP determined a significant increase in the compliance that reached a level higher than baseline; the slopes of volume-pressure relationships in supine postures were less than that in RTP (Table 3). The increased compliance obtained by RTP and steeper volume/pressure regression lines suggest a recruitment of alveolar units and therefore an increase of FRC; thus the operating compliance could be improved by increasing end expiratory volume toward the more compliant range.
A limitation of our study might be the lack of the direct measure of FRC. However, because the reduction in FRC is most likely responsible for the decreased Ctot during general anesthesia, it is reasonable to argue that the increase of Ctot obtained with RTP can be related to an increased FRC (13,18). However, despite this significant increase in compliance, well above baseline values, P(A-a)o2 did not decrease much.
A weak correlation was found between compliance and P(A−a)o2 (r = −0.32;P <0.05; r = −0.45;P <0.05), meaning that other factors play a significant role in determining gas exchange efficiency.
As for hemodynamic factors, it is possible that RTP may compromise cardiovascular function by reducing venous return. Therefore, the beneficial effects of RTP on Pao2 could be offset by a decreased cardiac output. However, obese subjects show a reduced venous compliance and a smaller decrease of the central blood volume and of the cardiac stroke volume during orthostatic stress (21). Moreover, even if extensive hemodynamic monitoring has not been performed, no clinically relevant cardiovascular change was noted in our study.
Another reasonable explanation for this weak relationship between compliance and P(A−a)o2 could be the redistribution of pulmonary blood flow toward less-ventilated dependent zones along a gravitational gradient (22); so reversing the decrease in lung volume with RTP meets with limited success. In this regard, Heneghan et al. (23) showed that there was no improvement in oxygenation when lung volumes were increased significantly by RTP (head up 30°) in normal anesthetized subjects.
However, in our series, other than P(A−a)o2 values significantly wider than those found by Heneghan et al. (23) in normal subjects (P < 0.01), we found a correlation between P(A−a)o2 values of Phase 3 and the improvement in oxygenation obtained with RTP (r = −0.64;P < 0.001) (Figure 3); so the improvement in oxygenation obtained with RTP is greater when P(A−a)o2 is wider. This relationship, as well as the different type of patients, could explain the difference with Heneghan et al’s (23) study.
Moreover, in upper abdomen surgery of obese subjects, the exposure of the operative field creates major problems, and the use of a fixed-support retractor system greatly facilitates the surgeon (10). Like any procedure that increases the subdiaphragmatic pressure, it may make a further decrease in FRC and pulmonary compliance leading to hypoxemia (2). In this regard, the RTP offers potential advantages; it ameliorates the oxygenation indexes, exposes at best the subdiaphragmatic region, and allows mechanical ventilation with safe levels of airway pressures.
In conclusion, our data suggest that RTP is a simple and safe intraoperative posture for obese patients and offers some cardiorespiratory advantages during upper abdominal surgery.
We express our thanks to Dr. A. Ceccarelli for his statistical analysis of original data presented in this article.
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